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Quantum anomaly and 2D-3D crossover in strongly interacting Fermi gases: We present an experimental investigation of collective oscillations in
harmonically trapped Fermi gases through the crossover from two to three
dimensions. Specifically, we measure the frequency of the radial monopole or
breathing mode as a function of dimensionality in Fermi gases with tunable
interactions. The frequency of this mode is set by the adiabatic
compressibility and probes the thermodynamic equation of state. In 2D, a
dynamical scaling symmetry for atoms interacting via a {\delta}-potential
predicts the breathing mode to occur at exactly twice the harmonic confinement
frequency. However, a renormalized quantum treatment introduces a new length
scale which breaks this classical scale invariance resulting in a so-called
quantum anomaly. Our measurements deep in the 2D regime lie above the
scale-invariant prediction for a range of interaction strengths indicating the
breakdown of a {\delta}-potential model for atomic interactions. As the
dimensionality is tuned from 2D to 3D we see the breathing oscillation
frequency evolve smoothly towards the 3D limit. | cond-mat_quant-gas |
Notes on the Cluster Gutzwiller Method: Inhomogeneous Lattices,
Excitations, and Cluster Time Evolution: Several perspectives of the cluster Gutzwiller method are briefly discussed.
I show that the cluster mean-field method can be used for large inhomogeneous
lattices, for computing local excitations, and for the time evolution of
correlated quantum systems. | cond-mat_quant-gas |
Self-Assembled Chains and Solids of Dipolar Atoms in a Multilayer: We predict that ultracold bosonic dipolar gases, confined within a multilayer
geometry, may undergo self-assembling processes, leading to the formation of
chain gases and solids. These dipolar chains, with dipoles aligned across
different layers, emerge at low densities and resemble phases observed in
liquid crystals, such as nematic and smectic phases. We calculate the phase
diagram using quantum Monte Carlo methods, introducing a newly devised trial
wave function designed for describing the chain gas, where dipoles from
different layers form chains without in-plane long-range order. We find gas,
solid, and chain phases, along with quantum phase transitions between these
states. Specifically, we predict a quantum phase transition from a gaseous to a
self-ordered phase, which occurs at a critical interlayer distance. Remarkably,
in the self-organized phases, the mean interparticle distance can significantly
exceed the characteristic length of the interaction potential, yielding solids
and chain gases with densities several orders of magnitude lower than those of
conventional quantum solids. | cond-mat_quant-gas |
Chiral condensates in a polariton hexagonal ring: We model generation of vortex modes in exciton-polariton condensates in
semiconductor micropillars, arranged into a hexagonal ring molecule, in the
presence of TE-TM splitting. This splitting lifts the degeneracy of azimuthally
modulated vortex modes with opposite topological charges supported by this
structure, so that a number of non-degenerate vortex states characterized by
different combinations of topological charges in two polarization components
appears. We present a full bifurcation picture for such vortex modes and show
that because they have different energies, they can be selectively excited by
coherent pump beams with specific frequencies and spatial configurations. At
high pumping intensity, polariton-polariton interactions give rise to the
coupling of different vortex resonances and a bistable regime is achieved. | cond-mat_quant-gas |
Hidden vortices in a Bose-Einstein condensate in a rotating double-well
potential: We study vortex formation in a Bose-Einstein condensate in a rotating
double-well potential. Besides the ordinary quantized vortices and elusive
ghost vortices, "hidden" vortices are found distributing along the central
barrier. These hidden vortices are invisible like ghost vortex but carry
angular momentum. Moreover, their core size is not given by the healing length,
but is strongly influenced by the external potential. We find that the
Feynman's rule can be well satisfied only after including the hidden vortices.
There is no critical rotating frequency for the formation of hidden vortex
while there is one for the formation of ordinary visible vortices. Hidden
vortices can be revealed in the free expansion of the Bose-Einstein
condensates. In addition, the hidden vortices in a Bose-Einstein condensate can
appear in other external potentials, such as a rotating anisotropic toroidal
trap. | cond-mat_quant-gas |
Two types of dark solitons in a spin-orbit-coupled Fermi gas: Dark solitons in quantum fluids are well known nonlinear excitations that are
usually characterized by a single length scale associated with the underlying
background fluid. We show that in the presence of spin-orbit coupling and a
linear Zeeman field, superfluid Fermi gases support two different types of
nonlinear excitations featured by corresponding length scales related to the
existence of two Fermi surfaces. Only one of these types, which occurs for
finite spin-orbit coupling and a Zeeman field, survives to the topological
phase transition, and is therefore capable to sustain Majorana zero modes. At
the point of the emergence of this soliton for varying the Zeeman field, the
associated Andreev bound states present a minigap that vanishes for practical
purposes, in spite of lacking the reality condition of Majorana modes. | cond-mat_quant-gas |
Effects of thermal and quantum fluctuations on the phase diagram of a
spin-1 87Rb Bose-Einstein condensate: We investigate effects of thermal and quantum fluctuations on the phase
diagram of a spin-1 87Rb Bose-Einstein condensate (BEC) under a quadratic
Zeeman effect. Due to the large ratio of spinindependent to spin-dependent
interactions of 87Rb atoms, the effect of noncondensed atoms on the condensate
is much more significant than that in scalar BECs. We find that the condensate
and spontaneous magnetization emerge at different temperatures when the ground
state is in the brokenaxisymmetry phase. In this phase, a magnetized condensate
induces spin coherence of noncondensed atoms in different magnetic sublevels,
resulting in temperature-dependent magnetization of the noncondensate. We also
examine the effect of quantum fluctuations on the order parameter at absolute
zero, and find that the ground-state phase diagram is significantly altered by
quantum depletion. | cond-mat_quant-gas |
Effective dipole-dipole interactions in multilayered dipolar
Bose-Einstein condensates: We propose a two-dimensional model for a multilayer stack of dipolar
Bose-Einstein condensates formed by a strong optical lattice. We derive
effective intra- and interlayer dipole-dipole interaction potentials and
provide simple analytical approximations for a given number of lattice sites at
arbitrary polarization. We find that the interlayer dipole-dipole interaction
changes the transverse aspect ratio of the ground state in the central layers
depending on its polarization and the number of lattice sites. The changing
aspect ratio should be observable in time of flight images. Furthermore, we
show that the interlayer dipole-dipole interaction reduces the excitation
energy of local perturbations, affecting the development of a roton minimum. | cond-mat_quant-gas |
Particle and spin transports of spin-orbit coupled Fermi gas through a
Quantum Point Contact: The particle and spin transport through a quantum point contact between two
Fermi gases with Raman-induced spin-orbit coupling are investigated. We show
that the particle and spin conductances both demonstrate the structure of
plateau due to the mesoscopic scale of the quantum point contact. Compared with
the normal Fermi gases the particle conductance can be significantly enhanced
by the spin-orbit coupling effect. Furthermore, the conversion of the particle
and spin currents can take place in the spin-orbit coupled system, and we find
that it is controlled by the parameter of two-photon detuning. When the
parameter of two-photon detuning vanishes the particle and spin currents
decouple. | cond-mat_quant-gas |
Two-mode Dicke model from non-degenerate polarization modes: We realize a non-degenerate two-mode Dicke model with competing interactions
in a Bose-Einstein condensate (BEC) coupled to two orthogonal polarization
modes of a single optical cavity. The BEC is coupled to the cavity modes via
the scalar and vectorial part of the atomic polarizability. We can
independently change these couplings and determine their effect on a
self-organization phase transition. Measuring the phases of the system, we
characterize a crossover from a single-mode to a two-mode Dicke model. This
work provides perspectives for the realization of coupled phases of spin and
density. | cond-mat_quant-gas |
Gauge Violation Spectroscopy in Synthetic Gauge Theories: Recently synthetic gauge fields have been implemented on quantum simulators.
Unlike the gauge fields in the real world, in synthetic gauge fields, the gauge
charge can fluctuate and gauge invariance can be violated, which leading rich
physics unexplored before. In this work, we propose the gauge violation
spectroscopy as a useful experimentally accessible measurement in the synthetic
gauge theories. We show that the gauge violation spectroscopy exhibits no
dispersion. Using three models as examples, two of them can be exactly solved
by bosonization, and one has been realized in experiment, we further
demonstrate the gauge violation spectroscopy can be used to detect the
confinement and deconfinement phases. In the confinement phase, it shows a
delta function behavior, while in the deconfinement phase, it has a finite
width. | cond-mat_quant-gas |
From Nodal Ring Topological Superfluids to Spiral Majorana Modes in Cold
Atomic Systems: In this work, we consider a 3D cubic optical lattice composed of coupled 1D
wires with 1D spin-orbit coupling. When the s-wave pairing is induced through
Feshbach resonance, the system becomes a topological superfluid with ring
nodes, which are the ring nodal degeneracies in the bulk, and supports a large
number of surface Majorana zero energy modes. The large number of surface
Majorana modes remain at zero energy even in the presence of disorder due to
the protection from a chiral symmetry. When the chiral symmetry is broken, the
system becomes a Weyl topological superfluid with Majorana arcs. With 3D
spin-orbit coupling, the Weyl superfluid becomes a novel gapless phase with
spiral Majorana modes on the surface. The spatial resolved radio frequency
spectroscopy is suggested to detect this novel nodal ring topological
superfluid phase. | cond-mat_quant-gas |
Interacting quantum walk on a two-leg flux ladder: Emergence of
re-entrant dynamics: We study the quench dynamics of interacting bosons on a two-leg flux ladder
by implementing the continuous-time quantum walk and explore the combined
effect of the magnetic field and onsite interaction in the presence of uniform
flux. We show that in the regime of weak interaction, the magnetic field
substantially slows down the spreading of the particles' wavefunction during
the dynamics. However, in the presence of strong interaction, we obtain an
interesting re-entrant behaviour in the dynamics where the radial velocity
associated to the spreading first increases, then decreases, and increases
again as a function of the flux strength. We also find a re-entrant dynamics in
the chiral motion of the particles as a function of interaction for fixed flux
strengths. | cond-mat_quant-gas |
Induced interaction and crystallization of self-localized impurity
fields in a Bose-Einstein condensate: We model the behavior of N classical impurity fields immersed in a larger
Bose-Einstein condensate by N+1 coupled nonlinear Schrodinger equations in 1,
2, and 3 space dimensions. We discuss the stability of the uniform miscible
system and show the importance of surface tension for self localization of the
impurity fields. We derive analytically the attractive tail of
impurity-impurity interaction due to mediation by the underlying condensate.
Assuming all impurity fields interact with the same strength, we explore the
resulting phase diagram, which contains four phases: {\it I}) all fields are
miscible; {\it II}) the impurity fields are miscible with each other but phase
separate from the condensate as a single bubble; {\it III}) the localized
impurity fields stay miscible with the condensate, but not with each other; and
{\it IV}) the impurity fields phase separate from the condensate and each
other, forming a crystalline structure within a bubble. Thus, we show that a
crystal can be constructed solely from superfluid components. Finally, we argue
that the crystalline phases maintain their superfluid behavior, i.e. they
possess a nonclassical rotational inertia, which, combined with lattice order,
is a characteristic of supersolidity. | cond-mat_quant-gas |
Bose-Einstein condensation of light in a cavity: The paper considers Bose-Einstein condensation (BEC) of light in a cavity
with medium. In the framework of two-level model we show the effect of gaseous
medium on the critical temperature of light condensation in the system.
Transition of the system to the state with released light condensate is
illustrated in consequent stages. Analytical expressions for a typical spatial
extent of the condensed cloud of photons, as well for spectral characteristics
of the condensate peak are derived. Energy and heat capacity of photons as
functions of temperature are obtained. Finally, we demonstrate that the energy
of light can be accumulated in the BEC state. | cond-mat_quant-gas |
Thermalisation of Local Observables in Small Hubbard Lattices: We present a study of thermalisation of a small isolated Hubbard lattice
cluster prepared in a pure state with a well-defined energy. We examine how a
two-site subsystem of the lattice thermalises with the rest of the system as
its environment. We explore numerically the existence of thermalisation over a
range of system parameters, such as the interaction strength, system size and
the strength of the coupling between the subsystem and the rest of the lattice.
We find thermalisation over a wide range of parameters and that interactions
are crucial for efficient thermalisation of small systems. We relate this
thermalisation behaviour to the eigenstate thermalisation hypothesis and
quantify numerically the extent to which eigenstate thermalisation holds. We
also verify our numerical results theoretically with the help of previously
established results from random matrix theory for the local density of states,
particularly the finite-size scaling for the onset of thermalisation. | cond-mat_quant-gas |
Critical velocity of a mobile impurity in one-dimensional quantum
liquids: We study the notion of superfluid critical velocity in one spatial dimension.
It is shown that for heavy impurities with mass $M$ exceeding a critical mass
$M_\mathrm{c}$, the dispersion develops periodic metastable branches resulting
in dramatic changes of dynamics in the presence of an external driving force.
In contrast to smooth Bloch Oscillations for $M<M_\mathrm{c}$, a heavy impurity
climbs metastable branches until it reaches a branch termination point or
undergoes a random tunneling event, both leading to an abrupt change in
velocity and an energy loss. This is predicted to lead to a non-analytic
dependence of the impurity drift velocity on small forces. | cond-mat_quant-gas |
Polaronic atom-trimer continuity in three-component Fermi gases: Recently it has been proposed that three-component Fermi gases may exhibit a
new type of crossover physics in which an unpaired Fermi sea of atoms smoothly
evolves into that of trimers in addition to the ordinary BCS-BEC crossover of
condensed pairs. Here we study its corresponding polaron problem in which a
single impurity atom of one component interacts with condensed pairs of the
other two components with equal populations. By developing a variational
approach in the vicinity of a narrow Feshbach resonance, we show that the
impurity atom smoothly changes its character from atom to trimer with
increasing the attraction and eventually there is a sharp transition to dimer.
The emergent polaronic atom-trimer continuity can be probed in ultracold atoms
experiments by measuring the impurity spectral function. Our novel crossover
wave function properly incorporating the polaronic atom-trimer continuity will
provide a useful basis to further investigate the phase diagram of
three-component Fermi gases in more general situations. | cond-mat_quant-gas |
Spin-Orbit Coupling and Spin Textures in Optical Superlattices: We proposed and demonstrated a new approach for realizing spin orbit coupling
with ultracold atoms. We use orbital levels in a double well potential as
pseudospin states. Two-photon Raman transitions between left and right wells
induce spin-orbit coupling. This scheme does not require near resonant light,
features adjustable interactions by shaping the double well potential, and does
not depend on special properties of the atoms. A pseudospinor Bose-Einstein
condensate spontaneously acquires an antiferromagnetic pseudospin texture which
breaks the lattice symmetry similar to a supersolid. | cond-mat_quant-gas |
Dynamics of a quantum phase transition in the Bose-Hubbard model:
Kibble-Zurek mechanism and beyond: In this paper, we study the dynamics of the Bose-Hubbard model by using
time-dependent Gutzwiller methods. In particular, we vary the parameters in the
Hamiltonian as a function of time, and investigate the temporal behavior of the
system from the Mott insulator to the superfluid (SF) crossing a second-order
phase transition. We first solve a time-dependent Schr\"odinger equation for
the experimental setup recently done by Braun et.al. [Proc. Nat. Acad. Sci.
112, 3641 (2015)] and show that the numerical and experimental results are in
fairly good agreement. However, these results disagree with the Kibble-Zurek
scaling. From our numerical study, we reveal a possible source of the
discrepancy. Next, we calculate the critical exponents of the correlation
length and vortex density in addition to the SF order parameter for a
Kibble-Zurek protocol. We show that beside the "freeze" time $\hat{t}$, there
exists another important time, $t_{\rm eq}$, at which an oscillating behavior
of the SF amplitude starts. From calculations of the exponents of the
correlation length and vortex density with respect to a quench time $\tQ$, we
obtain a physical picture of a coarsening process. Finally, we study how the
system evolves after the quench. We give a global picture of dynamics of the
Bose-Hubbard model. | cond-mat_quant-gas |
Stability of supercurrents in a superfluid phase of spin-1 bosons in an
optical lattice: We study collective modes and superfluidity of spin-1 bosons with
antiferromagnetic interactions in an optical lattice based on the
time-dependent Ginzburg-Landau (TDGL) equation derived from the spin-1
Bose-Hubbard model. Specifically, we examine the stability of supercurrents in
the polar phase in the vicinity of the Mott insulating phase with even filling
factors. Solving the linearized TDGL equation, we obtain gapless spin-nematic
modes and gapful spin-wave modes in the polar phase that arise due to the
breaking of $S^2$ symmetry in spin space. Supercurrents exhibit dynamical
instabilities induced by growing collective modes. In contrast to the
second-order phase transition, the critical momentum of mass currents is finite
at the phase boundary of the first-order superfluid-Mott insulator (SF-MI)
phase transition. Furthermore, the critical momentum remains finite throughout
the metastable SF phase and approaches zero towards the phase boundary, at
which the metastable SF state disappears. We also study the stability of spin
currents motivated by recent experiments for spinor gases. The critical
momentum of spin currents is found to be zero, where a spin-nematic mode causes
the dynamical instability. We investigate the origin of the zero critical
momentum of spin currents and find it attributed to the fact that the polar
state becomes energetically unstable even in the presence of an infinitesimal
spin current. We discuss implications of the zero critical momentum of spin
currents for the stability of the polar state. | cond-mat_quant-gas |
Fermionic Superradiance in a Transversely Pumped Optical Cavity: Following the experimental realization of Dicke superradiance in Bose gases
coupled to cavity light fields, we investigate the behavior of ultra cold
fermions in a transversely pumped cavity. We focus on the equilibrium phase
diagram of spinless fermions coupled to a single cavity mode and establish a
zero temperature transition to a superradiant state. In contrast to the bosonic
case, Pauli blocking leads to lattice commensuration effects that influence
self-organization in the cavity light field. This includes a sequence of
discontinuous transitions with increasing atomic density and tricritical
superradiance. We discuss the implications for experiment. | cond-mat_quant-gas |
Self-consistent theory of Bose-Einstein condensate with impurity at
finite temperature: We study the properties of Bose-Einstein condensate (BEC)-impurity mixtures
at finite temperature employing the Balian-V\'en\'eroni (BV) variational
principle. The method leads to a set of coupled nonlinear equations of motion
for the condensate and its normal and anomalous fluctuations on the one hand,
and for the impurity on the other. We show that the obtained equations satisfy
the energy and number conserving laws. Useful analytic expressions for the
chemical potential and the radius of both condensate and anomalous components
are derived in the framework of the Thomas-Fermi (TF) approximation in
$d$-dimensional regime. Effects of the impurity on these quantities are
discussed. | cond-mat_quant-gas |
$N$-coherence vs. $t$-coherence: An alternative route to the
Gross-Pitaevskii equation: We show how a candidate mean-field amplitude can be constructed from the
exact wave function of an externally forced $N$-Boson system. The construction
makes use of subsidiary $(N-1)$-particle states which are propagated in time in
addition to the true $N$-particle state, but does not involve spontaneous
breaking of the $U(1)$ symmetry associated with particle number conservation.
Provided the flow in Fock space possesses a property which we call maximum
stiffness, or $t$-coherence, the candidate amplitude actually satisfies the
time-dependent Gross-Pitaevskii equation, and then serves as macroscopic wave
function of the forced $N$-particle system. The general procedure is
illustrated in detail by numerical calculations performed for the model of a
driven bosonic Josephson junction, which allows one to keep track of all
contributions which usually are subject to uncontrolled assumptions. These
calculations indicate that macroscopic wave functions can persist even under
conditions of strong forcing, but are rapidly destroyed upon entering a regime
of chaotic dynamics. Our results provide a foundation for future attempts to
manipulate, and actively control, macroscopic wave functions by means of
purposefully designed force protocols. | cond-mat_quant-gas |
Relaxation and hysteresis near Shapiro resonances in a driven spinor
condensate: We study the coherent and dissipative aspects of a driven spin-1
Bose-Einstein condensate (BEC) when the Zeeman energy is modulated around a
static bias value. Resonances appear when the bias energy matches an integer
number of modulation quanta. They constitute the atomic counterpart of Shapiro
resonances observed in microwave-driven superconducting Josephson junctions.
The population dynamics near each resonance corresponds to slow and non-linear
secular oscillations on top of a rapid `micromotion'. At long times and in a
narrow window of modulation frequencies around each resonance, we observe a
relaxation to asymptotic states that are unstable without drive. These
stationary states correspond to phase-locked solutions of the Josephson
equations generalized to include dissipation, and are analogous to the
stationary states of driven superconducting junctions. We find that dissipation
is essential to understand this long-time behavior, and we propose a
phenomenological model to explain quantitatively the experimental results.
Finally, we demonstrate hysteresis in the asymptotic state of the driven spinor
BEC when sweeping the modulation frequency across a Shapiro resonance. | cond-mat_quant-gas |
Density correlations from analogue Hawking radiation in the presence of
atom losses: The sonic analogue of Hawking radiation can now be experimentally recreated
in Bose-Einstein Condensates that contain an acoustic black hole. In these
experiments the signal strength and analogue Hawking temperature increase for
denser condensates, which however also suffer increased atom losses from
inelastic collisions. To determine how these affect analogue Hawking radiation,
we numerically simulate creation of the latter in a Bose-Einstein Condensate in
the presence of atomic losses. In particular we explore modifications of
density-density correlations through which the radiation has been analyzed so
far. We find that losses increase the contrast of the correlation signal, which
we attribute to heating that in turn leads to a component of stimulated
radiation in addition to the spontaneous one. Another indirect consequence is
the modification of the white hole instability pattern. | cond-mat_quant-gas |
Thermalization and localization of an oscillating Bose-Einstein
condensate in a disordered trap: We numerically simulate an oscillating Bose-Einstein condensate in a
disordered trap [Phys. Rev. A 82, 033603 (2010)] and the results are in good
agreement with the experiment. It allows us to verify that total energy and
particle number are conserved in this quantum system. The disorder acts as a
medium, which results in a relaxation from nonequilibrium to equilibrium, i.e.,
thermalization. An algebraic localization is realized when the system
approaches the equilibrium, and if the system falls into the regime when the
healing length of the condensate exceeds the correlation length of the
disorder, exponential Anderson localization is to be observed. | cond-mat_quant-gas |
Momentum distribution of a dilute unitary Bose gas with three-body
losses: Using Boltzmann's equation, we study the effect of three-body losses on the
momentum distribution of a homogeneous unitary Bose gas in the dilute limit
where quantum correlations are negligible. We calculate the momentum
distribution of the gas and show that inelastic collisions are quantitatively
as important as a second order virial correction. | cond-mat_quant-gas |
Phases of driven two-level systems with nonlocal dissipation: We study an array of two-level systems arranged on a lattice and illuminated
by an external plane wave which drives a dipolar transition between the two
energy levels. In this set up, the two-level systems are coupled by dipolar
interactions and subject to nonlocal dissipation, so behave as an open
many-body quantum system. We investigate the long-time dynamics of the system
at the mean-field level, and use this to determine a phase diagram as a
function of external drive and detuning. We find a multitude of phases
including antiferromagnetism, spin density waves, oscillations and phase
bistabilities. We investigate these phases in more detail and explain how
nonlocal dissipation plays a role in the long-time dynamics. Furthermore, we
discuss what features would survive in the full quantum description. | cond-mat_quant-gas |
Physical Realization of von Neumann Lattices in Rotating
Dipole-blockaded Bose Gases: A mathematical lattice, called the von Neumann lattice, is a subset of
coherent states and exists periodically in the phase space. It is unlike solids
or Abrikosov lattices that are observable in physical systems. Abrikosov
lattices are vortices closely packed into a lattice with a flux quantum through
a unit cell. Although Abrikosov lattices appear generally in various physical
systems, vortex lattices with multiple-flux quantums through a unit cell are
more stable than Abrikosov lattices in some physical regimes of the systems
with non-local interactions between particles. No theory is able to describe
these vortex lattices today. Here, we develop a theory for these vortex
lattices by extending von Neumann lattices to the coordinate space with a unit
cell of area that is proportional to flux quantums through a unit cell. The von
Neumann lattices not only show the same physical properties as the Abrikosov
lattice, but also describe vortex lattices with multiple-flux quantums through
a unit cell. From numerical simulations of a rapidly rotating dipole-blockaded
gas, we confirm that vortex lattices showed in our simulations are the
representation of von Neumann lattices in the coordinate space. We anticipate
our theory to be a starting point for developing more sophisticated
vortex-lattice models. For example, the effect of Landau-level mixing on vortex
lattice structures, vortices formed inside superfluid droplets and structural
phase transitions of vortex matter in two-component Bose-Einstein condensates
will be relevant for such developments. | cond-mat_quant-gas |
Realization of a distributed Bragg reflector for propagating guided
matter waves: We report on the experimental realization of a Bragg reflector for guided
matter waves. A Bose-Einstein condensate with controlled velocity distribution
impinges onto an attractive optical lattice of finite length and directly
probes its band structure. We study the dynamics of the scattering by this
potential and compare the results with simple one-dimensional models. We
emphasize the importance of taking into account the gaussian envelope of the
optical lattice which gives rise to Bragg cavity effects. Our results are a
further step towards integrated atom optics setups for quasi-cw matter waves. | cond-mat_quant-gas |
On quantum time crystals and interacting gauge theories in atomic
Bose-Einstein condensates: We study the dynamics of a Bose-Einstein condensate trapped circumferentially
on a ring, and which is governed by an interacting gauge theory. We show that
the associated density-dependent gauge potential and concomitant current
nonlinearity permits a ground state in the form of a rotating chiral bright
soliton. This chiral soliton is constrained to move in one direction by virtue
of the current nonlinearity, and represents a time crystal in the same vein as
Wilczek's original proposal. | cond-mat_quant-gas |
Quantum Many-Body Scars in Optical Lattices: The concept of quantum many-body scars has recently been put forward as a
route to describe weak ergodicity breaking and violation of the Eigenstate
Thermalization Hypothesis. We propose a simple setup to generate quantum
many-body scars in a doubly modulated Bose-Hubbard system which can be readily
implemented in cold atomic gases. The dynamics are shown to be governed by
kinetic constraints which appear via density assisted tunneling in a
high-frequency expansion. We find the optimal driving parameters for the
kinetically constrained hopping which leads to small isolated subspaces of
scared eigenstates. The experimental signatures and the transition to fully
thermalizing behavior as a function of driving frequency are analyzed. | cond-mat_quant-gas |
Fractional angular momentum in cold atom systems: The quantum statistics of bosons or fermions are manifest through even or odd
relative angular momentum of a pair. We show theoretically that, under certain
conditions, a pair of certain test particles immersed in a fractional quantum
Hall state possesses, effectively, a fractional relative angular momentum,
which can be interpreted in terms of fractional braid statistics. We propose
that the fractionalization of the angular momentum can be detected directly
through the measurement of the pair correlation function in rotating ultra-cold
atomic systems in the fractional quantum Hall regime. Such a measurement will
also provide direct evidence for the effective magnetic field, resulting from
Berry phases arising from attached vortices, and of excitations with fractional
particle number, analogous to fractional charge of electron fractional quantum
Hall effect. | cond-mat_quant-gas |
Propagating wave-packets and quantised currents in coherently driven
polariton superfluids: We study the properties of propagating polariton wave-packets and their
connection to the stability of doubly charged vortices. Wave-packet propagation
and related photoluminescence spectra exhibit a rich behaviour dependent on the
excitation regime. We show that, because of the non-quadratic polariton
dispersion, doubly charged vortices are stable only when initiated in
wave-packets propagating at small velocities. Vortices propagating at larger
velocities, or those imprinted directly into the polariton optical parametric
oscillator (OPO) signal and idler are always unstable to splitting. | cond-mat_quant-gas |
Thermodynamic signatures of the polaron-molecule transition in a Fermi
gas: We consider the highly spin-imbalanced limit of a two-component Fermi gas,
where there is a small density of $\downarrow$ impurities attractively
interacting with a sea of $\uparrow$ fermions. In the single-impurity limit at
zero temperature, there exists the so-called polaron-molecule transition, where
the impurity sharply changes its character by binding a $\uparrow$ fermion at
sufficiently strong attraction. Using a recently developed variational
approach, we calculate the thermodynamic properties of the impurity, and we
show that the transition becomes a smooth crossover at finite temperature due
to the thermal occupation of excited states in the impurity spectral function.
However, remnants of the single-impurity transition are apparent in the
momentum-resolved spectral function, which can in principle be probed with
Raman spectroscopy. We furthermore show that the Tan contact exhibits a
characteristic non-monotonic dependence on temperature that provides a
signature of the zero-temperature polaron-molecule transition. For a finite
impurity density, we argue that descriptions purely based on the behavior of
the Fermi polaron are invalid near the polaron-molecule transition, since
correlations between impurities cannot be ignored. In particular, we show that
the spin-imbalanced system undergoes phase separation at low temperatures due
to the strong attraction between $\uparrow\downarrow$ molecules induced by the
Fermi sea. Thus, we find that the impurity spectrum and the induced
impurity-impurity interactions are key to understanding the phase diagram of
the spin-imbalanced Fermi gas. | cond-mat_quant-gas |
Direct observation of coherent inter-orbital spin-exchange dynamics: We report on the first direct observation of fast spin-exchange coherent
oscillations between different long-lived electronic orbitals of ultracold
$^{173}$Yb fermions. We measure, in a model-independent way, the strength of
the exchange interaction driving this coherent process. This observation allows
us to retrieve important information on the inter-orbital collisional
properties of $^{173}$Yb atoms and paves the way to novel quantum simulations
of paradigmatic models of two-orbital quantum magnetism. | cond-mat_quant-gas |
Modulational Instabity of Spin-Orbit Coupled Bose-Einstein Condensates
in Discrete Media: We address the impact of intra-site spin-orbit (SO) coupling and associated
inter-component Rabi coupling on the modulational instability (MI) of
plane-wave states in two-component discrete Bose-Einstein condensates (BECs).
Conditions for the onset of the MI and the respective instability are found
analytically. SO coupling allows us to produce the MI even for a small initial
wavenumber (q < {\pi}/2) for miscible states. In particular, SO coupling
introduces MI even in the absence of hopping coefficient, a concept which may
have wider ramifications in heavy atomic BECs. Our investigations predict that
the impact of Rabi coupling is more pronounced compared to the other system
parameters. We have also shown how our results of the linear stability analysis
can be corroborated numerically. The fact that we have brought out the
stability criteria in different domains of system parameters means that our
model is tailor made experiments. | cond-mat_quant-gas |
In Situ Momentum Distribution Measurement of a Quantum Degenerate Fermi
Gas using Raman Spectroscopy: The ability to directly measure the momentum distribution of quantum gases is
both unique to these systems and pivotal in extracting many other important
observables. Here we use Raman transitions to measure the momentum distribution
of a weakly-interacting Fermi gas in a harmonic trap. For narrow atomic
dispersions, momentum and energy conservation imply a linear relation between
the two-photon detuning and the atomic momentum. We detect the number of atoms
transferred by the Raman beams using sensitive fluorescence detection in a
magneto-optical trap. We employ this technique to a degenerate
weakly-interacting Fermi gas at different temperatures. The measured momentum
distributions match theoretical curves over two decades, and the extracted
temperatures are in very good agreement with the ones obtained from a
conventional time-of-flight technique. The main advantages of our measurement
scheme are that it can be spatially selective and applied to a trapped gas, it
can be completed in a relatively short time, and due to its high sensitivity,
it can be used with very small clouds. | cond-mat_quant-gas |
The Peierls substitution in an engineered lattice potential: Artificial gauge fields open new possibilities to realize quantum many-body
systems with ultracold atoms, by engineering Hamiltonians usually associated
with electronic systems. In the presence of a periodic potential, artificial
gauge fields may bring ultracold atoms closer to the quantum Hall regime. Here,
we describe a one-dimensional lattice derived purely from effective
Zeeman-shifts resulting from a combination of Raman coupling and radiofrequency
magnetic fields. In this lattice, the tunneling matrix element is generally
complex. We control both the amplitude and the phase of this tunneling
parameter, experimentally realizing the Peierls substitution for ultracold
neutral atoms. | cond-mat_quant-gas |
Anomalous Behavior of Spin Systems with Dipolar Interactions: We study the properties of spin systems realized by cold polar molecules
interacting via dipole-dipole interactions in two dimensions. Using a spin wave
theory, that allows for the full treatment of the characteristic long-distance
tail of the dipolar interaction, we find several anomalous features in the
ground state correlations and the spin wave excitation spectrum, which are
absent in their counterparts with short range interaction. The most striking
consequence is the existence of true long-range order at finite temperature for
a two-dimensional phase with a broken U(1) symmetry. | cond-mat_quant-gas |
Liquid crystal phases of two-dimensional dipolar gases and
Berezinskii-Kosterlitz-Thouless melting: Liquid crystals are phases of matter intermediate between crystals and
liquids. Whereas classical liquid crystals have been known for a long time and
are used in electro-optical displays, much less is known about their quantum
counterparts. There is growing evidence that quantum liquid crystals play a
central role in many electron systems including high temperature
superconductors, but a quantitative understanding is lacking due to disorder
and other complications. Here, we analyse the quantum phase diagram of a
two-dimensional dipolar gas, which exhibits stripe, nematic and supersolid
phases. We calculate the stiffness constants determining the stability of the
nematic and stripe phases, and the melting of the stripes set by the
proliferation of topological defects is analysed microscopically. Our results
for the critical temperatures of these phases demonstrate that a controlled
study of the interplay between quantum liquid and superfluid phases is within
experimental reach for the first time, using dipolar gases. | cond-mat_quant-gas |
Manipulating multimer propagation using lattice modulation: We propose a scheme for controlling the movement of dimers, trimers, and
other multimers in optical lattices by modulating the lattice potential. In
deep optical lattices the propagation of deeply bound atomic clusters is slowed
down by the high energy cost of virtual intermediate states. Adapting the
well-known method of lattice modulation spectroscopy, the movement of the
clusters can be made resonant by utilizing sequences of bound-bound
transitions. Using the scheme, the mobility of each specific cluster can be
selectively controlled by tuning the modulation frequency. We formulate a
simple and intuitive model and confirm the validity of the model by numerical
simulations of dimers and trimers in a one-dimensional optical lattice. | cond-mat_quant-gas |
Light scattering in inhomogeneous Tomonaga-Luttinger liquids: We derive the dynamical structure factor for an inhomogeneous
Tomonaga-Luttinger liquid as can be formed in a confined strongly interacting
one-dimensional gas. In view of current experimental progress in the field, we
provide a simple analytic expression for the light-scattering cross section,
requiring only the knowledge of the density dependence of the ground-state
energy, as they can be extracted e.g. from exact or Quantum Monte Carlo
techniques, and a Thomas-Fermi description. We apply the result to the case of
one-dimensional quantum bosonic gases with dipolar interaction in a harmonic
trap, using an energy functional deduced from Quantum Monte Carlo computations.
We find an universal scaling behavior peculiar of the Tomonaga-Luttinger
liquid, a signature that can be eventually probed by Bragg spectroscopy in
experimental realizations of such systems. | cond-mat_quant-gas |
Short note on the Rabi model: The spectral density of the Rabi model is calculated exactly within a
continued fraction approach. It is shown that the method yields a convergent
solution. | cond-mat_quant-gas |
Prethermal Floquet Steady States and Instabilities in the Periodically
Driven, Weakly Interacting Bose-Hubbard Model: We explore prethermal Floquet steady states and instabilities of the weakly
interacting two-dimensional Bose-Hubbard model subject to periodic driving. We
develop a description of the nonequilibrium dynamics, at arbitrary drive
strength and frequency, using a weak-coupling conserving approximation. We
establish the regimes in which conventional (zero-momentum) and unconventional
[$(\pi,\pi)$-momentum] condensates are stable on intermediate time scales. We
find that condensate stability is \emph{enhanced} by increasing the drive
strength, because this decreases the bandwidth of quasiparticle excitations and
thus impedes resonant absorption and heating. Our results are directly relevant
to a number of current experiments with ultracold bosons. | cond-mat_quant-gas |
Experimental realization of a non-magnetic one-way spin switch: Controlling magnetism through non-magnetic means is highly desirable for
future electronic devices, as such means typically have ultra-low power
requirements and can provide coherent control. In recent years, great
experimental progress has been made in the field of electrical manipulation of
magnetism in numerous material systems. These studies generally do not consider
the directionality of the applied non-magnetic potentials and/or magnetism
switching. Here, we theoretically conceive and experimentally demonstrate a
non-magnetic one-way spin switch device using a spin-orbit coupled
Bose-Einstein condensate subjected to a moving spin-independent dipole
potential. The physical foundation of this unidirectional device is based on
the breakdown of Galilean invariance in the presence of spin-orbit coupling.
Such a one-way spin switch opens an avenue for designing novel quantum devices
with unique functionalities and may facilitate further experimental
investigations of other one-way spintronic and atomtronic devices. | cond-mat_quant-gas |
Asymptotic Bound-state Model for Feshbach Resonances: We present an Asymptotic Bound-state Model which can be used to accurately
describe all Feshbach resonance positions and widths in a two-body system. With
this model we determine the coupled bound states of a particular two-body
system. The model is based on analytic properties of the two-body Hamiltonian,
and on asymptotic properties of uncoupled bound states in the interaction
potentials. In its most simple version, the only necessary parameters are the
least bound state energies and actual potentials are not used. The complexity
of the model can be stepwise increased by introducing threshold effects,
multiple vibrational levels and additional potential parameters. The model is
extensively tested on the 6Li-40K system and additional calculations on the
40K-87Rb system are presented. | cond-mat_quant-gas |
Feedback-enhanced algorithm for aberration correction of holographic
atom traps: We show that a phase-only spatial light modulator can be used to generate
non-trivial light distributions suitable for trapping ultracold atoms, when the
hologram calculation is included within a simple and robust feedback loop that
corrects for imperfect device response and optical aberrations. This correction
reduces the discrepancy between target and experimental light distribution to
the level of a few percent (RMS error). We prove the generality of this
algorithm by applying it to a variety of target light distributions of
relevance for cold atomic physics. | cond-mat_quant-gas |
Exotic photonic molecules via Lennard-Jones-like potentials: Ultracold systems offer an unprecedented level of control of interactions
between atoms. An important challenge is to achieve a similar level of control
of the interactions between photons. Towards this goal, we propose a
realization of a novel Lennard-Jones-like potential between photons coupled to
the Rydberg states via electromagnetically induced transparency (EIT). This
potential is achieved by tuning Rydberg states to a F{\"o}rster resonance with
other Rydberg states. We consider few-body problems in 1D and 2D geometries and
show the existence of self-bound clusters ("molecules") of photons. We
demonstrate that for a few-body problem, the multi-body interactions have a
significant impact on the geometry of the molecular ground state. This leads to
phenomena without counterparts in conventional systems: For example, three
photons in 2D preferentially arrange themselves in a line-configuration rather
than in an equilateral-triangle configuration. Our result opens a new avenue
for studies of many-body phenomena with strongly interacting photons. | cond-mat_quant-gas |
Time scale for adiabaticity breakdown in driven many-body systems and
orthogonality catastrophe: The adiabatic theorem is a fundamental result established in the early days
of quantum mechanics, which states that a system can be kept arbitrarily close
to the instantaneous ground state of its Hamiltonian if the latter varies in
time slowly enough. The theorem has an impressive record of applications
ranging from foundations of quantum field theory to computational recipes in
molecular dynamics. In light of this success it is remarkable that a
practicable quantitative understanding of what "slowly enough" means is limited
to a modest set of systems mostly having a small Hilbert space. Here we show
how this gap can be bridged for a broad natural class of physical systems,
namely many-body systems where a small move in the parameter space induces an
orthogonality catastrophe. In this class, the conditions for adiabaticity are
derived from the scaling properties of the parameter dependent ground state
without a reference to the excitation spectrum. This finding constitutes a
major simplification of a complex problem, which otherwise requires solving
non-autonomous time evolution in a large Hilbert space. We illustrate our
general results by analyzing conditions for the transport quantization in a
topological Thouless pump. | cond-mat_quant-gas |
All-optical transport and compression of ytterbium atoms into the
surface of a solid immersion lens: We present an all-optical method to load 174Yb atoms into a single layer of
an optical trap near the surface of a solid immersion lens which improves the
numerical aperture of a microscope system. Atoms are transported to a region 20
um below the surface using a system comprised by three optical dipole traps.
The "optical accordion" technique is used to create a condensate and compress
the atoms to a width of 120 nm and a distance of 1.8 um away from the surface.
Moreover, we are able to verify that after compression the condensate behaves
as a two-dimensional quantum gas. | cond-mat_quant-gas |
Large-scale characterization of Cu2O monocrystals via Rydberg excitons: Rydberg states of excitons can reach microns in size and require extremely
pure crystals. We introduce an experimental method for the rapid and
spatially-resolved characterization of Rydberg excitons in copper oxide (Cu2O)
with sub-micron resolution over large zones. Our approach involves illuminating
and imaging the entire sample on a camera to realize a spatially-resolved
version of resonant absorption spectroscopy, without any mobile part. This
yields spatial maps of Rydberg exciton properties, including their energy,
linewidth and peak absorption, providing a comprehensive quality assessment of
the entire sample in a single shot. Furthermore, by imaging the sample
photoluminescence over the same zone, we establish a strong relationship
between the spectral quality map and the photoluminescence map of charged
oxygen vacancies. This results in an independent, luminescence-based quality
map that closely matches the results obtained through resonant spectroscopy.
Our findings reveal that Rydberg excitons in natural Cu2O crystals are
predominantly influenced by optically-active charged oxygen vacancies, which
can be easily mapped. Together, these two complementary methods provide
valuable insights into Cu2O crystal properties. | cond-mat_quant-gas |
Leggett collective excitations in a two-band Fermi superfluid at finite
temperatures: The Leggett collective excitations for a two-band Fermi gas with s-wave
pairing and Josephson interband coupling in the BCS-BEC crossover at finite
temperatures are investigated within the Gaussian pair fluctuation approach.
Eigenfrequencies and damping factors for Leggett modes are determined in a
nonperturbative way, using the analytic continuation of the fluctuation
propagator through a branch cut in the complex frequency plane, as in Phys.
Rev. Lett. 122, 093403 (2019). The treatment is performed beyond the low-energy
expansion, which is necessary when the collective excitation energy reaches the
pair-breaking continuum edge. The results are applied in particular to cold
atomic gases at the orbital Feshbach resonance and in a regime far from BEC,
which can be relevant for future experiments. | cond-mat_quant-gas |
Quantum Charge Pumps with Topological Phases in Creutz Ladder: Quantum charge pumping phenomenon connects band topology through the dynamics
of a one-dimensional quantum system. In terms of a microscopic model, the
Su-Schrieffer-Heeger/Rice-Mele quantum pump continues to serve as a fruitful
starting point for many considerations of topological physics. Here we present
a generalized Creutz scheme as a distinct two-band quantum pump model. By
noting that it undergoes two kinds of topological band transitions accompanying
with a Zak-phase-difference of $\pi$ and $2\pi$, respectively, various charge
pumping schemes are studied by applying an elaborate Peierl's phase
substitution. Translating into real space, the transportation of quantized
charges is a result of cooperative quantum interference effect. In particular,
an all-flux quantum pump emerges which operates with time-varying fluxes only
and transports two charge units. This puts cold atoms with artificial gauge
fields as an unique system where this kind of phenomena can be realized. | cond-mat_quant-gas |
Abnormal Superfluid Fraction of Harmonically Trapped Few-Fermion Systems: Superfluidity is a fascinating phenomenon that, at the macroscopic scale,
leads to dissipationless flow and the emergence of vortices. While these
macroscopic manifestations of superfluidity are well described by theories that
have their origin in Landau's two-fluid model, our microscopic understanding of
superfluidity is far from complete. Using analytical and numerical \textit{ab
initio} approaches, this paper determines the superfluid fraction and local
superfluid density of small harmonically trapped two-component Fermi gases as a
function of the interaction strength and temperature. At low temperature, we
find that the superfluid fraction is, in certain regions of the parameter
space, negative. This counterintuitive finding is traced back to the symmetry
of the system's ground state wave function, which gives rise to a diverging
quantum moment of inertia $I_{\text{q}}$. Analogous abnormal behavior of
$I_{\text{q}}$ has been observed in even-odd nuclei at low temperature. Our
predictions can be tested in modern cold atom experiments. | cond-mat_quant-gas |
Twin matter waves for interferometry beyond the classical limit: Interferometers with atomic ensembles constitute an integral part of modern
precision metrology. However, these interferometers are fundamentally
restricted by the shot noise limit, which can only be overcome by creating
quantum entanglement among the atoms. We used spin dynamics in Bose-Einstein
condensates to create large ensembles of up to $10^4$ pair-correlated atoms
with an interferometric sensitivity $-1.61^{+0.98}_{-1.1}$ dB beyond the shot
noise limit. Our proof-of-principle results point the way toward a new
generation of atom interferometers. | cond-mat_quant-gas |
Non-thermal fixed points of universal sine-Gordon coarsening dynamics: We examine coarsening of field-excitation patterns of the sine-Gordon (SG)
model, in two and three spatial dimensions, identifying it as universal
dynamics near non-thermal fixed points. The SG model is relevant in many
different contexts, from solitons in quantum fluids to structure formation in
the universe. The coarsening process entails anomalously slow self-similar
transport of the spectral distribution of excitations towards low energies,
induced by the collisional interactions between the field modes. The focus is
set on the non-relativistic limit exhibiting particle excitations only,
governed by a Schr\"odinger-type equation with Bessel-function non-linearity.
The results of our classical statistical simulations suggest that, in contrast
to wave turbulent cascades, in which the transport is local in momentum space,
the coarsening is dominated by rather non-local processes corresponding to a
spatial containment in position space. The scaling analysis of a kinetic
equation obtained with path-integral techniques corroborates this numerical
observation and suggests that the non-locality is directly related to the
slowness of the scaling in space and time. Our methods, which we expect to be
applicable to more general types of models, could open a long-sought path to
analytically describing universality classes behind domain coarsening and
phase-ordering kinetics from first principles, which are usually modelled in a
near-equilibrium setting by a phenomenological diffusion-type equation in
combination with conservation laws. | cond-mat_quant-gas |
Lifetime of Single-Particle Excitations in a Dilute Bose-Einstein
Condensate at Zero Temperature: We study the lifetime of single-particle excitations in a dilute homogeneous
Bose-Einstein condensate at zero temperature based on a self-consistent
perturbation expansion of satisfying Goldstone's theorem and conservation laws
simultaneously.It is shown that every excitation for each momentum ${\bf p}$
should have a finite lifetime proportional to the inverse $a^{-1}$ of the
$s$-wave scattering length $a$, instead of $a^{-2}$ for the normal state, due
to a new class of Feynman diagrams for the self-energy that emerges upon
condensation. We calculate the lifetime as a function of $|{\bf p}|$
approximately. | cond-mat_quant-gas |
Symmetric and asymmetric solitons trapped in H-shaped potentials: We report results of numerical and analytical studies of the spontaneous
symmetry breaking in solitons, both two- and one-dimensional, which are trapped
in H-shaped potential profiles, built of two parallel potential troughs linked
by a narrow rung in the transverse direction. This system can be implemented in
self-attractive Bose-Einstein condensates (BECs), as well as in a nonlinear
bulk optical waveguide.We demonstrate that the introduction of the transverse
link changes the character of the symmetry-breaking bifurcation (SBB) in the
system from subcritical to supercritical (in terms of the corresponding phase
transition, it is a change between the first and second kinds). A noteworthy
feature of the SBB in this setting is a non-monotonous dependence of the
soliton's norm at the bifurcation point on the strength of the transverse link.
In the full 2D system, the results are obtained in a numerical form. An exact
analytical solution is found for the bifurcation in the 1D version of the
model, with the transverse rung modeled by the local linear coupling between
the parallel troughs with the Delta-functional longitudinal profile. Replacing
the Delta-function by its finite-width Gaussian counterpart, similar results
are obtained by means of the variational approximation (VA). The VA is also
applied to the 1D system with a mixed linear and nonlinear transverse localized
coupling. Comparison of the results produced by the different varieties of the
system clearly reveals basic features of the symmetry-breaking transition in
it. | cond-mat_quant-gas |
Mobile Impurity in a Two-Leg Bosonic Ladder: We study the dynamics of a mobile impurity in a two-leg bosonic ladder. The
impurity moves both along and across the legs and interacts with a bath of
interacting bosonic particles present in the ladder. We use both analytical
(Tomonaga-Luttinger liquid - TLL) and numerical (Density Matrix Renormalization
Group - DMRG) methods to compute the Green's function of the impurity. We find
that for a small impurity-bath interaction, the bonding mode of the impurity
effectively couples only to the gapless mode of the bath while the anti-bonding
mode of the impurity couples to both gapped and gapless mode of the bath. We
compute the time dependence of the Green's function of the impurity, for
impurity created either in the anti-bonding or bonding mode with a given
momentum. The later case leads to a decay as a power-law below a critical
momentum and exponential above, while the former case always decays
exponentially. We compare the DMRG results with analytical results using the
linked cluster expansion and find a good agreement. In addition we use DMRG to
extract the lifetime of the quasi-particle, when the Green's function decays
exponentially. We also treat the case of an infinite bath-impurity coupling for
which both the bonding and antibonding modes are systematically affected. For
this case the impurity Green's function in the bonding mode decays as a
power-law at zero momentum.The corresponding exponent increases with increasing
transverse-tunneling of the impurity. We compare our results with the other
impurity problems for which the motion of either the impurity or the bath is
limited to a single chain. Finally we comments on the consequences of our
findings for experiments with the ultracold gasses. | cond-mat_quant-gas |
Spin modulation instabilities and phase separation dynamics in trapped
two-component Bose condensates: In the study of trapped two-component Bose gases, a widely used dynamical
protocol is to start from the ground state of a one-component condensate and
then switch half the atoms into another hyperfine state. The slightly different
intra-component and inter-component interactions can then lead to highly
nontrivial dynamics. We study and classify the possible subsequent dynamics,
over a wide variety of parameters spanned by the trap strength and by the
inter- to intra-component interaction ratio. A stability analysis suited to the
trapped situation provides us with a framework to explain the various types of
dynamics in different regimes. | cond-mat_quant-gas |
Universal subdiffusion in strongly tilted many-body systems: The quantum dynamics away from equilibrium is of fundamental interest for
interacting many-body systems. In this letter, we study tilted many-body
systems using the effective Hamiltonian derived from the microscopic
description. We first give general arguments for the density relaxation rate
satisfying $1/\tau\propto k^4$ for a large class of systems, including the
Fermi Hubbard model case as observed in the the recent experiment [1]. Here $k$
is the wave vector of the density wave. The main ingredients are the emergence
of the reflection symmetry and dipole moment conservation to the leading
non-trivial order of the large tilted strength. To support our analysis, we
then construct a solvable model with large local Hilbert space dimension by
coupling sites discribed by the Sachdev-Ye-Kitaev models, where the density
response can be computed explicitly. The the tilt strength and the temperature
dependence of the subdiffusion constant are also discussed. | cond-mat_quant-gas |
Classical fields and quantum measurement for Bose-Einstein condensate: We analyze a process of splitting of the Bose-Einstein condensate and the
mutual coherence of two separated atomic clouds. Within the classical fields
approximation we show that coherence between clouds is degraded if atoms
interact and if we account for the sufficiently long observation time. We also
show, that upon recombination, the coherence across the sample is restored. The
coherence is not fully degraded if the splitting potential remains sufficiently
penetrable. We calculate the variance of atom number difference for this
time-averaging measurement and show that for low temperatures it can be well
below Poissonian limit like it was observed in the experiments. | cond-mat_quant-gas |
Acoustic Superradiance from a Bose-Einstein Condensate Vortex with a
Self-Consistent Background Density Profile: The axisymmetric acoustic perturbations in the velocity potential of a
Bose-Einstein condensate in the presence of a single vortex behave like
minimally coupled massless scalar fields propagating in a curved (1+1)
dimensional Lorentzian space-time, governed by the Klein-Gordon wave equation.
Thus far, the amplified scattering of these perturbations from the vortex, as a
manifestation of the acoustic superradiance, has been investigated with a
constant background density. This paper goes beyond by employing a
self-consistent condensate density profile that is obtained by solving the
Gross-Pitaevskii equation for an unbound BEC. Consequently, the loci of the
event horizon and the ergosphere of the acoustic black hole are modified
according to the radially varying speed of sound. The superradiance is
investigated both for transient features in the time-domain and for spectral
features in the frequency domain. In particular, an effective energy-potential
function defined in the spectral formulation correlates with the existence and
the frequency dependence of the acoustic superradiance. The numerical results
indicate that the constant background density approximation underestimates the
maximum superradiance and the frequency at which this maximum occurs. | cond-mat_quant-gas |
Photon Bose-Condensate as a Tunable Terahertz Laser Source without
Inversion: We develop a theoretical model for a tunable coherent terahertz radiation
source based on the long-lived Bose condensate of photons. In the device we
propose, the original photon pumping is performed incoherently by a blackbody
radiation emitter. The photons thus produced Bose-condense by the inelastic
relaxation on a two-dimensional electron gas in a perpendicular magnetostatic
field. The process involves neither population inversion nor light wave
amplification the standard laser sources are built on. The coherence and
tunability of the light emitted by such a photon condensate are provided and
supported by the discrete spectrum of the electron gas in the quantizing
magnetic field. The device is a compact-size semiconductor crystal. We propose
the design and perform the realistic calculations of the physical properties
and limiting factors for the terahertz photon Bose-condensate resonator. We
show that our terahertz source can deliver the highly coherent light emission
in the frequency range of 3-30 THz for the magnetic field induction of the
order of 2 T, with the upper emission frequency limit adjustable by the
strength of the magnetic field applied. | cond-mat_quant-gas |
The improved Gaussian approximation Calculation of Bogoliubov Mode in
One Dimensional Bosonic Gas: In this paper, we study the homogeneous one-dimensional bosonic gas
interacting via a repulsive contact potential by using the improved Gaussian
approximation. We obtain the gapless excitation spectrum of Bogoliubov mode.
Our result is in good agreement with the exact numerical calculation based on
the Bethe ansatz. We speculate that the improved Gaussian approximation could
be a quantitatively good approximation for higher dimensional systems. | cond-mat_quant-gas |
Full and fractional defects across the Berezinskii-Kosterlitz-Thouless
transition in a driven-dissipative spinor quantum fluid: We investigate the properties of a two-dimensional \emph{spinor} microcavity
polariton system driven by a linearly polarised continuous pump. In particular,
we establish the role of the elementary excitations, namely the so-called
half-vortices and full-vortices; these objects carry a quantum rotation only in
one of the two, or both, spin components respectively. Our numerical analysis
of the steady-state shows that it is only the half-vortices that are present in
the vortex-antivortex pairing/dissociation responsible for the
Berezinskii-Kosterlitz-Thouless transition. These are the relevant elementary
excitations close to the critical point. However, by exploring the
phase-ordering dynamics following a sudden quench across the transition we
prove that full-vortices become the relevant excitations away from the critical
point in a deep quasi-ordered state at late times. The time-scales for
half-vortices binding into full vortices are much faster than the
vortex-antivortex annihilations. | cond-mat_quant-gas |
Parametric instability of oscillations of a vortex ring in a
$z$-periodic Bose-Einstein condensate and the recurrence to starting state: The dynamics of deformations of a quantum vortex ring in a Bose-Einstein
condensate with periodic equilibrium density $\rho(z)= 1-\epsilon\cos z$ has
been considered within the local induction approximation. Parametric
instabilities of the normal modes with azimuthal numbers $\pm m$ have been
revealed at the energy integral $E$ near values $E_m^{(p)}=2m\sqrt{m^2-1}/p$,
where $p$ is the resonance order. Numerical simulations have shown that already
at $\epsilon\sim 0.03$ a rapid growth of unstable modes with $m=2$, $p=1$ to
magnitudes of order of unity is typical, which is then followed, after a few
large oscillations, by fast return to a weakly excited state. Such behavior
corresponds to an integrable Hamiltonian of the form $H\propto
\sigma(E_2^{(1)}-E)(|b_+|^2 + |b_-|^2) -\epsilon(b_+ b_- + b_+^* b_-^*)
+u(|b_+|^4 +|b_-|^4) + w |b_+|^2|b_-|^2$ for two complex envelopes $b_\pm(t)$.
The results have been compared to parametric instabilities of vortex ring in
condensate with density $\rho(z,r)=1-r^2-\alpha z^2$, which take place at
$\alpha\approx 8/5$ and at $\alpha\approx 16/7$. | cond-mat_quant-gas |
Propagation of phase-imprinted solitons from superfluid core to
Mott-insulator shell and superfluid shell: We study phase-imprinted solitons of ultracold bosons in an optical lattice
with a harmonic trap, which shows the superfluid (SF) and Mott-insulator (MI)
shell structures. The earlier study [Konstantin V. Krutitsky, J. Larson, and M.
Lewenstein, Phys. Rev. A 82, 033618 (2010).] reported three types of
phase-imprinted solitons in the Bose-Hubbard model: in-phase soliton,
out-of-phase soliton, and wavelet. In this paper, we uncover the dynamical
phase diagram of these phase-imprinted solitons, and find another type of the
phase-imprinted soliton namely, the hybrid soliton. In the harmonically trapped
system, the solitonic excitations created at the SF core cannot penetrate into
the outer SF shell. This repulsion at the surface of the outer SF shell can be
cured by inposing a repulsive potential at the center of the trap. These
results can be interpreted as a kind of the impedance matching of excitations
in BECs in terms of the effective chemical potentials or the local particle
numbers in the shell, and the analogous results can be observed also in the
sound wave created by the local single-shot pulse potential. | cond-mat_quant-gas |
Drag in Bose-Fermi Mixtures: We use kinetic theory to model the dynamics of a small Bose condensed cloud
of heavy particles moving through a larger degenerate Fermi gas of light
particles. Varying the Bose-Fermi interaction, we find a crossover between bulk
and surface dominated regimes -- where scattering occurs throughout the Bose
cloud, or solely on the surface. We calculate the damping and frequency shift
of the dipole mode in a harmonic trap as a function of the magnetic field
controlling an inter-species Feshbach resonance. We find excellent agreement
between our stochastic model and the experimental studies of Cs-Li mixtures. | cond-mat_quant-gas |
Statics and dynamics of atomic dark-bright solitons in the presence of
delta-like impurities: Adopting a mean-field description for a two-component atomic Bose-Einstein
condensate, we study the stat- ics and dynamics of dark-bright solitons in the
presence of localized impurities. We use adiabatic perturbation theory to
derive an equation of motion for the dark-bright soliton center. We show that,
counter-intuitively, an attractive (repulsive) delta-like impurity, acting
solely on the bright soliton component, induces an effective localized barrier
(well) in the effective potential felt by the soliton; this way, dark-bright
solitons are reflected from (transmitted through) attractive (repulsive)
impurities. Our analytical results for the small-amplitude oscil- lations of
solitons are found to be in good agreement with results obtained via a
Bogoliubov-de Gennes analysis and direct numerical simulations. | cond-mat_quant-gas |
Parametric Excitation of a 1D Gas in Integrable and Nonintegrable Cases: We study the response of a highly excited 1D gas with pointlike interactions
to a periodic modulation of the coupling constant. We calculate the
corresponding dynamic structure factors and show that their low-frequency
behavior differs dramatically for integrable and nonintegrable models.
Nonintegrable systems are sensitive to excitations with frequencies as low as
the mean level spacing, whereas much higher frequencies are required to excite
an integrable system. This effect can be used as a probe of integrability for
mesoscopic 1D systems and can be observed experimentally by measuring the
heating rate of a parametrically excited gas. | cond-mat_quant-gas |
Dimensional crossover in non-relativistic effective field theory: Isotropic scattering in various spatial dimensions is considered for
arbitrary finite-range potentials using non-relativistic effective field
theory. With periodic boundary conditions, compactifications from a box to a
plane and to a wire, and from a plane to a wire, are considered by matching
S-matrix elements. The problem is greatly simplified by regulating the
ultraviolet divergences using dimensional regularization with minimal
subtraction. General relations among (all) effective-range parameters in the
various dimensions are derived, and the dependence of bound states on changing
dimensionality are considered. Generally, it is found that compactification
binds the two-body system, even if the uncompactified system is unbound. For
instance, compactification from a box to a plane gives rise to a bound state
with binding momentum given by $\ln \left({\scriptstyle
\frac{1}{2}}\left(3+\sqrt{5} \right) \right)$ in units of the inverse
compactification length. This binding momentum is universal in the sense that
it does not depend on the two-body interaction in the box. When the two-body
system in the box is at unitarity, the S-matrices of the compactified two-body
system on the plane and on the wire are given exactly as universal functions of
the compactification length | cond-mat_quant-gas |
On the hydrodynamic canonical formalism of the Gross-Pitaevskii field: We derive a canonical formalism for the hydrodynamic representation of the
Gross-Pitaevskii field (nonlinear Schr\"odinger field), where the density and
the phase of the condensate form a canonical pair of conjugate field variables.
To do so, we treat the meanfield as a singular Lagrangian system and apply both
the Dirac-Bergmann and Faddeev-Jackiw methods. The Faddeev-Jackiw method is
found to be a more direct approach to the problem. | cond-mat_quant-gas |
Skyrmion crystals in the pseudo-spin-1/2 Bose-Einstein condensates: Exact two-dimensional solutions are constructed for the pseudo-spin-1/2
Bose-Einstein condensates which are described by the coupled nonlinear
Gross-Pitaevskii equations where the intraspecies and interspecies coupling
constants are assumed to be equal. The equations are decoupled by means of
re-combinations of the nonlinear terms of the hyperfine states according to the
spatial dimensions. These stationary solutions form various spin textures which
are identified as skyrmion crystals. In a special case, the crystal of
skyrmion-antiskyrmion pairs is formed in the soliton limit. | cond-mat_quant-gas |
Phonon-mediated Casimir interaction between finite mass impurities: The Casimir effect, a two-body interaction via vacuum fluctuations, is a
fundamental property of quantum systems. In solid state physics it emerges as a
long-range interaction between two impurity atoms via virtual phonons. In the
classical limit for the impurity atoms in $D$ dimensions the interaction is
known to follow the universal power-law $U(r)\sim r^{-D}$. However, for finite
masses of the impurity atoms on a lattice, it was predicted to be $U(r)\sim
r^{-2D-1}$ at large distances. We examine how one power-law can change into
another with increase of the impurity mass and in presence of an external
potential. We provide the exact solution for the system in one-dimension. At
large distances indeed $U(r)\sim r^{-3}$ for finite impurity masses, while for
the infinite impurity masses or in an external potential it crosses over to
$U(r)\sim r^{-1}$ . At short distances the Casimir interaction is not universal
and depends on the impurity mass and the external potential. | cond-mat_quant-gas |
Theory of a resonantly interacting impurity in a Bose-Einstein
condensate: We investigate a Bose-Einstein condensate in strong interaction with a single
impurity particle. While this situation has received considerable interest in
recent years, the regime of strong coupling remained inaccessible to most
approaches due to an instability in Bogoliubov theory arising near the
resonance. We present a nonlocal extension of Gross-Pitaevskii theory that is
free of such divergences and does not require the use of the Born approximation
in any of the interaction potentials. We find a new dynamical transition regime
between attractive and repulsive polarons, where an interaction quench results
in a finite number of coherent oscillations in the density profiles of the
medium and in the contact parameter before equilibrium is reached. | cond-mat_quant-gas |
Active Learning Algorithm for Computational Physics: In large-scale computation of physics problems, one often encounters the
problem of determining a multi-dimensional function, which can be
time-consuming when computing each point in this multi-dimensional space is
already time-demanding. In the work, we propose that the active learning
algorithm can speed up such calculations. The basic idea is to fit a
multi-dimensional function by neural networks, and the key point is to make the
query of labeled data economically by using a stratagem called "query by
committee". We present the general protocol of this fitting scheme, as well as
the procedure of how to further compute physical observables with the fitted
functions. We show that this method can work well with two examples, which are
quantum three-body problem in atomic physics and the anomalous Hall
conductivity in condensed matter physics, respectively. In these examples, we
show that one reaches an accuracy of few percent error for computing physical
observables with less than $10\%$ of total data points compared with uniform
sampling. With these two examples, we also visualize that by using the active
learning algorithm, the required data are added mostly in the regime where the
function varies most rapidly, which explains the mechanism for the efficiency
of the algorithm. We expect broad applications of our method on various kind of
computational physics problems. | cond-mat_quant-gas |
Complex scaling flows in the quench dynamics of interacting particles: Many-body systems driven out of equilibrium can exhibit scaling flows of the
quantum state. For a sudden quench to resonant interactions between particles
we construct a new class of analytical scaling solutions for the time evolved
wave function with a complex scale parameter. These solutions determine the
exact dynamical scaling of observables such as the pair correlation function,
the contact and the fidelity. We give explicit examples of the nonequilibrium
dynamics for two trapped fermions or bosons quenched to unitarity, for ideal
Bose polarons, and for resonantly interacting, Borromean three-body systems.
These solutions reveal universal scaling properties of interacting many-body
systems that arise from the buildup of correlations at short times after the
quench. | cond-mat_quant-gas |
Mott Insulators of Ultracold Fermionic Alkaline Earth Atoms:
Underconstrained Magnetism and Chiral Spin Liquid: We study Mott insulators of fermionic alkaline earth atoms, described by
Heisenberg spin models with enhanced SU(N) symmetry. In dramatic contrast to
SU(2) magnetism, more than two spins are required to form a singlet. On the
square lattice, the classical ground state is highly degenerate and magnetic
order is thus unlikely. In a large-N limit, we find a chiral spin liquid ground
state with topological order and Abelian fractional statistics. We discuss its
experimental detection. Chiral spin liquids with non-Abelian anyons may also be
realizable with alkaline earth atoms. | cond-mat_quant-gas |
Simultaneous Readout of Noncommuting Collective Spin Observables beyond
the Standard Quantum Limit: We augment the information extractable from a single absorption image of a
spinor Bose-Einstein condensate by coupling to initially empty auxiliary
hyperfine states. Performing unitary transformations in both, the original and
auxiliary hyperfine manifold, enables the simultaneous measurement of multiple
spin-1 observables. We apply this scheme to an elongated atomic cloud of $
^{87} $Rb to simultaneously read out three orthogonal spin directions and with
that directly access the spatial spin structure. The readout even allows the
extraction of quantum correlations which we demonstrate by detecting spin
nematic squeezing without state tomography. | cond-mat_quant-gas |
On quantum melting of superfluid vortex crystals: from Lifshitz scalar
to dual gravity: Despite a long history of studies of vortex crystals in rotating superfluids,
their melting due to quantum fluctuations is poorly understood. Here we develop
a fracton-elasticity duality to investigate a two-dimensional vortex lattice
within the fast rotation regime, where the Lifshitz model of the collective
Tkachenko mode serves as the leading-order low-energy effective theory. We
incorporate topological defects and discuss several quantum melting scenarios
triggered by their proliferation. Furthermore, we lay the groundwork for a dual
non-linear emergent gravity description of the superfluid vortex crystals. | cond-mat_quant-gas |
Spatial Patterns of Rydberg Excitations from Logarithmic Pair
Interactions: The collective excitations in ensembles of dissipative, laser driven
ultracold atoms exhibit crystal-like patterns, a many-body effect of the
Rydberg blockade mechanism. These crystalline structure are revealed in
experiment from a post-selection of configurations with fixed numbers of
excitations. Here, we show that these sub-ensemble can be well represented by
ensembles of effective particles that interact via logarithmic pair potentials.
This allows one to study the emergent patterns with a small number of effective
particles to determine the phases of Rydberg crystals and to systematically
study contributions from $N$-body terms. | cond-mat_quant-gas |
Bose Einstein condensation and superfluidity in an open system: theory: We propose a new theoretical formalism which describes the Bose Einstein
condensation of weakly interacting bosons with finite life time interacting
with a thermal bath. We show that if a quasi-thermal distribution function of
particles is achieved, the elementary excitations of the condensate show a
linear spectrum characteristic of a superfluid, with a renormalized sound
velocity with respect to the equilibrium case. | cond-mat_quant-gas |
The multiconfigurational time-dependent Hartree method for bosons with
internal degrees of freedom: Theory and composite fragmentation of
multi-component Bose-Einstein condensates: In this paper the multiconfigurational time-dependent Hartree for bosons
method (MCTDHB) is derived for the case of $N$ identical bosons with internal
degrees of freedom. The theory for bosons with internal degrees of freedom
constitutes a generalization of the MCTDHB method that substantially enriches
the many-body physics that can be described. We demonstrate that the
numerically exact solution of the time-dependent many-body Schr\"odinger
equation for interacting bosonic particles with internal degrees of freedom is
now feasible. We report on the MCTDHB equations of motion for bosons with
internal degrees of freedom and their implementation for a general many-body
Hamiltonian with one-body and two-body terms that, both, may depend on the
internal states of the considered particles. To demonstrate the capabilities of
the theory and its software implementation integrated in the MCTDH-X software,
we apply MCTDHB to the emergence of fragmentation of parabolically trapped
bosons with two internal states: we study the groundstate of $N=100$
parabolically confined bosons as a function of the splitting between the
state-dependent minima of the two parabolic potentials. To quantify the
coherence of the system we compute its normalized one-body correlation
function. We find that the coherence within each internal state of the atoms is
maintained, while it is lost between the different internal states. This is a
hallmark of a new kind of fragmentation which is absent in bosons without
internal structure. We term the emergent phenomenon "composite fragmentation". | cond-mat_quant-gas |
Density wave instability in a 2D dipolar Fermi gas: We consider a uniform dipolar Fermi gas in two-dimensions (2D) where the
dipole moments of fermions are aligned by an orientable external field. We
obtain the ground state of the gas in Hartree-Fock approximation and
investigate RPA stability against density fluctuations of finite momentum. It
is shown that the density wave instability takes place in a broad region where
the system is stable against collapse. We also find that the critical
temperature can be a significant fraction of Fermi temperature for a realistic
system of polar molecules. | cond-mat_quant-gas |
Mobile impurity probing a two-dimensional superfluid phase transition: The use of atomically sized quantum systems as highly sensitive measuring
devices represents an exciting and quickly growing research field. Here, we
explore the properties of a quasiparticle formed by a mobile impurity
interacting with a two-dimensional fermionic superfluid. The energy of the
quasiparticle is shown to be lowered by superfluid pairing as this increases
the compressibility of the Fermi gas, thereby making it easier for the impurity
to perturb its surroundings. We demonstrate that the fundamentally
discontinuous nature of the superfluid to normal phase transition of a
two-dimensional system, leads to a rapid increase in the quasiparticle energy
around the critical temperature. The magnitude of this increase exhibits a
nonmonotonic behavior as a function of the pairing strength with a sizable
maximum in the cross-over region, where the spatial extend of the Cooper pairs
is comparable to the interparticle spacing.
Since the quasiparticle energy is measurable with present experimental
techniques, our results illustrate how impurities entangled with their
environment can serve as useful probes for non-trivial thermal and quantum
correlations. | cond-mat_quant-gas |
Effect of Rare Fluctuations on the Thermalization of Isolated Quantum
Systems: We consider the question of thermalization for isolated quantum systems after
a sudden parameter change, a so-called quantum quench. In part icular we
investigate the pre-requisites for thermalization focusing on the statistical
properties of the time-averaged density matrix and o f the expectation values
of observables in the final eigenstates. We find that eigenstates, which are
rare compared to the typical ones sampled by the micro-canonical distribution,
are responsible for the absence of thermalization of some infinite integrable
models and play an important role for some non-integrable systems of finite
size, such as the Bose-Hubbard model. We stress the importance of finite size
effects for the thermalization of isolated quantum systems and discuss two
alternative scenarios for thermalization, as well as ways to prune down the
correct one. | cond-mat_quant-gas |
Fractional topological states of dipolar fermions in one-dimensional
optical superlattices: We study the properties of dipolar fermions trapped in one-dimensional
bichromatic optical lattices and show the existence of fractional topological
states in the presence of strong dipole-dipole interactions. We find some
interesting connections between fractional topological states in
one-dimensional superlattices and the fractional quantum Hall states: (i) the
one-dimensional fractional topological states for systems at filling factor
\nu=1/p have p-fold degeneracy, (ii) the quasihole excitations fulfill the same
counting rule as that of fractional quantum Hall states, and (iii) the total
Chern number of p-fold degenerate states is a nonzero integer. The existence of
crystalline order in our system is also consistent with the thin-torus limit of
the fractional quantum Hall state on a torus. The possible experimental
realization in cold atomic systems offers a new platform for the study of
fractional topological phases in one-dimensional superlattice systems. | cond-mat_quant-gas |
Precise measurements on a quantum phase transition in antiferromagnetic
spinor Bose-Einstein condensates: We investigate, both experimentally and theoretically, the quench dynamics of
antiferromagnetic spinor Bose-Einstein condensates in the vicinity of a zero
temperature quantum phase transition at zero quadratic Zeeman shift q. Both the
rate of instability and the associated finite wavevector of the unstable modes
- show good agreement with predictions based upon numerical solutions to the
Bogoliubov de-Gennes equations. A key feature of this work is inclusion of
magnetic field inhomogeneities that smooth the phase transition. Once these
were removed, we observed a dramatic sharpening of the transition point, which
could then be resolved within a quadratic Zeeman shift of only 1-2 Hz. Our
results point to the use of dynamics, rather than equilibrium quantities for
high precision measurements of phase transitions in quantum gases. | cond-mat_quant-gas |
Unstable Avoided Crossing in Coupled Spinor Condensates: We consider the dynamics of a Bose-Einstein condensate with two internal
states, coupled through a coherent drive. We focus on a specific quench
protocol, in which the sign of the coupling field is suddenly changed. At a
mean-field level, the system is transferred from a minimum to a maximum of the
coupling energy and can remain dynamically stable, in spite of the development
of negative- frequency modes. In the presence of a non-zero detuning between
the two states, the "charge" and "spin" modes couple, giving rise to an
unstable avoided crossing. This phenomenon is generic to systems with two
dispersing modes away from equilibrium and constitutes an example of
class-$I_o$ non-equilibrium pattern formation in quantum systems. | cond-mat_quant-gas |
Comment on: `Single-shot simulations of dynamic quantum many-body
systems' [arXiv:1501.03224]: In their recent paper [Nature Physics 15, 451 (2006)], Sakmann and Kasevich
study the formation of fringe patterns in ultra-cold Bose gases and claim:
`Here, we show how single shots can be simulated from numerical solutions of
the time-dependent many-body Schr\"odinger equation.' It would be remarkable if
they had solved this exponentially complex equation. Instead they solve
nonlinear equations with the aim to approximate the solution of the
Schr\"odinger equation. The authors proceed to criticize phase-space approaches
to simulating quantum dynamics and claim the impossibility of interpreting
single trajectories of the truncated Wigner (tW) method as single-shot
experimental outcomes. Here we aim to provide relevant context and elaborate
why we disagree with the authors' claims. | cond-mat_quant-gas |
Tuning an effective spin chain of three strongly interacting
one-dimensional fermions with the transversal confinement: Strongly interacting one-dimensional fermions form an effective spin chain in
the absence of an external lattice potential. We show that the exchange
coefficients of such a chain may be locally tuned by properly tailoring the
transversal confinement. In particular, in the vicinity of a
confinement-induced resonance (CIR) the exchange coefficients may have
simultaneously opposite ferromagnetic and antiferromagnetic characters at
different locations along the trap axis. Moreover, the local exchanges may be
engineered to induce avoided crossings between spin states at the CIR, and
hence a ramp across the resonance may be employed to create different spin
states and to induce spin dynamics in the chain. We show that such unusual spin
chains have already been realized in the experiment of Murmann et al. [Phys.
Rev. Lett. 115, 215301 (2015)]. | cond-mat_quant-gas |
Dynamics of a Polariton Condensate in an Organic Semiconducting
Microcavity: Recent experiments on thin-film microcavities give evidence of Bose
condensation of exciton-polariton states. Inspired by these observations, we
consider the possibility that such exotic "half-light/half matter" states could
be observed in thin-film organic semiconductors where the oscillator strength
is generally stronger than in inorganic systems. Here we present a theoretical
model and simulations of macroscopic exciton-polartiton condensates in thracene
thin films sandwiched within a micro-meter scale resonant cavity and establish
criteria for the conditions under which BEC could be achieved in these systems.
We consider the effect of lattice disorder on the threshold intensities
necessary to create polartion superfluid states and conclude that even allowing
for up to 5% angular disorder of the molecules within the crystal lattice, the
superfluid transition remains sharp. | cond-mat_quant-gas |
Real-Time Dynamics of an Impurity in an Ideal Bose Gas in a Trap: We investigate the behavior of a harmonically trapped system consisting of an
impurity in a dilute ideal Bose gas after the boson-impurity interaction is
suddenly switched on. As theoretical framework, we use a field theory approach
in the space-time domain within the T-matrix approximation. We establish the
form of the corresponding T-matrix and address the dynamical properties of the
system. As a numerical application, we consider a simple system of a weakly
interacting impurity in one dimension where the interaction leads to
oscillations of the impurity density. Moreover, we show that the amplitude of
the oscillations can be driven by periodically switching the interaction on and
off. | cond-mat_quant-gas |
Unitary dynamics of strongly-interacting Bose gases with time-dependent
variational Monte Carlo in continuous space: We introduce time-dependent variational Monte Carlo for continuous-space Bose
gases. Our approach is based on the systematic expansion of the many-body
wave-function in terms of multi-body correlations and is essentially exact up
to adaptive truncation. The method is benchmarked by comparison to exact
Bethe-ansatz or existing numerical results for the integrable Lieb-Liniger
model. We first show that the many-body wave-function achieves high precision
for ground-state properties, including energy and first-order as well as
second-order correlation functions. Then, we study the out-of-equilibrium,
unitary dynamics induced by a quantum quench in the interaction strength. Our
time-dependent variational Monte Carlo results are benchmarked by comparison to
exact Bethe ansatz results available for a small number of particles, and also
compared to quench action results available for non-interacting initial states.
Moreover, our approach allows us to study large particle numbers and general
quench protocols, previously inaccessible beyond the mean-field level. Our
results suggest that it is possible to find correlated initial states for which
the long-term dynamics of local density fluctuations is close to the
predictions of a simple Boltzmann ensemble. | cond-mat_quant-gas |
Controlling Correlated Tunneling and Superexchange Interactions with
AC-Driven Optical Lattices: The dynamical control of tunneling processes of single particles plays a
major role in science ranging from Shapiro steps in Josephson junctions to the
control of chemical reactions via light in molecules. Here we show how such
control can be extended to the regime of strongly interacting particles.
Through a weak modulation of a biased tunnel contact, we have been able to
coherently control single particle and correlated two-particle hopping
processes. We have furthermore been able to extend this control to
superexchange spin interactions in the presence of a magnetic-field gradient.
We show how such photon assisted superexchange processes constitute a novel
approach to realize arbitrary XXZ spin models in ultracold quantum gases, where
transverse and Ising type spin couplings can be fully controlled in magnitude
and sign. | cond-mat_quant-gas |
Preparing and probing Chern bands with cold atoms: The present Chapter discusses methods by which topological Bloch bands can be
prepared in cold-atom setups. Focusing on the case of Chern bands for
two-dimensional systems, we describe how topological properties can be
triggered by driving atomic gases, either by dressing internal levels with
light or through time-periodic modulations. We illustrate these methods with
concrete examples, and we discuss recent experiments where geometrical and
topological band properties have been identified. | cond-mat_quant-gas |
Dissipative Transport of Trapped Bose-Einstein Condensates through
Disorder: After almost half a century since the work of Anderson [Phys. Rev. {\bf 109},
1492 (1958)], at present there is no well established theoretical framework for
understanding the dynamics of interacting particles in the presence of
disorder. Here, we address this problem for interacting bosons near $T=0$, a
situation that has been realized in trapped atomic experiments with an optical
speckle disorder. We develop a theoretical model for understanding the
hydrodynamic transport of \emph{finite-size} Bose-Einstein condensates through
disorder potentials. The goal has been to set up a simple model that will
retain all the richness of the system, yet provide analytic expressions,
allowing deeper insight into the physical mechanism. Comparison of our
theoretical predictions with the experimental data on large-amplitude dipole
oscillations of a condensate in an optical-speckle disorder shows striking
agreement. We are able to quantify various dissipative regimes of slow and fast
damping. Our calculations provide a clear evidence of reduction in disorder
strength due to interactions. The analytic treatment presented here allows us
to predict the power law governing the interaction dependance of damping. The
corresponding exponents are found to depend sensitively on the dimensionality
and are in excellent agreement with experimental observations. Thus, the
adeptness of our model, to correctly capture the essential physics of
dissipation in such transport experiments, is established. | cond-mat_quant-gas |
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