
ON2.6 Recent research highlights
December 2015
Realspace collapse of a polariton condensate
Microcavity polaritons are twodimensional bosonic fluids with strong nonlinearities, composed of coupled photonic and electronic excitations. Here we report the unique phenomenology that is observed when a pulse of light impacts the polariton vacuum: the fluid which is suddenly created does not splash but instead coheres into a very bright spot.
Caption:
Experimental images of a collapsing condensate.
Realspace collapse of a polariton condensate
L. Dominici, M. Petrov, M. Matuszewski, D. Ballarini, M. De Giorgi, D. Colas, E. Cancellieri, B. Silva Fernández, A. Bramati, G. Gigli, A. Kavokin, F. Laussy, D. Sanvitto
Nature Commun. 6, 8993 (Dec 2015)
 
February 2016
Entanglement storage by classical fixed points in the twoaxis countertwisting model
We analyzed a scheme for storage of entanglement quantified by the quantum Fisher information in the twoaxis countertwisting model. The entangled state is generated dynamically from an initial spincoherent state located around an unstable saddle fixed point. At an optimal moment of time the state is shifted to a position around the stable center fixed points by a single rotation, where its dynamics and properties are approximately frozen.
Caption:
Time evolution of the Husimi function with rotation at an optimal moment of time.
 
February 2016
A tractable prescription for largescale free flight expansion of wavefunctions
Carrying out the free flight expansion of an ultracold bosonic cloud released from the trap is a pain when the needed numerical lattice doesn't fit in the memory of your computer.
An algorithm has been found to avoid this memory problem, and carry our even very large expansions.
Caption:
Free expansion of a thermal cloud containing large density fluctuations and solitons.
A tractable prescription for largescale free flight expansion of wavefunctions
P. Deuar
arXiv:1602.03395
 
February 2016
Spin squeezing in dipolar spinor condensates
We studied the effect of dipolar interactions on the level of spin squeezing in a spin1 BoseEinstein condensates by using the single mode approximation. The biaxial nature of dipolar interactions allows for dynamical generation of spinsqueezed states in the system. We calculated numerically the spin squeezing parameter showing that it is possible to reach the strongest squeezing set by the twoaxis countertwisting model.
Caption:
The inverse of the squeezing parameter ξ^{2}. The qualitative change of the squeezing parameter around 3α/β = 1 corresponds to changing stability of the meanfield fixed points between a stable center to an unstable saddle, or vice versa.
 
January 2016
Twoflavour mixture of a few fermions of different mass in a onedimensional harmonic trap
We study a system of two species of fermions of different mass confined
in a onedimensional harmonic trap. It is shown that a mass difference
between fermionic species induces a separation in the density of the
lighter flavour independently of the number of particles. The separation
is robust to the interactions with a thermal environment.
Caption:
Single particle density of heavy (blue) and light (red)
fermions calculated for the ground state for different numbers of
particles.
 
December 2015
Accuracy of classical field calculations and optimal cutoffs
We have determined the regime of acuracy for classical field descriptions of ideal and some weakly interacting gases, based on the criterion that many observables should be accurately reproduced, including density fluctuations and kinetic energy. Optimal cutoffs and accuracy are also found as a function fo temperature.
Caption:
Optimal cutoffs (green) and maximum error RMS (blue) as a function of dimensionless temperature (tau) and cutoff (fc).
 
September 2015
The Wigner Stochastic GrossPitaevskii Equation
A fuller version of the popularly used Stochastic GrossPitaevskii Equation (SGPE) has been derived, that takes into account also quantum fluctuatons in the gas, and is directly sensitive to the absolute number of atoms in the cloud. Antibunching, atomic shot noise and quantum phase fluctuations can be obtained. Unlike some earlier attempts in this direction, the equations are stable and allow one to obtain stationary thermal states.
Caption:
Soliton evolution in a thermal gas described by the WSGPE.
P. Deuar, N. Proukakis, T. Świsłocki
In preparation.
 
July 2015
Quantum dynamics generated by the twoaxis countertwisting Hamiltonian
We studied the quantum dynamics generated by the twoaxis countertwisting Hamiltonian from an initial spin coherent state in a spin1/2 ensemble. A characteristic feature of the twoaxis countertwisting Hamiltonian is the existence of four neutrally stable and two saddle unstable fixed points. The presence of the latter is responsible for a high level of squeezing.
Caption:
Time evolution of the Quantum Fisher Information (top), Husimi function (bottom left) and Wigner function (bottom right).
 
March 2010
Einstein  de Haas effect on a plaquette of optical lattice sites
When several vortices localised in optical lattice sites in a ^{87}Rb spinor gas in the F=1 state interact via dipoledipole magnetic spin interactions
we observe the formation of a global intersite vortex on a 2x2 plaquette when the lattice potential is partially lowered.
Caption:
Formation of a global vortex in a 2x2 plaquette after partial lowering of the lattice height.
Tunable dipolar resonances and Einsteinde Haas effect in a ^{87}Rbatom condensate
T. Świsłocki, T. Sowiński, J. Pietraszewicz, M. Brewczyk, M. Lewenstein, J. Zakrzewski, M. Gajda
Phys. Rev. A 83, 063617 (Jun 2011)
 
January 2010
Scattering of radiation by a system of point dipoles on a ring. Exact solution
The problem of scattering of radiation by a system on N quasipoint like dipoles located on a circle (equidistantly in the polar angle) in two spatial
dimensions has been solved. A closedform expression for the electric fields at the points where the dipoles are located has been given via a partial wave
decomposition. The scattering and focusing properties have been investigated for both dielectric and metallic scatterers.
Caption:
Radiation patterns of plane waves scattered by a system of metallic wires located on a circle. (left) 18 000 wires, (right) 1200 wires.
 
November 2009
Precision calculations of the incoherent halo of atoms in a supersonic BEC collision
The incoherently scattered atoms are found to lie in a spheroidal shell at smaller velocities than the incoming BECs.
In a broad collaboration with the experiment in Orsay, France, and theorists in Warsaw and Brisbane, Australia, the details of the processes
have been understood and verified by comparing the experimental data with mircoscopic manybody calculations using the positiveP method, and
a timeadaptive Bogoliubov approach that is used to investigate the removal or addition of chosen subprocesses.
Caption:
(topleft) experimental data for a slice through the scattered atom halo
(topright) simulation
(bottom) matching of experimental (black) and simulated (red) shell radii
Spontaneous FourWave Mixing of de Broglie Waves: Beyond Optics
V. Krachmalnicoff, J.C. Jaskula, M. Bonneau, G.B. Partridge, D. Boiron, C.I. Westbrook, P. Deuar, P. Ziń, M. Trippenbach, K.V. Kheruntsyan
Phys. Rev. Lett. 104, 150402 (Apr 15 2010)
 
May 2009
Collective diffusion of a onedimensional lattice gas adsorbate
The collective diffusion coefficient in a onedimensional lattice gas adsorbate was calculated using a variational
approach, for the case of longrange interparticle interactions. The diffusion coefficient depends strongly on the relationship between the substrate lattice
constant and the characteristic length of the interparticle interaction potential. At fixed density, it also has an oscillating character as a function of the
characteristic interaction length due to the interplay between the distances allowed by the substrate lattice structure those that minimize the total interaction
energy.
Caption:
Density dependence of the diffusion coefficient in terms of θ=1/l the inverse of the nearestneighbor pair length l, and lattice constant a,
for several values of the Fermi wavevector q_{F} of the surface electrons.
 
March 2009
Controlled Quantum/Semiclassical hybrid methods for simulating manybody quantum dynamics
A controlled hybridization between full quantum dynamics and semiclassical approaches (meanfield
and truncated Wigner) is implemented for interacting manyboson systems. It is then demonstrated how
simulating the resulting hybrid evolution equations allows one to obtain the full quantum dynamics for
much longer times than is possible using an exact treatment directly, while remaining reliable. A demonstration is made for the collision of sodium BECs with
150 000 atoms.
Caption:
A comparison of predictions of scattered particle number using various methods (independent hybrid predictions are labeled H_{A} and H_{B}).
 
January 2009
Dipolar interactions dominate the ^{87}Rb spinor gas
The thermalisation of the ^{87}Rb spinor bose gas in the F=1 state was simulated using the classicalfield method. It was found that
even at small magnetic fields of the order of 1mG or less, the process is dominated by the dipoledipole magnetic spin interactions, rather than spin mixing contact interactions.
Caption:
A sequence of timeframes showing fluctuations of the spin textures in the gas in thermal equilibrium.
 
January 2009
Optimal highenergy cutoff in cfield methods found
The quantitative details of the different cfield simulation methods for Bose gases are noticeably dependent on details of the highenergy cutoff employed  a technical parameter. We have determined the optimal cutoff for trapped Bose gases, in the sense that choosing this value and letting the gas thermalise very closely reproduces the true thermal distribution. Surprisingly, this cutoff corresponds to about half a particle on average per mode at the highenergy end of the scale, a value much lower than heretofore used in most variants of the cfield method.
Caption:
Probability distribution of having N_{ex} bosons outside the ground state in a 1D trap. Solid black: exact distribution; Red: classical field with
optimised cutoff; dashed black: classical field with 0.9 and 1.1 times optimal cutoff.
 
September 2008
Scattering of narrow beams and short pulses on dielectric spheres
The scattering of gaussian beam electromagnetic pulses by a hard dielectric sphere is shown to be well approximated by a superposition of plane waves.
Furthermore, the scattering of short pulses composed of discrete frequency waves on a hard or dielectric sphere is described. This extends the
LorenzMie theory for scattering of plane waves.
Caption:
Time evolution of very long and very narrow electromagnetic pulse.
 
September 2008
Gainloss asymmetry for emerging stock markets
Stock indices for European emerging markets are analyzed using the investment horizon approach. Gainloss asymmetry is observed for all analyzed data, but with
a different character for emerging and for established markets. When optimal investment horizon is plotted as a function of absolute return value, for
established markets gain curve lies typically above loss curve, whereas in the case of emerging markets the situation is the opposite. In the latter case one has
to wait longer for loss than for gain of the same return value. We propose a measure quantifying the gainloss asymmetry that clearly exhibits a difference
between emerging and established markets.
Caption:
Distribution of investment horizons (with fitted function) for return rate 0.04 of one of the polish mutual funds.
 
August 2008
Dynamical Casimir effect in oscillating media
It was shown that pairs of photons are produced in an oscillating homogenous medium with constant material coefficients, and that there are
parametric resonance regions where there is an exponentially growing number of photons. This process may be viewed as another manifestation of the dynamical
Casimir effect. However, in contrast to the standard dynamical Casimir effect, photon production here takes place in the entire volume and is not due to time
dependence of the boundary conditions or material constants.
Caption:
Behavior of the real and imaginary parts of the characteristic exponent ν for photons produced at frequency ω in the first two resonance regions.
Medium oscillation frequency is Ω.
 
June 2008
Decay of multiply charged vortices in a BEC
The decay of multiply charged vortices in the presence of thermal atoms in a BEC were studied using the classical field method.
The onset of the decay of a vortex is always preceded by the increase of a number of thermal (uncondensed) atoms in the system
and manifests itself by the sudden rise of the amplitude of the oscillations of the quadrupole moment.
Caption:
(topleft) Isodensity plots for two typical vortex configurations,
(botleft) zintegrated densities,
(topright) condensate density,
(botright) noncondensate density.
 
June 2008
Distillation of a BoseEinstein condensate
In a twotrap system, with the atoms initially in a BEc in the higherenergy trap, experiments have shown that
eventually the atoms make their way into the lower energy trap even when the potential barrier between the two traps is far too high for tunelling from the condensate to occur. Using classical field simulations, we have shown that this can occur via a process where the thermal cloud of the initial condensate rises out of the trap, moves over the barrier, and condenses eventually in the lower energy trap. This is highly analogous to distillation, or in a colder setting to the Helium fountain.
Caption:
Condensate fractions in the highpotential (green) and lowpotential (red) wells, as a function of time. Note the lower condensate fraction in the
lowpotential well.
 
March 2008
Segregation in a noninteracting binary mixture
The process of stripe formation in a system consisting of particles of two sizes on sites is analyzed numerically, for the case when there are no direct
interactions. However, the sites can be occupied by up to four neighboring large particles but smaller objects cannot overlap. The system orders, forming
stripes, under external, constant, or oscillating driving forces if its density is large enough. It is shown that this process happens with logarithmic like time
dependence.
Caption:
Successive stages of separation of large (red) and small (green) particles. Free sites are visible as white spots.

VIDEO 


February 2008
Spontaneous collective effects in a FabryPerot cavity
The spontaneous emission of radiation from two twolevel atoms in a FabryPerot cavity is considered, with a modulated transition frequency of the atoms. This
modulation leads to a radical change of the spontaneous emission dynamics, including decoherence properties of entangled atomic states. It could be used to
dynamically control the collective effects in spontaneous emission in cavities. To achieve this, the modulation should (1) be able to periodically detune the
atoms from the resonance with the cavity, and (2) fulfil a resonant condition between the modulation frequency and modulation depth.
Caption:
A parametric plot of the evolution of R+iI = α_{1}α_{2} for α_{1}e>g> and α_{2}g>e> amplitudes that
illustrates the dynamics of degradation of the relative phase between the atoms. a is a detuning from the resonance frequency modulation value.
 
October 2007
Light localization in lefthanded media
Localized waves in disordered lefthanded materials are studied using a generalized coupleddipole model. Resonances in an open system consisting
of randomly distributed electric and magnetic dipoles are investigated. A new type of longlived resonance modes localized at the boundary of the
system is found. They resemble evanescent waves responsible for a superfocusing phenomenon by a lefthanded lens.
Caption:
Field energy density in a resonance mode of the twodimensional medium consisting of randomly distributed dipoles.
 
March 2007
Domain growth in the interacting adsorbate
Domain growth dynamics is studied in a onedimensional Potts model. We show that an exponent of a powerlike time domain growth is determined not only by the
conservation or nonconservation of the order parameter, but also by the asymmetry of single particle jumps. Domains that have an anisotropic pattern, such as
(2X1), have a tendency to grow faster in a certain direction than they do in others. As a result, dynamical behavior of systems which start in the same
configurations and eventually reach the same equilibrium states is completely different.
Caption:
Examples of the domain pattern resulting from (2x1) ordering.
Domain growth in the interacting adsorbate: Nonsymmetric particle jump model
M.A. ZałuskaKotur, S. Krukowski, A. Łusakowski, Ł.A. Turski
Phys. Rev. B 75, 115412 (Mar 2007)
 
