mgr Luis Carnevale da Cunha
(Instytut Fizyki PAN)

MDPD Simulations of liquid thread break-up and formation of droplets

Seminarium stacjonarne: Sala D

The breakup of liquid threads is a fundamental process in nature and relevant for many industrial applications, such as drug manufacturing, inkjet printing, and nanowire fabrication. The classical theory used to describe the breakup of liquid threads has its origins in the works of Rayleigh and Plateau. By means of linear stability analysis, they have shown that a fluid cylinder is un stable for perturbations characterised by a wavelength larger than its circumference. Moreover, there is a characteristic wavelength that the perturbations are the most unstable. In our study, we have employed mesoscale simulations of a coarse-grained model (MDPD) to determine the characteristic wavelength of liquid nanothreads within the statistical uncertainty for a range of fluids, characterised by different Ohnesorge numbers and accessible to our model. Also, we have identified the mechanisms of satellite-droplets formation and characterised their properties. We anticipate that our study contributes to the understanding of a fundamental process in nature and paves the way for further developments in this area for relevant applications.

mgr Russell Kajouri
(Instytut Fizyki PAN)

Durotaxis; an example of spontaneous processes

Seminarium stacjonarne: Sala D

Using extensive molecular dynamics simulation of a coarse-grained model, we demonstrate the possibility of sustained unidirectional motion (durotaxis) of droplets without external energy supply when placed on a polymer brush substrate with a stiffness gradient in a certain direction. The governing key parameters for the specific substrate design studied, which determine the durotaxis efficiency, are found to be the grafting density of the brush and the droplet adhesion to the brush surface, whereas the strength of the stiffness gradient, the viscosity of the droplet, or the length of the polymer chains of the brush have only a minor effect on the process. It is shown that this durotaxis motion is driven by the steady increase of the interfacial energy between droplet and brush as the droplet moves from softer to stiffer parts of the substrate whereby the mean driving force gradually declines with decreasing roughness of the brush surface. We anticipate that our findings indicate further possibilities in the area of nanoscale motion without an external energy supply.

mgr Soheil Arbabi
(Instytut Fizyki PAN)

Molecular dynamics simulation of surfactant-laden droplets

Seminarium stacjonarne: Sala D

Droplet coalescence is commonly encountered in nature and is also relevant for various technologies, such as Microfluidics. In this presentation, I will discuss the details of molecular dynamics simulation and the Coarse-Grained method. Moreover, I will present our results on the coalescence of surfactant-laden water droplets, which have been obtained by means of a coarse-grained (CG) force-field. In particular, I will discuss the details of the coalescence mechanism and the bridge growth dynamics.

mgr Damian Włodzyński
(Instytut Fizyki PAN)

Lee-Low-Pine transformation and its generalization

Seminarium stacjonarne: Sala D

In general, the analysis of few-body systems is a difficult task. Fortunately, some of these systems can be simplified through center-of-mass separation. While the center-of-mass coordinates are uniquely determined, there are multiple ways to define the coordinates for the relative motion. Whichever of them is best suited depends on the analyzed system. In the case of the impurity problem, a beneficial change of the coordinates can be performed using the Lee-Low-Pine (LLP) transformation. Unfortunately, this transformation works only for systems without external trapping potential. In the talk, I will present a modified version of LLP transformation, which can be used for any system with a separable center of mass.

mgr Saeed Samadi
(Instytut Fizyki PAN)

Topological electronic structure of twin boundaries and twinning superlattices in the SnTe material class

Seminarium stacjonarne: Sala D

In this talk I will first review the geometric phase known as Berry phase, and its important role in the band theory of the electronic structure of crystalline materials. I will briefly discuss how the Berry phase, Berry curvature and topological invariants came to be potentially applicable tools to probe topological phases of materials. Next, I would like to give an overview of key topological properties of IV-VI semiconductors and their low dimensional structures such as surfaces, thin films and step defects. In addition, I will present the band topology of (111) planar defect (twin boundary) in SnTe class of material [1].

[1] S. Samadi, R. Rechciński, and R. Buczko, arXiv:2212.14640 (2022)

mgr Filip Gampel
(Instytut Fizyki PAN)

Tracing a quantum particle - an introduction to repeated measurements

Seminarium stacjonarne: Sala D

Ever since the invention of quantum mechanics roughly a century ago, the problem of measurement has received unwavering attention from physicists. In the talk I will review its relation to the foundations of the theory and present some key historical developments. This will lead us to the modern formulation, laying the groundwork in our quest for a model of a continouosly monitored quantum particle.

mgr Quyen Vu Van
(Instytut Fizyki PAN)

The Driving Force for Co-Translational Protein Folding Is Weaker in the Ribosome Vestibule Due to Greater Water Ordering

Seminarium stacjonarne: Sala D

Proteins are synthesized by macromolecular machines called ribosomes, which are found in living cells across all species, from bacteria to humans. Proteins are the workhorses of the cell, performing all manner of tasks needed to support life, but each protein must first self-assemble into a specific structure before it can carry out its function. Some proteins are starting to fold co-translationally (folding during translation) when the biosynthesis process is not completed and it can lead to protein misfolding.

In this talk, we will discuss the effects of the ribosome on a nascent protein (newly synthesized protein), specifically, how the ribosome exit tunnel modulates the co-translational protein folding pathways. The interactions between the ribosome and nascent protein can destabilize folded domains in the ribosome exit tunnel's vestibule, the last 3 nm of the exit tunnel where tertiary folding can occur. Here, we test if a contribution to this destabilization is a weakening of hydrophobic association, the driving force for protein folding. Using all-atom molecular dynamics simulations, we calculate the potential-of-mean force between two methane molecules along the center line of the ribosome exit tunnel and in bulk solution. Associated methanes, we find, are half as stable in the ribosome's vestibule as compared to bulk solution, demonstrating that the hydrophobic effect is weakened by the presence of the ribosome. These findings mean that nascent proteins pass through a ribosome vestibule environment that can destabilize folded structures, which has the potential to influence co-translational protein folding pathways, energetics, and kinetics.

References:
(1) Vu, Q. V; Jiang, Y.; Li, M. S.; O Brien, E. The Driving Force for Co-Translational Protein Folding Is Weaker In the Ribosome Vestibule Due to Greater Water Ordering. Chem. Sci. 2021. https://doi.org/10.1039/d1sc01008e

dr Piotr Szańkowski
(Instytut Fizyki PAN)

Different perspectives on the theory of open quantum systems, cz. II

Seminarium stacjonarne: Sala D

From one point of view, the open system theory is about describing the dynamics of system S induced by its coupling with the environment E. Within this interpretation, the goal of the theory is to derive from "first principles" the dynamical map of S density matrices; achieving computability of the map while maintaining accuracy and minimizing the necessary input of information about E, becomes then the main practical concern.

In an alternative interpretation, the focus is placed on the effects dynamical maps can have on the state of S rather than their physical origins; instead of deriving the map from first principles, the goal is to determine what kind of map, treated as an abstract transformation, produces desired effects. Then, the main practical concern is to define the form of valid transformations that is consistent with the fundamental physical laws.

Historically, the latter interpretation---the utilitarian approach---has dominated the mainstream, while the former approach mostly fell on the wayside. The concepts such as complete positivity, Kraus representations, GKLS equation, etc., are just a few examples of achievements of the utilitarian approach; it is not easy to point out similarly influential developments in the other approach.

Here, we wish to give an overview of key results of open system theory and discuss some of its shortcomings caused by the historical imbalance between the two interpretations.

dr Piotr Szańkowski
(Instytut Fizyki PAN)

Different perspectives on the theory of open quantum systems

Seminarium stacjonarne: Sala D

From one point of view, the open system theory is about describing the dynamics of system S induced by its coupling with the environment E. Within this interpretation, the goal of the theory is to derive from "first principles" the dynamical map of S density matrices; achieving computability of the map while maintaining accuracy and minimizing the necessary input of information about E, becomes then the main practical concern.

In an alternative interpretation, the focus is placed on the effects dynamical maps can have on the state of S rather than their physical origins; instead of deriving the map from first principles, the goal is to determine what kind of map, treated as an abstract transformation, produces desired effects. Then, the main practical concern is to define the form of valid transformations that is consistent with the fundamental physical laws.

Historically, the latter interpretation---the utilitarian approach---has dominated the mainstream, while the former approach mostly fell on the wayside. The concepts such as complete positivity, Kraus representations, GKLS equation, etc., are just a few examples of achievements of the utilitarian approach; it is not easy to point out similarly influential developments in the other approach.

Here, we wish to give an overview of key results of open system theory and discuss some of its shortcomings caused by the historical imbalance between the two interpretations.

Prof. Gediminas Juzeliūnas
(Institute of Theoretical Physics and Astronomy, Vilnius University, Vilnius, Lithuania)

Periodically driven sub-wavelength lattices

Seminarium stacjonarne: Sala D

Traditionally, optical lattices are created by interfering two or more light beams, so that atoms are trapped at minima or maxima of the emerging interference pattern depending on the sign of the atomic polarizability [1]. The characteristic distances over which such lattice potentials change are limited by diffraction and thus cannot be smaller than half of the optical wavelength . The diffraction limitation can be overcome and subwavelength lattices can be created using coherent coupling between atomic internal states [2-8]. In particular, recent experiments demonstrated deeply subwavelength lattices using atoms with N internal states Raman-coupled with lasers of wavelength λ [7]. The resulting unit cell was an N fold smaller compared to the usual λ/2 periodicity of an optical lattice.

In the present talk we will discuss various ways to produce subwavelength lattices and effects of the periodic driving on these lattices. In particular, we will present our recent work on periodic driving of subwavelength lattices leading to formation of pair of coupled subwavelength Rice--Mele chains which operate as a novel topological charge pump [8].

[1] I. Bloch, Nature Physics 1, 23 (2005)
[2] M. Ła cki et al., Phys. Rev. Lett. 117, 233001 (2016)
[3] F. Jendrzejewski et al., Phys. Rev. A 94, 063422 (2016)
[4] Y. Wang et al, Phys. Rev. Lett. 120, 083601 (2018)
[5] E. Gvozdiovas, P. Račkauskas, G. Juzeli nas, SciPost Phys. 11, 100 (2021)
[6] P. Kubala, J. Zakrzewski and M. Łącki, Phys. Rev. A 104, 053312 (2021)
[7] R. P. Anderson et al, Physical Review Research 2, 013149 (2020)
[8] D. Burba, M. Rači nas, I. B. Spielman and G. Juzeli nas, arXiv:2210.05515

Prof. Toan T. Nguyen
(University of Science, Vietnam National University)

Multidisciplinary computational biomolecules: some applications to molecular understanding of drug interaction in Gout and Covid-19 diseases

Seminarium on-line

In this talk, I will give an overview of Multidisciplinary computational biomolecules research at the Key Laboratory for Multiscale Simulation of Complex Systems. By using and integrating an extensive array of bioinformatics, computational quantum and classical modelling and simulation methods, we can have comprehensive understanding of biomolecules, their structure and functions. Particular focus in this talk will be new Covid-19 research being done at the KeyLab. Thermodynamic structural properties of SARS-CoV-2 Spike and Mpro proteins are investigated. These proteins play important roles in the viral interaction and entry to the host cells, replication and transcription. It is found that (i) the SARS-CoV-2 Spike is more structural stable and has higher binding energy to the human ACE2 receptor than SARS-CoV Spike; (ii) SARS-CoV-2 RBD-ACE2 binding interface is more stable, has higher binding area, and has more interactions than in SARS-CoV; (iii) the mutation -PPA469-471/GVEG482-485 in Spike structure has most important and favorable impact for SARS-CoV-2 Spike binding to the ACE2 receptor; (iv) the SARS-CoV-2 Mpro has hot-spots in the binding pocket of both covalent and non-covalent compounds: His41, Cys145, His163, Glu166 and GLN189. The results assist in in-silico screening of phytocompounds found in Vietnam plants for finding potential natural compounds against SARS-CoV-2 virus. Some potential phytocompounds are found and in-vitro and in-vivo testing being conducted by our experimental colleagues. Other active researches are also presented such as those involving Gout diseases treatment, biased and unbiased painkiller targeting mu-opioid, .

Beyond these biomedical applications, we also briefly present some other aspects of research carried out at the KeyLAB in the field of material science, and quantum information.

mgr Tymoteusz Salamon
(ICFO)

Quantum simulators of 2D materials

Seminarium on-line

Quantum materials form one of the most prominent and perspective fields of quantum technologies. They make use of Interparticle correlations fostering new, highly correlated phases of matter, enhanced transport properties, etc. Successful realisation of Magic Angle Twisted Bilayer Graphene (MATBG) has paved a new branch quantum materials - so called flat band systems. In these systems kinetic energy of the carriers is suppressed with respect to interactions allowing interactions to play a major role in system s behaviour. However, new geometry of the system resulting from the twisting of one layer with respect to the other - known as Moiré pattern - effectively disables any attempts to study microscopic properties due to the huge size of the new supercell. In this talk I will present a quantum simulator - a platform based on ultra-cold neutral atoms in optical lattice that mimics some of the fundamental properties of these highly complex 2d materials such as MATBG but without a necessity of real twisting - hence no Moire patterns [2] !

[1] Cao, Y., Fatemi, V., Fang, S. et al. Unconventional superconductivity in magic-angle graphene superlattices. Nature 556, 43 50 (2018)
[2] T. Salamon, A. Celi, R. W. Chhajlany, et. al. Simulating Twistronics without a Twist. Phys. Rev. Lett. 125, 030504

mgr Damian Włodzyński
(Instytut Fizyki PAN)

New approach to a small Fermi-polaron system in a harmonic trap

Seminarium on-line

Recent experimental realization of an ultracold mixture with a single impurity immersed in several fermions in a one-dimensional harmonic trap motivated theoretical studies of this problem. Unfortunately, for strong inter-component interactions, typical numerical methods perform poorly in simulating this system. Therefore analyses are usually limited to very small mixtures. In my seminar, I will present an alternative numerically exact approach to this problem. The method is especially effective in case of heavy impurity.

mgr Tanausu Hernández Yanes
(Instytut Fizyki PAN)

One- and two-axis squeezing via laser coupling in an atomic Fermi-Hubbard model

Seminarium on-line

We study the Ramsey-type spectroscopy and the production of spin-squeezed states with ultra-cold atomic fermions described by the Fermi-Hubbard model in the Mott insulating regime. We show activation of two twisting mechanisms by a position-dependent laser coupling between internal degrees of freedom of atoms. A single laser coupling simulates the one-axis twisting model with the axis and direction of squeezing determined by the value of phase defining the atom-laser coupling. As such, adding a second laser beam with an adequately chosen phase paves the way to simulate the two-axis counter-twisting model, allowing reaching the Heisenberg-limited level of squeezing. The scheme can be implemented readily in state-of-the-art optical lattice clocks.

Link to arxiv: https://arxiv.org/abs/2204.06065

dr Marcin M. Wysokiński
(Instytut Fizyki PAN)

Phases and phase transitions in correlated fermion systems at and out of equilibrium

Seminarium on-line

In my seminar I will present my scientific achievement being the basis for commencing the procedure for the conferment of the post-doctoral degree of doctor habilitated. I will discuss main results obtained in my works [1-7] offering theoretical insight into the nature of selected phases (also topological) and phase transitions (also dynamic) observed experimentally or realized by simplified many-body models in systems of correlated fermions in and out of equilibrium. Specifically, I will discuss phenomena related to itinerant magnetism [1-3], Mott [5-7] and Kondo [4] physics. The special emphasis will be put on the adjustment or development of the proper manybody technique allowing to tackle a given problem in an efficient manner.

[1] M. M. Wysokiński, Sci. Rep. 9, 19461 (2019)
[2] G. Cuono, C. Autieri, M.M. Wysokiński, Phys. Rev. B 104, 024428 (2021)
[3] M.M. Wysokiński, Phys. Rev. B: R. Comm. 97, 041107 (2018)
[4] M.M. Wysokiński, M. Fabrizio, Phys. Rev. B: R. Comm. 94, 121102 (2016)
[5] M.M. Wysokiński, M. Fabrizio, Phys. Rev. B: R. Comm. 95, 161106 (2017)
[6] M.M. Wysokiński, M. Fabrizio, Phys. Rev. B: R. Comm. 96, 201115 (2017)
[7] M. Płodzień, M.M. Wysokiński, Phys. Rev. B: R. Comm. 100, 041116 (2019)

dr hab. Adam Sieradzan, prof. UG
(Uniwersytet Gdański)

Do we need physics in the biomolecular force fields?

Seminarium on-line

With the progress of the protein structure prediction techniques and AI methods, many scientists doubt if there is a need for further development of physics-based force fields. Is it possible to substitute the "universal" force fields with structure-based ones? Can we base force fields on statistical potentials instead of deriving them in a rigorous manner? What type of approximations should be used, and which are not justified? In this presentation I will try to walk through several types of force fields and explain when approximations are justified and when they lead to oversimplification of the phenomenon.

prof. dr hab. Dariusz Chruściński
(Uniwersytet Mikołaja Kopernika w Toruniu)

Universal Constraint for Relaxation Rates for Quantum Dynamical Semigroup

Seminarium on-line

A general property of relaxation rates in open quantum systems is discussed. We derive a constraint for relaxation rates that universally holds in fairly large classes of quantum dynamics, e.g., weak coupling regimes, as well as for entropy nondecreasing evolutions. It is conjectured that this constraint is universal, i.e., it is valid for all quantum dynamical semigroups. The conjecture is supported by numerical analysis. This universality marks an essential step toward the physical characterization of complete positivity as the constraint is directly verifiable in experiments. It provides, therefore, a physical manifestation of complete positivity. Some implications of the constraint are discussed as well.

dr hab. Tomasz Sowiński
(Instytut Fizyki PAN)

Few fermions in a harmonic trap: collectivity vs. temperature

Seminarium on-line

Recently, it has been revealed that small quantum systems of attractively interacting fermions can host inter-particle correlations that are very similar to those known from bulk systems - the conventional Cooper pairs for spin-balanced systems and the more exotic Fulde-Ferrel-Larkin-Ovchinnikov pairs with non-zero net momentum when the opposite-spin particle numbers are not equal. However, until now, the question of the impact of the environment on the detectability of these correlations has not been sufficiently explored. In my talk, using the most trivial way of incorporating the thermal environment, I will attempt to formulate the first insights in this direction and try to open the way to establish another theoretical link between few-body problems and the fine-temperature results obtained for a large number of particles.

Prof. Dr. Philipp Hauke
(University of Trento)

Quantum simulation of lattice gauge theories: probing many-body phenomena and gauge-symmetry protection in laboratory devices

Seminarium on-line

We are currently witnessing a tantalizing progress in the control over quantum devices, handing us novel microscopic approaches to complex many-particle problems. In this talk, I will highlight how this leads to rapid progress in the quantum simulation of lattice gauge theories. First, I will discuss the quantum simulations of a quantum phase transition in a U(1) gauge theory in an optical lattice [1]. Second, I will present very recent works performed on the google Sycamore chips within the Google Early Access Program, where we tested confinement and gauge protection in a Z2 gauge theory [2]. I will also illustrate salient challenges such as protecting desired symmetries and increasing error resilience. With these examples, I aim at giving a glimpse on the current exciting progress, the fascinating future potential, and the outstanding challenges of gauge quantum simulation.

[1] Thermalization dynamics of a gauge theory on a quantum simulator, Z.Y. Zhou, G.X. Su, J.C. Halimeh, R. Ott,
H. Sun, P. Hauke, B. Yang, Z.S. Yuan, J. Berges, J.-W. Pan, arXiv:2107.13563 (2021)
[2] Probing confinement in a Z2 lattice gauge theory on a quantum computer, J. Mildenberger, W. Mruczkiewicz,
J. C. Halimeh, Z. Jiang, P. Hauke, arXiv:2203.08905 (2022)

prof. dr hab. Magdalena Załuska-Kotur
(Instytut Fizyki PAN)

Modelling of nanowires with special shapes and properties

Seminarium on-line

Among the various surface formations that arise as a result of surface dynamics processes, nanowires are one of the most interesting due to their specific shapes and the expected application possibilities. From a theoretical point of view, they are challenging because they are the result of many different processes taking place on the surface. Flag-shaped nanowires nucleate from craters on a (001) InAs surface and first grow with a pure wurtzite structure, then, by experiencing low temperature and high supersaturation they are forced to kink and change to a zinc-blende tapered rectangular nanoplate, which gradually converges into a very thin square tip. Recent theoretical predictions suggest that such nanoflag structures should be very well suited for the search and manipulation of Majorana zero modes. Indeed, the tapered global structure of the nanoflag InAs nanowires and their microscopic superstructure may provide two complementary methods for engineering the Kramer s degeneracy within the vicinity of the chemical potential. We have shown how Monte Carlo simulations and Molecular Dynamics can be used in the analysis of the subsequent steps of the formation of flag-shaped nanowires. At first the crater formation on the substrate surface was modelled. Then, the Monte Carlo approach was again used to explain the shape and structure of the kinked InAs nanowires. Finally, the hypothesis that the patterns observed at the InAs (110) surface of the nanoflags are related to As adatoms, was proven by a minimization procedure within the Lammps Molecular Dynamics Simulator.

dr Mateusz Denys
(Instytut Fizyki PAN)

How surfactants affect cloud droplet activation?

Seminarium on-line

Atmospheric aerosols usually consist of inorganic salts and organic substances, where a significant part is surfactants, that is, amphiphilic molecules containing a hydrophilic and a hydrophobic part. Due to their amphiphilic character, surfactants preferentially adsorb at the surface of liquids lowering the surface tension of the interfaces, thus affecting processes, such as droplet coalescence, development of precipitation and (ultimately) cloud lifetime.

We created a numerical model of cloud droplet formation with a presence of surfactant and salt. The model is based on the Lagrangian particle-based microphysics scheme Super-Droplet Method (SDM) [1]. By means of the SDM, we provided an evidence that surfactants influence cloud formation.

prof. dr hab. Marcin Mierzejewski
(Politechnika Wrocławska)

Thermalization of macroscopic quantum systems. Is it unavoidable?

Seminarium on-line

Dynamics of quantum systems are typically a few orders of magnitude faster than the time-scales which are relevant for our everyday experience. In particular, the relaxation times in solids with strong electronic correlations may be of the order of a few tens of femtoseconds. At longer time, quantum systems typically approach their thermal equilibrium.

Thermalization may be avoided only in integrable models and, possibly, also in strongly disordered systems which are many-body localized. However, recent studies of disordered spin chains have experienced a renewed interest, inspired by the question to which extent the exact numerical calculations comply with the existence of a many-body localization in macroscopic systems. During the talk I will discuss the basic concepts concerning thermalization in quantum systems and discuss transport in disordered chains with many-body interactions. In the case of the latter systems I will focus on the problem whether the thermalization may be completely eliminated or only slowed down.

dr hab. Maciej Lisicki
(Wydział Fizyki, UW)

Elastohydrodynamics of microscale swimming

Seminarium on-line

Swimming microorganisms and engineered artificial swimmers use multiple strategies to achieve propulsion in the viscosity-dominated microworld. A number of them use long, filamentous appendages called cilia or flagella. The motion of these slender objects is governed by a complex interplay between the driving forces, the elastic properties of the fibres, and the resistance forces of fluid. In my talk, I will describe the basic ideas behind microscale swimming and highlight the role of elastic flagella in swimming. I will show examples of both natural swimmers and artificial systems which can be described using elastohydrodynamics.
A recently studied system involving an emulsion of microscopic droplets of oil in water exhibits swimming induced by an extrusion of elastic fibres by the droplets [1]. The extrusion is controlled by a surface phase transition of the surfactant, and it drives the motion of droplets. Extruded fibres undergo dynamic buckling and produce complex shapes, which we describe by a combination of theoretical modelling and numerical simulations, which serve as the basis for interpretation of experimental data.

[1] D. Cholakova, M. Lisicki, S.K. Smoukov, S. Tcholakova, E. Lin, J. Chen, G. De Canio, E. Lauga, N. Denkov, Rechargeable self-assembled droplet microswimmers driven by surface phase transitions, Nature Physics (2021)

dr hab. Tomasz Karpiuk
(Uniwersytet w Białymstoku)

Disruption of a Bose-Fermi droplet by an artificial black hole

Seminarium on-line

We study the final stages of the evolution of a binary system consisting of a black hole and a white dwarf star. We implement the quantum hydrodynamic equations and carry out numerical simulations. As a model of a white dwarf star we consider a zero temperature droplet of attractively interacting degenerate atomic bosons and spin-polarized atomic fermions. Such mixtures are investigated experimentally nowadays.

prof. André Eckardt
(Technische Universität Berlin)

Prethermal memory loss as a dynamical quantum phase transition in the relaxation dynamics of open quantum systems

Seminarium on-line

We study the relaxation dynamics of tilted (Wannier-Stark-type) quantum chains coupled to thermal environments. We find that for a large set of pure initial states, the entropy evolves in a non-monotonous fashion. Starting from zero, it first approaches its maximal possible value, before decreasing to its equilibrium value defined by the bath temperature. Since the maximum entropy state is unique, reaching the peak implies that the system has lost memory of its precise initial state, long before reaching thermal equilibrium. This behaviour is confirmed by the observation that expectation values computed for different initial conditions converge at the peak time to subsequently follow the same dynamics. Moreover, by applying finite-size scaling, we argue that this prethermal memory loss corresponds to a phase transition with respect to time. The effect is confirmed experimentally in Artur Widera s group in Kaiserslautern for a large spin in contact with a bath.

References:
[1] "Prethermal memory loss and universal non-equilibrium dynamics in interacting quantum systems coupled to thermal baths", Ling-Na Wu and André Eckardt, Phys. Rev. B, 101, 220302(R) (2020)
[2] "Dynamical phase transition in an open quantum system", in preparation, Ling-Na Wu, Alexander Schnell and André Eckardt in collaboration with Artur Widera's group.

dr Krzysztof Jachymski
(Faculty of Physics, University of Warsaw)

Complex collisions and strong correlations in the ultracold domain

Seminarium on-line

Atomic gases cooled to quantum degeneracy offer exciting opportunities for quantum simulations and precision measurements. Since the first realization of the Bose-Einstein condensate in a weakly interacting gas, many new systems emerged with different types of interactions. In particular, creation of hybrid ion-atom systems enabled the studies of impurity physics in the strongly interacting regime. In this talk, I will demonstrate how long-range interactions can introduce new beyond mean-field effects, focusing on mixtures of cold ions and atoms as the main example. Then I will discuss a recent breakthrough experiment demonstrating control over the ion-atom interaction in Ba⁺ - Li system by means of Feshbach resonances and present the theoretical treatment of collisions in the presence of strong short-range interactions mixing the different channels.

dr Tomasz Wasak
(Max Planck Institute for the Physics of Complex Systems)

Fermi polarons in non-equilibrium systems

Seminarium on-line

A mobile impurity immersed into a degenerate Fermi system is a paradigmatic problem in many-body physics. The dressing of the impurity by particle-hole excitations of the environment leads to formation of a new quasi-particle called the polaron. Recent observations of polarons in experiments with ultracold atomic gases and with two-dimensional monolayer semiconductors renewed interest in the field. The description of polaron physics in these systems was based on equilibrium quantum field theory or wave function techniques. However, in semiconductors, due to finite lifetime of excitons, the system is non-equilibrium in nature, and a proper treatment is required that includes gain and loss into the theoretical description on equal footing with coherent processes of quasi-particle formation.

In this talk I will describe our approach to non-equilibrium polaron physics. Based on non-equilibrium quantum field theory, we derived a kinetic equation that consistently includes dissipation and drive, and it allows for studying the transitions between polaron branches and their non-equilibrium distributions. In particular, in the context of two-dimensional materials, we found that due to the bosonic nature of polarons, around the point where the decay into the attractive polaron overcomes the radiative and non-radiative loss, stimulated processes lead to a transition towards a lasing regime. Many-body polaronic effects reduce the emission linewidth below the bare exciton linewidth set by nonradiative loss, as the polaronic excitation is partially stored in the environment.

dr hab. Piotr Setny
(CeNT UW)

Structural determinants of influenza virus fusion peptides activity in lipid membranes

Seminarium on-line

The entrance of influenza virus particle into a host cell requires fusion of lipid membranes. This proces is mediated by viral protein hemagglutinin (HA). It has long been known that isolated terminal HA fragments consisting of merely 20 amino acids, called fusion peptides, are capable of inducing membrane fusion on their own, without the support of the entire protein. Despite many years of research, however, the details of peptides-lipid interactions as well as mechanistic understanding of their activity remain elusive.

Using the combination of extensive molecular dynamics simulations with experimental methods we investigate configurational free energy landscape of peptides in membrane environment, lipid order perturbation, and fusion efficacy. We suggest that peptides activity may be directly related to their ability to penetrate into&nsbp;the membrane core. We also demonstrate why this ability is lost as an effect of a known, function-abolishing point mutation. Our findings provide physical interpretation for structure-activity relationships for variants of influenza fusion peptides, thus contributing to mechanistic understanding of the fusion mechanism.

dr Sanjib Ghosh
(Beijing Academy of Quantum Information Sciences)

Quantum reservoir computing

Seminarium on-line

This talk will present quantum reservoir computing which is a quantum neural network. Quantum neural networks combine ideas from quantum mechanics and artificial neural networks. Among various forms of neural networks, reservoir computers, which are types of recurrent neural networks, can operate with random network connections. This is why reservoir computers are suitable for physical implementation in a wide range of systems. I will present that reservoir computing can be brought into quantum domain, where a random quantum network can be used for various quantum tasks, such as, quantum state recognition, quantum estimation, quantum state tomography and state preparation. Finally, I will present a scheme for realising universal quantum gates based on a network of nonlinear quantum modes. Potential physical systems for implementation will also be discussed.

dr Marek Tylutki
(Politechnika Warszawska)

One-Dimensional Quantum Droplets

Seminarium on-line

I will discuss a one-dimensional self-bound quantum droplet in a two-component BEC mixture described by the Gross-Pitaevskii equation (GPE) with cubic and quadratic nonlinearities. The cubic term originates from the mean-field energy of the mixture, whereas the quadratic nonlinearity corresponds to the attractive beyond-mean-field contribution. The droplet properties are governed by a single control parameter that depends on the particle number. For large values of this parameter the solution features the flat-top, droplet-like shape with the discrete part of its spectrum consisting of plane-wave Bogoliubov phonons propagating through the flat-density bulk. With decreasing control parameter these modes move to the continuum, sequentially crossing the particle-emission threshold. A notable exception is the breathing mode which is found to be always bound. As the control parameter tends to minus infinity, this ratio tends to one and the droplet transforms into the soliton solution of the integrable cubic GPE. For reference, see: Phys. Rev. A 101, 051601(R) (2020).

dr Mateusz Łącki
(Uniwersytet Jagielloński)

Dark state potentials and bands for ultracold atoms

Seminarium on-line

A proposal [Phys. Rev. Lett. 117, 233001 (2016)] to use a three-level Lambda system to create optical potentials for ultracold atoms has led to effective lattice with subwavelength potential peaks. This potential is for atoms that occupy the dark state manifold. Recently, the proposal was extended by using bands of Hofstadter-Harper modes in Lambda system configuration. When coupled properly the three-band system hosts a dark state band, which is flat and has the Chern number, |C| > 1. Another extension of the original work was achieved by considering a four level atomic setup in the tripod configuration with two-dimensional dark-space with 1D motion and the presence of external gauge fields and an interesting tight-binding model with long range hopping and interaction. In the talk I will present the two above extensions of the original Lambda system.

dr Piotr Szańkowski
(Instytut Fizyki PAN)

Surrogate field representations

Seminarium on-line

In classical mechanics, a natural way to simplify a many-body problem is to "replace" some of the elements of the composite system with a surrogate force fields. In the realm of quantum mechanics, however, such a description is rarely compatible with the formalism of the theory. Nevertheless, the quantum version of external field models - the surrogate field representations - can be employed in certain circumstances. Here, I present the explanation of the mechanism behind working surrogate field representation and I formulate the sufficient conditions for the representation to be valid.

dr Panagiotis Theodorakis
(Instytut Fizyki PAN)

Nanodroplets and Nanobubbles on Solid Substrates: Molecular Dynamics Simulation of Interfacial Phenomena

Seminarium on-line

mgr Filip Gampel
(Instytut Fizyki PAN)

On multipoint temporal and spatial correlations in several atoms systems

Seminarium on-line

We first study the influence of frequent observation on the temporal evolution of a single or several quantum particles. To this end we introduce a model of detectors on a grid measuring position and momentum. Using the Monte Carlo wave function (MCWF) method allows us to develop a framework to predict single possible trajectories of the particle(s). High-order spatial correlations are at the heart of Pauli crystals. These geometric structures emerge in systems of ultracold non-interacting fermions. Measuring all particles at once, their positions tend to be close to vertices of non-trivial polyhedrons. We investigate this for various geometries.

mgr Jan Krzywda
(Instytut Fizyki PAN)

Shuttling of electron in Silicon nanostructure as an adiabatic transition in the open quantum system

Seminarium on-line

As predicted by the Landau-Zener model, an occupation of ground state of a closed quantum system, driven though avoided crossing, remains constant provided external drive is sufficiently slow. We show the opposite is true in&bsp;the presence of coupling between the system and its environment, since longer transition time allows for energy transfer between them. As an example, relevant for scaling of future quantum computers, we consider electron shuttling in realistic Silicon nanostructure in presence of charge noise and electron-phonon coupling.

dr Paweł Krupa
(Instytut Fizyki PAN)

Use of force fields in studies of biomacromolecules

Seminarium on-line

Molecular dynamics (MD) in all-atom and coarse-grained force fields is the most popular computational method used to study biomacromolecules: proteins, peptides, nucleic acids, sugars, and lipids. Although results obtained by MD have to be validated by comparison with available data, especially experimental, MD provides valuable information when other methods cannot be effectively used. One example is computational study of intrinsically disordered peptides and proteins (IDPs), such as monomeric amyloid beta, for which experimental methods cannot be effectively used mostly due to transient nature of the peptide, while computational simulations are invaluable source of information about structure and dynamics. Presentation will focus on showing possible applications and limitations of all-atom and coarse-grained force fields in studies of various biomacromolecules and their interactions with other compounds based on recent works [1-8].

References:

1. Krupa P, Quoc Huy PD, Li MS (2019) Properties of monomeric Aβ42 probed by different sampling methods and force fields: Role of energy components. J Chem Phys 151:055101.
2. Nguyen HL, Krupa P, Hai NM, et al (2019) Structure and Physicochemical Properties of the Aβ42 Tetramer: Multiscale Molecular Dynamics Simulations. J Phys Chem B 123:7253-7269.
3. Huy Pham DQ, Krupa P, Nguyen HL, et al (2020) Computational Model to Unravel the Function of Amyloid-β Peptides in Contact with a Phospholipid Membrane. J Phys Chem B 124:3300-3314.
4. He Y, Mozolewska MA, Krupa P, et al (2013) Lessons from application of the UNRES force field to predictions of structures of CASP10 targets. Proc Natl Acad Sci U S A.
5. Krupa P, Mozolewska MA, Wiśniewska M, et al (2016) Performance of protein-structure predictions with the physics-based UNRES force field in CASP11. Bioinformatics 32:3270-3278.
6. Krupa P, Karczyśska AS, Mozolewska MA, et al (2020) UNRES-Dock - protein-protein and peptide-protein docking by coarse-grained replica-exchange MD simulations. Bioinformatics.
7. Ahmed L, Rasulev B, Kar S, et al (2017) Inhibitors or toxins? Large library target-specific screening of fullerene-based nanoparticles for drug design purpose. Nanoscale 9:10263-10276.
8. Maisuradze G, Medina J, Kachlishvili K, et al (2015) Preventing fibril formation of a protein by selective mutation. Proc Natl Acad Sci 112:13549-13554.

dr hab. Piotr Deuar
(Instytut Fizyki PAN)

Simulating the complete quantum mechanics of very large driven-dissipative Bose-Hubbard systems

Seminarium on-line

Phase-space descriptions of open quantum systems allow for linear or log-linear scaling of computational difficulty with system size, and adaptability to non-uniform and time-dependent systems. They also allow for straightforward calculation of many of the multi-time correlations of interest by replacing Heisenberg operators with the bare time-dependent stochastic variables. Examples are simulations using the truncated Wigner and positive-P distributions of quantum optical and ultracold atom systems that can produce complete or close to complete quantum dynamics of systems with millions of interacting particles and modes under the right conditions.
I will discuss this framework in general terms, and then describe recent work in which we demonstrated that the positive-P method captures the complete many-body quantum dynamics of the driven-dissipative Bose-Hubbard model across a wide range of parameters. Notably, these parameters include intermediate regimes where interactions and dissipation are comparable, and especially cases with low occupations for which common semiclassical approximations can break down. We demonstrate its use in several examples with non-trivial quantum correlations, including a nonuniform 2d system with tens of thousands of sites, larger than contemporary experimental setups. It is also found that the positive-P's region of applicability is complementary to that of the truncated Wigner method, and together both methods allow one to cover the majority of parameter space of dissipative Bose-Hubbard models.

References:
P. Deuar, A. Ferrier, M. Matuszewski, G. Orso, M. H. Szymańska, Fully Quantum Scalable Description of Driven-Dissipative Lattice Models, PRX Quantum 2, 010319 (2021)
P. Deuar, Multi-time correlations in the positive-P, Q, and doubled phase-space representations, Quantum 5, 455 (2021)

dr Giorgos Kritikos
(Instytut Fizyki PAN)

Bound Polymer Layer Dynamics and the Peculiar Arrhenius-like Behavior

Seminarium on-line

Macromolecules depict the common dynamical behavior of all glassy materials. At high enough temperatures (liquid state) the dynamics is described by the trend of constant activation energy and the Arrhenius equation. As the temperature drops the polymer enters a region of cooperative diffusion, where the Arrhenius equation is inadequate and the description of the relaxation times (α mechanism) follows the Vogel-Fulcher-Tammann (VFT) equation [1]. At a certain temperature, known as the glass transition temperature (T_g), the polymeric material undergoes a heat capacity step, while below T_g, an aging (enthalpy relaxation) process takes place. At the sub-Tg region the dynamics (β mechanism) follow again the constant activation energy trend. Especially, when a bound polymer layer is present (e.g. polymer nanocomposites), above T_g, the dynamics follows an Arrhenius-like temperature dependence (α' mechanism).

Recently, a unified description of this behavior was attempted by us [2]. According to the new model, the cooperative dynamics can be explained by a bimodal (free volume rearrangement) process, with one component being Arrhenius. The idea was formulated in a generalized Arrhenius equation and has been tested successfully in various glassy materials [2]. The Solid to Liquid Glass (SLG) transition was characterized by a sigmoidal shape in the cohesion energy and the extension of this transition described adequately the structural and dynamic behavior. The α', Arrhenius-like dynamics of the bound layer has been explained by the extension of the super-Arrhenius region. Furthermore, by incorporating this temperature dependence of the cohesion energy into the Sanchez-Lacombe theory a new SLG Equation of State (EoS) was derived, that described accurately the change in the slope of specific volume, at T_g, over various polymers [3]. The new mean-field idea allowed for an improvement of the numerical Self Consistent Field (nSCF) method for polymers and provided new tools (analytical equations) for the extraction of information concerning thermodynamic, mechanical and dynamic properties, in all-atom simulations on macromolecular systems [4]. The EoS-based model will be used for the derivation of Coarse-Grained (CG) force fields, for the performance of simulations at the mesoscale level, in Soft Matter systems.

References:
1. Vogel, H. The temperature Dependence Law of the Viscosity of Fluids. Phys. Zeit. 22, 645-646 (1921).
2. Kritikos, G. Exploring a unified description of the super-Arrhenius region above and below the glass transition temperature. Soft Matter 16, 6902-6913 (2020).
3. Kritikos, G. Mean field description of the structural heterogeneities in the region of cooperative diffusion. Comput. Mater. Sci. 179, 109682 (2020).
4. Kritikos, G. & Karatasos, K. K. Effect of Nanofiller's Size on the Mechanical Properties of Poly(acrylic acid)/Graphene Oxide Nanocomposites. Macromolecules (2021).

dr Daniel Pęcak
(Wydział Fizyki, Politechnika Warszawska)

Quantum vortex in neutron matter

Seminarium on-line

Neutron stars are remains of stars that reveal one of the most extreme conditions in the universe: the rotation of an entire star typically takes less than a second. Moreover, it is known since the 50s that superfluidity plays a vital role in the description of those objects. Therefore, quantum vortices are building blocks essential for neutron star modeling and understanding neutron star physics.

In this talk, I will present recent results on the structure and properties of a vortex in neutron matter. We use state-of-the-art numerical techniques based on density functional theory that employs a functional specifically designed to tackle astrophysical problems. We systematically analyze how the length scales change in various layers of the neutron star's crust as a function of temperature. Moreover, I will discuss the differences and similarities between vortices in Bose and Fermi systems. Finally, I will present how these differences translate into an additional contribution to the heat capacity.

mgr Damian Włodzyński
(Instytut Fizyki PAN)

Structural transition in a mass-imbalance few-fermion mixture

Seminarium on-line

In a mass-imbalance mixture of a few ultracold fermionic atoms with strong repulsive interactions, a spatial arrangement of the components depends on the shape of the external confinement. When the mixture is initially prepared in a one-dimensional box trap and then the harmonic potential is slowly turned on, the system undergoes a structural transition. In my talk, I will show the results of our analysis of this transition, in both adiabatic and non-adiabatic cases, with a particular emphasis on inter-component correlations.

dr Krzysztof Ptaszyński
(Institute of Molecular Physics, Polish Academy of Sciences, Poznań, Poland)

Nature of entropy production in open systems

Seminarium on-line

Starting from the 19th century much effort has been made to explain the emergence of thermodynamic irreversibility from the reversible laws of classical or quantum mechanics. Addressing this issue, Ref. [1] provided a microscopic derivation of the second law of thermodynamics applicable to open quantum system attached to a thermal environment. It has been demonstrated that the entropy production - a standard quantity characterizing the irreversibility of the thermodynamic process - is equal to the sum of two microscopic contributions described by means of the quantum information theory. The first of these contributions is related to the generation of correlations between the system and the environment, while the second one describes the displacement of the environment from equilibrium. In our work [2] we show that the latter term, rather than the former one, is predominant for systems strongly driven out of equilibrium (contrary to some previous claims). We further demonstrate that in the thermodynamic limit this predominant contribution to the entropy production is related to the generation of correlations between the microscopic degrees of freedom in the environment rather than the displacement of their individual states from equilibrium. This clarifies the microscopic origin of the second law of thermodynamics.

[1] M. Esposito, K. Lindenberg, C. Van den Broeck, New J. Phys. 12, 013013 (2010)
[2] K. Ptaszyński, M. Esposito, Phys. Rev. Lett. 123, 200603 (2019)

dr Jacek Herbryc
(Wrocław University of Science and Technology)

Magnetism of the orbital-selective Mott phase

Seminarium on-line

Iron-based superconductors display various phases originating in the competition between electronic, orbital, and spin degrees of freedom. Prominent among these novel effects is the orbital-selective Mott phase (OSMP), where interactions acting on a multiorbital Fermi surface cause the selective localization of electrons on one of the orbitals. As a consequence, the system is in a mixed state with coexisting metallic and Mott-insulating bands. In my talk, I will show that the magnetic ordering associated with the OSMP is significantly different from that observed in cuprates. The competing energy scales present in the low-dimensional OSMP induce an exotic spin arrangement without any apparent frustration in the system. I will also discuss the electronic properties of the OSMP, i.e., the emergence of the new quasiparticles and topological properties associated with them.

mgr Saeed Samadi
(Instytut Fizyki PAN)

One-dimensional Dirac modes in the core of a pentagonal topological crystalline insulator nanowire

Seminarium on-line

We study topological properties of IV-VI semiconductor nanostructures: thin films, heterostructures and nanowires (NWs). During the talk I will present their various topological properties. Mainly, however, I will discuss properties of the wires with pentagonal cross-section. They grow along [011] direction and host five, radially diverging, twin boundaries which can be either cationic or anionic. We use tight binding method and perform band structure calculations for various thicknesses. In cationic case, there exist topological states at the core with their counterparts in the entire surface. We investigate the properties of these states, particularly how their localization depends on the wire thickness and bulk gap energy. Moreover, we discuss the issue of topological protection against symmetry breaking.

mgr Artur Niezgoda
(Warsaw University)

Many-body nonlocality as a resource for quantum-enhanced metrology

Seminarium on-line

In this talk we will demonstrate that the many-body nonlocality is a resource for ultra-precise metrology. We formulate a general scheme which allows one to track how the sensitivity growths with the nonlocality extending over an increasing number of particles.
We illustrate our findings with some prominent examples - a collection of spins forming an Ising chain and a gas of ultra-cold atoms in any two-mode configuration. We show that in the vicinity of a quantum critical point the rapid increase of the sensitivity is accompanied by the emergence of the many-body Bell nonlocality. The method described in our work allows for a systematic study of highly quantum phenomena in complex systems, and also extends the understanding of the beneficial role played by fundamental non-classical effects in implementations of quantum-enhanced protocols.

dr Marcin Mińkowski
(Computational Physics Laboratory, Tampere University, Finland)

Rate-dependent predictability of crystal plasticity by machine learning

Seminarium on-line

Stress-strain curves of crystalline solids with dislocations exhibit a significant sample-to-sample variation. Recently it has been shown that those curves can be predicted reasonably well by means of machine learning tools using as input the initial configurations of the dislocations [1]. During the talk I will focus on strain-rate controlled plastic deformations of a system of hundreds of dislocations performed by discrete dislocation dynamics (DDD) simulations. As a result stress-strain curves for different strain rates and stiffness values of the system are obtained. Subsequently several machine learning algorithms to predict those curves are trained: linear regression, ordinary neural networks and convolutional neural networks (CNNs), and their predictive power is compared. The first two approaches use a set of manually defined features of the initial configuration as the input, while the last method uses directly the image of the dislocations in that configuration. All three approaches generally exhibit improvement of the predictability with the increase of the strain rate. While the linear regression and the ordinary neural networks have comparable predictive power, CNNs usually outperform the other two. However, linear regression and ordinary neural networks are found to outperform the CNN for certain values of strain, stiffness and strain rate.

[1] H. Salmenjoki, M. J. Alava, L. Laurson, Nature Communications 9, 5307 (2018)

dr Jan Kołodyński
(Centre of New Technologies, University of Warsaw)

Noisy atomic magnetometry in the linear-Gaussian regime

Seminarium on-line

Continuously monitored atomic spin-ensembles ideally allow to sense external magnetic fields beyond classical limits in real time. Thanks to the phenomenon of measurement-induced spin-squeezing, they may attain a quantum-enhanced scaling of sensitivity within the linear-Gaussian regime, both as a function of time, t, and the number of atoms involved, N. In our work, we rigorously study how such conclusions change when inevitable imperfections are taken into account: in the form of collective noise, as well as stochastic fluctuations of the field in time. We prove that even an infinitesimal amount of noise disallows the error to be arbitrarily diminished by simply increasing N, and forces it to eventually follow a classical-like behaviour in t. However, we also demonstrate that, ``thanks'' to the presence of noise, in some regimes the model based on a homodyne-like continuous measurement actually achieves the ultimate sensitivity allowed by the decoherence, yielding then the optimal quantum-enhancement. We can do so by constructing a noise-induced lower bound on the error that stems from a general method of classically simulating a noisy quantum evolution, during which the stochastic parameter to be estimated---here, the magnetic field---is encoded. The method naturally extends to schemes beyond the linear-Gaussian regime, in particular, also to ones involving feedback or active control.

prof. dr hab. Marek Czachor
(Gdańsk University of Technology)

Arithmetic loophole in Bell's theorem: An overlooked threat for entangled-state quantum cryptography

Seminarium on-line

Bell's theorem is supposed to exclude all local hidden-variable models of quantum correlations. However, an explicit counterexample shows that a new class of local realistic models, based on generalized arithmetic and calculus, can exactly reconstruct rotationally symmetric quantum probabilities typical of two-electron singlet states. Observable probabilities are consistent with the usual arithmetic employed by macroscopic observers, but counterfactual aspects of Bell's theorem are sensitive to the choice of hidden-variable arithmetic and calculus. The model is classical in the sense of Einstein, Podolsky, Rosen, and Bell: elements of reality exist and probabilities are modeled by integrals of hidden-variable probability densities. Probability densities have a Clauser-Horne product form typical of local realistic theories. However, neither the product nor the integral nor the representation of rotations are the usual ones. The integral has all the standard properties but only with respect to the arithmetic that defines the product. Certain formal transformations of integral expressions one finds in the usual proofs Ă  la Bell do not work, so standard Bell-type inequalities cannot be proved. The system we consider is deterministic, local-realistic, rotationally invariant, observers have free will, detectors are perfect, so is free of all the canonical loopholes discussed in the literature.

prof. Alice Sinatra
(Laboratoire Kastler Brossel, ENS-Universit e PSL, CNRS)

Nuclear spin-squeezing by continuous quantum non demolition measurement

Seminarium on-line

The nuclear spin of Helium-3 is very well isolated from the environment and has coherence times measured to be hundreds of hours. We propose a method to manipulate at the quantum level the collective nuclear spin of a Helium vapor in a cell at room temperature, by means of a continuous quantum non demolition measurement. A discharge is temporarily switched-on in the vapor which populates the metastable state of Helium. The nuclear collective spin then slightly hybridizes with the collective spin of metastable atoms thanks to metastability exchange collisions. The metastable atoms are then made to interact with light in an optical cavity, and the field leaking out from the cavity is continuously measured. Nuclear spin-squeezing could provide a metrological gain for all measuring instruments based on the precession of a collective nuclear spin such as magnetometers or gyroscopes. It also opens up fascinating perspectives on the possibility of creating and maintaining a macroscopic quantum state over very long periods of time.

prof. Gediminas Juzeliunas
(Institute of Theoretical Physics and Astronomy, Vilnius University, Vilnius, Lithuania)

Periodically driven systems and geometric phases

Seminarium on-line

Periodic driving of quantum systems can enrich their topological and many-body properties. For example topologically trivial systems can become topologically non-trivial due to the periodic driving. To deal with the periodically driven quantum systems, it is convenient to describe their long-term dynamics in terms of an effective time-independent Floquet Hamiltonian. In doing so, the periodic driving is assumed to be stationary. On the other hand, it is quite common that the periodic driving changes in time. For example, in typical ultracold atom experiments the periodic driving is often ramped up from zero to a stationary regime [1].

In this talk we will explain ways of describing the periodically driven systems with slowly changing driving. We show that the system can acquire non-Abelian geometric phases due to such changes in the driving [2,3]. We will also talk briefly about our recent work on co-existence of the Weyl physics and the chiral hinge states for a periodically driven three dimensional lattice [4].

[1] R. Desbuquois, M. Messer, F. Görg, K. Sandholzer, G. Jotzu, T. Esslinger, PRA 96, 053602 (2017).
[2] V. Novicenko, E. Anisimovas, G. Juzeliunas, Phys. Rev. A 95, 023615 (2017).
[3] V. Novicenko, G. Juzeliunas, Phys. Rev. A 100, 012127 (2019).
[4] B. Huang, V. Novienko, A.'Eckardt and G.Juzeliunas, to appear in arXiv soon.

prof. Marzena H. Szymańska
(Department of Physics and Astronomy, University College London, U.K.)

Critical and superfluid properties of polariton condensates

Seminarium on-line

Driven-dissipative polariton fluid can exhibit a novel non-equilibrium order, where superfluidity is accompanied by stretched exponential decay of correlations [1]. This celebrated Kardar-Parisi-Zhang (KPZ) phase has not been achieved in any system in 2D and even 1D realisations are not conclusive. We show analytically [1] and confirm numerically that polaritons in the OPO configuration can be fine-tuned to realise the so far experimentally elusive KPZ phase in two dimensions for realistic experimental parameters. Further, we study the phase-ordering after a sudden quench across the critical region [2] and show that the unique interplay between non-equilibrium and the variable degree of spatial anisotropy leads to different critical regimes. By providing an analytical expression for the vortex evolution, based on scaling arguments, which is in agreement with the numerical results, we confirm the form of the interaction potential between vortices in this KPZ system.

At the same time, we obtain that for typical experimental conditions polariton system, despite its driven-dissipative nature, fulfils dynamical scaling hypothesis by exhibiting self-similar patterns for the two-point correlator at late times of the phase ordering [3]. We show that polaritons are characterised by the dynamical critical exponent z ≈ 2 and that the Kibble-Zurek mechanism holds [4]. Thus, our findings reinforce the idea that the concepts of universality and critical dynamics are not restricted to systems at or close to thermodynamic equilibrium, but extend to driven-dissipative ones that do not conserve energy nor particle number, nor they satisfy a detailed balance condition.

Finally, we determine that the superfluid response - the difference between responses to longitudinal and transverse forces - is zero for coherently driven polaritons [5]. This is a consequence of the gapped excitation spectrum caused by external phase locking. Furthermore, while a normal component exists at finite pump momentum, the remainder forms a rigid state that is unresponsive to either longitudinal or transverse perturbations and suggests that the suppression of scattering observed in experiments should be interpreted as a sign of a new rigid state of matter and not a superfluid.

[1] A. Zamora, L. M. Sieberer, K. Dunnett, S. Diehl, and M. H. Szymanska, "Tuning across Universalities with a Driven Open Condensate", Phys. Rev. X 7, 041006 (2017)
[2] A. Zamora, N. Lada, M. H. Szymanska, "Vortex dynamics in a compact KPZ system", arXiv:2003.01743
[3] P. Comaron, G. Dagvadorj, A. Zamora, I. Carusotto, N. P. Proukakis, M. H. Szymańska, "Dynamical critical exponents in driven-dissipative quantum systems", Phys. Rev. Lett. 121, 095302 (2018)
[4] A. Zamora, G. Dagvadorj, P. Comaron, I. Carusotto, N. P. Proukakis, M. H. Szymanska, "Kibble-Zurek mechanism in driven-dissipative systems crossing a non-equilibrium phase transition", Phys. Rev. Lett. 125, 095401 (2020)
[5] R. T. Juggins, J. Keeling and M. H. Szymańska, "Coherently driven microcavity-polaritons and the question of superfluidity", Nature Comms. 9, 4062 (2018)

dr Florian Vigneau
(University of Oxford, Department of Materials)

Quantum thermodynamics with carbon nanotube nanomechanical devices

Seminarium on-line

Quantum information thermodynamics studies the links between information, entropy and energy in a regime when fluctuations are important and quantum effects arise. Thermodynamics of small systems have long been restricted to conjectures and thought experiments. With current technologies miniaturizing devices, various ideas emerge to verify or challenge these principles.
We use a carbon nanotube electromechanical resonator as a piston at the nanoscale. Its displacement is detected with a radiofrequency cavity [1-2] using superconducting amplifiers [3] to boost sensitivity giving a direct access to the mechanical work stored in the motion. We couple the carbon nanotube motion to a single electron transistor to encode one bit of information in the electronic occupation. I will show how we can use our device to study the relationship between work and information. For example, with the proper conditions, this device can be turned into an autonomous engine powered by the transport of single electrons, which have been revealed by the detection of self-oscillation [4].
Another exciting idea, to probe the frontier of thermodynamics, is almost one century old. In 1929 Leo Szilard devised a thought experiment in which k_BT log(2) of work was extracted from the information acquired on a system composed of one particle in a box connected to a heat bath. Following this idea, Rolf Landauer deduced a minimum energy of k_BT log(2) required to process one bit of information, establishing a fundamental limit to the energy cost of information processing. I will develop this idea and present an experimental protocol that could allow us to measure directly the work involved in these cycles through our device. I will support our ideas with numerical simulations and present our means to realize them experimentally.
[1] N. Ares et al. Phys. Rev. Lett. 117, 170801
[2] Y. Wen et al. Appl. Phys. Lett. 113, 153101 (2018)
[3] F. J. Schupp et al. Journal of Applied Physics 127, 244503 (2020)
[4] Y. Wen et al. Nature Physics 16, 75-82(2020).

dr inż. Maciej Pieczarka
(Wrocław University of Science and Technology & The Australian National University, Canberra)

Bose-Einstein condensation of exciton polaritons in optical traps

Seminarium on-line

Exciton polaritons are quasiparticles resulting from the strong coupling between excitons and photons confined inside an optical microcavity. Because of their bosonic nature, they can undergo a transition to a Bose-Einstein condensate (BEC) - a macroscopically coherent quantum state of matter. This talk will overview our experimental research on the fundamental properties exciton-polariton condensates in ultra-pure microcavities based on GaAs, which serve as a workhorse for experiments in exciton-polariton BECs. Most specifically, I will focus on optical trapping, which is achieved by spatial shaping of the laser beam pumping the semiconductor sample allowing for arbitrarily shaped two-dimensional potentials. In the first part of the seminar is devoted to the high-density condensation in the Thomas-Fermi regime in a circular trap, where we have observed for the first time an interaction-driven effect beyond the mean-field description in an exciton-polariton BEC, the effect of quantum depletion of the condensate [1]. Furthermore, the improved experimental approach unveiled the peculiarities of the exciton-polariton BECs, strong influence of the uncondensed part, which we explain in the improved theory based on Ref. [2]. I will briefly present our first direct measurement of collective excitations on top of a top-hat profile exciton-polariton condensate, which probed the long-wavelength part of the excitations and the hydrodynamic sound [3]. If there is enough time left, I will switch gears at the end of the seminar and present polariton condensation in a more complex geometry, an all optical Su-Schrieffer-Heeger chain, where we fully controlled the intra- and intercell couplings to induce a topological phase transition, by measuring the presence or absence of topological invariants in the chain [4].

[1] M. Pieczarka et al., Observation of quantum depletion in a nonequilibrium exciton-polariton condensate, Nat. Commun. 11, 429 (2020);
[2] O. Bleu et al., Polariton interactions in microcavities with atomically thin semiconductor layers, arXiv: 2004.01336 (2020);
[3] E. Estrecho, M. Pieczarka et al., Low-energy collective oscillations and Bogoliubov sound in an exciton-polariton condensate, arXiv:2004.12339 (2020);
[4] M. Pieczarka et al., Topological phase transition in all-optical polariton lattice, in preparation

mgr Rafał Rechciński
(Instytut Fizyki PAN)

Topologically-protected electronic states in thin PbSnSe films - theory and experiment

Seminarium on-line

Three-dimensional (3D) topological crystalline insulators in the SnTe class (including PbSnSe) have attracted tremendous interest due to their unconventional electronic properties. These result from the non-trivial topology of electronic band structure that leads to the emergence of gapless states of zero-mass Dirac electrons at the surfaces with the electron spin locked to the momentum.

In ultra thin films, these so-called topological surface states located on opposing surfaces hybridize due to the overlap of their wave functions. As a result, an energy gap opens up near the Dirac point and the electrons gain mass. The hybridization gap depends on the film thickness and may even change sign at certain thicknesses leading to a transition from the topologically trivial to the non-trivial 2D electronic structure.

In my talk I will show a theoretical description of the electronic structure of thin PbSnSe films and explain the relation between the topologies of the band structures of the 3D bulk and the 2D film. I will also present experimental ARPES results obtained by our collaborators at JKU Linz for heterostructures consisting of thin PbSnSe films grown on top of PbEuSe barrier layers. Based on these I will discuss how the geometry of the sample design and the atomic details of the surface termination may affect the film's electronic spectrum.

mgr Nguyen Truong Co
(Instytut Fizyki PAN)

Heat-induced degradation of fibrils: Exponential vs logistic kinetics

Seminarium on-line

The degradation of fibrils under the influence of thermal fluctuations was studied experimentally by various groups around the world. In the first set of experiments, it was shown that the decay of fibril content, which can be measured by the ThT fluorescence assay, obeys a bi-exponential function. In the second series of experiments, it was demonstrated that when the monomers separated from the aggregate are not recyclable, the time dependence of the number of monomers belonging to the dominant cluster is described by a single-exponential function if the fraction of bound chains becomes less than a certain threshold. Note that the time dependence of the fraction of bound chains can be measured by tryptophan fluorescence. To understand these interesting experimental results, we developed a phenomenological theory and performed molecular simulation. According to our theory and simulations using the lattice and all-atom models, the time dependence of bound chains is described by a logistic function, which slowly decreases at short time scales but becomes a single exponential function at large time scales. The results, obtained by using lattice and all-atom simulations, ascertained that the time dependence of the fibril content can be described by a bi-exponential function that decays faster than the logistic function on short time scales. We have uncovered the molecular mechanism for the distinction between the logistic and bi-exponential behaviors.

dr Panagiotis Theodorakis
(Instytut Fizyki PAN)

On the mechanisms of superspreading

Seminarium on-line

Superspreading is the rapid and complete spreading of surfactant-laden droplets on hydrophobic substrates, which is caused by certain surfactant molecules known as superspreaders. Despite significant experimental efforts, the precise mechanisms of this phenomenon and the characteristic properties of superspreading surfactants had remained elusive. Here, we report on extensive molecular dynamics simulations of a coarse-grained model based on the SAFT force-field. Our studies highlight the mechanisms of superspreading, the features of superspreading surfactants and a range of parameters that affect the spreading efficiency of surfactant-laden droplets. We anticipate that our investigations will pave the way for the design of molecular architectures tailored specifically for applications that rely on the control of wetting.

dr hab. Bartosz Różycki
(Instytut Fizyki PAN)

Binding cooperativity of membrane adhesion molecules: From statistical mechanics to reconstituting cell biology

Seminarium on-line
Zoom Meeting (https://zoom.us/j/94980733829?pwd=WWRNNEdoMThmRmZmRVpQV2xuaUhUUT09)

Adhesion of membranes that enclose biological cells is essential for immune responses, tissue formation, and cell signaling. These adhesion processes result from the binding of receptor and ligand proteins that are anchored in cell membranes. A central question is how to characterize the binding affinity of these membrane-anchored proteins. For soluble molecules, the binding affinity is typically quantified by the binding equilibrium constant K in the linear relation [RL] = K[R][L] between the volume concentration [RL] of molecular complexes and the volume concentrations [R] and [L] of unbound molecules. For membrane-anchored molecules, it is often assumed by analogy that the area concentration of receptor-ligand complexes [RL] i s proportional to the product [R][L] of the area concentrations for the unbound receptor and ligand molecules. Using the Helfrich theory and equilibrium statistical mechanics, we have shown (i) that this analogy is only valid for planar membranes immobilized on rigid surfaces, (ii) that membrane conformational fluctuations on nanoscales lead to cooperative binding of receptors and ligands and (iii) that, in the limit of weak adhesion, the area concentration ratio [R][L]/[RL] is proportional to the roughness of thermally fluctuating membranes [1]. Our theoretical predictions of membrane-mediated binding cooperativity have been quantitatively confirmed in molecular dynamics simulations [2]and, recently, in fluorescence experiments on giant plasma membrane vesicles that adhere via the SIRPÎą-CD47 protein complexes [3]. The experiments have shown that slight acidity (pH=6) stiffens membranes, diminishes cooperative interactions, and also reduces 'self' signaling of cancer cells in phagocytosis. Our results demonstrate that sensitivity of cell-cell interactions to microenvironmental factors (such as acidity of tumors) can arise from collective, cooperative properties of cell membranes.

References:
[1] H. Krobath, B. Różycki, R. Lipowsky, T.R. Weikl. Binding cooperativity of membrane adhesion receptors. Soft Matter 17, 3354-3361 (2009).
[2] J. Hu, R. Lipowsky, T.R. Weikl. Binding constants of membrane-anchored receptors and ligands depend strongly on the nanoscale roughness of membranes. PNAS 110, 15283-15288 (2013).
[3] J. Steinkuehler, B. Różycki, C. Alvey, R. Lipowsky, T.R. Weikl, R. Dimova, D.E. Discher. Membrane fluctuations and acidosis regulate cooperative binding of 'marker of self' protein CD47 with the macrophage checkpoint receptor SIRPÎą. J. Cell Sci. 132, 216770 (2019).

prof. Selim Jochim
(Heidelberg University)

Juggling with single atoms to understand many-body physics

Seminarium on-line
Zoom Meeting (https://zoom.us/j/94980733829?pwd=WWRNNEdoMThmRmZmRVpQV2xuaUhUUT09)

The talk will be related to recent experimental work on Pauli crystals, arXiv.2005.03929.

mgr Damian Kwiatkowski
(Instytut Fizyki PAN)

Observing a natural environment around nitrogen-vacancy centers through spin echo decoherence

Seminarium on-line
Zoom Meeting (https://zoom.us/j/94980733829?pwd=WWRNNEdoMThmRmZmRVpQV2xuaUhUUT09)

Nitrogen-vacancy center is a deep defect in diamond. Its energy level structure helps to initialise its pure spin state in room temperature through optical pumping and rotate its state on a Bloch sphere through application of controlled microwave pulses [1]. Lifetime of a superposition state is limited by pure dephasing caused by interactions with a bath of a few hundred randomly distributed C-13 nuclear spins, naturally present in diamond and coupled to the central qubit with dipolar interaction.

In the first part of my talk, I will show that the shape of decoherence in time strongly depends on distribution of those nuclei in the lattice. A number of effects, observed in these systems, can be attributed to either few tens of spins closest to the NV center or the remainder of weakly coupled bath spins treated as a source of noise.

In room temperature, state of the nuclear bath is completely mixed. However, recently a number of experimental results showed successful polarisation of few spins closest to the NV center (e.g. [2]). Such a partially polarised state can be investigated in a variety of fundamentally interesting directions around the physics of decoherence, such as qubit-environment entanglement [3] or quantum Darwinism [4] observed through formation of Spectrum Broadcast Structures.

Another interesting application of NV centers in diamond, is initialisation and control of two-qubit entangled states, which also was proven to be experimentally viable [5]. Correlations, stored in multi-qubit coherence of initial Bell entangled states, can be helpful to register spatial correlations in the bath [6]. In the final part, I will summarise how can we exploit such multi-qubit control to learn about the nature of couplings in the bath and whether or not they can be substituted by a classical noise of Gaussian statistics [7].

[1] Doherty, M. W., Manson, N. B., Delaney, P., Jelezko, F., Wrachtrup, J., & Hollenberg, L. C. L., The nitrogen-vacancy colour centre in diamond. Physics Reports, 528, 1-45 (2013)
[2] London, P., Scheuer, J., Cai, J.-M., Schwarz, I., Retzker, A., Plenio, M. B., Jelezko, F., Detecting and Polarizing Nuclear Spins with Double Resonance on a Single Electron Spin. Physical Review Letters, 111, 067601 (2013)
[3] Roszak, K., Kwiatkowski, D., Cywiński, Ł., How to detect qubit-environment entanglement generated during qubit dephasing. Physical Review A, 100, 022318 (2019)
[4] Kwiatkowski, D., Cywiński, Ł. , Korbicz, J., Emergence of classicality for NV centers interacting with dynamically polarised nuclear environment, to appear on arXiv in June 2020.
[5] Dolde, F., Jakobi, I., Naydenov, B., Zhao, N., Pezzagna, S., Trautmann, C., Wrachtrup, J., Room-temperature entanglement between single defect spins in diamond. Nature Physics, 9(3), 139 - 143 (2013)
[6] Szańkowski, P., Trippenbach, M., Cywiński, Ł., Spectroscopy of cross correlations of environmental noises with two qubits. Physical Review A, 94 (1), 012109 (2016)
[7] Kwiatkowski, D., Cywiński, Ł., Decoherence of two entangled spin qubits coupled to an interacting sparse nuclear spin bath: Application to nitrogen vacancy centers. Physical Review B, 98(15), 155202 (2018)

mgr inż. Andrzej Opala
(Instytut Fizyki PAN)

Neuromorphic computing with quantum fluid of light

Seminarium on-line
Wiecej informacji udziela dr hab. Łukasz Cywiński (lcyw@ifpan.edu.pl)

In recent years, progress in miniaturization of microelectronics has slowed down, rendering Moore's law obsolete. Technological solutions based on the two-dimensional CMOS technology reach the physical limits, achieving quantum effects. Progress in digitization and communication enforces the necessity to process larger and larger data sets in an ever shorter time. Unfortunately, the performance of commonly used computers based on the von Neumann architecture reaches its limit. Physical limits of the miniaturization of integrated circuits make it impossible to solve this problem in a traditional way. Arguably, the best solution to overcome the technological impasse is using optoelectronic systems with architecture inspired by the structure of brain, called neuromorphic systems. The features which are crucial for a neuromorphic computing system are the non-linearity of the active medium, the possibility of precise input state manipulation, scalability, energy efficiency and speed of operation. All of the above criteria are fulfilled by exciton-polariton quantum fluids of light [1,2].

In this presentation, the concepts of polariton neuromorphic computing systems [3] and their first experimental realization, will be described [4]. The performance of the polariton artificial intelligence system in the MNIST pattern recognition task will be compared with other optoelectronic neuromorphic systems.

[1] H. Deng, H. Haug, and Y. Yamamoto, Rev. Mod. Phys. 82, 1489 (2010)
[2] I. Carusotto and C. Ciuti, Rev. Mod. Phys. 85, 299 (2013)
[3] A. Opala, S. Ghosh, T. C. H. Liew and M. Matuszewski, Phys. Rev. Applied 11, 064029 (2019)
[4] D. Ballarini, A. Gianfrate, R. Panico, A. Opala, S. Ghosh, L. Dominici, V. Ardizzone, M. De Giorgi, G. Lerario, G. Gigli, T.C.H. Liew, M. Matuszewski, D. Sanvitto, “Polaritonic Neuromorphic Computing Outperforms Linear Classifiers”, accepted in Nano Letters. (2020)

dr Maciej Bieniek
(Politechnika Wrocławska)

Role of band nesting and topology in optical and electronic properties of novel 2D semiconductors

There is currently a great interest in van der Waals materials, including semiconductors, topological insulators, semimetals, superconductors and ferromagnets. In the following presentation the effect of band nesting and topology on both fine structure of excitons and single-particle spectrum of states in gate-defined quantum dots in monolayer transition metal dichalcogenides (TMDs) family of 2D semiconductors will be discussed.

First, ab-initio based electronic structure obtained within tight-binding model of family of MX₂ (M=Mo,W, X=S,Se,Te) direct gap semiconductors will be described [1]. Next, Bethe-Salpeter theory of excitonic fine structure will be presented. Effects of electron-hole dispersion, details of band structure on Coulomb intra/inter - valley interactions, topology of wavefunctions, screening and dielectric environment will be discussed [2].

In the second part, atomistic theory of electrons confined by metallic gates in a single layer of TMDs will be shown. The electronic states will be described by the tight-binding model [1] and computed using a computational box including up to million atoms with periodic boundary conditions and parabolic confining potential [3]. Twofold degenerate energy spectrum of electrons confined in the two non-equivalent K-valleys by the metallic gates as well as six-fold degenerate spectrum associated with Q-valleys is found, resulting in SU(3) symmetric quantum states. Finally, discussion of the role of spin splitting and topological moments on the K and Q valley electronic states will be shown.

[1] M. Bieniek, M. Korkusiński, L. Szulakowska, P. Potasz, I. Ozfidan, and P. Hawrylak, Band nesting, massive Dirac fermions, and valley Landé and Zeeman effects in transition metal dichalcogenides: A tight-binding model, Phys. Rev. B 97, 085153 (2018).
[2] M. Bieniek, L. Szulakowska, and P. Hawrylak, Band nesting and exciton spectrum in monolayer MoS₂, arxiv:2001.00443 (2020) (PRB 2020, to be published)
[3] M. Bieniek, L. Szulakowska, and P. Hawrylak, Effect of valley, spin, and band nesting on the electronic properties of gated quantum dots in a single layer of transition metal dichalcogenides, Phys. Rev. B 101, 035401 (2020).

prof. Giuliano Orso
(Paris Diderot University)

Boson-enhanced visibility of the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state in one dimension

Bosons are routinely used in cold-atoms experiments to cool fermions down to the quantum regime, where superfluidity can emerge. Can bosons be also useful to detect exotic pairing states?

In this talk, I present a recent work [1] on one-dimensional mixtures of repulsive bosons and spin-imbalanced attractive fermions, the latter being in the FFLO state, where Cooper pairs condense at a finite momentum k=k_FFLO. Based on DMRG numerics, we find that, when the Bose-Fermi mixture is close to the phase separation point, the density profiles of both species develop a self-induced spatial oscillations with wave-vector 2k_FFLO, leading to sharp kinks in the corresponding static structure factors.

We show that the mechanism is very robust and provides a direct way to detect the FFLO state in current experiments.

[1] Enhanced visibility of the Fulde-Ferrel-Larkin-Ovchinnikov state in one dimensional Bose-Fermi mixtures near the immiscibility point, Manpreet Singh, Giuliano Orso, arXiv:1911.03448

dr Daniel Pęcak
(Politechnika Warszawska)

FFLO-like pairing in few-body systems

A system of a few attractively interacting atoms of lithium in one-dimensional harmonic confinement is investigated. Non-trivial interparticle correlations induced by interactions in a particle-imbalanced system are studied in the framework of the noise correlation. In this way, it is shown that evident signatures of strongly correlated fermionic pairs in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state are present in the system and they can be detected by measurements directly accessible within state-of-the-art techniques. The results convincingly show that the exotic pairing mechanism is a very universal phenomenon and can be captured in systems being essentially non-uniform and far from the many-body limit.

mgr Filip Gampel
(Instytut Fizyki PAN)

Continuously monitored quantum particles - a preliminary study

Conventionally, quantum mechanical predictions consider values of observables averaged over many repeated realizations of an experimental setup. Nowadays continouos observation of an individual quantum system is within experimental reach. However, the implementation of the measuring process with its influence on the monitored system remains an open problem. In my talk I will present a simplistic attempt to describe such situations within the standard quantum mechanical framework.

prof. Antun Balaž
(Institute of Physics Belgrade)

Vortices and droplets in dipolar Bose-Einstein condensates

In recent experiments, the Rosensweig instability was observed in a quantum ferrofluid of a strongly dipolar BEC, leading to a formation of atomic droplets, which represent a new state of quantum matter. Surprisingly, it was demonstrated that the stability of such droplets is only due to a quantum fluctuation correction of the ground-state energy. Here we extend previous theoretical description and develop a full Bogoliubov-Popov theory for such systems, which also takes into account the condensate depletion due to quantum fluctuations. We apply this approach and use extensive numerical simulations to study formation of quantum droplets and their properties in 164Dy BEC in a ring geometry, as well as formation of vortices in a rotating system in 3D geometry, including the droplet phase.

dr hab. Marek Wolf, prof. UKSW
(Uniwersytet Kardynała Stefana Wyszyńskiego w Warszawie)

Will a physicist prove the Riemann Hypothesis?

In the first part I will present the number theoretical properties of the Riemann zeta function and formulate the Riemann Hypothesis. In the second part I will review some physical problems related to this hypothesis: the Polya-Hilbert conjecture, the links with Random Matrix Theory, relation with the Lee-Yang theorem on the zeros of the partition function and phase transitions, random walks, billiards etc.

dr hab. Michał Zieliński
(Uniwersytet Mikołaja Kopernika w Toruniu)

From nanostructures to dopant based devices: theory and challenges

Efficient theoretical modeling of nanostructures is a necessary step towards control of their spectral properties aiming for applications in quantum information and cryptography. In my talk I will discuss the key role of alloy randomness in various nanostructures, and show how external electric field can tune optical spectra of quantum dots and quantum dot molecules to compensate for detrimental effects of low-symmetry. Moreover, I will discuss our recent methodological progress allowing us to bridge between different computational methods, in particular between atomistic approaches and the continuous media approximation. Further, I will describe demanding computer simulations for ultimate nanodevices formed by single dopants in silicon. Here, the challenging problem of STM images analysis is resolved with the help of machine learning.

dr Daniel Benedicto-Orenes
(ICFO, Barcelona)

Single domain Spinor BEC and ultra-cold magnetometry

During this talk, I will present our recent results on the first co-magnetometer using a mixture of spin-1,2 Bose-Einstein condensates [1]. We exploit our all-optical trap and the two ground state hyperfine manifolds of 87Rb to perform continuous Faraday measurements of the two collective spin-F systems. The two independent measurements of co-located spins, and the nearly opposite gyromagnetic ratios in the F=1,2 manifolds allow a differential readout that is highly insensitive to the common mode magnetic field noise, demonstrating a noise reduction of ~ 44dB. These results open the way for new applications of um length-scale co-magnetometers. Then, I will present the current status of the experiment and the external cavity that we are working on to bring the experiment into the strong light-coupling regime. This will enable the possibility of performing QND measurements in this experiment, and opens the way to create measurement-induced spin squeezing for precision metrology.

[1] arXiv:1910.06642

dr Thomas Sturges
(Instytut Fizyki Teoretycznej, Uniwersytet Warszawaski)

Topological Polaritons and the Bulk-Edge Correspondence

Metamaterials are notorious for providing fascinating optical properties not found in nature, by virtue of designing the underlying geometry of arrays of optical components. However it is not quite as well appreciated that one can also induce non-trivial, even topological, changes within an optical medium simply by tuning the strength of the light-matter coupling. We highlight this key fact with a very simple example. First, we consider a dimerised chain of meta-atoms that experience Coloumb interactions with their nearest neighbours. In this case, the famous bulk-edge correspondence holds: there is a direct correspondence between the topological Zak phase (a certain integral over eigenstates performed in the continuum), and the presence of edge states in a finite chain. This correspondence holds when we include next-nearest neighbours. It holds when we include all neighbours. It holds when we include non-resonant terms, when we break chirality, when we include retardation effects and radiation damping, and when we include disorder. Thus the bulk-edge correspondence indeed appears to be a very useful predictor of edge states. However, strong light-matter coupling spoils it all. In the strong light-matter coupling limit, we show that by simply changing the height of an enclosing photonic cavity, we can reach a critical coupling strength where we break the bulk-edge correspondence. Namely, we observe an apparently topologically nontrivial phase, which unexpectedly does not exhibit edge states.

dr Behnam Tonekaboni Faghihnasiri
(Griffith University, Australia)

Quantum Noise Spectroscopy Beyond Frequency Domain

Quantum information processing relies on the robust control of quantum systems which are always influenced by external degrees of freedom, i.e. the environment. The presence of the environment effectively generates noise, which in turn causes decoherence in the quantum system of interest, say a qubit. Many methods have been developed to fight the decoherence in the qubit, such as dynamical decoupling (DD) techniques and quantum error correction methods, which apply in different noise regimes, i.e. under different assumptions. Achieving optimal suppression of arbitrary noise, i.e., mitigating the effect of a generic environment, however, requires precise knowledge of the noise.

Quantum noise spectroscopy (QNS) is a tool to systematically infer information about the statistical properties of the noise generated by the environment. State of the art QNS protocols are usually based on applying π-pulses and using the filter functions in frequency domain to facilitate the extraction of information. A crucial limitation is that the environment noise is assumed to be stationary, which means that bath auto-correlation depends only on the time difference. However, there are many classical and quantum environments which are non-stationary.

In this presentation we report our progress on a new type of noise spectroscopy protocol which lets us to relax the stationary assumption and analyse a broader range of environment noises. Furthermore we use Walsh bases (instead of Fourier bases), which give us more flexibility to choose required control pulses in our QNS protocol.

Wil Jones
(University of Edinburgh)

An application of machine learning in flow regime identification

The complex nature of multiphase flow makes fully understanding their behaviour very challenging even to this day. This means that for the purpose of industrial designs, generalizations of multiphase flow phenomena need to be made, this is typically done in the form of flow regime identification. In this talk I will be presenting a new, robust technique for identifying multiphase flow regimes using machine intelligence and image recognition. I will also be talking about some of the fundamentals that makes multiphase flow so challenging, flow regimes maps and why we need to identify these regimes correctly so long as we don't have a unique numerical solution to multiphase flow problem.

mgr Jan Krzywda
(Instytut Fizyki PAN)

Theory of coherent spin transfer between two silicon quantum dots

Despite its additional degree of freedom (the valley state), the spin of an electron in silicon nanostructures is recently being tested as a flying qubit, which is necessary for future quantum computer architecture. In my talk I will explain the principles of valley physics in silicon and introduce a model describing decoherence caused by charge noise during the electron electron shuttling in Si quantum dots.

dr Alexander Lau
(Kavli Institute of Nanoscience, TU Delft, Netherlands)

Topological insulators and semimetals from structural distortions in tellurides

The search for novel topological phases of matter and topological materials is a rapidly progressing field in today's condensed matter physics community. Tellurium-based materials have always provided promising routes in this endeavor: ranging from quantum spin Hall insulators in HgTe quantum wells over strong topological insulators in alloys of Bi₂Te₃ to topological crystalline insulators in SnTe, tellurides realize a wide variety of topological phases. In this talk, I discuss recent theoretical results on two directions in this area where a structural distortion gives rise to a topological phase. First, I will show how the ferroelectric distortion in group-IV tellurides can lead to semimetallic topological phases, namely, Weyl semimetals and topological nodal-line semimetals [1]. In particular, based on miminal low-energy k·p models, I will present a step-by-step analysis of the structural symmetry reduction from the rocksalt to the rhombohedral crystal structure. Experimentally, the proposed semimetals could potentially be realized in the low-temperature ferroelectric phase of SnTe, GeTe and related alloys.

Second, I will look into the recently discovered quantum spin Hall phase in monolayer 1T'-WTe2 [2] where the characteristic band inversion originates from a structural distortion. I will demonstrate how an accurate low-energy tight-binding model of this material can be constructed by combining density-functional theory, symmetry considerations, and data from photoemission spectroscopy. Based on this model, I investigate the influence of sample termination on the electronic properties of the material. In particular, I will present computational results of edge state dispersions and 2-terminal conductance spectra for various device geometries with different terminations, with and without magnetic field. Finally, I will discuss how the roughness of a disordered edge affects its transport properties.

[1] A. Lau, C. Ortix, arXiv:1804.09574 (2018).
[2] A. Lau, R. Ray, D. Varjas, A. Akhmerov, arXiv:1812.05693 (2018).

dr Paolo Comaron
(Instytut Fizyki PAN)

Equilibrium and Nonequilibrium Condensates: Phase Transitions and Quench Dynamics

We investigate universal features of equilibrium and nonequilibrium condensates, for experimentally-relevant parameters, when quenching the systems from a disordered to an ordered configuration. We evaluate the dynamical critical exponent from the long-time scaling behavior of the dynamics of spontaneously-generated topological defects (vortices) and the characteristic lengths of the systems. In the case of polariton (nonequilibrium) systems, we report strong numerical evidence that critical behavior is characterized by the dynamical critical exponent z≈2 with logarithmic correction to the diffusive dynamics. We find that the universal properties of the system, also in the dynamical case, are dominated by BKT-type of physics. Thus, we consider finite-duration linear quenches across the critical point. We demonstrate explicitly that our findings agree with the general Kibble-Zurek mechanism scenario, which provides a characterization of the relation of topological defect density to quench rate, through the critical exponents of the system. A detailed study of the steady-state system of an incoherently-driven condensate is also performed in order to facilitate our studies. Finally, for the case of two-dimensional Bose gases (equilibrium system), we demonstrate that the system undergoes phase ordering kinetics and fulfils dynamical scaling hypothesis at late-time dynamics. Focussing on experimentlly-accessible finite-size box-like geometries, we show that observation of the dynamical critical exponent z≈2 is within current experimental reach.

dr Konrad J. Kapcia
(Department of Computational Materials Science, Institute of Nuclear Physics PAS, Kraków, Poland)

Exact analytic solution of the extended Falicov-Kimball model in infinite dimensions

The Falicov-Kimball model [1] is a simplified version of the Hubbard model [2], where only electrons with, e.g., spin down are itinerant and the other are localized. During the seminar, we will discuss results for the extended Falicov-Kimball model at half-filling on the Bethe lattice in the limit of large dimensions derived within the DMFT formalism [3-7], which is a rigorous approach in this limit. The on-site and the intersite density-density interactions between particles occupying neighboring sites are included in the hamiltonian [5-7]. We determined the exact phase diagrams of the model both in the ground state [6] and at finite temperatures [7]. Using analytical formulas we showed that in the ground state the system is an insulator for any non-zero values of the interaction couplings and we detected the discontinuous transition between two different charge-ordered phases [6]. The finite temperature diagrams are a generalization of the diagram previously obtained in Ref. [4] for the standard Falicov-Kimball model, but they are much richer than it. In particular, it turns out that these diagrams contain more types of ordered phases, both conductive and insulating, and phase transitions between them are either continuous or discontinuous [7]. What is more, it turns out that in a certain range of interaction parameters the order-disorder phase transition may be discontinuous.

[1] L.M. Falicov, J.C. Kimball, Phys. Rev. Lett. 22, 997 (1969); R. Ramirez, L.M. Falicov, J.C. Kimball, Phys. Rev. B 2, 3383 (1970).
[2] J. Hubbard, Proc. R. Soc. London, Ser. A 276, 238 (1963).
[3] J.K. Freericks, V. Zlatic, Rev. Mod. Phys. 75, 1333 (2003); A. Georges, G. Kotliar, W. Krauth, M.J. Rozenberg, Rev. Mod. Phys. 68, 13 (1996).
[4] R. Lemański, K. Ziegler, Phys. Rev. B 89, 075104 (2014); S.R. Hassan, H.R. Krishnamurthy, Phys. Rev. B 76, 205109 (2007).
[5] P.G.J. van Dongen, D. Vollhardt, Phys. Rev. Lett. 65, 1663 (1990); P.G.J. van Dongen, Phys. Rev. B 45, 2267 (1992).
[6] R. Lemański, K.J. Kapcia, S. Robaszkiewicz, Phys. Rev B 96, 205102 (2017).
[7] K.J. Kapcia, R. Lemański, S. Robaszkiewicz, preprint, arXiv: 1903.08092 (2019).

dr Mircea Trif
(MagTop, Instytut Fizyki PAN)

Cavity quantum electrodynamics with quantum transport

The versatility of the cQED method relies on the fact that it allows to monitor in a non-invasive fashion the states in quantum conductors, both in equilibrium and non-equilibrium situations, to affect and manipulate the quantum transport, and opens the pathway to create non-classical states of light by means of both electronic and/or spin transport. In this talk, I will discuss some of these aspects for various types of quantum conductors out of equilibrium. I will focus on tunnel junctions, magnetic tunnel junctions, Josephson junctions, and quantum magnets, respectively. I will show that one can reveal properties that are invisible in electronic transport (via the conductance), in particular in out-of-equilibrium situations pertaining to a large voltage bias applied over the quantum conductor. For the case of voltage biased Josephson junction, I will show that the emitted radiation is non-classical in the sense that the photonic correlators violate some Cauchy-Schwarz inequalities. Finally, I will present a method by which to directly measure spin current noise created at a junction between two out of equilibrium quantum magnets using cQED methods.

dr Marcin Mińkowski
(University of Vienna)

Neural-network-based molecular dynamics of point defects in PbTe and CdTe

Lead telluride (PbTe) and cadmium telluride (CdTe) constitute an immiscible system, which, as shown in the growth experiments, can be used to manufacture nanostructures such as nanowires and quantum dots through morphological transformations of an initially multilayer structure at high temperatures. Those transformations occur entirely in the solid state, therefore it is expected that the underlying mechanism is based on the exchange processes in the microscopic scale such as diffusion of point defects like interstitials and vacancies.

During the talk I will show the recent results of molecular dynamics (MD) simulations of PbTe and CdTe based on a neural network potential trained with ab initio data. That approach allows to study large systems in long time scales with the accuracy comparable to ab initio MD. The simulations performed in finite temperatures reproduce the bulk properties of the materials studied and also can be used to analyse the kinetics of the defects introduced into the sample.

dr Vanik Shahnazaryan
(Instytut Fizyki PAN)

Strong light-matter coupling in carbon nanotubes as a route to exciton brightening

In this talk I will show that strong light-matter coupling can be used to overcome the long-standing dark exciton problem that has prevented efficient optical emission from carbon nanotubes. Despite the significant potential for optoelectronic technologies and in quantum optics, take-up of carbon nanotubes in optoelectronic applications is so far hindered due to technological issues and the presence of dark exciton states. I will present a theoretical analysis to show that by placing carbon nanotubes in an optical microcavity the bright excitonic state may be split into two hybrid exciton-polariton states, while the dark state remains unaltered. For sufficiently strong coupling between the bright exciton and the cavity the energy of the lower polariton may be pushed below that of the dark exciton. This overturning of the relative energies of the bright and dark excitons prevents the dark exciton from quenching the emission.

dr Julia Stasińska
(Instytut Fizyki PAN)

Many-body localization: a generic mechanism for the breakdown of thermalization

In recent years there is a huge growth of interest in quantum systems that would escape thermalization, and thus preserve information encoded in their initial state in local observables. Since a precondition for thermalization is the possibility to establish some kind of transport in the system, insulators become natural candidates for systems that would fail to thermalize. A prominent example of a universal transport suppression mechanism for non-interacting particles is the Anderson localization, i.e., exponential localization of the wave function in the presence of disorder. Recently, the possibility of localization in an interacting setting, a trully many-body localization (MBL), was established providing a generic scenario for the breakdown of thermalization in a closed quantum system. In my talk I will introduce the concept of many-body localization and review recent developments in the field. I will also discuss a specific example of a system which undergoes a transition between a thermal and an MBL phase.

dr Dillip Nandy
(Instytut Fizyki PAN)

Relativistic coupled-cluster method, a route to high precision atomic calculations

Exact treatment of electronic correlation in basic particles like atoms and molecules is still a challenging and an exciting topic of research in modern science. Demands of highly accurate atomic calculations in many precision experiments lead to the development of new advanced many-body methods in physics and chemistry. As for instance, the accurate estimations of various detrimental systematic involved in the atomic clock experiments are quite urge for the precise measurement of the clock frequencies. Such ultra precision frequency measurements are extremely useful for probing the temporal variation of some dimensionless fundamental constant such as fine structure constant α of the electromagnetic interaction between the charge particles. In this talk, I will discuss the results of various systematic effects, for some useful atomic clock candidates, obtained using the relativistic coupled-cluster (RCC) method. We will see that it is indeed necessary to employ some higher order method like RCC for determining the above atomic properties more accurately. Finally, I will briefly explain the requirements of such high precision calculations for studying temporal variation in α using atoms as a basic tools. I will conclude the talk with my future directions of developments along with their importance.

Giovanni Pireddu
(University of Sassari, Department of Chemistry and Pharmacy)

Generalized spatial coarse-graining of host-guest systems

Nowadays, multiscale modelling techniques hold great promise for the representation of physical systems in relatively broad spatial and temporal scales. Starting from a microscopic model, reaching larger scales often requires performing a coarse-graining procedure in order to obtain a simpler but more computationally efficient model. This approach is necessary for the representation of adsorption and diffusion occurring in macroscopic scales, such as industrial gas-separation involving microporous materials.
In this context, we investigated the spatial coarse-graining of interactions in host-guest systems within the framework of the recently proposed Interacting Pair Approximation (IPA). Essentially, the IPA method derives local effective interactions from the knowledge of local occupancy statistics.
We tested our method to represent the adsorption isotherm the occupancy fluctuations and spatial correlations on a variety of systems including particles in abstract lattice gases, methane in microporous materials and methane in graphene-based materials.
Once established this procedure for the representation of static properties, we intend to map the reference dynamic properties such as occupancy life-times, time-correlation functions and collective diffusion, into the new coarse-grained representation.

mgr Rafał Rechciński
(Instytut Fizyki PAN)

Topological electronic states on uneven surfaces of (Pb,Sn)Se crystals

Some of the group IV-VI crystals with a three-dimensional rock-salt type lattice (e.g., SnTe, as well as certain solid solutions (Pb,Sn)Te, and (Pb,Sn)Se) belong to the category of topological crystalline insulators. In this class of materials inverted band ordering leads to emergence of in-gap surface states with massless Dirac-like energy dispersion and helical spin structure. Provided that the mirror symmetries of the crystalline rock-salt lattice are not broken, these states cannot be removed from the surface neither by weak nor local perturbations due to the so-called topological protection. Still, the properties of these surface states can be considerably altered by modification of the surface structure. This results from the fact that in these materials the nontrivial band topology coincides with multivalley effects, and often also strong effective mass anisotropy. In this talk I will focus on two phenomena recently observed in (Pb,Sn)Se: the emergence of one-dimensional states bound to atomic steps on the surface [1], as well as the strong dependence of the dispersion of the surface states on the surface roughness [2]. We have developed a simple envelope function model [3] that allowed us to explain both these effects through the fact that valley mixing, the phenomenon responsible for the peculiar "double Dirac cone" shape of the surface state dispersion, depends crucially on the structure of the surface. In particular, our analysis shows that the puzzling one-dimensional states can be thought of as boundary modes protected by the winding number topological invariant.

[1] P. Sessi et al., Science 354, 1269 (2016)
[2] C. Polley et al., ACS Nano 12, 617 (2018)
[3] R. Rechcinski, R. Buczko, Phys. Rev. B 98, 245302 (2018)

dr Barbara Piętka
(Institute of Experimental Physics, Faculty of Physics, University of Warsaw)

Semimagnetic exciton-polariton condensates in external magnetic fields

Exciton-polaritons are quasiparticles that are formed in a microcavity due to the strong coupling between quantum well excitons and cavity photons. Polaritons inherit the spin degree of freedom from excitonic component and from the point of view of spin structure are therefore bosons with a 1/2 pseudospin. The polariton non-equilibrium Bose-Einstein condensation is a well established phenomena and is widely studied for its remarkable properties. The spin property of exciton-polaritons is however difficult to explore due to small polariton Zeeman splitting in standard semiconductor microcavities.
We propose structures based on semimagnetic semiconductors with magnetic ions incorporated into the crystal lattice that offer a unique opportunity to study in detail spin related phenomena in exciton-polariton condensates. In the CdMgZnTe microcavity and CdMnTe material of the quantum well, the s,p-d exchange interactions between localized d-shell electrons of the magnetic ions and delocalized electrons and holes allows to enhance magneto-optical properties of polaritons. The application of external magnetic field modifies strongly the excitonic component of semimagnetic polariton and the effects of giant Zeeman splitting and giant Faraday rotations are observed. These effects allow also to tune the spin-related interactions in non-equilibrium polariton condensate.
I'll discuss many spectacular effect, such condensation induced by external magnetic field, giant spin Meissner effect, or spin polarized vortices. I'll also demonstrate the creation of a spin multicomponent condensate and the method to tune it smoothly to a single component condensate by an external parameter, i.e. excitation power and/or magnetic field.

dr hab. Michał Matuszewski, prof. IF PAN
(Instytut Fizyki PAN)

Neuromorphic computing in Ginzburg-Landau polariton lattice systems

The availability of large amounts of data and the necessity to process it efficiently have led to rapid development of machine learning techniques. To name a few examples, artificial neural network architectures are commonly used for financial forecasting, speech and image recognition, robotics, medicine, and even research. However, efficient hardware implementation is still lacking, since the most developed computing technologies available have been designed for the von Neumann architecture. Reservoir computing (RC) is a recent and increasingly popular bio-inspired computing scheme which holds promise for an efficient temporal information processing. We demonstrate the applicability and performance of reservoir computing in a complex Ginzburg-Landau lattice model, which adequately describes dynamics of a wide class of systems, including coherent photonic devices. In particular, we propose that the concept can be readily applied in exciton-polariton lattices, which are characterized by unprecedented photonic nonlinearity, opening the way to signal processing at rates of the order of 1 Tbit/s.

dr hab. Michał Zawada
(Uniwersytet Mikołaja Kopernika w Toruniu)

Dark matter searches within the intercontinental optical atomic clock network

We (NIST, NICT, SYRTE, and KL FAMO) report preliminary results of dark mater searches within the worldwide network made of our laboratories. We demonstrate that data routinely collected by our currently operating optical atomic clocks without any further developments of the experimental set-ups may be used to run a global program aimed on searches of dark matter.

dr Jagoda Sławińska
(University of North Texas)

Novel tunable materials for spintronics applications

Employing the electron's spin to carry information emerges as a technological revolution to increase the computing capabilities of devices while Moore's law seems to approach its limit of validity. Despite their success in the field of memories, spintronics devices so far has been lacked efficient mechanisms of switching that enable and disable the flow of spin current, thus preventing logic applications. Recent progress in the materials science, both from computational and experimental side, allows us to design new advanced materials with spin properties on demand which might be tuned in situ, bringing spintronics closer to the area of computations. In this talk, I will discuss several aspects of the spin-orbit interaction in solids focusing on the ways of its external control in a variety of systems ranging from 2D materials, surfaces and complex interfaces to Rashba ferroelectrics and multiferroics. Finally, I will address further challenges for spin-based classical and quantum computing as well as the roadmap for computational discovery of suitable materials.

dr hab. Remigiusz Augusiak
(Centrum Fizyki Teoretycznej PAN)

Towards self-testing of qudit quantum states and measurements

Self-testing is a device-independent way of testing whether a quantum device functions according to its specification. More precisely, it allows one to verify whether the device operates on a certain quantum state, and performs certain measurements on it, solely from the correlations this device generates. In this talk we present our recent results concerning self-testing of quantum states and measurements in Hilbert spaces of dimension higher than 2.

dr hab. Andrzej Łusakowski, prof. IF PAN
(Instytut Fizyki PAN)

Alloy broadening of the transition to the nontrivial topological phase of Pb_1-xSn_xTe

Transition between the topologically trivial and nontrivial phase of Pb_1-xSn_xTe alloy is driven by the increasing content x of Sn, or by the hydrostatic pressure for x < 0.3. We show that a sharp border between these two topologies exists in the virtual crystal approximation only. In more realistic models, the special quasirandom structure method and the supercell method (with averaging over various atomic configurations), the transitions are broadened. We find a surprisingly large interval of alloy composition, 0.3 < x < 0.5, in which the energy gap is practically vanishing. A similar strong broadening is also obtained for transitions driven by hydrostatic pressure. Analysis of the band structure shows that the alloy broadening originates in splittings of the energy bands caused by the different chemical nature of Pb and Sn, and by the decreased crystal symmetry due to spatial disorder. Based on our results of ab initio and tight-binding calculations forPb_1-xSn_xTe we discuss different criteria of discrimination between trivial and nontrivial topology of the band structure of alloys.

dr Krzysztof Pawłowski
(Centrum Fizyki Teoretycznej PAN)

Roton in a few-body dipolar system - where the ideas of Landau, Lieb and Zakharov meet together

The work is inspired by the connections between different descriptions of the many-body system. In the long introduction, I will recall the basics of two models. One is the many-body model of N interacting particles, solved by E. Lieb. The second, valid for weak interaction and at low temperature, is the mean-field model. In the mean-field model one finds the famous Zakharov solitons. Their many-body counter-parts are the lowest energy eigenstate with fixed total momentum. We show that, in the system with dipolar interaction soliton may be smoothly transformed to so called roton state (notion introduced by L. Landau in the context of liquid Helium). The smooth transition is realized by simultaneous tuning short-range interactions and adjusting a trap geometry. With our methods we study the weakly interacting regime as well as the regime beyond the mean-field model.

dr hab. Tomasz Karpiuk
(Uniwersytet w Białymstoku)

Quantum Bose-Fermi 3D droplets

We study stability of a zero temperature mixture of attractively interacting degenerate bosons and spin-polarized fermions in the absence of confinement. We demonstrate that higher order corrections to the standard mean field energy of the system can lead to a formation of liquid droplets -- self-bound incompressible systems in a three-dimensional space. The stability analysis of the homogeneous system is supported by numerical simulations of finite systems by explicit inclusion of surface effects. Our results indicate that Bose-Fermi droplets can be realized experimentally.

prof. Vesselin Tonchev
(Wydział Fizyki Uniwersytetu Sofijskiego)

Three etudes on modelling the instabilities and patterning of crystal surfaces

The three parts of the talk are devoted to different physical phenomena but unified by the modelling approaches. The first one provides a modelling perspective on the measurements of fractal dimension of ice on the nanoscale. The increase of the fractal dimension of ice aggregates in certain range of temperatures is reproduced by a 2D lattice model based on a combination of Cellular Automaton and Monte Carlo (CA+MC), here the change is achieved by fine tuning the diffusion-to-kinetics rate. Accent is put on results that go beyond reproducing the experiments. In the main part a CA+MC model in (1+1)D is used to provide new results from modelling surface instabilities and patterning that were observed experimentally on semiconductor, Si(111) and metal, W(110), vicinal crystal surfaces. Here the change of the diffusion-to-kinetics rate is causing also increase of the step transparency. A unified perspective is used for both for the intermediate, in terms of time-scaling, and for the initial instability stages, in terms of stability diagrams. The third part presents a new model that is built with recent experiments and modelling in mind that are done in the IP-PAS on patterning in immiscible crystals. In each of the three parts attention is paid both on the appropriate retrospective on the experiments done by other groups and on the perspectives for future work.

The second two parts contain results obtained within the long term collaboration with the team from IP-PAS Prof. Magdalena Załuska-Kotur, Dr. Filip Krzyżewski and Dr. Marcin Minkowski.
Member of the team from Bulgarian side is also Dr. Hristina Popova from IPC - BAS.

Dr. Silas Hoffman
(University of Basel)

Majorana Fermions in Dots, Chains, and Wires

Majorana fermions are charge- and spinless states that reside at the end of one-dimensional topological superconductors. Their potential use as topologically stable quantum memory has compelled many experimentalists to synthesize topological superconductors in hyrbid nanostructures with strong spin orbit interactions by coupling nanowires or magnetic adatoms to superconductors. Although experiments have provided evidence for Majorana fermions, a smoking gun confirmation of their existence has been elusive. Even more challenging is the operation of a Majorana qubit which has not yet been experimentally realized. To overcome these challenges, we propose alternative nanostructures which utilize quantum dots, chains of magnetic atoms, and nanowires coupled to topological superconductors to realize, detect, and manipulate Majorana fermions.

prof. dr hab. Witold Bardyszewski
(Uniwersytet Warszawski)

Topological phase transition and magnetoconductance in In-rich InGaN/GaN quantum wells

A short overview of the key properties of a hypothetical topological insulator based on InGaN/GaN quantum wells will be presented. The discussion will cover the basic band structure properties as well as the influence of the magnetic field and disorder on the quantum transport in such systems.

dr Timo Hyart
(MagTop - Instytut Fizyki PAN)

Majorana fermions and topological quantum computing: An overview

I will try to describe the development of this field from abstract mathematical concepts to down-to-earth industry projects aiming for the demonstration of the non-Abelian braiding statistics and realization of a topological quantum computer.

prof. dr hab. inż. Paweł Machnikowski
(Politechnika Wrocławska)

Spontaneous emission and excitation diffusion in a system of dipole-coupled two-level atoms

In the first part of my talk I will present the results of modeling of the spontaneous emission from a system of coupled quantum emitters. I will start with some older results regarding the conditions of occurrence of the collectively accelerated emission (“superradiance”). Next, I will discuss the photon-photon correlation properties resulting from the collective nature of the emission. In the second part, I would like to present our latest results on the diffusion of excitation in such a system of quantum-coupled dipolar emitters. Both areas of research are motivated by problems in the field of optics of quantum dot systems, although it seems that the models themselves may be of interest from a more general point of view. In particular, the diffusion of excitation can be discussed in terms of Anderson localization in a system with 1/r coupling.

dr Paweł Zin
(Wydział Fizyki, Uniwersytet Warszawski)

Exact Quantum Dynamics of Bose-fermi Systems via Classical Field Stochastic Equations

In the last years, methods of mapping exact quantum dynamics of interacting bosons or fermions into classical field stochastic methods were invented. They were successfully used in numerical simulations of imaginary- and real- time dynamics of many systems. I will generalize these methods to treat Bose-Fermi systems.

dr inż. Jan Tuziemski
(Politechnika Gdańska)

Redundant information encoding during decoherence in a non-Markovian spin-boson model

In this talk I will discuss decoherence process of a two-level system interacting with non-Markovian bosonic environment. In particular I will focus on information about the system that is redundantly encoded in the environmental degrees of freedom. I will also comment on recent claims that non-Markovianity hinders emergence of objective classical reality out of quantum mechanical description within quantum Darwinism paradigm.

dr Marta Galicka
(Instytut Fizyki PAN)

Rashba splitting in low dimensional topological crystalline insulator structures

Exploiting the spin degree of freedom of the electrons is one of the primary goals in the rapidly growing field of spintronics. A promising candidate to achieve this goal is so-called Rashba effect observed in the inversion non-symmetric systems and with strong spin-orbit interaction. During my talk I will present the results of theoretical modeling of Rashba splitting in (111)-oriented topological crystalline insulator (TCI) PbSnTe films doped with Bi atoms as well as in TCI Pb_1-xSn_xSe surface quantum wells on Pb_1-yEu_ySe barriers with an additional Sn submonolayer on the surface. The presence of Bi atoms and Sn submonolayer affects the carrier concentration in the crystal and modifies the effective potential at the surface. The latter effect was simulated in the calculations by applying a potential described by Thomas-Fermi screening model. Using the tight-binding method we have calculated the surface spectral density of states of the materials. The theoretical tight-binding results will be compared with experimental angle resolved photoemission investigations of such structures grown by molecular beam epitaxy.

dr Ching-Kai Chiu
(University of Maryland)

Topological band crossings in 3D time-reversal symmetric hexagonal materials

Topological states of matter start from topological insulators and superconductors, which are fermionic systems with bulk energy gaps separating the valence and conduction bands. They possess gapless boundary states that are topologically protected and are related to physical quantities, such as quantized Hall conductivity. Topological semimetals, which are another type of topological states, exhibits band crossings near the Fermi energy, which are protected by the topological invariants. In many cases, these topological band degeneracies give rise to exotic surface states and unusual magneto-transport properties. In this talk, I will first review the family of topological phases by studying topologically-protected surface and bulk states. I further present a complete classification of all possible topological band degeneracies in 3D hexagonal materials with strong-orbit coupling under time-reversal and nonsymmorphic symmetries and identify existing material candidates where the topological band degeneracies are realized.

dr Tomasz Wasak
(Uniwersytet Warszawski)

Ultracold collisions and their application for magnetic field sensing

Feshbach resonances, which allow for tuning the interactions of ultracold atoms with an external magnetic field, have been widely used to control the properties of quantum gases. I will present a scheme for using scattering resonances as a probe for external fields, showing that by carefully tuning the parameters it is possible to reach a high level of precision with a single pair of atoms. The collisional setup allows to saturate the quantum precision bound with a simple measurement protocol.

prof. dr hab. Jacek Dziarmaga
(Uniwersytet Jagielloński)

Quantum tensor networks in 2D

After two successful decades of 1D methods like the celebrated DMRG, quantum tensor networks emerge as a competitive tool -- free of the notorious sign problem -- to study strongly correlatedcondensed matter systems in 2D. For instance, they provide the most accurate results on the ground state of the Hubbard model. It is thus the highest time to extend these methods to finite temperature. In my talk, after brief introduction to tensor networks in general, I will describe the concept of our 2D algorithm developed to simulate thermal states of strongly correlated systems and present its benchmark results in the compass model, the Hubbard model, the e_g model, and for fermions on a hexagonal lattice. Futhermore, I will show how -- with the help of scaling arguments -- the algorithm can be used to estimate critical temperature of a second order phase transition with high accuracy. Finally, as a highlight of a new line of research, I will mention our work on detecting topological order in a 2D tensor network describing the ground state of a strongly correlated system.

dr hab. Rafał Demkowicz-Dobrzański
(Uniwersytet Warszawski)

From Quantum Master Equation to Adaptive Quantum Metrology

A general model of frequency estimation in presence of Markovian noise is considered, where the parameter to be estimated is associated with the Hamiltonian part of the dynamics. In absence of noise, unitary parameter can be estimated with precision scaling as T^(-1), where T is the total probing time. We provide a simple algebraic condition involving solely the operators appearing in the quantum Master equation, implying at most T^(-1/2) scaling of precision under the most general adaptive quantum estimation strategies. A construction of a quantum error-correction like protocol is presented that provides 1/T precision scaling in case the above mentioned algebraiccondition is not satisfied.

mgr Andrzej Syrwid
(Uniwersytet Jagieloński)

Crystallization in time domain

In analogy to spontaneous breaking of continuous space translation symmetry in the process of space crystal formation, it was proposed that spontaneous breaking of continuous time translation symmetry could lead to time crystal formation. In other words, a time-independent system prepared in the energy ground state is expected to reveal periodic motion under infinitely weak perturbation. In the case of the system proposed originally by Frank Wilczek, spontaneous breaking of time translation symmetry can not be observed if one starts with the ground state. We point out that the symmetry breaking can take place if the system is prepared in an excited eigenstate. The latter can be realized experimentally in ultra-cold atomic gases. We simulate the process of the spontaneous symmetry breaking due to measurements of particle positions and analyze the lifetime of the resulting symmetry broken state.

dr Debraj Rakshit
(Instytut Fizyki PAN)

Quantum liquid droplets in Bose-Fermi mixture

Quantum droplets are self-bound systems in free space due to a subtle balance of attractive and repulsive interactions. They are incompressible and behave like liquids. Their equilibrium densities are determined by interactions. In this talk I shall discuss about formation of the zero-temperature stable droplets in a mixture of attractively interacting degenerate bosons and spin-polarized fermions. Our analysis indicates that quantum fluctuations play a crucial role in stabilization of these systems.

dr Mario Cuoco
(CNR-SPIN and Dipartimento di Fisica, Università di Salerno)

Exploring novel quantum platforms for spinorbitronics and superconducting spintronics

This talk will deal with the presentation of quantum materials platforms with potential impact in the area of spinorbitronics and superconducting spintronics which combine spin-orbit coupling, superconductivity and topological effects. We have theoretically considered the interplay between geometric curvature effects [1] on the electronic properties and the topological properties of the quantum states in low-dimensional nanomaterials. I will discuss how geometric effects in low-dimensional nanomaterials can lead to metal-insulator transition and promote the generation of topological states of matter [2], for instance upon the application of a rotating magnetic field one can realize the topological pumping protocol in an entirely novel fashion [3]. Then, I will present the intricate twist between spin texture and spin transport in shape deformed nanostructures including the superconducting case [4,5]. Finally, I will consider the interplay of superconductivity and magnetism in heterostructures with a special focus on topological superconductors [6,7,8,9].

[Acknowledgements - EU-FET OPEN project "CNTQC" , grant agreement N. 618083 (http://www.nano2qc.eu/) ]
1) P. Gentile, M. Cuoco, C. Ortix, SPIN, Vol. 3, No. 2, 1340002 (2013).
2) P. Gentile, M. Cuoco, C. Ortix, Phys. Rev. Lett. 115, 256801 (2015).
3) S. Pandey, N. Scopigno, P. Gentile, M. Cuoco, C. Ortix, arXiv:1707.08773 (2017).
4) Z.-J. Ying, P. Gentile, C. Ortix, and M. Cuoco, Phys. Rev. B 94, 081406(R) (2016).
5) Z.-J. Ying, M. Cuoco, C. Ortix, and P. Gentile, Phys. Rev. B 96, 100506(R) (2017).
6) P. Gentile, M. Cuoco, A. Romano, C. Noce, D. Manske, P. M. R. Brydon, Phys. Rev. Lett. 111, 097003 (2013).
7) A. Romano, P. Gentile, C. Noce, I. Vekhter, M. Cuoco, Phys. Rev. Lett. 110, 267002 (2013).
8) D. Terrade, D. Manske, and M. Cuoco, Phys. Rev. B 93, 104523 (2016).
9) M. T. Mercaldo, M. Cuoco, P. Kotetes, Phys. Rev. B 94, 140503(R) (2016).

prof. dr hab. Jan Mostowski
(Instytut Fizyki PAN)

Bell's inequalities - in search of Planck's constant

Violation of Bell's inequalities provides strong arguments that quantum mechanics cannot be modelled by a classical probabilistic theory. All physical quantities that appear in Bell's inequality are dimensionless. On the other hand it is expected that quantum theory should have a classical limit when Planck's constant tends to zero. I will show a generalization of quantities present in Bell's inequalities, explicitly depending on Planck's constant. I will show how the classical limit of relevant quantum systems is reached.

dr hab. Bogdan Damski
(Uniwersytet Jagielloński)

1D and 2D Bose-Hubbard models

Bose-Hubbard models capture physics of cold atoms in optical lattices. Despite their simplicity, they are not exactly analytically solvable in any dimension.
In the first part of the talk, I will discuss basic properties of the 1D Bose-Hubbard model focusing on physically meaningful observables such as the ground state energy, the variance of on-site atom number occupation, and the nearest-neighbor correlation function. I will present analytical results coming from very high-order perturbative expansions and carefully compare them to numerical simulations.
In the second part of the talk, I will show how the position of the critical point of the 2D Bose-Hubbard model can be efficiently obtained from the variance of on-site atom number occupation and the nearest-neighbor correlation function. The analogies between the experimental studies of the lambda transition in liquid helium-4 and our theoretical studies of the superfluid-Mott insulator transition of the 2D Bose-Hubbard model will be discussed.

dr Wojciech Brzezicki
(International Research Centre MagTop)

Exotic topological states in hybrid transition metal oxides

The interplay between localized 3d and more delocalized 4d states in hybrid transition metal oxides tunes the competition between metallic and insulating states. For instance, doping of the 3d^3/3d^2 ions in the 4d^4 host realizes orbital/charge doping scenario. For the d^3 doping the competition between localized antiferromagnetism and itinerant ferromagnetism can lead to one-dimensional (1D) zigzag magnetic structures [1] whose non-symmorphic symmetries stabilize exotic Dirac semi-metal phases with multiple topological protection [2]. On the other hand the d^2 doping in the $d^4$ system leads to a pairing mechanism and effective non-uniform Kitaev model [3] in 1D, with hidden Lorentz symmetry and hosting topological phases even in disordered cases.

W.B. acknowledges support by the EU Horizon 2020 grant No. 655515.

References:
[1] W. Brzezicki, C. Noce, A. Romano, M. Cuoco, Phys. Rev. Lett. 114,247002 (2015).
[2] W. Brzezicki and M. Cuoco, Physical Review B 95, 155108 (2017).
[3] W. Brzezicki, A.M. Oleś, and M. Cuoco, Phys. Rev. B 95, 140506(R) (2017).

dr inż. Michał Piotr Nowak
(AGH, Akademickie Centrum Materiałów i Nanotechnologii)

Hunting for Majorana bound states in hybrid nanostructures

Majorana fermions are engineered as emergent bound states in hybrid superconductor-semiconductor naonstructures. In this talk I will report on the measurements of the key ingredients necessary for the reliable detection of the Majorana modes in proximitized nanowires: ballistic transport through a normal-superconductor junction [1] and spin-momentum locking due to strong spin-orbit interaction [2]. Finally, I will explain how the orbital effects of a magnetic field help to achieve the topological zero-energy-modes [3].

[1] H. Zhang, Ö. Gül, S. Conesa-Boj, M. P. Nowak, M. Wimmer, K. Zuo,V. Mourik, F. K. de Vries, J. van Veen, M. W. A. de Moor, J. D. S.Bommer, D. J. van Woerkom, D. Car, S. R Plissard, E.P.A.M. Bakkers, M.Quintero-Pérez, M. C. Cassidy, S. Koelling, S. Goswami, K. Watanabe, T. Taniguchi, L. P. Kouwenhoven, Nat. Commun. 8, 16025 (2017).
[2] J. Kammhuber, M. C. Cassidy, F. Pei, M. P. Nowak, A. Vuik, D. Car, S. R. Plissard, E. P. A. M. Bakkers, M. Wimmer, and L. P. Kouwenhoven, Nat. Commun. 8, 478 (2017).
[3] M. P. Nowak, P. Wójcik, arXiv:1711.06643 (2017)

dr Marcin Płodzień
(Instytut Fizyki PAN)

Simulating polaron biophysics with Rydberg atoms

Transport of excitations along proteins can be formulated in a quantum physics context, based on the periodicity and vibrational modes of the structures. Exact solutions are very challenging to obtain on classical computers, however, approximate solutions based on the Davydov ansatz have demonstrated the possibility of stabilized solitonic excitations along the protein. We propose an alternative study based on a chain of ultracold atoms. We investigate the experimental parameters to control such a quantum simulator based on dressed Rydberg atoms. We show that there is a feasible range of parameters where a quantum simulator can directly mimic the Davydov equations and their solutions. Such a quantum simulator opens up new directions for the study of transport phenomena in a biophysical context.

mgr Jan Krzywda
(Instytut Fizyki PAN)

Localization of a magnetic moment using a two-qubit probe

Spin qubits such as NV centers in diamond are currently used as nanoscale-resolution sensors of magnetic field noise, generated by nuclear spins of molecules localized in the neighbourhood of the sensor [2,3]. Subjecting the qubit to a periodic sequence of rotations makes it sensitive only to fluctuations of given frequency [4]. When the magnitude of the fluctuating field associated with given source and the form of qubit-source interaction are known, measurement of the qubit decoherence allows one to find a surface on which the source is located.
Extending this idea to the multi-qubit sensor should enable to pin-point the exact location of the source (cf. the classical triangulation procedure). Here we show that already with two qubits, each subjected to a given sequence of rotations, one can localize the fluctuating magnetic moment. The method can be viewed as an application of a general protocol for performing noise cross-correlation spectroscopy to the case of two qubits interacting with a precessing magnetic moment. We consider not only the case of the sensors localized in predefined spots, interacting via dipolar coupling with the moment, but also situations with less a priori knowledge available, e.g. when the relative position of the two qubits is unknown or different coupling is present. We show that source localization is also possible then, provided we have control over external magnetic field direction.
Alternatively, when the form of the qubit-moment coupling is not known, we describe the idea of symmetry detector, capable of narrowing down the possible source location. We emphasize the existence of sensor-source arrangement leading to robust decoherence-free evolution, and provide its geometrical interpretation.

[1] J. Krzywda et al., Accepted to Phys. Rev. A, arXiv:1706.02948 (2017).
[2] T. Staudacher et al., Science 339, 561 (2013).
[3] I. Lovchinsky et al., Science. 351, 836 (2016).
[4] P. Szańkowski, G. Ramon, J. Krzywda, D. Kwiatkowski, and L. Cywiński, Journal of Physics Condensed Matter 29, 33 (2017).
[5] P. Szańkowski, M. Trippenbach, and L. Cywiński Phys. Rev. A 92, 123 (2016).

dr Marcin Wysokiński
(MagTop i Instytut Fizyki PAN)

Efficient variational approach to strongly correlated fermions at and far from equilibrium

I am going to present a new method combining Gutzwiller wave function with a variational Schrieffer-Wolff transformation that allows one to accurately describe correlated fermions, as yet in infinite dimensions, with a minimal computational effort. I will demonstrate application of the technique to the half-filled, single-band Hubbard model on the infinitely coordinated Bethe lattice for a description of a Mott transition at equilibrium [1] as well as a dynamical Mott transition when system is driven far from equilibrium via the interaction quench [2]. If time permits I will comment on the possibilities of the use of the developed in [2] new form of a non-equilibrium Schrieffer-Wolff transformation for a manipulation of magnitude and sign of the spin-exchange by electric field impulses or by a change of an interaction amplitude.

[1] M. M. Wysokiński, M. Fabrizio, Mott physics beyond the Brinkman-Rice scenario, Phys. Rev. B 95, 161106(R), 2017[2] M. M. Wysokiński, M. Fabrizio, Interplay of charge and spin dynamics after an interaction quench in the Hubbard model, arXiv:1711.01840

mgr inż. Paweł Miętki
(Instytut Fizyki PAN)

Magnetic self-trapping of exciton-polariton condensate

Bose Einstein condensation can be achieved in the solid state. An example is the exciton-polarioton condensate in a diluted magnetic semiconductor microcavity. Such system may exhibit magnetic self-trapping in the case of sufficiently strong coupling between polaritons and magnetic ions embedded in the semiconductor. I will show the simple theoretical model describing the nonequilibrium nature of this exciton-polaritons.

dr Krzysztof Zegadło
(Uniwersytet Jagielloński)

Optical processes in nanostructures with gain and loss

Gain and loss are omnipotent in the physical, chemical and biological systems. Their effects can in a convenient way be modelled by effective non-Hermitian Hamiltonians. Imaginary contributions to the potential introduce source and drain terms for the probability amplitude. A special class of non-Hermitian Hamiltonians are those which possess a parity-time symmetry. In spite of their non-Hermiticity these Hamiltonians allow for real energy eigenvalues, i.e. the existence of stationary states in the presence of balanced gain and loss. This effect has been identified theoretically in a large number of quantum systems. Its existence has also been proved experimentally in coupled optical waveguides. The dynamics can be very interesting and worth studying even if the parity-time symmetry is not conserved. The list of systems that belong to this class include whispering gallery modes in the micro-resonators, coupled waveguides, unidirectional reflectionless metamaterial at optical frequencies, polariton condensates and may, many others. In my talk I consider a nanostructure of two coupled ring waveguides with constant linear gain and nonlinear absorption - the system that can be implemented in various settings including polariton condensates, optical waveguides or atomic Bose-Einstein condensates. It was found that, depending on the parameters, this simple configuration allows for observing several complex nonlinear phenomena, which include spontaneous symmetry breaking, modulational instability leading to generation of stable circular flows with various vorticities, stable inhomogeneous states with interesting structure of currents flowing between rings, as well as dynamical regimes having signatures of chaotic behavior.

dr Julia Stasińska
(Instytut Fizyki PAN)

Exact diagonalization vs. cluster mean-field method for a non-standard bosonic model

Ultracold bosonic atoms in a one-dimensional, spin-dependent optical lattice are described by a non-standard Bose-Hubbard model with next-nearest-neighbour correlated hopping. The model was derived by analogy to the fermionic Hamiltonian studied in [R. W. Chhajlany et al., PRL 116, 225303 (2016)], known also as tha Bariev model [R. Z. Bariev, JPA 24, L919 (1991)], and reproduces it in the limit of hard-core bosons. Unlike in the fermionic case, however, the analysis relies heavily on numerical methods: exact diagonalization and a cluster Gutzwiller ansatz method. I will discuss both the phase diagram of the bosonic model and the performance of the used numerical methods.

dr hab. Piotr Szymczak, prof. UW
(Uniwersytet Warszawski)

In search of the ideal form: smooth wormholes and solution pipes

Dissolution of fractured and porous media introduces a positive feedback between fluid transport and chemical reactions at mineral surfaces leading to the formation of pronounced wormhole-like channels. Using the combination of experimental, numerical and analytical techniques, we study the shapes of these channels as a function of the flow rate, reaction rate and initial geometry of the system.
In particular, we are interested in finding stationary forms, which advance into the system with constant velocity and shape. The most interesting result is the appearance of a morphological phase transition in the channel shapes as the flow rate is increased. At high flows, well-separated, cylindrical shafts are formed, of a nearly uniform diameter all along their lengths. They advance quickly into the matrix, with velocities several times larger than that of a unperturbed, planar dissolution front. On the other hand, for small flow rates, the channels are funnel-shaped with parabolic tips and their advancement velocity is of the same order as that of a planar front. The transition between the two forms is abrupt, with no intermediate forms observed. We relate these results to the natural dissolution structures in karst landscape.

dr Piotr Szańkowski
(Instytut Fizyki PAN)

Principles of Dynamical Decoupling based spectroscopy

A qubit-probe subjected to pure dephasing due coupling with enviromental noise can be turned into a spectrometer of this signal. In an appropirate regime of evolution parameters, the measured decay rate of qubit coherence is given by the signal's spectral density spanned on frequency comb defined by the DD pulse sequences. Using properly chosen sequences allows for inverting this relation, and thus, the reconstruction of the spectrum. I will review the theory behind this DD-based spectroscopy technique: the assumptions, principles of operation, limitations, and possible extensions.

mgr Daniel Pęcak
(Instytut Fizyki PAN)

Qubit-environment entanglement generation during pure dephasing

A simple theoretical model of a few interacting ultracold fermions will be presented. I will show that the mass imbalance in a strongly interacting few-body system leads to spatial separation and that the mechanisms and the kind of the spatial separation are strongly dependent on the shape of an external potential. The change of the shape of the confinement induces a unique type of phenomenon which exhibits many features of phase transitions.

dr hab. Katarzyna Roszak
(Politechnika Wrocławska)

Qubit-environment entanglement generation during pure dephasing

The problem of detecting entanglement between a qubit and its environment is known to be complicated [1]. To simplify the issue, we study the class of Hamiltonians that describe a qubit interacting with its environment in such a way that the resulting evolution of the qubit alone is of pure dephasing type, i.e. when the Hamiltonian of the qubit commutes with the qubit-environment interaction term. This relation means that the eigenstates of the qubit Hamiltonian form a preferred basis - they are pointer states [2] - selected by the form of the qubit-environment coupling. When both the qubit and the environment are initially in a separable pure state, an interaction between them that leads to a pure dephasing of the qubit always leads to the creation of entanglement between the two. It is often assumed that a dephasing mechanism of this type must induce entanglement between the qubit and environment also when the environment is initially in a mixed state. In [3] we showed that while the creation of qubit-environment entanglement in the pure dephasing case is possible when the environment is initially in a mixed state, the occurrence of this entanglement is by no means guaranteed. I will discuss the criteria for initial state of the environment and for the relevant evolution operator under which the qubit-environment entanglement is generated. I will also show that for a certain class of initial environmental states, the detection of any change of the state of the environment is then equivalent to the detection of entanglement.

[1] B. Kraus, J. I. Cirac, S. Karnas, and M. Lewenstein, Phys. Rev. A 61, 062302 (2000).
[2] W. H. Żurek, Rev. Mod. Phys. 75, 715 (2003).
[3] K. Roszak and Ł. Cywiński, Phys. Rev. A 92, 032310 (2015).

dr Nataliya Bobrovska
(Instytut Fizyki PAN)

Interactive optomechanical coupling with nonlinear polaritonic systems

We study the systems of exciton-polaritons and dipolaritons inside the optomechanical cavity. In cavity optomechanics the optical properties of photonic systems are influenced by the motion of a mechanical oscillator. We show that the presence of the mechanical mode and the resonance frequency of the cavity results in a new type of optomechanical coupling - interactive coupling. This type of coupling generates the effective optical nonlinearity (high order nonlinear terms), which can exceed in magnitude the strength of Kerr nonlinear terms. Additionally, we show numerically the generation of the localized bright flat-top solitons in exciton-polariton condensates. The appearance of flat-top solitons results from the high order nonlinear terms.

prof. dr hab. Krzysztof Sacha
(Uniwersytet Jagielloński)

Condensed matter physics in time crystals

In condensed matter physics it is often assumed that space crystals are already formed and their properties are analyzed with the help of a space periodic Hamiltonian. Analogues of space periodic systems but in the time domain are time periodic systems. It will be shown that solid state phases can be realized in the time domain in periodically driven systems.

dr hab. Andrzej Łusakowski, prof. IF PAN
(Instytut Fizyki PAN)

Selected aspects of mathematics of topological insulators

Quantum Hall effect, topological insulators and crystalline topological insulators belong to the main topics in the field of condensed matter. Theoretical description of these phenomena uses ideas developed in topology. In my presentation I will introduce Chern numbers, Z2 index, and mirror and spin Chern numbers. I will discuss these concepts in a qualitative way and explain their meaning from the mathematical point of view. In particular I will show the difference between topological indices for systems with and without time reversal symmetry. I will also show the need of new index for systems with time reversal symmetry.

dr Krzysztof Pawłowski
(CFT PAN)

Towards ultracold Schrödinger cat

Theoretically, a cloud of ultracold atoms in the coherent state should itself, due to atom-atom interaction, evolves towards a Schrodinger cat state. We take this scheme seriously and we study effects of the important decoherence effects and practical limitations to estimate what kind of entangled states are possible to achieve in nowadays experiments with ultracold atoms.

dr hab. Paweł Jakubczyk
(Uniwersytet Warszawski)

Functional renormalization group in equilibrium condensed matter systems

I will introduce the functional renormalization group as a reformulation of the equilibrium many-body problem into a differential form. I will demonstrate its usefulness on four examples: 1. classical O(N) models; 2. effective field theory for quantum criticality; 3. the XY model in two dimensions; 4. the imbalanced Fermi mixture in two dimensions.

prof. dr hab. Jakub Tworzydło
(Wydział Fizyki UW)

From Thouless invariant to Weyl semimetal

The main purpose of this seminar is to review results of David Thouless, that earned him Nobel Prize in 2016. In particular, I intend to introduce and explain in pedagogical terms the Thouless invariant, which arises in a natural manner as the explanation of the quantum Hall effect. This invariant, which is also known as the Chern number, has recently been recognized to underlay the topological properties of some 3D metals. The low energy excitations of electrons are effectively described by the Weyl equation in such a topological metal, and so the material is known as a Weyl semimetal. Some of the unusual properties of Weyl semimetals, such as the presence of Fermi arcs and the chiral anomaly will be briefly discussed.

dr hab. Jan Chwedenczuk
(Wydział Fizyki UW)

Violation of Bell inequalities in a many-body system of massive particles

Entanglement between two separate systems is a necessary resource to violate a Bell inequality in a test of local realism. We demonstrate that to overcome the Bell bound, this correlation must be accompanied by the entanglement between the constituent particles. This happens whenever a super-selection rule imposes a constraint on feasible local operations. Next, we propose an experiment, where the violation of the Bell inequality occurs between two distant parts of a many-body system of massive particles. The source of correlated atoms is a spinor Bose-Einstein condensate residing in an optical lattice, as in: M. Bonneau, et al., Phys. Rev. A 87, 061603 (2013). We characterize the complete experimental procedure, together with the local operations and the measurements, necessary to run the Bell test. We show how the amplitude of the violation of the Bell inequality depends on the strengths of the two-body correlations and on the number of scattered pairs.

dr Michał Tomza
(Centrum Nowych Technologii UW)

Engineering ultracold quantum ion-atom systems

Hybrid systems of laser-cooled trapped ions and ultracold atoms combined in a single experimental setup are currently emerging as a new platform for fundamental research in quantum physics. In my talk, I will present a few approaches to prepare and manipulate ion-atom systems at ultralow temperatures. Particularly, I will show the prospects to avoid radiative charge exchange losses in quantum simulation and computation applications and to reach the s-wave quantum regime of ion-atom scattering by the ionization of weakly bound Rydberg molecules. At the end, I will discuss the unusual properties of molecular ions in weakly bound rovibrational states which have a giant permanent electric dipole moment and a giant electric dipole polarizability.

dr inż. Gabriel Wlazłowski
(Politechnika Warszawska)

Towards accurate description of non-equilibrium dynamics in superfluid Fermi systems

Presently, the most accurate microscopic approaches to the dynamics of Fermi systems are based on the density functional theory (DFT). In the past decade tremendous effort has been done in order to extend the DFT approach towards superfluid Fermi systems. Implementable of the method in realistic calculations has became possible only recently. Two factors played an crucial role: a) the development and validation against experiment of an appropriate microscopic framework for the structure and dynamics of fermionic superfluids, and b) the implementation of this framework using sophisticated numerical algorithms that fully utilize the advanced capabilities of modern leadership class supercomputers. The formulation is known as time-dependent superfluid local density approximation (TDSLDA) and it proved to be very accurate for describing dynamics of strongly correlated fermionic systems, like ultracold atomic gases and nuclear systems. In my talk I will summarize these developments and I show recent applications of the method to ultracold fermionic gases, neutron stars and nuclear reactions. Although, these applications looks as rather distinct problems it is fascinating that they can be simulated within a common theoretical framework.

prof. dr hab. Piotr Bogusławski
(Instytut Fizyki PAN)

Transition metal ions in semiconductors: theory vs experiment

After a brief introduction of theoretical methods based on density functional theory, I'll apply this approach to discuss the main physical effects that determine electronic and magnetic properties of transition metal ions semiconductors (Mn and Fe ions in GaN, ZnO, etc.). Of particular interest is the role of electron-electron interaction, revealed in recent experiments at IF PAN and University of Warsaw.

dr Emilia Witkowska
(Instytut Fizyki PAN)

Spin squeezing and entanglement in spinor condensates

In the seminar I will introduce concepts of spin squeezing and entanglement in spinor condensates. I will give an overview of recent experiments that demonstrated the squeezing and entanglement in a system that was generated dynamically from spin coherent state, discuss possible limits imposed by decoherence effects and show theoretically calculated relevant limits. In the end of my talk, I will present interesting entangled properties of spinor condensates at thermal equilibrium.

dr Filip Krzyżewski
(Instytut Fizyki PAN)

Various surface morphologies observed during simulations of GaN crystals growth

I will present 2-component kinetic Monte Carlo model of GaN polar planes which can also be used in simulations of other wurtzite structure crystals. I will describe possible applications of the model and show variety of surface patterns obtained at different growth conditions. Numerical results will be compared with experiments. I will also present mathematically calculated quantities describing observed patterns.

dr hab. Piotr Deuar, prof. IF PAN
(Instytut Fizyki PAN)

Quantum atom optics with Bose-Einstein condensates

In the quantum degenerate regime, atomic Bose gases are described well by a second-quantized Bose field. Formally, the main difference between the what happens in the basic ultracold Bose gas and what happens in quantum optics is a different single-particle energy spectrum. It becomes quadratic and non-superfluid at high enough velocity. Due to the similarities, many of the phenomena from quantum optics such as correlated pair production, squeezing, particle entanglement, reappear though often changed in interesting ways. I will discuss experiments with condensates of metastable Helium (He*) that create and detect such correlated atom pairs, and the ongoing quest to violate the Bell inequality with measurements that distinguish different distributions of rest mass. And of course, I will talk about the related theoretical calculations. This is a rather "fun" topic for theory and numerics, because none of the standard approaches (Gross-Pitaevskii, few-modes, classical field, Bogoliubov diagonalization) gives an accurate description, for reasons that I will go into.

Prof. Vesselin Tonchev
(Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria)

The different faces of the destabilized surfaces: What we see when we look at?

In the beginning is given a broader perspective on the studies of the minimal step-step distance in the bunch of steps with the size of the bunch. This still is a basic tool in the studies of step bunching on vicinal crystal surfaces[1]. Then it is shown that it is not sufficient to elucidate sufficient details for the evolution of the destabilized surfaces. Thus two different viewpoints are provided on their self-affinity - a self-similarity of special kind, in which two length-scales, the width and the size of the step bunches, evolve differently. One is based on extended Burton-Cabrera-Frank models, operating with systems of Ordinary Differential Equations, one for the velocity of each step from the vicinal surface[2]. The other one uses the recently designed atomistic scale model based on Cellular Automaton (vicCA). Both approaches depend strongly on the suitable monitoring protocols developed. In particular, we quantify the specific manifestation of self-affinity for the case of vicCA where there is no step-step repulsion to prevent the formation of macrosteps [3].

[1] K Siewierska, V Tonchev, Scaling of the minimal step-step distance with the step-bunch size: Theoretical predictions and experimental findings (a mini-review), arXiv:1611.09963.
[2] H Popova, A Krasteva, F Krzyżewski, M Załuska-Kotur, N Akutsu, V Tonchev, in preparation.
[3] F Krzyżewski, M Załuska-Kotur, A Krasteva, H Popova, V Tonchev, Journal of Crystal Growth, 2016, 10.1016/j.jcrysgro.2016.11.121.

prof. dr hab. Mariusz Gajda
(Instytut Fizyki PAN)

Quantum gases: experimental and theoretical background

In my talk I will present the advances in the field of ultracold atomic gases. I will start with a short description of experimental methods of cooling of dilute samples of bosonic atoms down to the nanokelvin range where the system reaches the state of quantum degeneracy and becomes Bose condensed. Next I will introduce a mean field approach and a working horse of the theory - the Gross-Pitaievskii equation. Based on the mean field description I will show how superfluidity, quantized vortices and solitons emerge in the system. Finally (if time permits) I will briefly discuss the issue of coherence in low dimensional systems, in particular the Berezinskii-Kosterlitz-Thouless phase in ultracold gasses in a flatland.

dr hab. Piotr Żuchowski
(Instytut Fizyki, Uniwersytet Mikołaja Kopernika, Toruń)

Cold collisions of metastable helium with atoms and molecules

Development of techniques for cooling and manipulating atoms and molecules was very impressive in past several years. One of the important outcomes of this amazing progress was emergence of new experiments on molecular and atomic collisions which provide information on interactions in unprecedented detail. It is usually believed that quantum chemistry methods are not accurate enough to provide interaction potentials good enough to predict collisional properties in ultracold regime, as the scattering is very sensitive for variation of the potential. I will revise this statement and discuss for what systems we might expect ab-initio methods to be accurate enough to quantitatively predict such parameters as the scattering lengths. I will give example of Penning-ionizing systems containing metastable helium (He^*) and closed-shell species. I will show how quantum chemistry methods can provide interaction potentials which predict low-energy shape resonances in He^*+molecular hydrogen system in recently performed experiments in Narevicius group (Weizmann Institute). I will discuss also other system for which theory provided crucial guidance: the ultracold mixture of metastable helium and rubidium atoms. This system was recently investigated experimentally by the group of Steven Knoop (VU Amsterdam) who determined the termalization rates of the He^*+Rb mixture in magnetic trap in which system experiences only the quartet potential. Theoretical calculations predicted very accurately the scattering lengths.

dr hab. Michał Matuszewski, prof. IFPAN
(Instytut Fizyki PAN)

Instability of an exciton-polariton condensate

The physics of quantum fluids of light and the related field of nonequilibrium condensation experience dynamic development in the recent years. The experimental realizations of exciton-polariton condensates ten years ago provided vast possibilities for investigating nonequilibrium quantum systems on an entirely new level, important for understanding of fundamentals of nonequilibrium physics.
At the same time, prototypes of devices for applications, such as low-threshold polariton lasers, quantum-enhanced interferometers or low-loss polariton circuits, have been realized. I will briefly describe the basic theory of exciton-polaritons as well as some of the most interesting experimental realizations and future directions. Commonly used theoretical models based on the mean-field generalized Gross-Pitaevskii equation and its extensions including the effects of quantum fluctuations will be discussed. In the second part of the talk I will describe our work related to the recent observations of instabilities of non-equilibrium exciton-polariton condensates. We demonstrate the first observation of the long-sought reservoir-induced instability in a nonresonantly pumpedcondensate. Without any free parameters, we find an excellent agreement between the experimental data and numerical simulations of the open-dissipative Gross-Pitaevskii equation, which allows us to dismiss the competing theoretical model based on the complex Ginzburg-Landau equation. In the case of resonant excitation, the observation of the real-space collapse of the polariton fluid was explained by the effective attractive interactions mediated by the lattice phonons. The observation of instabilities in polariton condensates may fundamentally change our understanding of superfluidity, coherence, and critical properties of these systems.

prof. dr hab. Magdalena Załuska-Kotur
(Instytut Fizyki PAN)

Surface pattern formation in crystal growth kinetics

Precise layer by layer crystal growth process became very important instrument used in production of nano-technological devices. On the basis of experimental experience it is clear that surface dynamics of growing crystal is one of the crucial factors that decide about stability of the process. From the other side formation of various geometric patterns during crystal is important phenomenon to study as an interesting example of far from equilibrium, stationary process. It remains a subject of continuous interests of many researchers. I will show how typical meandered or bunched step patterns at the surface can be studied by Kinetic Monte Carlo simulation method. They will be compared with experimental results obtained for GaN crystal growth. The main aim of the extended kMC study is to find proper model parameters - coupling constants between particles, energy barriers for each type of particle jump, such that describe the main characteristics of the studied system. The simplicity of the model and small number of control parameters allows studying systems of large particle numbers, performing long time simulations and describing the model behavior in various conditions. Larger systems can be analyzed by simpler, based on cellular automata model. Such models allow to study asymptotic time and size behavior. I will describe main features of this newly constructed model and show main results that were obtained by its means.

dr hab. Łukasz Cywiński, prof. IF PAN
(Instytut Fizyki PAN)

Kubity spinowe w półprzewodnikach: dekoherencja jako wróg i jako przyjaciel

W pierwszej części seminarium postaram się przedstawić obecny stan badań nad koherentną kontrolą dwupoziomowych układów opartych na spinach elektronów zlokalizowanych w półprzewodnikach. W drugiej części opowiem o teoretycznych zmaganiach ze zrozumieniem głównych mechanizmów dekoherencji spinu, a na zakończenie pokażę, jak z pomiaru dekoherencji wyciągnąć wiele informacji o własnościach otoczenia zaburzającego kubit.

dr hab. Tomasz Sowiński, prof. IF PAN
(Instytut Fizyki PAN)

Sieci optyczne: symulatory kwantowe dla fizyki materii skondensowanej

Metody doświadczalne współczesnej inżynierii kwantowej umożliwiają precyzyjne kontrolowanie atomów umieszczonych w tzw. sieciach optycznych - specjalnie wytwarzanych wiązek światła laserowego tworzących periodyczny potencjał. To sprawia, że szeroko rozumiane pojęcie sieci optycznej staje się nie tylko doskonałym narzędziem optyki kwantowej,ale również może stanowić milowy krok w zrozumieniu problemów fizyki materii skondensowanej. Wynika to z faktu, że możliwe staje się przygotowanie układów eksperymentalnych tak, aby były one niemalże idealną realizacją doświadczalną różnych rozszerzonych wariantów modelu Hubbarda. Tym sposobem sieci optyczne stają się niczym innym jak dedykowanymi symulatorami kwantowymi dla fizyki materii skondensowanej.

prof. dr hab. Ryszard Buczko
(Instytut Fizyki PAN)

Topologiczne Izolatory Krystaliczne. Od stanów powierzchniowych do Kwantowego Spinowego Efektu Halla.

Topologiczne Izolatory Krystaliczne (TIK) są to materiały o nietrywialnej strukturze pasmowej topologicznie chronionej przez symetrie grupy punktowej. Odkryto, że na powierzchniach krystalicznych roztworów stałych (Pb,Sn)Te oraz (Pb,Sn)Se istnieją metaliczne helikalne stany chronione przez symetrią zwierciadlaną. Pokazano, że spektrum energetyczne oraz tekstura spinowa stanów powierzchniowych silnie zależy od orientacji powierzchni. Dla przykładu, w przeciwieństwie do stanów powierzchniowych obserwowanych na powierzchni (001) stożki Diraca na powierzchni (111) sąod siebie odseparowane. Zredukowana wymiarowość cienkich warstw TIK wpływa na własności topologiczne układu i co za tym idzie na własności stanów powierzchniowych. Używając metody ciasnego wiąania pokazaliśmy, że w cienkich warstwach SnSe oraz SnTe zorientowanych w kierunku (111) przerwa energetyczna oscyluje w zależności od grubości warstwy. W obszarze grubości odpowiadającej około 20-40 warstw atomowych układ przechodzi do fazy dwuwymiarowego izolatora topologicznego. Pojawiają się spinowo spolaryzowane stany brzegowe typowe dla Kwantowego Spinowego Efektu Halla.

prof. Vesselin Tonchev
(Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria)

Diffusion-limited vs. kinetics-limited regime of step bunching: How to distinguish in between?

In this talk are presented numerical results from models of unstable vicinal crystal growth. In particular, they resolve the long-standing controversy between numerical results on the minimal step-step distance lmin in bunches and predictions of continuum theory - while the size-scaling exponent of lmin is found the same in diffusion-limited (DL) and kinetics-limited (KL) regime, and the time-scaling exponent of the bunch size N is predicted also to be the same and equal to 1/2, the scaling exponents of the bunch width W and lmin are predicted to distinguish in between. Using results from the model of unstable vicinal growth of Sato and Uwaha (SU) we propose a solution to that puzzle. Results from other models are discussed as well, and especially from vicinal Cellular Automata (vicCA). In particular, for the vicCA it is the time-scaling exponent of the macrostep size Nm that makes the difference, instead of that of W in the SU-model. In the prelude an account is given of the numerical results for the size-scaling of lmin in the two regimes for evaporation affected sublimation and the experimental evidence for DL regime. These are confronted with the predictions of the Pimpinelli et al. theory of universality classes in bunching and the correction of Krug et al. (KTSP) [1] to account for the differences in two regimes. Especially in the ρ = -1 universality class to which are expected to belong both vicinal evaporation and growth, only the time-scaling of W can distinguish between the two regimes. KTSP predict additionally that the time-scaling exponent of W in the KL regime is shifted with respect to the DL regime. Systematic numerical results for intermediate asymptotics from the SU-model [2] are obtained in the two regimes. These results show that the time-scaling exponents of W for the DL and for the KL are both shifted (differently) but the size-scaling exponent of lmin remains the same. Since SU model does not permit! studying the infinite Ehrlich-Schwoebel barrier we study his using vicCA[3]. In this model the time-scaling exponent of N is also invariant for DL and KL regimes (=1/3). Since there is no step-step repulsion incorporated in the model, besides bunching of mono-steps a macrostep formation is observed, thus lmin=0. The time-scaling exponent of Nm is 1/4 in the DL and 1/5 in the KL (see Fig. 1).

[1] Pimpinelli et al., Phys. Rev. Lett. 88, 206103 (2002); Krug et al., Phys. Rev. B 71, 206103 (2005).
[2] M. Sato and M. Uwaha, Surface science 493, 494 (2001).
[3] F. Krzyżewski, M. Załuska-Kotur, A. Krasteva, H. Popova, V. Tonchev, arXiv:1601.07371.

PDF file with full abstract and figures:   ( )

dr hab. Maciej Janowicz
(Szkoła Główna Gospodarstwa Wiejskiego)

Lower bounds for ground-state energy, overlap integrals and expectation values for driven Bose-Hubbard model

The problem of coupled resonators with nonlinear self-interaction of light in each resonator is essentially dynamical and such that the dissipation has to be taken into account. However, if the rotating wave approximation is justified, one obtains a well-defined time-independent Hamiltonian, the spectral properties of which are worth of attention. Using an old Lieb-Yamazaki technique and Weinhold's criteria, lower bounds for the energy of the ground and lowest excited states have been obtained for nearest-neighbors coupling between resonators. Those bounds have then been used to obtain lower bounds for the overlap integrals between the exact and a specially constructed trial wave function. The overlap integrals, in turn, have been employed to compute expectation values of some positive operators, including density-density correlation functions.

dr Krzysztof Wohlfeld
(Uniwersytet Warszawski)

Spin-orbital separation in the 1D cuprates

In contrast to collective magnetic excitations (such as e.g. magnons), the collective excitations which carry orbital degree of freedom (often called orbitons) are very hard to detect. Nevertheless, recent advancements in resonant inelastic x-ray scattering have allowed for a rather unambiguous detection of orbitons in various transition metal oxides -- in particular the quasi-1D copper oxides [1, 2]. Strikingly, a closer investigation of the observed 1D orbiton dispersion suggested that this dispersion could not be understood using a simple orbital wave picture [1, 3]. Instead it occurred that the orbitons are in general very strongly coupled to spin excitations. It is then only in 1D that they can decouple leading to a particularly strong dispersion which is due to a phenonemonen called spin-orbital separation: the initial orbital excitation fractionalizes into an independent orbiton and spinon excitation.

References:
[1] J. Schlappa, K. Wohlfeld et al.; Nature 485, 82 (2012).
[2] V. Bisogni, K. Wohlfeld, et al.; Phys. Rev. Lett. 114, 096402 (2015).
[3] K. Wohlfeld, M. Daghofer, S. Nishimoto, G. Khaliullin, and J. van den Brink; Phys. Rev. Lett. 107, 147201 (2011).

dr Emilia Witkowska
(Instytut Fizyki PAN)

Kibble-Zurek mechanism in antiferromagnetic spinor condensates

I will present our work concerning the dynamics and outcome of a quantum phase transition from an antiferromagnetic to a phase-separated ground state in a spin-1 Bose-Einstein condensate of ultracold atoms. We demonstrate the occurrence of two scaling laws, for the number of spin domain seeds just after the phase transition, and for the number of spin domains in the final, stable configuration. Only the first scaling can be explained by the standard Kibble-Zurek mechanism. We explain the occurrence of two scaling laws by a model including postselection of spin domains for the uniform system and transport of local magnetization for the trapped system.

Piotr Szańkowski
(Instytut Fizyki PAN)

Noise Spectroscopy with Qubit Probe

The discovery of Spin Echo in NMR led to development of Dynamical Decoupling (DD) techniques, where a qubit is undergoing a complex control scheme that effectively decouples it from the influence of the environmental noise. Initially, DD was conceived and applied as a way to extend the lifetime of qubit's quantum properties (its coherence), so that it could remain useful for various quantum-based task (like information processing or metrology) despite the unavoidable interference from the environment. Recently, there has been a paradigm shift in the literature. The environmental noise does not have to be merely a nuisance that stands in the way of "truly"interesting applications of qubits. Noise generated by a complex system bears useful information about its source - similarly how light from a distant star can be used to get a glimpse into it's innerworkings. Indeed, qubit under DD acts as a spectrometer which can analyze the fluctuation of the environmental signal; analogically to spectroscopy of light from celestial bodies. A single qubit driven by DD sequence can serve as a spectrometer of local noise. When two (or more) spatially separated qubits are used instead, one can reconstruct the spectrum of cross-correlations of noises acting at various locations. By adjusting the types of sequences used on each qubit it is possible, for example, to determine some transport properties of the environment, or to check whether its different part parts are causally correlated. These are just a few examples of potential applications stemming from DD-based noise spectroscopy.

dr hab. Janusz Stafiej
(Uniwersytet Kardynała Stefana Wyszyńskiego)

Korozja, pasywacja, nanopory w modelach typu automatów komórkowych

Przedstawione zostaną cztery zagadnienia z prac prelegenta dotyczących zjawisk korozji i pasywacji.

1. Przejście od wolnej do szybkiej korozji w modelu procesu korozyjnego, w którym następuje rozdział przestrzeni anodowych kwaśnych i katodowych zasadowych w wyniku autokatalitycznego charakteru procesu korozyjnego.
2. Model procesu pasywacji opartu na migracji anionów (wakansów, defektów) w warstwie pasywnej. Przejście od reżymu kontrolowanego reakcją do reżymu kontrolowanego dyfuzją.
3. Model procesu pasywacji oparty na asymetrycznym błądzeniu przypadkowym (Walker model). Przejście od reżymu gołej powierzchni do reżymu powierzchni pokrytej warstwą pasywną.
4. Model WOW (walker on walker) tworzenia się porów w warstwie pasywnej.

prof. Alexey Kavokin
(University of Southampton)

New quantum effect in a two-dimensional electron gas

We demonstrate that the temperature derivative of the chemical potential of a two-dimensional electron gas (2DEG) exhibits quantized dips where the chemical potential passes through the electron levels of size quantization. We find analytically the shape of the dips accounting for the elastic and inelastic scattering processes in 2DEG. In the limit of no scattering, at zero magnetic field, the dips depend only on the subband quantization number and are independent on material parameters, shape of the confining potential and temperature. The smearing of the dips is a direct measure of the disorder induced smearing of the electronic denisty of states. This peculiar quantum effect should manifest itself in various optical and electronic transport experiments [1].

[1] A.A. Varlamov, A.V. Kavokin and Yu.M. Galperin, Quantisation of entropy in a quasi-two-dimensional electron gas, Phys. Rev. B 93, 155404 (2016).

mgr Maciej Pieczarka
(Politechnika Wrocławska)

Wpływ nieporządku na propagację nierównowagowego kondensatu polarytonów ekscytonowych

Kondensaty polarytonów ekscytonowych są nowym typem kondensatów bozonowych, których nieodłączną właściwością jest brak równowagi termodynamicznej, wynikający z ich krótkiego czasu życia (krótszego niż czas termalizacji gazu). Pomimo tego faktu, zachowują one wiele właściwości charakterystycznych dla kondensatów arcyzimnych atomów, takich jak nadciekłość, czy generacja kwantowych wirów i solitonów. Charakterystyczną cechą optycznych mikrownęk półprzewodnikowych, w których generowane są polarytony, jest wbudowany naturalny nieporządek, wynikający z zastosowanego procesu technologicznego. Nieporządek może wpływać na stopień koherencji w kondensatach polarytonów ekscytonowych, jak również generować nietrywialne rozkłady prądów i wzbudzeń. W tym wystąpieniu przybliżę efekty nieporządku na kondensaty polarytonów propagujących z niezerowym pądem. Omówię zaobserwowane eksperymentalnie efekty rozpraszania polarytonów w silnym potencjale losowym w strukturze, co skutkowało generacją wzbudzeń typu Bogolubowa o liniowej dyspersji oraz obsadzeniem wirtualnej gałęzi wzbudzeń o energii negatywnej względem energii wytworzonego kondensatu. Przedstawione zostaną również efekty polaryzacji spinowej obu gałęzi wzbudzeń oraz ewolucje czasowe badanych zjawisk.

Jacek Dobrzyniecki
(Instytut Fizyki PAN)

Ścisła dynamika dwóch ultrazimnych bozonów umieszczonych
w jednowymiarowej podwójnej studni potencjału

Przeanalizowano ewolucję dwóch ultrazimnych bozonów umieszczonych w podwójnej studni potencjału początkowo umieszczonych w jednej ze studni. Porównano dynamikę przewidywaną przez ścisły dwuciałowy hamiltonian z dynamiką uzyskaną w ramach uproszczonych modeli dwumodowych. Pokazano, że dla niskiej bariery potencjału i odpowiednio silnego oddziaływania wszystkie modele dwumodowe załamują się i są niewystarczające do prawidłowego opisu dynamicznych własności układu. W szczególności źle opisują one korelacje pomiędzy bozonami.

prof. dr hab. Jan Mostowski
(Instytut Fizyki PAN)

Integral equation approach to electromagnetic field in dielectrics

We study static electric field and electromagnetic waves in dielectric media. As opposed to the standard approach we formulate and solve integral equations for the field. We discuss the case of electrostatic field of a point charge placed inside a dielectric. Integral equation approach allows to find and interpret the dielectric constant in terms of molecular polarizability. Next we discuss propagation of electromagnetic waves using the same integral equation approach. We derive dispersion relation and find reflection and transmission coefficients at the boundary between vacuum and the dielectric. The present approach supplements the standard approach based on macroscopic Maxwell's equations and contributes to better understanding of some electromagnetic effects.

dr Barbara Dietz
(Instytut Fizyki PAN)

Microwave Billiards, Graphene and Fullerene C60

We determined experimentally the eigenvalues of quantum billiards with the shapes of a rectangle and of Africa, respectively, that contained circular scatterers forming a triangular grid, so-called Dirac billiards. For this, high-precision measurements have been performed with superconducting microwave billiards. In the first part of my talk I will shortly review the salient properties of classical, quantum and microwave billiards. In the second part of my talk, I will present results concerning the particular features of the density of states (DOS) of Dirac billiards, which resembles that of a graphene flake, and their spectral fluctuation properties. I will demonstrate that the van Hove singularities, that show up as sharp peaks in the DOS, divide the band structure into regions where the system is governed by the non-relativistic Schrödinger equation of the quantum billiard and the Dirac equation of the graphene billiard of corresponding shape, respectively. In the third part of my talk I will present experiments that have been performed using a spherical superconducting microwave resonator with the geometric structure of the C60 fullerene molecule. Firstly, we studied the exceptional spectral properties emerging from the symmetries of the icosahedral structure of the carbon lattice. Secondly, we determined the number of zero modes with eigenvalues at the Dirac point to test the predictions of the Atiyah-Singer index theorem, which relates it to the topology of the curved carbon lattice. For this purpose, we performed numerical calculations in order to extend the experimental results to larger fullerene molecules.

* Supported by the DFG within the Collaborative Research Center CRC634

dr Krzysztof Jachymski
(Universität Stuttgart, Stuttgart)

Three-body interactions of slow light Rydberg polaritons

Rydberg polaritons have recently emerged as a new promising platform for studying quantum nonlinear optics and few-body physics. In this scheme photons in an atomic medium are tuned near the conditions of electromagnetically induced transparency with a large admixture of the Rydberg state. Strong interactions between Rydberg atoms can be mapped onto effective interactions between the polaritons. We show that in systems consisting of more than two photons, effective many-body interactions appear in addition to two-body ones. We focus on three-body systems in one-dimensional geometry and analyze how the three-body bound state is modified by these interactions and how the correlation functions of outgoing photons can be used to detect them.

dr inż. Paweł Potasz
(Katedra Fizyki Teoretycznej, Politechnika Wrocławska)

Ułamkowe izolatory Cherna przy przejściu pomiędzy siecią Checkerboard a siecią Lieba

Ułamkowe Izolatory Cherna (ang. Fractional Chern Insulators) to nowa klasa skorelowanych nieściśliwych cieczy kwantowych występujących dla ułamkowo zapełnionych płaskich pasm energetycznych o nietrywialnej topologii. W niniejszej pracy analizujemy stabilność fazy FCI ze względu na oddziaływanie z dodatkową podsiecią, której sprzężenie z układem jest kontrolowane za pomocą potencjału na węźle sieci. Odpowiada to przejściu pomiędzy siecią Checkerboard, a siecią Lieba. W układzie analizujemy wielkość wielocząstkowej przerwy energetycznej pomiędzy stanem podstawowym o trzykrotnej degeneracji, a stanami wzbudzonymi. Rozważamy modele z różną fazą odpowiadającą za nietrywialną topologię pasm oraz modelem pasma wypłaszczonego. Głównym wynikiem jest pokazanie silnej korelacji pomiędzy stałością tzw. krzywizny Berry'ego, a wielkością przerwy wielocząstkowej. Jest to związane z analogią pomiędzy fazą FCI, a cieczą Laughlina dla zapełnienia 1/3 najniższego poziomu Landaua.

dr inż. Paweł Potasz
(Katedra Fizyki Teoretycznej, Politechnika Wrocławska)

Ułamkowe izolatory Cherna przy przejściu pomiędzy siecią Checkerboard a siecią Lieba

Seminarium odwołane i przeniesione na późniejszy termin.

Prof. Vesselin Tonchev
("Rostislav Kaischew" Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria)

Phase transitions in 2D Ising-type model with essentially anisotropic interactions: Transfer Matrix study

In the introductory part is presented briefly the Transfer Matrix (TM) method - a powerful andelegant method of the statistical physics. Some technical details of the method, such asblock-diagonalization of the resulting matrices, are sketched.The machinery of the TM is applied to a 2D Ising-type model evolving from a real surface sciencesystem, Pb/Cu(110), and with essentially anisotropic interactions - attraction Jx between nearestneighbors along x-direction and repulsions Ji between all neighbors up to i-th one along they-direction, decreasing according to the power law, where a is the lattice constant along y. Thus, themodel parameters are: the external magnetic field H, the attraction energy Jx, the power p and thecut-off r of the potential along y. In the ground state analysis are accounted up to 200-th neighborswhen the interactions are long-ranged, p = 1, 2. In particular, it is shown that a sequence ofdistinguishable in the ground state phases (2x1), (3x1), .., (rx1), (1x1) exists when the cut off of theinteractions is r-1.When studying the thermal stability of the phases the cut-off r is restricted by the computationallyaffordable sizes of the TM - up to 9-th neighbors. The thermal behavior of the phases (2x1) and(3x1)+ is studied and the order-disorder transition lines for p = 2 and 3 are found using Nightingale'sphenomenological renormalization - comparing the correlation lengths obtained from the two largesteigenvalues of the TM of two systems with similar sizes. Additionally, the TM permits direct estimationof two of the critical indices, that of the correlation length and of the correlation function. A secondcorrelation length is obtained from the largest and the third largest eigenvalue of the TM expecting thatthe order-disorder transition may happen in two stages due to the interactions anisotropy. The universality of the numerically observed phase transitions is discussed in detail in the concludingpart of the talk.