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Driven-dissipative Ising model: Dynamical crossover at weak dissipation: Driven quantum systems coupled to an environment typically exhibit
effectively thermal behavior with relaxational dynamics near criticality.
However, a different qualitative behavior might be expected in the weakly
dissipative limit due to the competition between coherent dynamics and weak
dissipation. In this work, we investigate a driven-dissipative infinite-range
Ising model in the presence of individual atomic dissipation, a model that
emerges from the paradigmatic open Dicke model in the large-detuning limit. We
show that the system undergoes a dynamical crossover from relaxational
dynamics, with a characteristic dynamical exponent $\zeta=1/2$, to underdamped
critical dynamics governed by the exponent $\zeta=1/4$ in the weakly
dissipative regime; a behavior that is markedly distinct from that of
equilibrium. Finally, utilizing an exact diagrammatic representation, we
demonstrate that the dynamical crossover to underdamped criticality is not an
artifact of the mean-field nature of the model and persists even in the
presence of short-range perturbations. | cond-mat_quant-gas |
Quasi-one-dimensional flow of polariton condensate past an obstacle: Nonlinear wave patterns generated by the flow of polariton condensate past an
obstacle are studied for quasi-one-dimensional microcavity geometry. It is
shown that pumping and nonlinear damping play a crucial role in this process
leading to sharp differences in subsonic and supersonic regimes. Subsonic flows
result in a smooth disturbance of the equilibrium condensate around the
obstacle whereas supersonic flow generates a dispersive shock wave in the flow
upstream the obstacle and a long smooth downstream tail. Main characteristics
of the wave pattern are calculated analytically and analytical results are in
excellent agreement with the results of numerical simulations. The conditions
for existence of stationary wave patterns are determined numerically. | cond-mat_quant-gas |
Staggered superfluid phases of dipolar bosons in two-dimensional square
lattices: We study the quantum ground state of ultracold bosons in a two-dimensional
square lattice. The bosons interact via the repulsive dipolar interactions and
s-wave scattering. The dynamics is described by the extended Bose-Hubbard model
including correlated hopping due to the dipolar interactions, the coefficients
are found from the second quantized Hamiltonian using the Wannier expansion
with realistic parameters. We determine the phase diagram using the Gutzwiller
ansatz in the regime where the coefficients of the correlated hopping terms are
negative and can interfere with the tunneling due to single-particle effects.
We show that this interference gives rise to staggered superfluid and
supersolid phases at vanishing kinetic energy, while we identify parameter
regions at finite kinetic energy where the phases are incompressible. We
compare the results with the phase diagram obtained with the cluster Gutzwiller
approach and with the results found in one dimension using DMRG. | cond-mat_quant-gas |
BCS-BCS crossover between atomic and molecular superfluids in a
Bose-Fermi mixture: We theoretically examine a continuity between atomic and molecular Fermi
superfluids in a Bose-Fermi mixture near the Feshbach resonance. Considering a
two-channel model describing the Feshbach resonance between Fermi and Bose
atoms, we have constructed the mean-field framework based on the perturbative
expansion of the Feshbach atom-dimer coupling. The resulting effective
Hamiltonian exhibits not only the continuity between atom-atom to
molecule-molecule Cooper pairings but also becomes equivalent to the
two-band-superconductor model with Suhl-Matthias-Walker type pair-exchange
coupling. We demonstrate how these atomic and molecular Fermi superfluids
coexist within the two-band-like superfluid theory. The pair-exchange coupling
and resulting superfluid gaps are found to be strongly enhanced near the
Feshbach resonance due to the interplay between the infrared singularity of
Bogoliubov phonons and their Landau damping arising from the coupling with
fermions. The pair-exchange coupling can be probed via the observation of the
intrinsic Josephson effect between atomic and molecular superfluids. | cond-mat_quant-gas |
Fulde-Ferrell superfluids in spinless ultracold Fermi gases: The Fulde-Ferrell (FF) superfluid phase, in which fermions form
finite-momentum Cooper pairings, is well studied in spin-singlet superfluids in
past decades. Different from previous works that engineer the FF state in
spinful cold atoms, we show that the FF state can emerge in spinless Fermi
gases confined in optical lattice associated with nearest-neighbor
interactions. The mechanism of the spinless FF state relies on the split Fermi
surfaces by tuning the chemistry potential, which naturally gives rise to
finite-momentum Cooper pairings. The phase transition is accompanied by changed
Chern numbers, in which, different from the conventional picture, the band gap
does not close. By beyond-mean-field calculations, we find the finite-momentum
pairing is more robust, yielding the system promising for maintaining the FF
state at finite temperature. Finally we present the possible realization and
detection scheme of the spinless FF state. | cond-mat_quant-gas |
Reply to the correspondence of Drummond and Brand [arXiv:1610.07633]: In their correspondence [arXiv:1610.07633] Drummond and Brand criticize our
work [Nature Physics 12, 451-454 (2016) http://dx.doi.org/10.1038/nphys3631].
We show that their criticism is misleading and unfounded. | cond-mat_quant-gas |
Dynamics of Uniform Quantum Gases, I: Density and Current Correlations: A unified approach valid for any wavenumber, frequency, and temperature is
presented for uniform ideal quantum gases allowing for a comprehensive study of
number density and particle-current density response functions. Exact
analytical expressions are obtained for spectral functions in terms of
polylogarithms. Also, particle-number and particle-current static
susceptibilities are presented which, for fugacity less than unity,
additionally involve Kummer functions. The wavenumber and temperature dependent
transverse-current static susceptibility is used to show explicitly that
current correlations are of a long range in a Bose-condensed uniform ideal gas
but for bosons above the critical temperature and for Fermi and Boltzmann gases
at all temperatures these correlations are of short range. Contact repulsive
interactions for systems of neutral quantum particles are considered within the
random-phase approximation. The expressions for particle-number and
transverse-current susceptibilities are utilized to discuss the existence or
nonexistence of superfluidity in the systems under consideration. | cond-mat_quant-gas |
A long-lived Higgs mode in a two-dimensional confined Fermi gas: The Higgs mode corresponds to the collective motion of particles due to the
vibrations of an invisible field. It plays a fundamental role for our
understanding of both low and high energy physics, giving elementary particles
their mass and leading to collective modes in condensed matter and nuclear
systems. The Higgs mode has been observed in a limited number of table-top
systems, where it however is characterised by a short lifetime due to decay
into a continuum of modes. A major goal which has remained elusive so far, is
therefore to realise a long-lived Higgs mode in a controllable system. Here, we
show how an undamped Higgs mode can be observed unambiguously in a Fermi gas in
a two-dimensional trap, close to a quantum phase transition between a normal
and a superfluid phase. We develop a first-principles theory of the pairing and
the associated collective modes, which is quantitatively reliable when the
pairing energy is much smaller than the trap level spacing, yet simple enough
to allow the derivation of analytical results. The theory includes the trapping
potential exactly, which is demonstrated to stabilize the Higgs mode by making
its decay channels discrete. Our results show how atoms in micro-traps can
unravel properties of a long-lived Higgs mode, including the role of
confinement and finite size effects. | cond-mat_quant-gas |
Critical Dynamics of a Two-dimensional Superfluid near a Non-Thermal
Fixed Point: Critical dynamics of an ultracold Bose gas far from equilibrium is studied in
two spatial dimensions. Superfluid turbulence is created by quenching the
equilibrium state close to zero temperature. Instead of immediately
re-thermalizing, the system approaches a meta-stable transient state,
characterized as a non-thermal fixed point. A focus is set on the vortex
density and vortex-antivortex correlations which characterize the evolution
towards the non-thermal fixed point and the departure to final
(quasi-)condensation. Two distinct power-law regimes in the vortex-density
decay are found and discussed in terms of a vortex binding-unbinding transition
and a kinetic description of vortex scattering. A possible relation to decaying
turbulence in classical fluids is pointed out. By comparing the results to
equilibrium studies of a two-dimensional Bose gas, an intuitive understanding
of the location of the non-thermal fixed point in a reduced phase space is
developed. | cond-mat_quant-gas |
A superfluid-droplet crystal and a free-space supersolid in a
dipole-blockaded gas: A novel supersolid phase is predicted for an ensemble of Rydberg atoms in the
dipole-blockade regime, interacting via a repulsive dipolar potential
"softened" at short distances. Using exact numerical techniques, we study the
low temperature phase diagram of this system, and observe an intriguing phase
consisting of a crystal of mesoscopic superfluid droplets. At low temperature,
phase coherence throughout the whole system, and the ensuing bulk
superfluidity, are established through tunnelling of identical particles
between neighbouring droplets. | cond-mat_quant-gas |
Atom chips with free-standing two-dimensional electron gases: advantages
and challenges: In this work we consider the advantages and challenges of using free-standing
two-dimensional electron gases (2DEG) as active components in atom chips for
manipulating ultracold ensembles of alkali atoms. We calculate trapping
parameters achievable with typical high-mobility 2DEGs in an atom chip
configuration, and identify advantages of this system for trapping atoms at
sub-micron distances from the atom chip. We show how the sensitivity of atomic
gases to magnetic field inhomogeneity can be exploited for controlling the
atoms with quantum electronic devices and, conversely, using the atoms to probe
the structural and transport properties of semiconductor devices. | cond-mat_quant-gas |
Supersolid and charge density-wave states from anisotropic interaction
in an optical lattice: We show anisotropy of the dipole interaction between magnetic atoms or polar
molecules can stabilize new quantum phases in an optical lattice. Using a well
controlled numerical method based on the tensor network algorithm, we calculate
phase diagram of the resultant effective Hamiltonian in a two-dimensional
square lattice - an anisotropic Hubbard model of hard-core bosons with
attractive interaction in one direction and repulsive interaction in the other
direction. Besides the conventional superfluid and the Mott insulator states,
we find the striped and the checkerboard charge density wave states and the
supersolid phase that interconnect the superfluid and the striped solid states.
The transition to the supersolid phase has a mechanism different from the case
of the soft-core Bose Hubbard model. | cond-mat_quant-gas |
Bound states in a quasi-two-dimensional Fermi gas: We consider the problem of N identical fermions of mass M and one
distinguishable particle of mass m interacting via short-range interactions in
a confined quasi-two-dimensional (quasi-2D) geometry. For N=2 and mass ratios
M/m<13.6, we find non-Efimov trimers that smoothly evolve from 2D to 3D. In the
limit of strong 2D confinement, we show that the energy of the N+1 system can
be approximated by an effective two-channel model. We use this approximation to
solve the 3+1 problem and we find that a bound tetramer can exist for mass
ratios M/m as low as 5 for strong confinement, thus providing the first example
of a universal, non-Efimov tetramer involving three identical fermions. | cond-mat_quant-gas |
Collective excitations in two-dimensional harmonically trapped quantum
droplets: The collective excitation modes in quantum droplets trapped in a
two-dimensional harmonic potential in the context of symmetric weakly
interacting binary bosonic mixtures are studied. By utilizing the linearization
technique, the time-dependent extended Gross-Pitaevskii equation, and a
sum-rule approach with a variational approximation, the ground state properties
and collective excitations of such a two-dimensional quantum system are
investigated for various system parameters. We present comprehensive analysis
and calculations on the effect of the confinement strength and anisotropy of
the trapping potential, the number of atoms in the droplet, and the collective
excitation modes. The radius of the droplet, as well as the chemical potential,
is non-monotonically related to the number of atoms in the droplet, and the
confinement tends to shift the minimum values towards the ideal gas limit. The
excitation frequency peaks, which are prominent in a self-bounded droplet,
become less pronounced and smoother when subjected to a strong trapping
potential. The sum-rule approach fails to reproduce the breathing mode
frequency for a moderate number of atoms in a weak trapping potential, however,
works perfectly well in a strong confinement. It was found that the anisotropy
in the trap eliminates the degeneracy between the quadrupole and scissors modes
that occurs in an isotropic trap, causing the frequencies of these two modes to
immediately diverge from each other for any degree of anisotropy. These
findings provide valuable insights into the unique characteristics and behavior
of quantum droplets, offering potential implications for future research and
applications in the dynamic behaviors of intriguing quantum droplets. | cond-mat_quant-gas |
Loosely Bound Few-Body States in a Spin-1 Gas with Near-Degenerate
Continua: A distinguishing feature of ultracold collisions of bosonic lithium atoms is
the presence of two near-degenerate two-body continua. The influence of such a
near-degeneracy on the few-body physics in the vicinity of a narrow Feshbach
resonance is investigated within the framework of a minimal model with two
atomic continua and one closed molecular channel. The model allows analysis of
the spin composition of loosely bound dimers and trimers. In the two-body
sector the well-established coupled-channels calculations phenomenology of
lithium is qualitatively reproduced, and its particularities are emphasized and
clarified. In the three-body sector we find that the Efimov trimer energy
levels follow a different functional form as compared to a single continuum
scenario while the thresholds remain untouched. This three-channel model with
two atomic continua complements our earlier developed three-channel model with
two molecular channels [Y. Yudkin and L. Khaykovich, Phys. Rev. A 103, 063303
(2021)] and suggests that the experimentally observed exotic behavior of the
first excited Efimov energy level [Y. Yudkin, R. Elbaz and L. Khaykovich,
arXiv:2004.02723] is most probably caused by the short-range details of the
interaction potential. | cond-mat_quant-gas |
Quantum vortex instability and black hole superradiance: Vortices and black holes set the scene for many interesting dynamical
processes in physics. Here, we study the dynamical instability of quantised
vortices and rotational superradiance around rotating black holes, illustrating
in the process that the same physics is at play in these two seemingly
disparate phenomena. We also compare the instability of the vortex to the black
hole bomb instability, which occurs for massive scalar fields in the Kerr
spacetime. Taking inspiration from the analogy between black hole bomb modes
and the hydrogen spectrum, the vortex instability is compared with nuclear
resonances involved in $\alpha$-decay. | cond-mat_quant-gas |
An exact formalism for the quench dynamics of integrable models: We describe a formulation for studying the quench dynamics of integrable
systems generalizing an approach by Yudson. We study the evolution of the
Lieb-Liniger model, a gas of interacting bosons moving on the continuous
infinite line and interacting via a short range potential. The formalism allows
us to quench the system from any initial state. We find that for any value of
repulsive coupling independently of the initial state the system asymptotes
towards a strongly repulsive gas, while for any value of attractive coupling,
the system forms a maximal bound state that dominates at longer times. In
either case the system equilibrates but does not thermalize. We compare this to
quenches in a Bose-Hubbard lattice and show that there, initial states
determine long-time dynamics independent of the sign of the coupling. | cond-mat_quant-gas |
Open source Matrix Product States: Opening ways to simulate entangled
many-body quantum systems in one dimension: Numerical simulations are a powerful tool to study quantum systems beyond
exactly solvable systems lacking an analytic expression. For one-dimensional
entangled quantum systems, tensor network methods, amongst them Matrix Product
States (MPSs), have attracted interest from different fields of quantum physics
ranging from solid state systems to quantum simulators and quantum computing.
Our open source MPS code provides the community with a toolset to analyze the
statics and dynamics of one-dimensional quantum systems. Here, we present our
open source library, Open Source Matrix Product States (OSMPS), of MPS methods
implemented in Python and Fortran2003. The library includes tools for ground
state calculation and excited states via the variational ansatz. We also
support ground states for infinite systems with translational invariance.
Dynamics are simulated with different algorithms, including three algorithms
with support for long-range interactions. Convenient features include built-in
support for fermionic systems and number conservation with rotational
$\mathcal{U}(1)$ and discrete $\mathbb{Z}_2$ symmetries for finite systems, as
well as data parallelism with MPI. We explain the principles and techniques
used in this library along with examples of how to efficiently use the general
interfaces to analyze the Ising and Bose-Hubbard models. This description
includes the preparation of simulations as well as dispatching and
post-processing of them. | cond-mat_quant-gas |
Thermalization and Bose-Einstein condensation of quantum light in bulk
nonlinear media: We study the thermalization and the Bose-Einstein condensation of a paraxial,
spectrally narrow beam of quantum light propagating in a lossless bulk Kerr
medium. The spatiotemporal evolution of the quantum optical field is ruled by a
Heisenberg equation analogous to the quantum nonlinear Schr\"odinger equation
of dilute atomic Bose gases. Correspondingly, in the weak-nonlinearity regime,
the phase-space density evolves according to the Boltzmann equation.
Expressions for the thermalization time and for the temperature and the
chemical potential of the eventual Bose-Einstein distribution are found. After
discussing experimental issues, we introduce an optical setup allowing the
evaporative cooling of a guided beam of light towards Bose-Einstein
condensation. This might serve as a novel source of coherent light. | cond-mat_quant-gas |
Unitary $p$-wave Fermi gas in one dimension: We elucidate universal many-body properties of a one-dimensional,
two-component ultracold Fermi gas near the $p$-wave Feshbach resonance. The
low-energy scattering in this system can be characterized by two parameters,
that is, $p$-wave scattering length and effective range. At the unitarity limit
where the $p$-wave scattering length diverges and the effective range is
reduced to zero without conflicting with the causality bound, the system obeys
universal thermodynamics as observed in a unitary Fermi gas with contact
$s$-wave interaction in three dimensions. It is in contrast to a Fermi gas with
the $p$-wave resonance in three dimensions in which the effective range is
inevitably finite. We present the universal equation of state in this unitary
$p$-wave Fermi gas within the many-body $T$-matrix approach as well as the
virial expansion method. Moreover, we examine the single-particle spectral
function in the high-density regime where the virial expansion is no longer
valid. On the basis of the Hartree-like self-energy shift at the divergent
scattering length, we conjecture that the equivalence of the Bertsch parameter
across spatial dimensions holds even for a one-dimensional unitary $p$-wave
Fermi gas. | cond-mat_quant-gas |
Spatial separation of rotating binary Bose-Einstein condensate by tuning
the dipolar interactions: We are pointing out relevant anisotropic effects, related to spatial
separation, miscibility and mass-symmetry, due to dipole-dipole interactions in
rotating binary dipolar Bose-Einstein condensates, by considering symmetric
($^{164}$Dy-$^{162}$Dy) and asymmetric ($^{168}$Er-$^{164}$Dy,
$^{164}$Dy-$^{87}$Rb) dipolar mixtures. The binary mixtures are kept in strong
pancake-shaped trap, modeled by an effective two-dimensional coupled
Gross-Pitaevskii equation. The anisotropy of the dipolar interactions, on
miscibility and vortex-lattice structures, is studied by tuning the
polarization angle of the dipoles $\varphi$, which can enhance the attractive
part of the dipole-dipole interaction (DDI) for both inter- and intra-species.
Within this procedure of changing to attractive the DDI, a clear spatial
separation is verified in the densities at some critical polarization angle.
The spatial separations, being angular for symmetric mixtures and radial for
asymmetric ones, are verified for repulsive contact interactions when the
inter- to intra-species ratio $\delta$ is larger than one, implying the system
is less miscible. The corresponding result for the critical polarization angle
as a function of $\delta$ is shown in the particular dipolar symmetric case. A
striking outcome of the present study is the observed sensibility of the
vortex-pattern binary distributions due to the mass-asymmetry. This is
exemplified by the symmetric dipolar mixture, where the two isotopes are of the
same species. | cond-mat_quant-gas |
Spectrum of elementary excitations in Galilean-invariant integrable
models: The spectrum of elementary excitations in one-dimensional quantum liquids is
generically linear at low momenta. It is characterized by the sound velocity
that can be related to the ground state energy. Here we study the spectrum at
higher momenta in Galilean invariant integrable models. Somewhat surprisingly,
we show that the spectrum at arbitrary momentum is fully determined by the
properties of the ground state. We find general exact relations for the
coefficients of several terms in the expansion of the excitation energy at low
momenta and arbitrary interaction and express them in terms of the Luttinger
liquid parameter. We apply the obtained formulas to the Lieb-Liniger model and
obtain several new results. | cond-mat_quant-gas |
Dynamics of polar-core spin vortices in a ferromagnetic spin-1
Bose-Einstein condensate: A ferromagnetic spin-1 condensate supports polar-core spin vortices (PCVs) in
the easy-plane phase. We derive a model for the dynamics of these PCVs using a
variational Lagrangian approach. The PCVs behave as massive charged particles
interacting under the two dimensional Coulomb interaction, with the mass
arising from interaction effects within the vortex core. We compare this model
to numerical simulations of the spin-1 Gross-Pitaevskii equations and find
semi-quantitative agreement. In addition, the numerical results suggest that
the PCV core couples to spin waves, and this affects the PCV dynamics even far
from the core. We identify areas of further research that could extend the
model of PCV dynamics presented here. | cond-mat_quant-gas |
Cooperative scattering measurement of coherence in a spatially modulated
Bose gas: Correlations of a Bose gas released from an optical lattice are measured
using superradiant scattering. Conditions are chosen so that after initial
incident light pumping at the Bragg angle for diffraction, due to matter wave
amplification and mode competition, superradiant scattering into the Bragg
diffracted mode is preponderant. A temporal analysis of the superradiant
scattering gain reveals periodical oscillations and damping due to the initial
lack of coherence between lattice sites. Such damping is used for
characterizing first order spatial correlations in our system with a precision
of one lattice period. | cond-mat_quant-gas |
Light-cone effect and supersonic correlations in one- and
two-dimensional bosonic superfluids: We study the spreading of density-density correlations in Bose-Hubbard models
after a quench of the interaction strength, using time-dependent variational
Monte Carlo simulations. It gives access to unprecedented long propagation
times and to dimensions higher than one. In both one and two dimensions, we
find ballistic light-cone spreading of correlations and extract accurate values
of the light-cone velocity in the superfluid regime. We show that the spreading
of correlations is generally supersonic, with a light-cone propagating faster
than sound modes but slower than the maximum group velocity of density
excitations, except at the Mott transition, where all the characteristic
velocities are equal. Further, we show that in two dimensions the correlation
spreading is highly anisotropic and presents nontrivial interference effects. | cond-mat_quant-gas |
Two- and one-dimensional gap solitons in spin-orbit-coupled systems with
Zeeman splitting: We elaborate a mechanism for the formation of stable solitons of the
semi-vortex type (with vorticities 0 and 1 in their two components), populating
a finite bandgap in the spectrum of the spin-orbit-coupled binary Bose-Einstein
condensate with the Zeeman splitting, in the two-dimensional free space, under
conditions which make the kinetic-energy terms in the respective coupled
Gross-Pitaevskii equations negligible. Unlike a recent work which used
long-range dipole-dipole interactions to construct stable gap solitons in a
similar setting, we here demonstrate that stable solitons are supported by
generic local interactions of both attractive and repulsive signs, provided
that the relative strength of the cross/self interaction in the two-component
system does not exceed a critical value ~ 0.77. A boundary between stable and
unstable fundamental 2D gap solitons is precisely predicted by the
Vakhitov-Kolokolov criterion, while all excited states of the 2D solitons, with
vorticities (m, 1 + m) in the two components, m = 1, 2, ..., are unstable. The
analysis of the one-dimensional (1D) reduction of the system produces an exact
analytical solution for the family of gap solitons which populate the entire
bandgap, the family being fully stable. Motion of the 1D solitons in the
trapping potential is considered too, showing that their effective mass is
positive or negative if the cubic nonlinearity is attractive or repulsive,
respectively. | cond-mat_quant-gas |
A Quantum Gas Microscope for Fermionic Atoms: Strongly interacting fermions define the properties of complex matter at all
densities, from atomic nuclei to modern solid state materials and neutron
stars. Ultracold atomic Fermi gases have emerged as a pristine platform for the
study of many-fermion systems. Here we realize a quantum gas microscope for
fermionic $^{40}$K atoms trapped in an optical lattice, which allows one to
probe strongly correlated fermions at the single atom level. We combine 3D
Raman sideband cooling with high-resolution optics to simultaneously cool and
image individual atoms with single lattice site resolution at a detection
fidelity above $95\%$. The imaging process leaves each atom predominantly in
the 3D ground state of its lattice site, inviting the implementation of a
Maxwell's demon to assemble low-entropy many-body states. Single site resolved
imaging of fermions enables the direct observation of magnetic order, time
resolved measurements of the spread of particle correlations, and the detection
of many-fermion entanglement. | cond-mat_quant-gas |
Efimovian three-body potential from broad to narrow Feshbach resonances: We analyse the change in the hyperradial Efimovian three-body potential as
the two-body interaction is tuned from the broad to narrow Feshbach resonance
regime. Here, it is known from both theory and experiment that the three-body
dissociation scattering length $a_-$ shifts away from the universal value of
$-9.7 \ r_{\mathrm{vdW}}$, with $r_{\mathrm{vdW}} = \frac{1}{2} \left(m
C_6/\hbar^2 \right)^{1/4}$ the two-body van der Waals range. We model the
three-body system using a separable two-body interaction that takes into
account the full zero-energy behaviour of the multichannel wave function. We
find that the short-range repulsive barrier in the three-body potential
characteristic for single-channel models remains universal for narrow
resonances, whilst the change in the three-body parameter originates from a
strong decrease in the potential depth. From an analysis of the underlying spin
structure we further attribute this behavior to the dominance of the two-body
interaction in the resonant channel compared to other background interactions. | cond-mat_quant-gas |
Many-body Dynamics with Time-dependent Interaction: Recent advances in optical Feshbach resonance technique have enabled the
experimental investigation of atomic gases with time-dependent interaction. In
this work, we study the many-body dynamics of weakly interacting bosons subject
with an arbitrary time varying scattering length. By employing a variational
ansatz, we derive an effective Hamiltonian that governs the dynamics of thermal
particles. Crucially, we show that there exists a hidden symmetry in this
Hamiltonian that can map the many-body dynamics to the precession of an SU(1,1)
"spin". As a demonstration, we calculate the situation where the scattering
length is sinusoidally modulated. We show that the non-compactness of the
SU(1,1) group naturally leads to solutions with exponentially growth of
Bogoliubov modes and causes instabilities. | cond-mat_quant-gas |
Dynamical Zeeman resonance in spin-orbit-coupled spin-1 Bose gases: We predict a dynamical resonant effect, which is driven by externally applied
linear and quadratic Zeeman fields, in a spin-orbit-coupled spin-1
Bose-Einstein condensate. The Bose-Einstein condensate is assumed to be
initialized in some superposed state of Zeeman sublevels and subject to a
sudden shift of the trapping potential. It is shown that the time-averaged
center-of-mass oscillation and the spin polarizations of the Bose-Einstein
condensate exhibit remarkable resonant peaks when the Zeeman fields are tuned
to certain strengths. The underlying physics behind this resonance can be
traced back to the out-of-phase interference of the dynamical phases carried by
different spinorbit states. By analyzing the single particle spectrum, the
resonant condition is summarized as a simple algebraic relation, connecting the
strengths of the linear and quadratic Zeeman fields. This property is
potentially applicable in quantum information and quantum precision
measurement. | cond-mat_quant-gas |
Entanglement structure of a quantum simulator: the two-component
Bose-Hubbard model: We consider a quantum simulator of the Heisenberg chain with ferromagnetic
interactions based on the two-component 1D Bose-Hubbard model at filling equal
to two in the strong coupling regime. The entanglement properties of the ground
state are compared between the original spin model and the quantum simulator as
the interspecies interaction approaches the intraspecies one. A numerical study
of the entanglement properties of the quantum simulator state is supplemented
with analytical expressions derived from the simulated Hamiltonian. At the
isotropic point, the entanglement properties of the simulated system are not
properly predicted by the quantum simulator. | cond-mat_quant-gas |
Finite-momentum Bose-Einstein condensates in shaken 2D square optical
lattices: We consider ultracold bosons in a 2D square optical lattice described by the
Bose-Hubbard model. In addition, an external time-dependent sinusoidal force is
applied to the system, which shakes the lattice along one of the diagonals. The
effect of the shaking is to renormalize the nearest-neighbor hopping
coefficients, which can be arbitrarily reduced, can vanish, or can even change
sign, depending on the shaking parameter. It is therefore necessary to account
for higher-order hopping terms, which are renormalized differently by the
shaking, and introduce anisotropy into the problem. We show that the
competition between these different hopping terms leads to finite-momentum
condensates, with a momentum that may be tuned via the strength of the shaking.
We calculate the boundaries between the Mott-insulator and the different
superfluid phases, and present the time-of-flight images expected to be
observed experimentally. Our results open up new possibilities for the
realization of bosonic analogs of the FFLO phase describing inhomogeneous
superconductivity. | cond-mat_quant-gas |
Nonequilibrium Phase Diagram of a Driven-Dissipative Many-Body System: We study the nonequilibrium dynamics of a many-body bosonic system on a
lattice, subject to driving and dissipation. The time-evolution is described by
a master equation, which we treat within a generalized Gutzwiller mean field
approximation for density matrices. The dissipative processes are engineered
such that the system, in the absence of interaction between the bosons, is
driven into a homogeneous steady state with off-diagonal long range order. We
investigate how the coherent interaction affects qualitatively the properties
of the steady state of the system and derive a nonequilibrium phase diagram
featuring a phase transition into a steady state without long range order. The
phase diagram exhibits also an extended domain where an instability of the
homogeneous steady state gives rise to a persistent density pattern with
spontaneously broken translational symmetry. In the limit of small particle
density, we provide a precise analytical description of the time-evolution
during the instability. Moreover, we investigate the transient following a
quantum quench of the dissipative processes and we elucidate the prominent role
played by collective topological variables in this regime. | cond-mat_quant-gas |
Cooling and Near-equilibrium Dynamics of Atomic Gases Across the
Superfluid-Mott Insulator Transition: We study near-equilibrium thermodynamics of bosonic atoms in a
two-dimensional optical lattice by ramping up the lattice depth to convert a
superfluid into an inhomogeneous mixture of superfluid and Mott insulator.
Detailed study of in situ density profiles shows that, first, locally adiabatic
ramps do not guarantee global thermal equilibrium. Indeed, full thermalization
for typical parameters only occurs for experiment times which exceed one
second. Secondly, ramping non-adiabatically to the Mott insulator regime can
result in strong localized cooling at short times and global cooling once
equilibrated. For an initial temperature estimated as 20 nK, we observe local
temperatures as low as 1.5 nK, and a final global temperature of 9 nK. Possible
cooling mechanisms include adiabatic decompression, modification of the density
of states near the quantum critical regime, and the Joule-Thomson effect.
**NOTE: Following submission of arXiv:0910.1382v1, a systematic correction was
discovered in the density measurement, stemming from three-body losses during
the imaging process. New measurements were performed, and the result is in
support of the claim on the slow global dynamics. Due to the substantially
altered methods and analysis, a new text has been posted as arXiv:1003.0855. | cond-mat_quant-gas |
Pi-phases in balanced fermionic superfluids on spin-dependent optical
lattices: We study a balanced two-component system of ultracold fermions in one
dimension with attractive interactions and subject to a spin-dependent optical
lattice potential of opposite sign for the two components. We find states with
different types of modulated pairing order parameters which are conceptually
similar to pi-phases discussed for superconductor-ferromagnet heterostructures.
Increasing the lattice depth induces sharp transitions between states of
different parity. While the origin of the order parameter oscillations is
similar to the FFLO phase for paired states with spin imbalance, the current
system is intrinsically stable to phase separation. We discuss experimental
requirements for creating and probing these novel phases. | cond-mat_quant-gas |
Dipolar dark solitons: We numerically generate, and then study the basic properties of dark
soliton-like excitations in a dipolar gas confined in a quasi one dimensional
trap. These excitations, although very similar to dark solitons in a gas with
contact interaction, interact with each other and can form bound states. During
collisions these dipolar solitons emit phonons, loosing energy, but
accelerating. Even after thousands of subsequent collisions they survive as
gray solitons and finally reach dynamical equilibrium with background
quasiparticles. Finally, in the frame of classical field approximation, we
verified, that these solitons appear spontaneously in thermal samples,
analogously to the type II excitations in a gas of atoms with contact
interaction. | cond-mat_quant-gas |
Entanglement spectrum and quantum phase diagram of the long-range XXZ
chain: Entanglement is a central feature of many-body quantum systems and plays a
unique role in quantum phase transitions.
In many cases, the entanglement spectrum, which represents the spectrum of
the density matrix of a bipartite system, contains valuable information beyond
the sole entanglement entropy.
Here we investigate the entanglement spectrum of the long-range XXZ model. We
show that within the critical phase it exhibits a remarkable self-similarity.
The breakdown of self-similarity and the transition away from a Luttinger
liquid is consistent with renormalization group theory.
Combining the two, we are able to determine the quantum phase diagram of the
model and locate the corresponding phase transitions. Our results are confirmed
by numerically-exact calculations using tensor-network techniques.
Moreover, we show that the self-similar rescaling extends to the geometrical
entanglement as well as the Luttinger parameter in the critical phase.
Our results pave the way to further studies of entanglement properties in
long-range quantum models. | cond-mat_quant-gas |
Transition between vacuum and finite-density states in the
infinite-dimensional Bose-Hubbard model with spatially inhomogeneous
dissipation: We analyze dynamics of the infinite-dimensional Bose-Hubbard model with
spatially inhomogeneous dissipation in the hardcore boson limit by solving the
Lindblad master equation with use of the Gutzwiller variational method. We
consider dissipation processes that correspond to inelastic light scattering in
the case of Bose gases in optical lattices. We assume that the dissipation is
applied to a half of lattice sites in a spatially alternating manner. We focus
on steady states at which the system arrives after long-time evolution. We find
that when the average particle density is varied, the steady state exhibits a
transition between a state in which the sites without dissipation are vacuum
and that containing a finite number of particles at those sites. We associate
the transition with the tendency of the sites with dissipation towards a local
state at infinite temperature. | cond-mat_quant-gas |
Non-degenerate Bound State Solitons in Multi-component Bose-Einstein
Condensates: We investigate non-degenerate bound state solitons systematically in
multi-component Bose-Einstein condensates, through developing Darboux
transformation method to derive exact soliton solutions analytically. In
particular, we show that bright solitons with nodes correspond to the excited
bound eigen-states in the self-induced effective quantum wells, in sharp
contrast to the bright soliton and dark soliton reported before (which usually
correspond to ground state and free eigen-state respectively). We further
demonstrate that the bound state solitons with nodes are induced by incoherent
interactions between solitons in different components. Moreover, we reveal that
the interactions between these bound state solitons are usually inelastic,
caused by the incoherent interactions between solitons in different components
and the coherent interactions between solitons in same component. The bound
state solitons can be used to discuss many different physical problems, such as
beating dynamics, spin-orbital coupling effects, quantum fluctuations, and even
quantum entanglement states. | cond-mat_quant-gas |
Dynamics of polar-core spin vortices in a ferromagnetic spin-1
Bose-Einstein condensate: A ferromagnetic spin-1 condensate supports polar-core spin vortices (PCVs) in
the easy-plane phase. We derive a model for the dynamics of these PCVs using a
variational Lagrangian approach. The PCVs behave as massive charged particles
interacting under the two dimensional Coulomb interaction, with the mass
arising from interaction effects within the vortex core. We compare this model
to numerical simulations of the spin-1 Gross-Pitaevskii equations and find
semi-quantitative agreement. In addition, the numerical results suggest that
the PCV core couples to spin waves, and this affects the PCV dynamics even far
from the core. We identify areas of further research that could extend the
model of PCV dynamics presented here. | cond-mat_quant-gas |
Site-resolved imaging of a fermionic Mott insulator: The complexity of quantum many-body systems originates from the interplay of
strong interactions, quantum statistics, and the large number of
quantum-mechanical degrees of freedom. Probing these systems on a microscopic
level with single-site resolution offers important insights. Here we report
site-resolved imaging of two-component fermionic Mott insulators, metals, and
band insulators using ultracold atoms in a square lattice. For strong repulsive
interactions we observe two-dimensional Mott insulators containing over 400
atoms. For intermediate interactions, we observe a coexistence of phases. From
comparison to theory we find trap-averaged entropies per particle of
$1.0\,k_{\mathrm{B}}$. In the band-insulator we find local entropies as low as
$0.5\,k_{\mathrm{B}}$. Access to local observables will aid the understanding
of fermionic many-body systems in regimes inaccessible by modern theoretical
methods. | cond-mat_quant-gas |
Weyl points and topological nodal superfluids in a face-centered cubic
optical lattice: We point out that a face-centered cubic (FCC) optical lattice, which can be
realised by a simple scheme using three lasers, provides one a highly
controllable platform for creating Weyl points and topological nodal
superfluids in ultracold atoms. In non-interacting systems, Weyl points
automatically arise in the Floquet band structure when shaking such FCC
lattices, and sophisticated design of the tunnelling is not required. More
interestingly, in the presence of attractive interaction between two hyperfine
spin states, which experience the same shaken FCC lattice, a three-dimensional
topological nodal superfluid emerges, and Weyl points show up as the gapless
points in the quasiparticle spectrum. One could either create a double Weyl
point of charge 2, or split it to two Weyl points of charge 1, which can be
moved in the momentum space by tuning the interactions. Correspondingly, the
Fermi arcs at the surface may be linked with each other or separated as
individual ones. | 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 photon-mediated interactions in a multimode cavity: from
supersolid to insulating droplets hosting phononic excitations: Ultracold atoms trapped in laser-generated optical lattices serve as a
versatile platform for quantum simulations. However, as these lattices are
infinitely stiff, they do not allow to emulate phonon degrees of freedom. This
restriction can be lifted in emerged optical lattices inside multimode
cavities. Motivated by recent experimental progress in multimode cavity QED, we
propose a scheme to implement and study supersolid and droplet states with
phonon-like lattice excitations by coupling a Bose gas to many longitudinal
modes of a ring cavity. The interplay between contact collisional and
tunable-range cavity-mediated interactions leads to a rich phase diagram, which
includes elastic supersolid as well as insulating droplet phases exhibiting
roton-type mode softening for a continuous range of momenta across the
superradiant phase transition. The non-trivial dynamic response of the system
to local density perturbations further proves the existence of phonon-like
modes. | cond-mat_quant-gas |
Dark soliton oscillations in Bose-Einstein condensates with multi-body
interactions: We consider the dynamics of dark matter solitons moving through non-uniform
cigar-shaped Bose-Einstein condensates described by the mean field
Gross-Pitaevskii equation with generalized nonlinearities, in the case when the
condition for the modulation stability of the Bose-Einstein condensate is
fulfilled. The analytical expression for the frequency of the oscillations of a
deep dark soliton is derived for nonlinearities which are arbitrary functions
of the density, while specific results are discussed for the physically
relevant case of a cubic-quintic nonlinearity modeling two- and three-body
interactions, respectively. In contrast to the cubic Gross-Pitaevskii equation
for which the frequencies of the oscillations are known to be independent of
background density and interaction strengths, we find that in the presence of a
cubic-quintic nonlinearity an explicit dependence of the oscillations frequency
on the above quantities appears. This dependence gives rise to the possibility
of measuring these quantities directly from the dark soliton dynamics, or to
manage the oscillation via the changes of the scattering lengths by means of
Feshbach resonance. A comparison between analytical results and direct
numerical simulations of the cubic-quintic Gross-Pitaevskii equation shows good
agreement which confirms the validity of our approach. | cond-mat_quant-gas |
A Renormalization-Group Study of Interacting Bose-Einstein Condensates:
II. Anomalous Dimension $η$ for $d\lesssim 4$ at Finite Temperatures: We study the anomalous dimension $\eta$ of homogeneous interacting
single-component Bose-Einstein condensates at finite temperatures for
$d\lesssim 4$ dimensions. This $\eta$ is defined in terms of the one-particle
density matrix $\rho({\bf r})\equiv \langle \hat\psi^\dagger({\bf
r}_1)\hat\psi({\bf r}_1+{\bf r})\rangle$ through its asymptotic behavior
$\rho({\bf r})\rightarrow N_{\bf 0}/V+C r^{-d+2-\eta}$ for $r\rightarrow
\infty$, where $N_{\bf 0}/V$ is the condensate density and $C$ is a constant.
It is shown that the anomalous dimension is given by $\eta=0.181\epsilon^2$ to
the leading order in $\epsilon\equiv d-4$. The change of the prefactor $0.181$
from the value $0.02$ at the transition point of the ${\rm O}(2)$ symmetric
$\phi^4$ model is attributed to the emergence of three-point vertices and the
anomalous Green's function when $N_{\bf 0}$ acquires a finite value. | cond-mat_quant-gas |
Quasi-one-dimensional spin-orbit- and Rabi-coupled bright dipolar
Bose-Einstein-condensate solitons: We study the formation of stable bright solitons in quasi-one-dimensional
(quasi-1D) spin-orbit- (SO-) and Rabi-coupled two pseudospinor dipolar
Bose-Einstein condensates (BECs) of 164 Dy atoms in the presence of repulsive
contact interactions. As a result of the combined attraction-repulsion effect
of both interactions and the addition of SO and Rabi couplings, two kinds of
ground states in the form of self-trapped bright solitons can be formed, a
plane-wave soliton (PWS) and a stripe soliton (SS). These quasi-1D solitons
cannot exist in a condensate with purely repulsive contact interactions and SO
and Rabi couplings (no dipole). Neglecting the repulsive contact interactions,
our findings also show the possibility of creating PWSs and SSs. When the
strengths of the two interactions are close to each other, the SS develops an
oscillatory instability indicating a possibility of a breather solution,
eventually leading to its destruction. We also obtain a phase diagram showing
regions where the solution is a PWS or SS. | cond-mat_quant-gas |
Transition state theory for wave packet dynamics. II. Thermal decay of
Bose-Einstein condensates with long-range interaction: We apply transition state theory to coupled Gaussian wave packets and
calculate thermal decay rates of Bose-Einstein condensates with additional
long-range interaction. The ground state of such a condensate is metastable if
the contact interaction is attractive and a sufficient thermal excitation may
lead to its collapse. The use of transition state theory is made possible by
describing the condensate within a variational framework and locally mapping
the variational parameters to classical phase space as has been demonstrated in
the preceding paper [A. Junginger, J. Main, and G. Wunner, submitted to J.
Phys. A]. We apply this procedure to Gaussian wave packets and present results
for condensates with monopolar 1/r-interaction comparing decay rates obtained
by using different numbers of coupled Gaussian trial wave functions as well as
different normal form orders. | cond-mat_quant-gas |
Many-particle Quantum Hydrodynamics of Spin-1 Bose-Einstein Condensates: We develop a novel model of the magnetized spin-1 Bose-Einstein condensate
(BEC) of neutral atoms, using the method of many-particle quantum hydrodynamic
(QHD) and propose an original derivation of the system of continual equations.
We consider bosons with a spin-spin interaction and a short range interaction
in the first order in the interaction radius, on the of basis of the
self-consistent field approximation of the QHD equations. We demonstrate that
the dynamics of the fluid velocity and magnetization is determined by a
nontrivial modification of the Euler and Landau-Lifshitz equation, and show
that a nontrivial modification of the spin density evolution equation contains
the spin torque effect that arises from the self-interactions between spins of
the bosons. The properties of the dispersion spectrum of collective excitations
are described. We obtain the new contribution of the self-interaction of spins
in the spin wave spectrum together with the influence of an external magnetic
field and spin-spin interactions between polarized particles. The anisotropic
spin wave instability is predicted. | cond-mat_quant-gas |
Interference dynamics of matter-waves of SU($N$) fermions: We analyze the two main physical observables related to the momenta of
strongly correlated SU($N$) fermions in ring-shaped lattices pierced by an
effective magnetic flux: homodyne (momentum distribution) and self-heterodyne
interference patterns. We demonstrate how their analysis allows us to monitor
the persistent current pattern. We find that both homodyne and self-heterodyne
interference display a specific dependence on the structure of the Fermi
distribution and particles' correlations. For homodyne protocols, the momentum
distribution is affected by the particle statistics in two distinctive ways.
The first effect is a purely statistical one: at zero interactions, the
characteristic hole in the momentum distribution around the momentum
$\mathbf{k}=0$ opens up once half of the SU($N$) Fermi sphere is displaced. The
second effect originates from interaction: the fractionalization in the
interacting system manifests itself by an additional `delay' in the flux for
the occurrence of the hole, that now becomes a depression at $\mathbf{k}=0$. In
the case of self-heterodyne interference patterns, we are not only able to
monitor, but also observe the fractionalization. Indeed, the fractionalized
angular momenta, due to level crossings in the system, are reflected in
dislocations present in interferograms. Our analysis demonstrate how the study
of the interference fringes grants us access to both number of particles and
number of components of SU($N$) fermions. | cond-mat_quant-gas |
Counter-propagating edge modes and topological phases of a kicked
quantum Hall system: A periodically driven quantum Hall system in a fixed magnetic field is found
to exhibit a series of phases featuring anomalous edge modes with the "wrong"
chirality. This leads to pairs of counter-propagating chiral edge modes at each
edge, in sharp contrast to stationary quantum Hall systems. We show that the
pair of Floquet edge modes are protected by the chiral (sublattice) symmetry,
and that they are robust against static disorder. The existence of distinctive
phases with the same Chern and winding numbers but very different edge state
spectra points to the important role played by symmetry in classifying
topological properties of driven systems. We further explore the evolution of
the edge states with driving using a simplified model, and discuss their
experimental signatures. | cond-mat_quant-gas |
Mesoscopic quantum switching of a Bose-Einstein condensate in an optical
lattice governed by the parity of the number of atoms: It is shown that for a $N$-boson system the parity of $N$ can be responsible
for a qualitative difference in the system response to variation of a
parameter. The nonlinear boson model is considered, which describes tunneling
of boson pairs between two distinct modes $X_{1,2}$ of the same energy and
applies to a Bose-Einstein condensate in an optical lattice. By varying the
lattice depth one induces the parity-dependent quantum switching, i.e. $X_1\to
X_2$ for even $N$ and $X_1\to X_1$ for odd $N$, for arbitrarily large $N$. A
simple scheme is proposed for observation of the parity effect on the
\textit{mesoscopic scale} by using the bounce switching regime, which is
insensitive to the initial state preparation (as long as only one of the two
$X_l$ modes is significantly populated), stable under small perturbations and
requires an experimentally accessible coherence time. | cond-mat_quant-gas |
Breaking of Goldstone modes in two component Bose-Einstein condensate: We study the decay rate $\Gamma(k)$ of density excitations of two-component
Bose-Einstein condensates at zero temperature. Those excitations, where the two
components oscillate in phase, include the Goldstone mode resulting from
condensation. While within Bogoliubov approximation the density sector and the
spin (out-of-phase) sector are independent, they couple at the three-phonon
level. For a Bose-Bose mixture we find that the Belyaev decay is slightly
modified due to the coupling with the gapless spin mode. At the phase
separation point the decay rate changes instead from the standard $k^5$ to a
$k^{5/2}$ behaviour due to the parabolic nature of the spin mode. In presence
of coherent coupling between the two components the spin sector is gapped and,
away from the ferromagnetic-like phase transition point, the decay of density
mode is not affected. On the other hand at the transition point, when the spin
fluctuations become critical, the Goldstone mode is not well defined anymore
since $\Gamma(k)\propto k$. As a consequence, we show that the friction induced
by a moving impurity is enahnced -- a feature which could be experimentally
tested. Our results apply to every non-linear 2-component quantum hydrodynamic
Hamiltonian which is time-reversal invariant, and possesses an $U(1)\times
{\mathbf Z}_2$ symmetry. | cond-mat_quant-gas |
Magnetic Phase Transition in the Ground-State Phase Diagram of Binary
Bose Gases in Optical Lattices: We investigate the ground-state phase diagram of interacting binary Bose
gases trapped in two-dimensional optical lattices by means of quantum Monte
Carlo simulations. Our simulations reveal a magnetic phase transition from a
$x-y$ ferromagnetic-order to a spin insulator inside the Mott insulating phase
with two particles per site for quasi-balanced on-site inter- and
intra-particle interactions, i.e., $U_{\uparrow \downarrow} \lesssim U$. This
3D-XY transition is characterized by the establishment of a finite local
magnetic moment along the $z$-axis, ferromagnetic correlations in the $x-y$
plan and by counterflow superfluidity inside the Mott phase. When decreasing
$U_{\uparrow \downarrow}/U$, this transition merges with the Mott-superfluid
transition and becomes first-order. The merging of the two transitions is
investigated with respect to $U_{\uparrow \downarrow}/U$ parameter. | cond-mat_quant-gas |
Charge density wave and charge pump of interacting fermions in
circularly shaken hexagonal optical lattices: We analyze strong correlation effects and topological properties of
interacting fermions with a Falicov-Kimball type interaction in circularly
shaken hexagonal optical lattices, which can be effectively described by the
Haldane-Falicov-Kimball model, using the real-space Floquet dynamical
mean-field theory (DMFT). The Haldane model, a paradigmatic model of the Chern
insulator, is experimentally relevant, because it has been realized using
circularly shaken hexagonal optical lattices. We show that in the presence of
staggering a charge density wave emerges, which is affected by interactions and
resonant tunneling. We demonstrate that interactions smear out the edge states
by introducing a finite life time of quasiparticles. Even though a general
method for calculating the topological invariant of a nonequilibrium steady
state is lacking, we extract the topological invariant using a Laughlin charge
pump set-up. We find and attribute to the dissipations into the bath connected
to every lattice site, which is intrinsic to real-space Floquet DMFT methods,
that the pumped charge is not an integer even for the non-interacting case at
very low reservoir temperatures. Furthermore, using the rate equation based on
the Floquet-Born-Markov approximation, we calculate the charge pump from the
rate equations for the non-interacting case to identify the role of the
spectral properties of the bath. Starting from this approach we propose an
experimental protocol for measuring quantized charge pumping. | cond-mat_quant-gas |
Zero-temperature equation of state of mass-imbalanced resonant Fermi
gases: We calculate the zero-temperature equation of state of mass-imbalanced
resonant Fermi gases in an ab initio fashion, by implementing the recent
proposal of imaginary-valued mass difference to bypass the sign problem in
lattice Monte Carlo calculations. The fully non-perturbative results thus
obtained are analytically continued to real mass imbalance to yield the
physical equation of state, providing predictions for upcoming experiments with
mass-imbalanced atomic Fermi gases. In addition, we present an exact relation
for the rate of change of the equation of state at small mass imbalances,
showing that it is fully determined by the energy of the mass-balanced system. | cond-mat_quant-gas |
Quantum Field Theory of Correlated Bose-Einstein condensates: I. Basic
Formalism: Quantum field theory of equilibrium and nonequilibrium Bose-Einstein
condensates is formulated so as to satisfy three basic requirements: the
Hugenholtz-Pines relation; conservation laws; identities among vertices
originating from Goldstone's theorem I. The key inputs are irreducible
four-point vertices, in terms of which we derive a closed system of equations
for Green's functions, three- and four-point vertices, and two-particle Green's
functions. It enables us to study correlated Bose-Einstein condensates with a
gapless branch of single-particle excitations without encountering any infrared
divergence. The single- and two-particle Green's functions are found to share
poles, i.e., the structure of the two-particle Green's functions predicted by
Gavoret and Nozi\`eres for a homogeneous condensate at $T=0$ is also shown to
persist at finite temperatures, in the presence of inhomogeneity, and also in
nonequilibrium situations. | cond-mat_quant-gas |
Stability and dynamics across magnetic phases of vortex-bright type
excitations in spinor Bose-Einstein condensates: The static properties, i.e., existence and stability, as well as the
quench-induced dynamics of vortex-bright type excitations in two-dimensional
harmonically confined spin-1 Bose-Einstein condensates are investigated.
Linearly stable vortex-bright-vortex and bright-vortex-bright solutions arise
in both antiferromagnetic and ferromagnetic spinor gases upon quadratic Zeeman
energy shift variations. Their deformations across the relevant transitions are
exposed and discussed in detail evincing also that emergent instabilities can
lead to pattern formation. Spatial elongations, precessional motion and
spiraling of the nonlinear excitations when exposed to finite temperatures and
upon crossing the distinct phase boundaries, via quenching of the quadratic
Zeeman coefficient, are unveiled. Spin-mixing processes triggered by the quench
lead, among others, to changes in the waveform of the ensuing configurations.
Our findings reveal an interplay between pattern formation and spin-mixing
processes being accessible in contemporary cold atom experiments. | cond-mat_quant-gas |
A Strongly Interacting Polaritonic Quantum Dot: Polaritons are an emerging platform for exploration of synthetic materials
[1] and quantum information processing [2] that draw properties from two
disparate particles: a photon and an atom. Cavity polaritons are particularly
promising, as they are long-lived and their dispersion and mass are
controllable through cavity geometry [3]. To date, studies of cavity polaritons
have operated in the mean-field regime, using short-range interactions between
their matter components [4]. Rydberg excitations have recently been
demonstrated as a promising matter-component of polaritons [5], due to their
strong interactions over distances large compared to an optical wavelength. In
this work we explore, for the first time, the cavity quantum electrodynamics of
Rydberg polaritons, combining the non-linearity of polaritonic quantum wires
with the zero-dimensional strong coupling of an optical resonator. We assemble
a quantum dot composed of $\sim 150$ strongly interacting, Rydberg-dressed
$^{87}$Rb atoms in a cavity, and observe blockaded polariton transport as well
as coherent quantum dynamics of a single polaritonic super-atom. This work
establishes a new generation of photonic quantum information processors and
quantum materials, along with a clear path to topological quantum matter [6]. | cond-mat_quant-gas |
Dark-Bright Solitons in a Superfluid Bose-Fermi Mixture: The recent experimental realization of Bose-Fermi superfluid mixtures of
dilute ultracold atomic gases has opened new perspectives in the study of
quantum many-body systems. Depending on the values of the scattering lengths
and the amount of bosons and fermions, a uniform Bose-Fermi mixture is
predicted to exhibit a fully mixed phase, a fully separated phase or, in
addition, a purely fermionic phase coexisting with a mixed phase. The
occurrence of this intermediate configuration has interesting consequences when
the system is nonuniform. In this work we theoretically investigate the case of
solitonic solutions of coupled Bogoliubov-de Gennes and Gross-Pitaevskii
equations for the fermionic and bosonic components, respectively. We show that,
in the partially separated phase, a dark soliton in Fermi superfluid is
accompanied by a broad bosonic component in the soliton, forming a dark-bright
soliton which keeps full spatial coherence. | cond-mat_quant-gas |
Two-dimensional scattering and bound states of polar molecules in
bilayers: Low-energy two-dimensional scattering is particularly sensitive to the
existence and properties of weakly-bound states. We show that interaction
potentials $V(r)$ with vanishing zero-momentum Born approximation $\int d^2r
V(r)=0$ lead to an anomalously weak bound state which crucially modifies the
two-dimensional scattering properties. This anomalous case is especially
relevant in the context of polar molecules in bilayer arrangements. | cond-mat_quant-gas |
First-order spatial coherence measurements in a thermalized
two-dimensional photonic quantum gas: Phase transitions between different states of matter can profoundly modify
the order in physical systems, with the emergence of ferromagnetic or
topological order constituting important examples. Correlations allow to
quantify the degree of order and classify different phases. Here we report
measurements of first-order spatial correlations in a harmonically trapped
two-dimensional photon gas below, at, and above the critical particle number
for Bose-Einstein condensation, using interferometric measurements of the
emission of a dye-filled optical microcavity. For the uncondensed gas, the
transverse coherence decays on a length scale determined by the thermal de
Broglie wavelength of the photons, which shows the expected scaling with
temperature. At the onset of Bose-Einstein condensation true long-range order
emerges, and we observe quantum statistical effects as the thermal wave packets
overlap. The excellent agreement with equilibrium Bose gas theory prompts
microcavity photons as promising candidates for studies of critical scaling and
universality in optical quantum gases. | cond-mat_quant-gas |
Dark-soliton-like excitations in the Yang-Gaudin gas of attractively
interacting fermions: Yrast states are the lowest energy states at given non-zero momentum and
provide a natural extension of the concept of dark solitons to
strongly-interacting one-dimensional quantum gases. Here we study the yrast
states of the balanced spin-$\frac{1}{2}$ Fermi gas with attractive
delta-function interactions in one dimension with the exactly solvable
Yang-Gaudin model. The corresponding Bethe-ansatz equations are solved for
finite particle number and in the thermodynamic limit. Properties corresponding
to the soliton-like nature of the yrast excitations are calculated including
the missing particle number, phase step, and inertial and physical masses. The
inertial to physical mass ratio, which is related to the frequency of
oscillations in a trapped gas, is found to be unity in the limits of strong and
weak attraction and falls to $\approx 0.78$ in the crossover regime. This
result is contrasted by one-dimensional mean field theory, which predicts a
divergent mass ratio in the weakly attractive limit. By means of an exact
mapping our results also predict the existence and properties of
dark-soliton-like excitations in the super Tonks-Girardeau gas. The prospects
for experimental observations are briefly discussed. | cond-mat_quant-gas |
Superfluidity and Stabilities of a Bose-Einstein condensate with
periodically modulated interatomic interaction: We study theoretically the superfluidity and stability of a Bose-Einstein
condensate (BEC) whose interatomic scattering length is periodically modulated
with optical Feshbach resonance. Our numerical study finds that the properties
of this periodic BEC are strongly influenced by the modulation strength. When
the modulation strength is small, only the Bloch waves close to the Brillouin
zone edge suffer both Landau and dynamical instabilities. When the modulation
strength is strong enough, all Bloch waves become dynamically unstable. In
other words, the periodic BEC loses its superfluidity completely. | cond-mat_quant-gas |
Bose-Einstein Condensates in Non-abelian Gauge Fields: The recent success of the NIST group in generating abelian gauge field in
cold atoms has created opportunities to simulate electronic transports in
solids using atomic gases. Very recently, the NIST group has also announced in
a DARPA Meeting the creation of non-abelian gauge fields in a pseudo spin-1/2
Bose gas. While there have been considerable theoretical activities in
synthetic gauge fields, non-abelian fields have not been generated until now.
Here, we show that in a non-abelian gauge field, a spinor condensate will
develop a spontaneous stripe structure in each spin component, reflecting a
ground state made up of two non-orthogonal dressed states with different
momenta. Depending on interactions, this ground state can reduce back to a
single dressed state. These momentum carrying stripes are the {\em macroscopic}
bosonic counterpart of the spin-orbit phenomena in fermions that are being
actively studied in electron physics today. | cond-mat_quant-gas |
Three-level Haldane-like model on dice optical lattice: We consider ultracold atoms in a two-dimensional optical lattice of the dice
geometry in a tight-binding regime. The atoms experience a laser-assisted
tunneling between the nearest neighbour sites of the dice lattice accompanied
by the momentum recoil. This allows one to engineer staggered synthetic
magnetic fluxes over plaquettes, and thus pave a way towards a realization of
topologically nontrivial band structures. In such a lattice the real-valued
next-neighbour transitions are not needed to reach a topological regime. Yet,
such transitions can increase a variety of the obtained topological phases. The
dice lattice represents a triangular Bravais lattice with a three-site basis
consisting of a hub site connected to two rim sites. As a consequence, the dice
lattice supports three dispersion bands. From this point of view, our model can
be interpreted as a generalization of the paradigmatic Haldane model which is
reproduced if one of the two rim sub-lattices is eliminated. We demonstrate
that the proposed upgrade of the Haldane model creates a significant added
value, including an easy access to topological semimetal phases relying only on
the nearest neighbour coupling, as well as enhanced topological band structures
featuring Chern numbers higher than one. The numerical investigation is
supported and complemented by an analytical scheme based on the study of
singularities in the Berry connection. | cond-mat_quant-gas |
(Inverse) Magnetic Catalysis in Bose-Einstein Condensation of Neutral
Bound Pairs: The Bose-Einstein condensation of bound pairs made of oppositely charged
fermions in a magnetic field is investigated. We find that the condensation
temperature shows the magnetic catalysis effect in weak coupling and the
inverse magnetic catalysis effect in strong coupling. The different responses
to the magnetic field can be attributed to the competition between the
dimensional reduction by Landau orbitals in pairing dynamics and the anisotropy
of the kinetic spectrum of fluctuations (bound pairs in the normal phase) | cond-mat_quant-gas |
Synchronization transition in dipole-coupled two-level systems with
positional disorder: We study the decoherence dynamics of dipole-coupled two-level quantum systems
in Ramsey-type experiments. We focus on large networks of two-level systems,
confined to two spatial dimensions and with positional disorder giving rise to
disordered dipolar couplings. This setting is relevant for modeling the
decoherence dynamics of the rotational excitations of polar molecules confined
to deep optical lattices, where disorder arises from the random filling of
lattice sites with occupation probability $p$. We show that the decoherence
dynamics exhibits a phase transition at a critical filling $p_c\simeq 0.15$.
For $p<p_c$ the dynamics is disorder-dominated and the Ramsey interference
signal decays on a timescale $T_2 \propto p^{-3/2}$. For $p>p_c$ the dipolar
interactions dominate the disorder, and the system behaves as a collective
spin-ordered phase, representing synchronization of the two-level systems and
persistent Ramsey oscillations with divergent $T_2$ for large systems. For a
finite number of two-level systems, $N$, the spin-ordered phase at $p> p_c$
undergoes a crossover to a collective spin-squeezed state on a timescale
$\tau_{\rm sq} \propto \sqrt{N}$. We develop a self-consistent mean-field
theory that is capable of capturing the synchronization transition at $p_c$,
and provide an intuitive theoretical picture that describes the phase
transition in the long-time dynamics. We also show that the decoherence
dynamics appear to be ergodic in the vicinity of $p_c$, the long-time behaviour
being well described by the predictions of equilibrium thermodynamics. The
results are supported by the results of exact diagonalization studies of small
systems. | cond-mat_quant-gas |
Strong optical self-focusing effect in coherent light scattering with
condensates: We present a theoretical investigation of optical self-focusing effects in
light scattering with condensates. Using long (>200 \mu s), red-detuned pulses
we show numerically that a non-negligible self-focusing effect is present that
causes rapid optical beam width reduction as the scattered field propagates
through a medium with an inhomogeneous density distribution. The rapid growth
of the scattered field intensity and significant local density feedback
positively to further enhance the wave generation process and condensate
compression, leading to highly efficient collective atomic recoil motion. | cond-mat_quant-gas |
Confinement-induced collapse of a dipolar Bose-Einstein condensate: We report on the observation of the confinement-induced collapse dynamics of
a dipolar Bose-Einstein condensate (dBEC) in a one-dimensional optical lattice.
We show that for a fixed interaction strength the collapse can be initiated
in-trap by lowering the lattice depth below a critical value. Moreover, a
stable dBEC in the lattice may become unstable during the time-of-flight
dynamics upon release, due to the combined effect of the anisotropy of the
dipolar interactions and inter-site coherence in the lattice. | cond-mat_quant-gas |
Design and construction of a quantum matter synthesizer: The quantum matter synthesizer (QMS) is a new quantum simulation platform in
which individual particles in a lattice can be resolved and re-arranged into
arbitrary patterns. The ability to spatially manipulate ultracold atoms and
control their tunneling and interactions at the single-particle level allows
full control of a many-body quantum system. We present the design and
characterization of the QMS, which integrates into a single ultra-stable
apparatus a two-dimensional optical lattice, a moving optical tweezer array
formed by a digital micromirror device, and site-resolved atomic imaging. We
demonstrate excellent mechanical stability between the lattice and tweezer
array with relative fluctuations below 10 nm, high-speed real-time control of
the tweezer array at a 2.52 kHz refresh rate, and diffraction-limited imaging
at a resolution of 655 nm. The QMS also features new technologies and schemes
such as nanotextured anti-reflective windows and all-optical long-distance
transport of atoms. | cond-mat_quant-gas |
Excitation spectrum of a mixture of two Bose gases confined in a ring
potential with interaction asymmetry: We study the rotational properties of a two-component Bose-Einstein condensed
gas of distinguishable atoms which are confined in a ring potential using both
the mean-field approximation, as well as the method of diagonalization of the
many-body Hamiltonian. We demonstrate that the angular momentum may be given to
the system either via single-particle, or "collective" excitation. Furthermore,
despite the complexity of this problem, under rather typical conditions the
dispersion relation takes a remarkably simple and regular form. Finally, we
argue that under certain conditions the dispersion relation is determined via
collective excitation. The corresponding many-body state, which, in addition to
the interaction energy minimizes also the kinetic energy, is dictated by
elementary number theory. | cond-mat_quant-gas |
Disordered structures in ultracold spin-imbalanced Fermi gas: We investigate properties of spin-imbalanced ultracold Fermi gas in a large
range of spin polarizations at low temperatures. We present results of
microscopic calculations based on mean-field and density functional theory
approaches, with no symmetry constraints. At low polarization values we predict
the structure of the system as consisting of several spin-polarized droplets.
As the polarization increases, the system self-organizes into a disordered
structures similar to liquid crystals, and energetically they can compete with
ordered structures such as grid-like domain walls. At higher polarizations the
system starts to develop regularities that, in principle, can be called
supersolid, where periodic density modulation and pairing correlations coexist.
The robustness of the results has been checked with respect to temperature
effects, dimensionality, and the presence of a trapping potential. Dynamical
stability has also been investigated. | cond-mat_quant-gas |
Thermal suppression of demixing dynamics in a binary condensate: We investigate the demixing dynamics in a binary two-dimensional (2D) Bose
superfluid using classical-field dynamics. By quenching the interspecies
interaction parameter, we identify a strong and weak separation regime
depending on the system temperature and the quench parameter. In the strong
separation regime our results are in agreement with the inertial hydrodynamic
domain growth law of binary fluids and a Porod scaling law for the structure
factor at zero temperature is found. In the weak separation regime thermal
fluctuations modify both the domain growth law and the Porod tail of the
structure factor. Near the superfluid transition temperature the scaling
dynamics approaches the diffusive growth law of a 2D conserved field. We then
analyze the demixing dynamics in a box cloud. For low quench we find
distinctive domain dynamics dictated by the boundary condition. Otherwise, the
dynamics are qualitatively similar to those of systems with periodic boundary
conditions. | cond-mat_quant-gas |
Dynamical transitions and quantum quenches in mean-field models: We develop a generic method to compute the dynamics induced by quenches in
completely connected quantum systems. These models are expected to provide a
mean-field description at least of the short time dynamics of finite
dimensional system. We apply our method to the Bose-Hubbard model, to a
generalized Jaynes-Cummings model, and to the Ising model in a transverse
field. We find that the quantum evolution can be mapped onto a classical
effective dynamics, which involves only a few intensive observables. For some
special parameters of the quench, peculiar dynamical transitions occur. They
result from singularities of the classical effective dynamics and are
reminiscent of the transition recently found in the fermionic Hubbard model.
Finally, we discuss the generality of our results and possible extensions. | cond-mat_quant-gas |
Cavity-enhanced optical lattices for scaling neutral atom quantum
technologies to higher qubit numbers: We demonstrate a cavity-based solution to scale up experiments with ultracold
atoms in optical lattices by an order of magnitude over state-of-the-art free
space lattices. Our two-dimensional optical lattices are created by power
enhancement cavities with large mode waists of 489(8) $\mu$m and allow us to
trap ultracold strontium atoms at a lattice depth of 60 $\mu$K by using only 80
mW of input light per cavity axis. We characterize these lattices using
high-resolution clock spectroscopy and resolve carrier transitions between
different vibrational levels. With these spectral features, we locally measure
the lattice potential envelope and the sample temperature with a spatial
resolution limited only by the optical resolution of the imaging system. The
measured ground-band and trap lifetimes are 18(3) s and 59(2) s, respectively,
and the lattice frequency (depth) is long-term stable on the MHz (0.1\%) level.
Our results show that large, deep, and stable two-dimensional cavity-enhanced
lattices can be created at any wavelength and can be used to scale up
neutral-atom-based quantum simulators, quantum computers, sensors, and optical
lattice clocks. | cond-mat_quant-gas |
Goldstone mode and pair-breaking excitations in atomic Fermi superfluids: Spontaneous symmetry breaking is a central paradigm of elementary particle
physics, magnetism, superfluidity and superconductivity. According to
Goldstone's theorem, phase transitions that break continuous symmetries lead to
the existence of gapless excitations in the long-wavelength limit. These
Goldstone modes generally dominate the low-energy excitations, showing that
symmetry breaking has a profound impact on the physical properties of matter.
Here, we present the first comprehensive study of the elementary excitations in
a homogeneous strongly interacting Fermi gas through the crossover from a
Bardeen-Cooper-Schrieffer (BCS) superfluid to a Bose-Einstein condensate (BEC)
of molecules using two-photon Bragg spectroscopy. The spectra exhibit a
discrete Goldstone mode, associated with the broken symmetry superfluid phase,
as well as pair breaking single-particle excitations. Our techniques yield a
direct determination of the superfluid pairing gap and speed of sound in close
agreement with a strong-coupling theory. | cond-mat_quant-gas |
Energy Cascade in Quantum Gases: Energy cascade is ubiquitous in systems far from equilibrium. Facilitated by
particle interactions and external forces, it can lead to highly complex
phenomena like fully developed turbulence, characterized by power law velocity
correlation functions. Yet despite decades of research, how these power laws
emerge from first principle remains unclear. Recently, experiments show that
when a Bose condensate is subjected to periodic shaking, its momentum
distribution exhibits a power law behavior. The flexibility of cold atom
experiments has provided new opportunities to explore the emergence of these
power laws, and to disentangle different sources of energy cascade. Here, we
point out that recent experiments in cold atoms imply that classical turbulence
is part of a larger family of scale invariant phenomena that include ideal
gases. Moreover, the property of the entire family is contained in the
structure of its Floquet states. For ideal gases, we show analytically that its
momentum distribution acquires a $1/q^2$ tail in each dimension when it is
shaken periodically. | cond-mat_quant-gas |
Induced supersolidity in a Dy-Er mixture: Recent experimental realization of the heteronuclear dipolar mixture of Dy
and Er atoms opens fascinating prospects for creating intriguing novel phases
in dipolar quantum gases. The experimentally measured value of intra-species
$s$-wave scattering length of $^{166}$Er condensate in a $^{164}$Dy-$^{166}$Er
mixture is larger than its intra-species dipolar length, implies that the
$^{166}$Er condensate itself will not be in a regime of dominated dipole-dipole
interaction (DDI). However, we find that the presence of $^{164}$Dy atoms with
high magnetic moment induces droplet nucleation and supersolidity in $^{166}$Er
condensate via the long-range and anisotropic inter-species DDI. Remarkably, we
find that the imbalance in the magnetic dipole moment combined with its strong
anisotropic coupling led to the emergence of unique ground state phases. The
emerging phases include doubly superfluid states, a mixture of insulating
droplets and supersolid states, binary supersolids with uniform and alternating
domains and a combination of supersolid-superfluid mixed states. We delineate
the properties of all these ground state phases and construct a phase diagram.
We also explore the dynamical evolution across these phase boundaries via a
linear quench of inter-species scattering length. Although we have demonstrated
the result for the $^{164}$Dy-$^{166}$Er mixture, our results are generally
valid for other dipolar bosonic mixtures of different Dy-Er isotope
combinations and may become an important benchmark for future experimental
scenarios. | cond-mat_quant-gas |
A continuum of compass spin models on the honeycomb lattice: Quantum spin models with spatially dependent interactions, known as compass
models, play an important role in the study of frustrated quantum magnetism.
One example is the Kitaev model on the honeycomb lattice with spin-liquid
ground states and anyonic excitations. Another example is the geometrically
frustrated quantum $120^\circ$ model on the same lattice whose ground state has
not been unambiguously established. To generalize the Kitaev model beyond the
exactly solvable limit and connect it with other compass models, we propose a
new model, dubbed "the tripod model", which contains a continuum of
compass-type models. It smoothly interpolates the Ising model, the Kitaev
model, and the quantum $120^\circ$ model by tuning a single parameter
$\theta'$, the angle between the three legs of a tripod in the spin space.
Hence it not only unifies three paradigmatic spin models, but also enables the
study of their quantum phase transitions. We obtain the phase diagram of the
tripod model numerically by tensor networks in the thermodynamic limit. We show
that the ground state of the quantum $120^\circ$ model has long-range dimer
order. Moreover, we find an extended spin-disordered (spin-liquid) phase
between the dimer phase and an antiferromagnetic phase. The unification and
solution of a continuum of frustrated spin models as outline here may be useful
to exploring new domains of other quantum spin or orbital models. | cond-mat_quant-gas |
Finite temperature theory of superfluid bosons in optical lattices: A practical finite temperature theory is developed for the superfluid regime
of a weakly interacting Bose gas in an optical lattice with additional harmonic
confinement. We derive an extended Bose-Hubbard model that is valid for shallow
lattices and when excited bands are occupied. Using the
Hartree-Fock-Bogoliubov-Popov mean-field approach, and applying local density
and coarse-grained envelope approximations, we arrive at a theory that can be
numerically implemented accurately and efficiently. We present results for a
three-dimensional system, characterizing the importance of the features of the
extended Bose-Hubbard model and compare against other theoretical results and
show an improved agreement with experimental data. | cond-mat_quant-gas |
The one-dimensional Bose gas with strong two-body losses: the effect of
the harmonic confinement: We study the dynamics of a one-dimensional Bose gas in presence of strong
two-body losses. In this dissipative quantum Zeno regime, the gas fermionises
and its dynamics can be described with a simple set of rate equations.
Employing the local density approximation and a Boltzmann-like dynamical
equation, the description is easily extended to take into account an external
potential. We show that in the absence of confinement the population is
depleted in an anomalous way and that the gas behaves as a low-temperature
classical gas. The harmonic confinement accelerates the depopulation of the gas
and introduces a novel decay regime, which we thoroughly characterise. | cond-mat_quant-gas |
Dynamics of spin-polarized impurity in ultracold Fermi gas: We show that the motion of spin-polarized impurity (ferron) in ultracold
atomic gas is characterized by a certain critical velocity which can be traced
back to the amount of spin imbalance inside the impurity. We have calculated
the effective mass of ferron in two dimensions. We show that the effective mass
scales with the surface of the ferron. We discuss the impact of these findings;
in particular, we demonstrate that ferrons become unstable in the vicinity of a
vortex. | cond-mat_quant-gas |
Dimensional crossover and cold-atom realization of topological Mott
insulators: We propose a cold-atom setup which allows for a dimensional crossover from a
two-dimensional quantum spin Hall insulating phase to a three-dimensional
strong topological insulator by tuning the hopping between the layers. We
further show that additional Hubbard onsite interactions can give rise to spin
liquid-like phases: weak and strong topological Mott insulators. They represent
the celebrated paradigm of a quantum state of matter which merely exists
because of the interplay of the non-trivial topology of the band structure and
strong interactions. While the theoretical understanding of this phase has
remained elusive, our proposal shall help to shed some light on this exotic
state of matter by paving the way for a controlled experimental investigation
in optical lattices. | cond-mat_quant-gas |
Spontaneous Magnetic Ordering in a Ferromagnetic Spinor Dipolar
Bose-Einstein Condensate: We study the spin dynamics in a spin-1 ferromagnetic Bose-Einstein condensate
with magnetic dipole-dipole interaction (MDDI) based on the Gross-Pitaevskii
and Bogoliubov theories. We find that various magnetic structures such as
checkerboards and stripes emerge in the course of the dynamics due to the
combined effects of spin-exchange interaction, MDDI, quadratic Zeeman and
finite-size effects, and non-stationary initial conditions. However, the
short-range magnetic order observed by the Berkeley group [Phys. Rev. Lett.
{\bf 100}, 170403 (2008)] is not fully reproduced in our calculations; the
periodicity of the order differs by a factor of three and the checkerboard
pattern eventually dissolves in our numerical simulations. Possible reasons for
the discrepancy are discussed. | cond-mat_quant-gas |
Three-body scattering area for particles with infinite or zero
scattering length in two dimensions: We derive the asymptotic expansions of the wave function of three particles
having equal mass with finite-range interactions and infinite or zero
two-dimensional scattering length colliding at zero energy and zero orbital
angular momentum, from which a three-body parameter $D$ is defined. The
dimension of $D$ is length squared, and we call $D$ three-body scattering area.
We find that the ground state energy per particle of a zero-temperature dilute
Bose gas with these interactions is approximately $\frac{\hbar^2 D
}{6m}\rho^2$, where $\rho$ is the number density of the bosons, $m$ is the mass
of each boson, and $\hbar$ is Planck's constant over $2\pi$. Such a Bose gas is
stable at $D\geq 0$ in the thermodynamic limit, and metastable at $D<0$ in the
harmonic trap if the number of bosons is less than $N_{cr}\approx 3.6413
\sqrt{\frac{\hbar}{m\omega |D|}}$, where $\omega$ is the angular frequency of
the harmonic trap. If the two-body interaction supports bound states, $D$
typically acquires a negative imaginary part, and we find the relation between
this imaginary part and the amplitudes of the pair-boson production processes.
We derive a formula for the three-body recombination rate constant of the
many-boson system in terms of the imaginary part of $D$. | cond-mat_quant-gas |
Nature of polaron-molecule transition in Fermi polarons: In this work, we explore the polaron and molecule physics by utilizing a
unified variational ansatz with up to two particle-hole(p-h)
excitations(V-2ph). We confirm the existence of a first-order transition in 3D
and 2D Fermi polarons, and show that the nature of such transition lies in an
energy competition between systems with different momenta ${\mathbf Q}=0$ and
$|{\mathbf Q}|=k_F$, here ${\mathbf Q}$ is defined as the momentum of Fermi
polaron system with respect to the Fermi sea of majority fermions (with Fermi
momentum $k_F$). The literally proposed molecule ansatz is identified as an
asymptotic limit of $|{\mathbf Q}|=k_F$ state in strong coupling regime, which
implies a huge $SO(3)$(for 3D) or $SO(2)$ (for 2D) ground state degeneracy in
this regime. The recognization of such degeneracy is crucially important for
evaluating the molecule occupation in realistic systems with finite impurity
density and at finite temperature. To compare with recent experiment of 3D
Fermi polarons, we have calculated various physical quantities under the V-2ph
framework and obtained results that are in good agreements with experimental
data in the weak coupling and near resonance regime. Further, to check the
validity of our conclusion in 2D, we have adopted a different variational
method based on the Gaussian sample of high-order p-h excitations(V-Gph), and
found the same conclusion on the nature of polaron-molecule transition therein.
For 1D system, the V-2ph method predicts no sharp transition and the ground
state is always at ${\mathbf Q}=0$ sector, consistent with exact Bethe ansatz
solution. The presence/absence of polaron-molecule transition is analyzed to be
closely related to the interplay effect of Pauli-blocking and p-h excitations
in different dimensions. | cond-mat_quant-gas |
Observation of a two-dimensional Fermi gas of atoms: We have prepared a degenerate gas of fermionic atoms which move in two
dimensions while the motion in the third dimension is "frozen" by tight
confinement and low temperature. {\it In situ} imaging provides direct
measurement of the density profile and temperature. The gas is confined in a
defect-free optical potential, and the interactions are widely tunable by means
of a Fano--Feshbach resonance. This system can be a starting point for
exploration of 2D Fermi physics and critical phenomena in a pure, controllable
environment. | cond-mat_quant-gas |
Long-range sound-mediated dark soliton interactions in trapped atomic
condensates: A long-range soliton interaction is discussed whereby two or more dark
solitons interact in an inhomogeneous atomic condensate, modifying their
respective dynamics via the exchange of sound waves without ever coming into
direct contact. An idealized double well geometry is shown to yield perfect
energy transfer and complete periodic identity reversal of the two solitons.
Two experimentally relevant geometries are analyzed which should enable the
observation of this long-range interaction. | cond-mat_quant-gas |
Supersolid-like square- and honeycomb-lattice crystallization of
droplets in a dipolar condensate: We demonstrate a supersolid-like spatially-periodic square- and
honeycomb-lattice crystallization of droplets, in addition to the
commonly-studied triangular-lattice crystallization, in a
cylindrically-symmetric quasi-two-dimensional trapped dipolar condensate, using
a beyond-mean-field model including a quantum-fluctuation Lee-Huang-Yang-type
interaction. These three types of crystallization of droplets may appear for
the same atomic interactions and the same trap frequencies. The energy $E$ of
all three crystallization as a function of number $N$ of atoms satisfy the
universal scaling relation $E\sim N^{0.4}$ indicating that all three
arrangements of the droplets should be energetically probable processes of
phenomenological interest. The state of square-lattice crystallization may have
the central site occupied or unoccupied, corresponding to a parity-symmetric or
parity-antisymmetric state, respectively. The state of square-lattice
crystallization with the occupied central site and the state of
triangular-lattice crystallization, for a fixed $N$, constitute two
quasi-degenerate ground states while the other states are low-lying excited
states. This makes the square-lattice crystallization with the occupied central
site an ideal candidate for future experimental observation. | cond-mat_quant-gas |
The self-energy of an impurity in an ideal Fermi gas to second order in
the interaction strength: We study in three dimensions the problem of a spatially homogeneous
zero-temperature ideal Fermi gas of spin-polarized particles of mass $m$
perturbed by the presence of a single distinguishable impurity of mass $M$. The
interaction between the impurity and the fermions involves only the partial
$s$-wave through the scattering length $a$, and has negligible range $b$
compared to the inverse Fermi wave number $1/\kf$ of the gas. Through the
interactions with the Fermi gas the impurity gives birth to a quasi-particle,
which will be here a Fermi polaron (or more precisely a {\sl monomeron}). We
consider the general case of an impurity moving with wave vector $\KK\neq\OO$:
Then the quasi-particle acquires a finite lifetime in its initial momentum
channel because it can radiate particle-hole pairs in the Fermi sea. A
description of the system using a variational approach, based on a finite
number of particle-hole excitations of the Fermi sea, then becomes
inappropriate around $\KK=\mathbf{0}$. We rely thus upon perturbation theory,
where the small and negative parameter $\kf a\to0^-$ excludes any branches
other than the monomeronic one in the ground state (as e.g.\ the dimeronic
one), and allows us a systematic study of the system. We calculate the impurity
self-energy $\Sigma^{(2)}(\KK,\omega)$ up to second order included in $a$.
Remarkably, we obtain an analytical explicit expression for
$\Sigma^{(2)}(\KK,\omega)$ allowing us to study its derivatives in the plane
$(K,\omega)$. These present interesting singularities, which in general appear
in the third order derivatives $\partial^3 \Sigma^{(2)}(\KK,\omega)$. In the
special case of equal masses, $M=m$, singularities appear already in the
physically more accessible second order derivatives $\partial^2
\Sigma^{(2)}(\KK,\omega)$; using a self-consistent heuristic approach based on
$\Sigma^{(2)}$ we then regularise the divergence of the second order derivative
$\partial\_K^2 \Delta E(\KK)$ of the complex energy of the quasi-particle found
in reference [C. Trefzger, Y. Castin, Europhys. Lett. {\bf 104}, 50005 (2013)]
at $K=\kf$, and we predict an interesting scaling law in the neighborhood of
$K=\kf$. As a by product of our theory we have access to all moments of the
momentum of the particle-hole pair emitted by the impurity while damping its
motion in the Fermi sea, at the level of Fermi's golden rule. | cond-mat_quant-gas |
Many-body tunneling dynamics of Bose-Einstein condensates and vortex
states in two spatial dimensions: In this work, we study the out-of-equilibrium many-body tunneling dynamics of
a Bose-Einstein condensate in a two-dimensional radial double well. We
investigate the impact of interparticle repulsion and compare the influence of
angular momentum on the many-body tunneling dynamics. Accurate many-body
dynamics are obtained by solving the full many-body Schr\"odinger equation. We
demonstrate that macroscopic vortex states of definite total angular momentum
indeed tunnel and that, even in the regime of weak repulsions, a many-body
treatment is necessary to capture the correct tunneling dynamics. As a general
rule, many-body effects set in at weaker interactions when the tunneling system
carries angular momentum. | cond-mat_quant-gas |
Vortex annihilation and inverse cascades in two dimensional superfluid
turbulence: We study two dimensional superfluid turbulence by employing an effective
description valid in the limit where the density of superfluid vortices is
parametrically small. At sufficiently low temperatures the effective
description yields an inverse cascade with Kolmogorov energy spectrum $E(k)
\sim k^{-5/3}$. Denoting the number of vortices as a function of time by
$N(t)$, we find that the vortex annihilation rate scales like $\dot N \sim
N^{5/3}$ in states with an inverse cascade and $\dot N \sim N^2$ for laminar
flow. | cond-mat_quant-gas |
Quantum phase diagram for two species hardcore bosons in one-dimensional
optical lattices with the resonantly driven Rabi frequency: We propose an experimental realization of the time-periodically modulated
Rabi frequency and suggest density-dependent hoppings of two species hardcore
bosons in a one-dimensional optical lattice. Distinct from the previous work
[Phys. Rev. Research {\bf 2}, 013275 (2020)], we study effects in the first
resonance region. In the effective Hamiltonian, the intra-species hopping
occurs only if the density discrepancy of the other species on these sites is
zero, while the inter-species one is allowed once the relevant density
discrepancy becomes nonzero. At integer-$1$ filling, the quantum phase diagram
of the effective Hamiltonian is determined by the perturbation analysis
together with numerical calculations. We find that in the limit of dominant
$J_{1}$, the system becomes a double-degenerate dimerized state, with
spontaneously breaking the translation symmetry. The interplay of $J_{0}$,
$J_{1}$ and the fixed ${\bar U}=1$ leads to three BKT transition lines and a
tricritical BKT point. Exact transition lines are obtained by the level
spectroscopic technique. Besides, general physical properties, including the
charge gap, neutral gap, superfluid density and dimerization strength, are
investigated as well. | cond-mat_quant-gas |
Bogoliubov excitation spectrum of an elongated condensate from
quasi-one-dimensional to three-dimensional transition: The quasiparticle excitation spectra of a Bose gas trapped in a highly
anisotropic trap is studied with respect to varying total number of particles
by numerically solving the effective one-dimensional (1D) Gross-Pitaevskii (GP)
equation proposed recently by Mateo \textit{et al.}. We obtain the static
properties and Bogoliubov spectra of the system in the high energy domain. This
method is computationally efficient and highly accurate for a condensate system
undergoing a 1D to three-dimensional (3D) cigar-shaped transition, as is shown
through a comparison our results with both those calculated by the 3D-GP
equation and analytical results obtained in limiting cases. We identify the
applicable parameter space for the effective 1D-GP equation and find that this
equation fails to describe a system with large number of atoms. We also
identify that the description of the transition from 1D Bose-Einstein
condensate (BEC) to 3D cigar-shaped BEC using this equation is not smooth,
which highlights the fact that for a finite value of $a_\perp/a_s$ the junction
between the 1D and 3D crossover is not perfect. | cond-mat_quant-gas |
Universal Five- and Six-Body Droplets Tied to an Efimov Trimer: We explore the properties of weakly bound bosonic states in the strongly
interacting regime. Combining a correlated-Gaussian (CG) basis set expansion
with a complex scaling method, we extract the energies and structural
properties of bosonic cluster states with $N\le6$ for different two-body
potentials. The identification of five- and six-body resonances attached to an
excited Efimov trimer provides strong support to the premise of Efimov
universality in bosonic systems. Our study also reveals a rich structure of
bosonic cluster states. Besides the lowest cluster states which behave as
bosonic droplets, we identify cluster states weakly bound to one or two atoms
forming effective cluster-atom "dimers" and cluster-atom-atom "trimers." The
experimental signatures of these cluster states are discussed. | cond-mat_quant-gas |
Optical Flux Lattices for Two-Photon Dressed States: We describe a simple scheme by which "optical flux lattices" can be
implemented in ultracold atomic gases using two-photon dressed states. This
scheme can be applied, for example, to the ground state hyperfine levels of
commonly used atomic species. The resulting flux lattices simulate a magnetic
field with high mean flux density, and have low energy bands analogous to the
lowest Landau level. We show that in practical cases the atomic motion
significantly deviates from the adiabatic following of one dressed state, and
that this can lead to significant interactions even for fermions occupying a
single band. Our scheme allows experiments on cold atomic gases to explore
strong correlation phenomena related to the fractional quantum Hall effect,
both for fermions and bosons. | cond-mat_quant-gas |
Topological bands with Chern number C=2 by dipolar exchange interactions: We demonstrate the realization of topological band structures by exploiting
the intrinsic spin-orbit coupling of dipolar interactions in combination with
broken time-reversal symmetry. The system is based on polar molecules trapped
in a deep optical lattice, where the dynamics of rotational excitations follows
a hopping Hamiltonian which is determined by the dipolar exchange interactions.
We find topological bands with Chern number $C=2$ on the square lattice, while
a very rich structure of different topological bands appears on the honeycomb
lattice. We show that the system is robust against missing molecules. For
certain parameters we obtain flat bands, providing a promising candidate for
the realization of hard-core bosonic fractional Chern insulators. | cond-mat_quant-gas |
Z_2 Topological Insulators in Ultracold Atomic Gases: We describe how optical dressing can be used to generate bandstructures for
ultracold atoms with non-trivial Z_2 topological order. Time reversal symmetry
is preserved by simple conditions on the optical fields. We first show how to
construct optical lattices that give rise to Z_2 topological insulators in two
dimensions. We then describe a general method for the construction of
three-dimensional Z_2 topological insulators. A central feature of our approach
is a new way to understand Z_2 topological insulators starting from the
nearly-free electron limit. | cond-mat_quant-gas |
High-precision numerical solution of the Fermi polaron problem and
large-order behavior of its diagrammatic series: We introduce a simple determinant diagrammatic Monte Carlo algorithm to
compute the ground-state properties of a particle interacting with a Fermi sea
through a zero-range interaction. The fermionic sign does not cause any
fundamental problem when going to high diagram orders, and we reach order
$N=30$. The data reveal that the diagrammatic series diverges exponentially as
$(-1/R)^{N}$ with a radius of convergence $R<1$. Furthermore, on the polaron
side of the polaron-dimeron transition, the value of $R$ is determined by a
special class of three-body diagrams, corresponding to repeated scattering of
the impurity between two particles of the Fermi sea. A power-counting argument
explains why finite $R$ is possible for zero-range interactions in three
dimensions. Resumming the divergent series through a conformal mapping yields
the polaron energy with record accuracy. | cond-mat_quant-gas |
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