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Feshbach resonances of harmonically trapped atoms: Employing a short-range two-channel description we derive an analytic model
of atoms in isotropic and anisotropic harmonic traps at a Feshbach resonance.
On this basis we obtain a new parameterization of the energy-dependent
scattering length which differs from the one previously employed. We validate
the model by comparison to full numerical calculations for Li-Rb and explain
quantitatively the experimental observation of a resonance shift and
trap-induced molecules in exited bands. Finally, we analyze the bound state
admixture and Landau-Zener transition probabilities. | cond-mat_quant-gas |
Curving the space by non-Hermiticity: Quantum systems are often classified into Hermitian and non-Hermitian ones.
Extraordinary non-Hermitian phenomena, ranging from the non-Hermitian skin
effect to the supersensitivity to boundary conditions, have been widely
explored. Whereas these intriguing phenomena have been considered peculiar to
non-Hermitian systems, we show that they can be naturally explained by a
duality between non-Hermitian models in flat spaces and their counterparts,
which could be Hermitian, in curved spaces. For instance, prototypical
one-dimensional (1D) chains with uniform chiral tunnelings are equivalent to
their duals in two-dimensional (2D) hyperbolic spaces with or without magnetic
fields, and non-uniform tunnelings could further tailor local curvatures. Such
a duality unfolds deep geometric roots of non-Hermitian phenomena, delivers an
unprecedented routine connecting Hermitian and non-Hermitian physics, and gives
rise to a theoretical perspective reformulating our understandings of
curvatures and distance. In practice, it provides experimentalists with a
powerful two-fold application, using non-Hermiticity as a new protocol to
engineer curvatures or implementing synthetic curved spaces to explore
non-Hermitian quantum physics. | cond-mat_quant-gas |
Two-dimensional dynamics of expansion of a degenerate Bose gas: Expansion of a degenerate Bose gas released from a pancakelike trap is
numerically simulated under the assumption of separation of the motion in the
plane of the loose initial trapping and the motion in the direction of the
initial tight trapping. The initial conditions for the phase fluctuations are
generated using the extension to the two-dimensional case of the description of
the phase noise by the Ornstein-Uhlenbeck stochastic process. The numerical
simulations, taking into account both the finite size of the two-dimensional
system and the atomic interactions, which cannot be neglected on the early
stage of expansion, did not reproduce the scaling law for the peaks in the
density fluctuation spectra experimentally observed by Choi, Seo, Kwon, and
Shin [Phys. Rev. Lett. 109, 125301 (2012)]. The latter experimental results may
thus require an explanation beyond our current assumptions. | cond-mat_quant-gas |
Spin 1 microcondensate in a magnetic field: semiclassics and exact
solution: We study a spin 1 Bose condensate small enough to be treated as a single
magnetic `domain': a system that we term a microcondensate. Because all
particles occupy a single spatial mode, this quantum many body system has a
well defined classical limit consisting of three degrees of freedom,
corresponding to the three macroscopically occupied spin states. We study both
the classical limit and its quantization, finding an integrable system in both
cases. Depending on the sign of the ratio of the spin interaction energy and
the quadratic Zeeman energy, the classical limit displays either a separartrix
in phase space, or Hamiltonian monodromy corresponding to non-trivial phase
space topology. We discuss the quantum signatures of these classical phenomena
using semiclassical quantization as well as an exact solution using the Bethe
ansatz. | cond-mat_quant-gas |
Multi-wavelength holography with a single Spatial Light Modulator for
ultracold atom experiments: We demonstrate a method to create arbitrary intensity distributions of
multiple wavelengths of light, which can be useful for ultracold atom
experiments, by using regional phase-calculation algorithms to find a single
hologram which is illuminated with overlapped laser beams. The regionality of
the algorithms is used to program spatially distinct features in the calculated
intensity distribution, which then overlap in the Fourier plane due to the
dependence of diffraction angle on wavelength. This technique is easily
integrated into cold atom experiments, requiring little optical access. We
demonstrate the method and two possible experimental scenarios by generating
light patterns with 670nm, 780nm and 1064nm laser light which are accurate to
the level of a few percent. | cond-mat_quant-gas |
Thermalized Abrikosov lattices from decaying turbulence in rotating BECs: We study the long-time decay of rotating turbulence in Bose-Einstein
condensates (BECs). We consider the Gross-Pitaevskii equation in a rotating
frame of reference, and review different formulations for the Hamiltonian of a
rotating BEC. We discuss how the energy can be decomposed, and present a method
to generate out-of-equilibrium initial conditions. We also present a method to
generate finite-temperature states of rotating BECs compatible with the
Canonical or the Grand canonical ensembles. Finally, we integrate numerically
rotating BECs in cigar-shaped traps. A transition is found in the system
dynamics as the rotation rate is increased, with a final state of the decay of
the turbulent flow compatible with an Abrikosov lattice in a finite-temperature
thermalized state. | cond-mat_quant-gas |
Ultracold Atomic Gases: Novel States of Matter: Article to appear in the Encyclopedia of Complexity and Systems Science, Dr.
R. A. Meyers (Ed.) (Springer Heidelberg, 2009). | cond-mat_quant-gas |
Faraday patterns in spin-orbit coupled Bose-Einstein condensates: We study the Faraday patterns generated by spin-orbit-coupling induced
parametric resonance in a spinor Bose-Einstein condensate with repulsive
interaction. The collective elementary excitations of the Bose-Einstein
condensate, including density waves and spin waves, are coupled as the result
of the Raman-induced spin-orbit coupling and a quench of the relative phase of
two Raman lasers without the modulation of any of the system's parameters. We
observed several higher parametric resonance tongues at integer multiples of
the driving frequency and investigated the interplay between Faraday
instabilities and modulation instabilities when we quench the
spin-orbit-coupled Bose-Einstein condensate from zero-momentum phase to
plane-wave phase. If the detuning is equal to zero, the wave number of
combination resonance barely changes as the strength of spin-orbit coupling
increases. If the detuning is not equal to zero after a quench, a single
combination resonance tongue will split into two parts. | cond-mat_quant-gas |
Entanglement prethermalization: Locally thermal but non-locally
non-thermal states in a one-dimensional Bose gas: A well-isolated system often shows relaxation to a quasi-stationary state
before reaching thermal equilibrium. Such a prethermalization has attracted
considerable interest recently in association with closely related fundamental
problems of relaxation and thermalization of isolated quantum systems.
Motivated by the recent experiment in ultracold atoms, we study the dynamics of
a one-dimensional Bose gas which is split into two subsystems, and find that
individual subsystems relax to Gibbs states, yet the entire system does not due
to quantum entanglement. In view of recent experimental realization on a small
well-defined number of ultracold atoms, our prediction based on exact few-body
calculations is amenable to experimental test. | cond-mat_quant-gas |
Persistent oscillations of the order parameter and interaction quench
phase diagram for a confined Bardeen-Cooper-Schrieffer Fermi gas: We present a numerical study of the interaction quench dynamics in a
superfluid ultracold Fermi gas confined in a three-dimensional cigar-shaped
harmonic trap. In the present paper we investigate the amplitude mode of the
superfluid order parameter after interaction quenches which start deep in the
BCS phase and end in the BCS-BEC crossover regime. To this end, we exploit the
Bogoliubov-de Gennes formalism which takes the confinement potential explicitly
into account and provides a microscopic fully coherent description of the
system. We find an anharmonic nonlinear oscillation of the modulus of the
superfluid order parameter, i.e., of the Higgs mode. This oscillation persists
for large times with only a small amplitude modulation being visible. We
connect the frequency and the mean value of this oscillation with the breaking
of Cooper pairs in the superfluid phase. Additionally, we demonstrate that the
occurrence of this persistent oscillation is connected to the onset of chaotic
dynamics in our model. Finally, we calculate an interaction quench phase
diagram of the Higgs mode for quenches on the BCS side of the BCS-BEC crossover
and discuss its properties as a function of the aspect ratio of the
cigar-shaped trap. | cond-mat_quant-gas |
Stability of Excited Dressed States with Spin-Orbit Coupling: We study the decay behaviors of ultracold atoms in metastable states with
spin-orbit coupling (SOC), and demonstrate that there are two SOC-induced decay
mechanisms. One arises from the trapping potential and the other is due to
interatomic collision. We present general schemes for calculating decay rates
from these two mechanisms, and illustrate how the decay rates can be controlled
by experimental parameters.We experimentally measure the decay rates over a
broad parameter region, and the results agree well with theoretical
calculations. This work provides an insight for both quantum simulation
involving metastable dressed states and studies on few-body problems with SO
coupling. | cond-mat_quant-gas |
Quantum theory of bright matter wave solitons in harmonic confinement: This paper investigates bright quantum-matter-wave solitons beyond the
Gross-Pitaevskii equation (GPE). As proposals for interferometry and creating
nonlocal quantum superpositions have been formed, it has become necessary to
investigate effects not present in mean-field models. We investigate the effect
of harmonic confinement on the internal degrees of freedom, as the ratio of
zero-point harmonic oscillator length to classical soliton length, for
different numbers of atoms. We derive a first-order energy correction for the
addition of a harmonic potential to the many-body wave function and use this to
create a variational technique based on energy minimization of this wave
function for an arbitrary number of atoms, and include numerics based on
diagonalization of the Hamiltonian in a basis of harmonic oscillator Fock
states. Finally we compare agreement between a Hartree product ground state and
the Bethe ansatz solution with a Gaussian envelope localizing the center of
mass and show a region of good agreement. | cond-mat_quant-gas |
Exact solutions to the four Goldstone modes around a dark soliton of the
nonlinear Schroedinger equation: This article is concerned with the linearisation around a dark soliton
solution of the nonlinear Schr\"odinger equation. Crucially, we present
analytic expressions for the four linearly-independent zero eigenvalue
solutions (also known as Goldstone modes) to the linearised problem. These
solutions are then used to construct a Greens matrix which gives the
first-order spatial response due to some perturbation. Finally we apply this
Greens matrix to find the correction to the dark-soliton wavefunction of a
Bose-Einstein condensate in the presence of fluctuations. | cond-mat_quant-gas |
Interaction induced dynamical $\mathcal{PT}$ symmetry breaking in
dissipative Fermi-Hubbard models: We investigate the dynamical properties of one-dimensional dissipative
Fermi-Hubbard models, which are described by the Lindblad master equations with
site-dependent jump operators. The corresponding non-Hermitian effective
Hamiltonians with pure loss terms possess parity-time ($\mathcal{PT}$) symmetry
if we compensate the system additionally an overall gain term. By solving the
two-site Lindblad equation with fixed dissipation exactly, we find that the
dynamics of rescaled density matrix shows an instability as the interaction
increases over a threshold, which can be equivalently described in the scheme
of non-Hermitian effective Hamiltonians. This instability is also observed in
multi-site systems and closely related to the $\mathcal{PT}$ symmetry breaking
accompanied by appearance of complex eigenvalues of the effective Hamiltonian.
Moreover, we unveil that the dynamical instability of the anti-ferromagnetic
Mott phase comes from the $\mathcal{PT}$ symmetry breaking in highly excited
bands, although the low-energy effective model of the non-Hermitian Hubbard
model in the strongly interacting regime is always Hermitian. We also provide a
quantitative estimation of the time for the observation of dynamical
$\mathcal{PT}$ symmetry breaking which could be probed in experiments. | cond-mat_quant-gas |
Coherent Interaction of a Single Fermion with a Small Bosonic Field: We have experimentally studied few-body impurity systems consisting of a
single fermionic atom and a small bosonic field on the sites of an optical
lattice. Quantum phase revival spectroscopy has allowed us to accurately
measure the absolute strength of Bose-Fermi interactions as a function of the
interspecies scattering length. Furthermore, we observe the modification of
Bose-Bose interactions that is induced by the interacting fermion. Because of
an interference between Bose-Bose and Bose-Fermi phase dynamics, we can infer
the mean fermionic filling of the mixture and quantify its increase (decrease)
when the lattice is loaded with attractive (repulsive) interspecies
interactions. | cond-mat_quant-gas |
Asymmetric Particle Transport and Light-Cone Dynamics Induced by Anyonic
Statistics: We study the non-equilibrium dynamics of Abelian anyons in a one-dimensional
system. We find that the interplay of anyonic statistics and interactions gives
rise to spatially asymmetric particle transport together with a novel dynamical
symmetry that depends on the anyonic statistical angle and the sign of
interactions. Moreover, we show that anyonic statistics induces asymmetric
spreading of quantum information, characterized by asymmetric light cones of
out-of-time-ordered correlators. Such asymmetric dynamics is in sharp contrast
with the dynamics of conventional fermions or bosons, where both the transport
and information dynamics are spatially symmetric. We further discuss
experiments with cold atoms where the predicted phenomena can be observed using
state-of-the-art technologies. Our results pave the way toward experimentally
probing anyonic statistics through non-equilibrium dynamics. | cond-mat_quant-gas |
Spin and mass currents near a moving magnetic obstacle in a
two-component Bose-Einstein condensate: We study the spatial distributions of the spin and mass currents generated by
a moving Gaussian magnetic obstacle in a symmetric, two-component Bose-Einstein
condensate in two dimensions. We analytically describe the current
distributions for a slow obstacle and show that the spin and the mass currents
exhibit characteristic spatial structures resembling those of electromagnetic
fields around dipole moments. When the obstacle's velocity increases, we
numerically observe that the flow pattern maintains its overall structure while
the spin polarization induced by the obstacle is enhanced with an increased
spin current. We investigate the critical velocity of the magnetic obstacle
based on the local criterion of Landau energetic instability and find that it
decreases almost linearly as the magnitude of the obstacle's potential
increases, which can be directly tested in current experiments. | cond-mat_quant-gas |
Many-body approach to low-lying collective excitations in a BEC
approaching collapse: An approximate many-body theory incorporating two-body correlations has been
employed to calculate low-lying collective multipole frequencies in a
Bose-Einstein condensate containing $A$ bosons, for different values of the
interaction parameter $\lambda=\frac{Aa_{s}}{a_{ho}}$. Significant difference
from the variational estimate of the Gross-Pitaevskii equation has been found
near the collapse region. This is attributed to two-body correlations and
finite range attraction of the realistic interatomic interaction. A large
deviation from the hydrodynamic model is also seen for the second monopole
breathing mode and the quadrupole mode for large positive $\lambda$. | cond-mat_quant-gas |
Optical lattice for tripod-like atomic level structure: Standard optical potentials use off-resonant laser standing wave induced
AC-Stark shift. In a recent development [Phys. Rev. Lett. {\bf 117}, 233001
(2016)] a three-level scheme in $\Lambda$ configuration coupled coherently by
resonant laser fields was introduced leading to an effective lattice with
subwavelength potential peaks. Here as an extension of that work to a four
level atomic setup in the tripod configuration is used to create spin
$1/2$-like two-dimensional dark-space with 1D motion and the presence of
external gauge fields. Most interestingly for a possible application, the
lifetime for a dark subspace motion is up to two orders of magnitude larger
than for a similar $\Lambda$ system. The model is quite flexible leading to
lattices with significant nearest, next-nearest, or next-next-nearest hopping
rates, $J_1,J_2,J_3$ opening up new intriguing possibilities to study, e.g.
frustrated systems. The characteristic Wannier functions lead also to new type
of inter-site interactions not realizable in typical optical lattices. | cond-mat_quant-gas |
Quantum-granularity effect in the formation of supermixed solitons in
ring lattices: We investigate a notable class of states peculiar to a bosonic binary mixture
featuring repulsive intraspecies and attractive interspecies couplings. We
evidence that, for small values of the hopping amplitudes, one can access
particular regimes marked by the fact that the interwell boson transfer occurs
in a jerky fashion. This property is shown to be responsible for the emergence
of a staircase-like structure in the phase diagram of a mixture confined in a
ring trimer and to strongly resemble the mechanism of the superfluid-Mott
insulator transition. Under certain conditions, in fact, we show that it is
possible to interpret the interspecies attraction as an effective chemical
potential and the supermixed soliton as an effective particle reservoir. Our
investigation is developed both within a fully quantum approach based on the
analysis of several quantum indicators and by means of a simple analytical
approximation scheme capable of capturing the essential features of this
ultraquantum effect. | cond-mat_quant-gas |
Quantum-torque-induced breaking of magnetic interfaces in ultracold
gases: A rich variety of physical effects in spin dynamics arises at the interface
between different magnetic materials. Engineered systems with interlaced
magnetic structures have been used to implement spin transistors, memories and
other spintronic devices. However, experiments in solid state systems can be
difficult to interpret because of disorder and losses. Here, we realize
analogues of magnetic junctions using a coherently-coupled mixture of ultracold
bosonic gases. The spatial inhomogeneity of the atomic gas makes the system
change its behavior from regions with oscillating magnetization -- resembling a
magnetic material in the presence of an external transverse field -- to regions
with a defined magnetization, as in magnetic materials with a ferromagnetic
anisotropy stronger than external fields. Starting from a far-from-equilibrium
fully polarized state, magnetic interfaces rapidly form. At the interfaces, we
observe the formation of short-wavelength magnetic waves. They are generated by
a quantum torque contribution to the spin current and produce strong spatial
anticorrelations in the magnetization. Our results establish ultracold gases as
a platform for the study of far-from-equilibrium spin dynamics in regimes that
are not easily accessible in solid-state systems. | cond-mat_quant-gas |
Tunable Wigner States with Dipolar Atoms and Molecules: We study the few-body physics of trapped atoms or molecules with electric or
magnetic dipole moments aligned by an external field. Using exact numerical
diagonalization appropriate for the strongly correlated regime, as well as a
classical analysis, we show how Wigner localization emerges with increasing
coupling strength. The Wigner states exhibit non-trivial geometries due to the
anisotropy of the interaction. This leads to transitions between different
Wigner states as the tilt angle of the dipoles with the confining plane is
changed. Intriguingly, while the individual Wigner states are well described by
a classical analysis, the transitions between different Wigner states are
strongly affected by quantum statistics. This can be understood by considering
the interplay between quantum-mechanical and spatial symmetry properties.
Finally, we demonstrate that our results are relevant to experimentally
realistic systems. | cond-mat_quant-gas |
Dissipation-induced dynamical phase transition in postselected quantum
trajectories: It is known that effects of dissipation or measurement backreaction in
postselected quantum trajectories are described by non-Hermitian Hamiltonian,
but their consequences in real-time dynamics of many-body systems are yet to be
elucidated. Through a study of a non-Hermitian Hubbard model, we reveal a novel
dissipation-induced dynamical phase transition in postselected quantum
trajectories, where time controls the strength of postselection and becomes the
intrinsic parameter inducing the phase transition. Our findings are testable in
ultracold atom experiments and may open a new avenue in the dissipative
engineering of quantum systems. | cond-mat_quant-gas |
Feynman path-integral treatment of the Bose polaron beyond the
Fröhlich model: An impurity immersed in a Bose-Einstein condensate is no longer accurately
described by the Fr\"ohlich Hamiltonian as the coupling between the impurity
and the boson bath gets stronger. We study the dominant effects of the
two-phonon terms beyond the Fr\"ohlich model on the ground-state properties of
the polaron using Feynman's variational path-integral approach. The previously
reported discrepancy in the effective mass between the renormalization group
approach and this theory is shown to be absent in the beyond-Fr\"ohlich model
on the positive side of the Feshbach resonance. Self-trapping, characterized by
a sharp and dramatic increase of the effective mass, is no longer observed for
the repulsive polaron once the two-phonon interactions are included. For the
attractive polaron we find a divergence of the ground-state energy and
effective mass at weaker couplings than previously observed within the
Fr\"ohlich model. | cond-mat_quant-gas |
Measuring molecular electric dipoles using trapped atomic ions and
ultrafast laser pulses: We study a hybrid quantum system composed of an ion and an electric dipole.
We show how a trapped ion can be used to measure the small electric field
generated by a classical dipole. We discuss the application of this scheme to
measure the electric dipole moment of cold polar molecules, whose internal
state can be controlled with ultrafast laser pulses, by trapping them in the
vicinity of a trapped ion. | cond-mat_quant-gas |
Finite-size effects on the Bose-Einstein condensation critical
temperature in a harmonic trap: We obtain second and higher order corrections to the shift of the
Bose-Einstein critical temperature due to finite-size effects. The confinement
is that of a harmonic trap with general anisotropy. Numerical work shows the
high accuracy of our expressions. We draw attention to a subtlety involved in
the consideration of experimental values of the critical temperature in
connection with analytical expressions for the finite-size corrections. | cond-mat_quant-gas |
Controlling particle current in a many-body quantum system by external
driving: We propose a method to control the particle current of a one-dimensional
quantum system by resonating two many-body states through an external driving
field. We consider the Bose-Hubbard and spinless Fermi-Hubbard models with the
Peierls phase which induces net particle currents in the many-body eigenstates.
A driving field couples the ground state with one of the excited states having
large net currents, enabling us to control the system's current via Rabi
oscillation. Employing the Floquet analysis, we find that the resonate excited
states are determined by the symmetry of the driving field, which allows us to
selectively excite only certain states among the dense spectrum of a many-body
quantum system. | cond-mat_quant-gas |
Few-to-many vortex states of density-angular-momentum coupled
Bose-Einstein condensates: Motivated by recent experiments, we theoretically study a gas of atomic
bosons confined in an elliptical harmonic trap; forming a quasi-two-dimensional
atomic Bose-Einstein condensate subject to a density-dependent gauge potential
which realises an effective density-angular-momentum coupling. We present exact
Thomas-Fermi solutions which allows us to identify the stable regimes of the
full parameter space of the model. Accompanying numerical simulations reveal
the effect of the interplay of the rigid body and density-angular-momentum
coupling for the elliptically confined condensate. By varying the strength of
the gauge potential and trap anisotropy we explore how the superfluid state
emerges in different experimentally accessible geometries, while for large
rotation strengths dense vortex lattices and concentric vortex ring
arrangements are obtained. | cond-mat_quant-gas |
Orbital superfluidity in the $P$-band of a bipartite optical square
lattice: The successful emulation of the Hubbard model in optical lattices has
stimulated world wide efforts to extend their scope to also capture more
complex, incompletely understood scenarios of many-body physics. Unfortunately,
for bosons, Feynmans fundamental "no-node" theorem under very general
circumstances predicts a positive definite ground state wave function with
limited relevance for many-body systems of interest. A promising way around
Feynmans statement is to consider higher bands in optical lattices with more
than one dimension, where the orbital degree of freedom with its intrinsic
anisotropy due to multiple orbital orientations gives rise to a structural
diversity, highly relevant, for example, in the area of strongly correlated
electronic matter. In homogeneous two-dimensional optical lattices, lifetimes
of excited bands on the order of a hundred milliseconds are possible but the
tunneling dynamics appears not to support cross-dimensional coherence. Here we
report the first observation of a superfluid in the $P$-band of a bipartite
optical square lattice with $S$-orbits and $P$-orbits arranged in a
chequerboard pattern. This permits us to establish full cross-dimensional
coherence with a life-time of several ten milliseconds. Depending on a small
adjustable anisotropy of the lattice, we can realize real-valued striped
superfluid order parameters with different orientations $P_x \pm P_y$ or a
complex-valued $P_x \pm i P_y$ order parameter, which breaks time reversal
symmetry and resembles the $\pi$-flux model proposed in the context of high
temperature superconductors. Our experiment opens up the realms of orbital
superfluids to investigations with optical lattice models. | cond-mat_quant-gas |
Buckling transitions and clock order of two-dimensional Coulomb crystals: Crystals of repulsively interacting ions in planar traps form hexagonal
lattices, which undergo a buckling instability towards a multi-layer structure
as the transverse trap frequency is reduced. Numerical and experimental results
indicate that the new structure is composed of three planes, whose separation
increases continuously from zero. We study the effects of thermal and quantum
fluctuations by mapping this structural instability to the six-state clock
model. A prominent implication of this mapping is that at finite temperature,
fluctuations split the buckling instability into two thermal transitions,
accompanied by the appearance of an intermediate critical phase. This phase is
characterized by quasi-long-range order in the spatial tripartite pattern. It
is manifested by broadened Bragg peaks at new wave vectors, whose line-shape
provides a direct measurement of the temperature dependent exponent $\eta(T)$
characteristic of the power-law correlations in the critical phase. A quantum
phase transition is found at the largest value of the critical transverse
frequency: here the critical intermediate phase shrinks to zero. Moreover,
within the ordered phase, we predict a crossover from classical to quantum
behavior, signifying the emergence of an additional characteristic scale for
clock order. We discuss experimental realizations with trapped ions and
polarized dipolar gases, and propose that within accessible technology, such
experiments can provide a direct probe of the rich phase diagram of the quantum
clock model, not easily observable in condensed matter analogues. Therefore,
this works highlights the potential for ionic and dipolar systems to serve as
simulators for complex models in statistical mechanics and condensed matter
physics. | cond-mat_quant-gas |
Spin-orbit coupling in the presence of strong atomic correlations: We explore the influence of contact interactions on a synthetically
spin-orbit coupled system of two ultracold trapped atoms. Even though the
system we consider is bosonic, we show that a regime exists in which the
competition between the contact and spin-orbit interactions results in the
emergence of a ground state that contains a significant contribution from the
anti-symmetric spin state. This ground state is unique to few-particle systems
and does not exist in the mean-field regime. The transition to this state is
signalled by an inversion in the average momentum from being dominated by
centre-of-mass momentum to relative momentum and also affects the global
entanglement shared between the real- and pseudo-spin spaces. Indeed,
competition between the interactions can also result in avoided crossings in
the groundstate which further enhances these correlations. However, we find
that correlations shared between the pseudo-spin states are strongly depressed
due to the spin-orbit coupling and therefore the system does not contain
spin-spin entanglement. | cond-mat_quant-gas |
Generalized Hydrodynamics in the 1D Bose gas: theory and experiments: We review the recent theoretical and experimental progress regarding the
Generalized Hydrodynamics (GHD) behavior of the one-dimensional Bose gas with
contact repulsive interactions, also known as the Lieb-Liniger gas. In the
first section, we review the theory of the Lieb-Liniger gas, introducing the
key notions of the rapidities and of the rapidity distribution. The latter
characterizes the Lieb-Liniger gas after relaxation and is at the heart of GHD.
We also present the asymptotic regimes of the Lieb-Liniger gas with their
dedicated approximate descriptions. In the second section we enter the core of
the subject and review the theoretical results on GHD in 1D Bose gases. The
third and fourth sections are dedicated to experimental results obtained in
cold atoms experiments: the experimental realization of the Lieb-Liniger model
is presented in section 3, with a selection of key results for systems at
equilibrium, and section 4 presents the experimental tests of the GHD theory.
In section 5 we review the effects of atom losses, which, assuming slow loss
processes, can be described within the GHD framework. We conclude with a few
open questions. | cond-mat_quant-gas |
Multiorder topological superfluid phase transitions in a two-dimensional
optical superlattice: Higher-order topological superfluids have gapped bulk and symmetry-protected
Majorana zero modes with various localizations. Motivated by recent advances,
we present a proposal for synthesizing multi-order topological superfluids that
support various Majorana zero modes in ultracold atomic gases. For this
purpose, we use the two-dimensional optical superlattice that introduces a
spatial modulation to the spin-orbit coupling in one direction, providing an
extra degree of freedom for the emergent higher-order topological state. We
find the topologically trivial superfluids, first-order and second-order
topological superfluids, as well as different topological phase transitions
among them with respect to the experimentally tunable parameters. Besides the
conventional transition characterized by the Chern number associated with the
bulk gap closing and reopening, we find the system can support the topological
superfluids with Majorana corner modes, but the topological phase transition
undergoes no gap-closing of bulk bands. Instead, the transition is refined by
the quadrupole moment and signaled out by the gap-closing of edge states. The
proposal is based on the $s$-wave interaction and is valid using existing
experimental techniques, which unifies multi-order topological phase
transitions in a simple but realistic system. | cond-mat_quant-gas |
Dipolar fermions in a multilayer geometry: We investigate the behavior of identical dipolar fermions with aligned dipole
moments in two-dimensional multilayers at zero temperature. We consider density
instabilities that are driven by the attractive part of the dipolar interaction
and, for the case of bilayers, we elucidate the properties of the stripe phase
recently predicted to exist in this interaction regime. When the number of
layers is increased, we find that this "attractive" stripe phase exists for an
increasingly larger range of dipole angles, and if the interlayer distance is
sufficiently small, the stripe phase eventually spans the full range of angles,
including the situation where the dipole moments are aligned perpendicular to
the planes. In the limit of an infinite number of layers, we derive an analytic
expression for the interlayer effects in the density-density response function
and, using this result, we find that the stripe phase is replaced by a collapse
of the dipolar system. | cond-mat_quant-gas |
Flowing bosonization in the nonperturbative functional
renormalization-group approach: Bosonization allows one to describe the low-energy physics of one-dimensional
quantum fluids within a bosonic effective field theory formulated in terms of
two fields: the "density" field $\varphi$ and its conjugate partner, the phase
$\vartheta$ of the superfluid order parameter. We discuss the implementation of
the nonperturbative functional renormalization group in this formalism,
considering a Luttinger liquid in a periodic potential as an example. We show
that in order for $\vartheta$ and $\varphi$ to remain conjugate variables at
all energy scales, one must dynamically redefine the field $\vartheta$ along
the renormalization-group flow. We derive explicit flow equations using a
derivative expansion of the scale-dependent effective action to second order
and show that they reproduce the flow equations of the sine-Gordon model
(obtained by integrating out the field $\vartheta$ from the outset) derived
within the same approximation. Only with the scale-dependent (flowing)
reparametrization of the phase field $\vartheta$ do we obtain the standard
phenomenology of the Luttinger liquid (when the periodic potential is
sufficiently weak so as to avoid the Mott-insulating phase) characterized by
two low-energy parameters, the velocity of the sound mode and the renormalized
Luttinger parameter. | cond-mat_quant-gas |
Emergent patterns in a spin-orbit coupled spin-2 Bose-Einstein
condensate: The ground-state phases of a spin-orbit (SO) coupled atomic spin-2
Bose-Einstein condensate (BEC) are studied. Interesting density patterns
spontaneously formed are widespread due to the competition between SO coupling
and spin-dependent interactions like in a SO coupled spin-1 condensate. Unlike
the case of spin-1 condensates, which are characterized by either ferromagnetic
or polar phase in the absence of SO, spin-2 condensates can take a cyclic
phase, where we find the patterns formed due to SO are square or triangular in
their spin component densities for axial symmetric SO interaction. Both
patterns are found to continuously evolve into striped forms with increased
asymmetry of the SO coupling. | cond-mat_quant-gas |
Coherent phase slips in coupled matter-wave circuits: Quantum Phase slips are dual process of particle tunneling in coherent
networks. Besides to be of central interest for condensed matter physics,
quantum phase slips are resources that are sought to be manipulated in quantum
circuits. Here, we devise a specific matter-wave circuit enlightening quantum
phase slips. Specifically, we investigate the quantum many body dynamics of two
side-by-side ring-shaped neutral bosonic systems coupled through a weak link.
By imparting a suitable magnetic flux, persistent currents flow in each ring
with given winding numbers. We demonstrate that coherent phase slips occur as
winding number transfer among the two rings, with the populations in each ring
remaining nearly constant. Such a phenomenon occurs as a result of a specific
entanglement of circulating states, that, as such cannot be captured by a mean
field treatment of the system. Our work can be relevant for the observation of
quantum phase slips in cold atoms experiments and their manipulation in
matter-wave circuits. To make contact with the field, we show that the
phenomenon has clear signatures in the momentum distribution of the system
providing the time of flight image of the condensate. | cond-mat_quant-gas |
BCS-BEC crossover at finite temperature in spin-orbit coupled Fermi
gases: By adopting a $T$-matrix-based method within the $G_0G$ approximation for the
pair susceptibility, we study the effects of the pairing fluctuation on the
three-dimensional spin-orbit coupled Fermi gases at finite temperature. The
critical temperatures of the superfluid/normal phase transition are determined
for three different types of spin-orbit coupling (SOC): (1) the extreme oblate
(EO) or Rashba SOC, (2) the extreme prolate (EP) or equal Rashba-Dresselhaus
SOC, and (3) the spherical (S) SOC. For EO- and S-type SOC, the SOC dependence
of the critical temperature signals a crossover from BCS to BEC state; at
strong SOC limit, the critical temperature recover those of ideal BEC of
rashbons. The pairing fluctuation induces a pseudogap in the fermionic
excitation spectrum in both superfluid and normal phases. We find that, for EO-
and S-type SOC, even at weak coupling, sufficiently strong SOC can induce
sizable pseudogap. Our research suggests that the spin-orbit coupled Fermi
gases may open new means to the study of the pseudogap formation in fermionic
systems. | cond-mat_quant-gas |
Superdiffusive nonequilibrium motion of an impurity in a Fermi sea: We treat the nonequilibrium motion of a single impurity atom in a
low-temperature single-species Fermi sea, interacting via a contact
interaction. In the nonequilibrium regime, the impurity does a superdiffusive
geometric random walk where the typical distance traveled grows with time as
$\sim t^{d/(d+1)}$ for the $d$-dimensional system with $d\geq 2$. For nonzero
temperature $T$, this crosses over to diffusive motion at long times with
diffusivity $D\sim T^{-(d-1)/2}$. These results apply also to a nonzero
concentration of impurity atoms as long as they remain dilute and
nondegenerate. | cond-mat_quant-gas |
Creating quantum many-body scars through topological pumping of a 1D
dipolar gas: Quantum many-body scars, long-lived excited states of correlated quantum
chaotic systems that evade thermalization, are of great fundamental and
technological interest. We create novel scar states in a bosonic 1D quantum gas
of dysprosium by stabilizing a super-Tonks-Girardeau gas against collapse and
thermalization with repulsive long-range dipolar interactions. Stiffness and
energy density measurements show that the system is dynamically stable
regardless of contact interaction strength. This enables us to cycle contact
interactions from weakly to strongly repulsive, then strongly attractive, and
finally weakly attractive. We show that this cycle is an energy-space
topological pump (due to a quantum holonomy). Iterating this cycle offers an
unexplored topological pumping method to create a hierarchy of quantum
many-body scar states. | cond-mat_quant-gas |
Multi-particle composites in density-imbalanced quantum fluids: We consider two-component one-dimensional quantum gases with density
imbalance. While generically such fluids are two-component Luttinger liquids,
we show that if the ratio of the densities is a rational number, p/q, and mass
asymmetry between components is sufficiently strong, one of the two eigenmodes
acquires a gap. The gapped phase corresponds to (algebraic) ordering of
(p+q)-particle composites. In particular, for attractive mixtures, this implies
that the superconducting correlations are destroyed. We illustrate our
predictions by numerical simulations of the fermionic Hubbard model with
hopping asymmetry. | cond-mat_quant-gas |
Flux enhanced localization and reentrant delocalization in the quantum
walk of interacting bosons on two-leg ladder: We study the quantum walk of two bosons possessing onsite repulsive
interaction on a two-leg ladder and show that the presence of uniform flux
piercing through the plaquettes of the ladder favors the localization of the
bound states in the dynamics. We find that when the two bosons are
symmetrically initialized on the edge rung of the ladder, they tend to
edge-localize in their quantum walk - a phenomenon which is not possible in the
absence of flux. On the other hand, when the bosons are initialized on the bulk
rung they never localize and exhibit linear spreading in their quantum walk.
Interestingly, however, we find that in the later case a finite flux favours
localization of the bulk bound states in the presence of sufficiently weak
quasiperiodic disorder which is otherwise insufficient to localize the
particles in the absence of flux. In both the cases, we obtain that the
localization in the dynamics strongly depends on the combined effect of the
flux and interaction strengths, as a result which we obtain a signature of
re-entrant delocalization as a function of flux (interaction) for fixed
interaction (flux) strengths. | cond-mat_quant-gas |
Dynamical Equilibration of Topological Properties: We study the dynamical process of equilibration of topological properties in
quantum many-body systems undergoing a parameter quench between two
topologically inequivalent Hamiltonians. This scenario is motivated by recent
experiments on ultracold atomic gases, where a trivial initial state is
prepared before the Hamiltonian is ramped into a topological insulator phase.
While the many-body wave function must stay topologically trivial in the
coherent post-quench dynamics, here we show how the topological properties of
the single particle density matrix dynamically change and equilibrate in the
presence of interactions. In this process, the single particle density matrix
goes through a characteristic level crossing as a function of time, which plays
an analogous role to the gap closing of a Hamiltonian in an equilibrium
topological quantum phase transition. As an exact case study exemplifying this
mechanism, we numerically solve the quench dynamics of an interacting
one-dimensional topological insulator. | cond-mat_quant-gas |
Controlling Dipolar Exchange Interactions in a Dense 3D Array of Large
Spin Fermions: Dipolar interactions are ubiquitous in nature and rule the behavior of a
broad range of systems spanning from energy transfer in biological systems to
quantum magnetism. Here, we study magnetization-conserving dipolar induced
spin-exchange dynamics in dense arrays of fermionic erbium atoms confined in a
deep three-dimensional lattice. Harnessing the special atomic properties of
erbium, we demonstrate control over the spin dynamics by tuning the dipole
orientation and changing the initial spin state within the large 20 spin
hyperfine manifold. Furthermore, we demonstrate the capability to quickly turn
on and off the dipolar exchange dynamics via optical control. The experimental
observations are in excellent quantitative agreement with numerical
calculations based on discrete phase-space methods, which capture entanglement
and beyond-mean field effects. Our experiment sets the stage for future
explorations of rich magnetic behaviors in long-range interacting dipoles,
including exotic phases of matter and applications for quantum information
processing. | cond-mat_quant-gas |
How is the density of quasi-two-dimensional uniform dipolar quantum Bose
gases affected by trap imperfections?: We theoretically investigate the impact of weak perturbations of a flat
potential on the density of a quasi-two-dimensional dipolar Bose gas. We use a
mean-field perturbative treatment of the potential defects and derive their
effects at first order in the mean-field stable regime. We first focus on
defects containing a single spatial frequency and study the wavevector
dependence of the density perturbation. A qualitative modification of the
wavenumber dependence with the interaction parameters and a sensitivity in the
excitation direction reveal the long-range and anisotropic dipolar effects.
These effects are found to be most important at intermediate wavenumbers and
can give rise to a local maximum in the density perturbation reminiscent of the
roton mode softening and local instabilities. The dependence on the gas and
interaction parameters is studied. The case of a flat potential perturbed with
white noise on a certain momentum range is then examined. Here it is found that
the strength perturbation becomes independent of the mean density when
sufficiently large. Our study touches upon experimentally relevant issues,
giving hints on how flat a uniform potential should be to achieve uniform
quasi-two-dimensional dipolar Bose gases. | cond-mat_quant-gas |
Spin conductivity spectrum and spin superfluidity in a binary Bose
mixture: We investigate the spectrum of spin conductivity for a miscible two-component
Bose-Einstein condensate (BEC) that exhibits spin superfluidity. By using the
Bogoliubov theory, the regular part being the spin conductivity at finite ac
frequency and the spin Drude weight characterizing the delta-function peak at
zero frequency are analytically computed. We demonstrate that the spectrum
exhibits a power-law behavior at low frequency, reflecting gapless density and
spin modes specific to the binary BEC. At the phase transition points into
immiscible and quantum-droplet states, the change in quasiparticle dispersion
relations modifies the power law. In addition, the spin Drude weight becomes
finite, indicating zero spin resistivity due to spin superfluidity. Our results
also suggest that the Andreev-Bashkin drag density is accessible by measuring
the spin conductivity spectrum. | cond-mat_quant-gas |
Singular mean-field states: A brief review of recent results: This article provides a focused review of recent findings which demonstrate,
in some cases quite counter-intuitively, the existence of bound states with a
singularity of the density pattern at the center, while the states are
physically meaningful because their total norm converges. One model of this
type is based on the 2D Gross-Pitaevskii equation (GPE) which combines the
attractive potential ~ 1/r^2 and the quartic self-repulsive nonlinearity,
induced by the Lee-Huang-Yang effect (quantum fluctuations around the
mean-field state). The GPE demonstrates suppression of the 2D quantum collapse,
driven by the attractive potential, and emergence of a stable ground state
(GS), whose density features an integrable singularity ~1/r^{4/3} at r --> 0.
Modes with embedded angular momentum exist too, and they have their stability
regions. A counter-intuitive peculiarity of the model is that the GS exists
even if the sign of the potential is reversed from attraction to repulsion,
provided that its strength is small enough. This peculiarity finds a relevant
explanation. The other model outlined in the review includes 1D, 2D, and 3D
GPEs, with the septimal (seventh-order), quintic, and cubic self-repulsive
terms, respectively. These equations give rise to stable singular solitons,
which represent the GS for each dimension D, with the density singularity
~1/r^{2/(4-D). Such states may be considered as a result of screening of a
"bare" delta-functional attractive potential by the respective nonlinearity. | cond-mat_quant-gas |
Dynamics of spatial coherence and momentum distribution of polaritons in
a semiconductor microcavity under conditions of Bose-Einstein condensation: The dynamics of spatial coherence and momentum distribution of polaritons in
the regime of Bose-Einstein condensation are investigated in a GaAs microcavity
with embedded quantum wells under nonresonant excitation with picosecond laser
pulses. It is shown that the onset of the condensate first order sparial
coherence is accompanied by narrowing of the polariton momentum distribution.
At the same time, at sufficiently high excitation densities, there is
significant qualitative discrepancy between the dynamic behavior of the width
of the polariton momentum distribution determined from direct measurements and
that calculated from the coherence spatial distribution. This discrepancy is
observed at the fast initial stage of the polariton system kinetics and,
apparently, results from the strong spatial nonuniformity of the phase of the
condensate wave function, which equilibrates on a much longer time scale. | cond-mat_quant-gas |
Quantum information theoretic measures to distinguish fermionized bosons
from non-interacting fermions: We study the dynamical fermionization of strongly interacting one-dimensional
bosons in Tonks-Girardeau limit by solving the time dependent many-boson
Schr\"odinger equation numerically exactly. We establish that the one-body
momentum distribution approaches the ideal Fermi gas distribution at the time
of dynamical fermionization. The analysis is further complemented by the
measures on two-body level. Investigation on two-body momentum distribution,
two-body local and non-local correlation clearly distinguish the fermionized
bosons from non-interacting fermions. The magnitude of distinguishablity
between the two systems is further discussed employing suitable measures of
information theory, i.e., the well known Kullback-Leibler relative entropy and
the Jensen-Shannon divergence entropy. We also observe very rich structure in
the higher-body density for strongly correlated bosons whereas non-interacting
fermions do not possess any higher order correlation beyond two-body. | cond-mat_quant-gas |
Dynamics of exciton-polaritons in a Josephson double dimer: We study the dynamics of exciton-polaritons in a double-well configuration.
The system consists of two weakly coupled Bose-Josephson junctions, each
corresponding to a different circular polarization of the polaritons, forming a
{\it Josephson double dimer}. We show that the Josephson oscillation between
the wells is strongly coupled to the polarization rotation and that
consequently Josephson excitation is periodically exchanged between the two
polarizations. Linearized analysis agrees well with numerical simulations using
typical experimental parameters. | cond-mat_quant-gas |
Evidence of a liquid phase in interacting Bosons at intermediate
densities: In this paper, we present evidence for a liquid-like phase in systems of many
interacting Bosons at intermediate densities. The interacting Bose gas has been
studied extensively in the low and high density regimes, in which interactions
do not play a physically significant role, and the system behaves similarly to
the ideal quantum gas. Instead, we will turn our attention to the intermediate
density regime, and report evidence that the system enters a strongly
correlated phase where its behavior is markedly different from that of the
ideal quantum gas. To do so, we use the Simplified approach to the Bose gas,
which was introduced by Lieb in 1963 and recently found to provide very
accurate predictions for many-Boson systems at all densities. Using this tool,
we will compute predictions for the radial distribution function, structure
factor, condensate fraction and momentum distribution, and show that they are
consistent with liquid-type behavior. | cond-mat_quant-gas |
Bidirectional dynamic scaling in an isolated Bose gas far from
equilibrium: Understanding and classifying nonequilibrium many-body phenomena, analogous
to the classification of equilibrium states of matter into universality
classes, is an outstanding problem in physics. Any many-body system, from
stellar matter to financial markets, can be out of equilibrium in a myriad of
ways; since many are also difficult to experiment on, it is a major goal to
establish universal principles that apply to different phenomena and physical
systems. At the heart of the classification of equilibrium states is the
universality seen in the self-similar spatial scaling of systems close to phase
transitions. Recent theoretical work, and first experimental evidence, suggest
that isolated many-body systems far from equilibrium generically exhibit
dynamic (spatiotemporal) self-similar scaling, akin to turbulent cascades and
the Family-Vicsek scaling in classical surface growth. Here we observe
bidirectional dynamic scaling in an isolated quench-cooled atomic Bose gas; as
the gas thermalises and undergoes Bose-Einstein condensation, it shows
self-similar net flows of particles towards the infrared (smaller momenta) and
energy towards the ultraviolet (smaller lengthscales). For both infrared (IR)
and ultraviolet (UV) dynamics we find that the scaling exponents are
independent of the strength of the interparticle interactions that drive the
thermalisation. | cond-mat_quant-gas |
Rotational pendulum dynamics of a vortex molecule in a channel geometry: A vortex molecule is a topological excitation in two coherently coupled
superfluids consisting of a vortex in each superfluid connected by a domain
wall of the relative phase, also known as a Josephson vortex. We investigate
the dynamics of this excitation in a quasi-two-dimensional geometry with slab
or channel boundary conditions using an extended point vortex framework
complemented by Gross-Pitaevskii simulations. Apart from translational motion
along the channel, the vortex molecule is found to exhibit intriguing internal
dynamics including rotation and rotational-pendulum-like dynamics. Trajectories
leading to a boundary-induced break-up of the vortex molecule are also
described qualitatively by the simplified model. We classify the stable and
unstable fixed points as well as separatrices that characterize the vortex
molecule dynamics. | cond-mat_quant-gas |
Bose-Einstein Condensation of 84-Sr: We report Bose-Einstein condensation of 84-Sr in an optical dipole trap.
Efficient laser cooling on the narrow intercombination line and an ideal s-wave
scattering length allow creation of large condensates (N0 ~ 3x10^5) even though
the natural abundance of this isotope is only 0.6%. Condensation is heralded by
the emergence of a low-velocity component in time-of-flight images. | cond-mat_quant-gas |
Dynamic Structure Factor of Normal Fermi Gas from Collisionless to
Hydrodynamic Regime: The dynamic structure factor of a normal Fermi gas is investigated by using
the moment method for the Boltzmann equation. We determine the spectral
function at finite temperatures over the full range of crossover from the
collisionless regime to the hydrodynamic regime. We find that the Brillouin
peak in the dynamic structure factor exhibits a smooth crossover from zero to
first sound as functions of temperature and interaction strength. The dynamic
structure factor obtained using the moment method also exhibits a definite
Rayleigh peak ($/omega /sim 0$), which is a characteristic of the hydrodynamic
regime. We compare the dynamic structure factor obtained by the moment method
with that obtained from the hydrodynamic equations. | cond-mat_quant-gas |
Small two-component Fermi gases in a cubic box with periodic boundary
conditions: The properties of two-component Fermi gases become universal if the
interspecies s-wave scattering length $a_s$ and the average interparticle
spacing are much larger than the range of the underlying two-body potential.
Using an explicitly correlated Gaussian basis set expansion approach, we
determine the eigen energies of two-component Fermi gases in a cubic box with
periodic boundary conditions as functions of the interspecies s-wave scattering
length and the effective range of the two-body potential. The universal
properties of systems consisting of up to four particles are determined by
extrapolating the finite-range energies to the zero-range limit. We determine
the eigen energies of states with vanishing and finite momentum. In the
weakly-attractive BCS regime, we analyze the energy spectra and degeneracies
using first-order degenerate perturbation theory. Excellent agreement between
the perturbative energy shifts and the numerically determined energies is
obtained. For the infinitely large scattering length case, we compare our
results - where available - with those presented in the literature. | cond-mat_quant-gas |
Driven-dissipative many-body pairing states for cold fermionic atoms in
an optical lattice: We discuss the preparation of many-body states of cold fermionic atoms in an
optical lattice via controlled dissipative processes induced by coupling the
system to a reservoir. Based on a mechanism combining Pauli blocking and phase
locking between adjacent sites, we construct complete sets of jump operators
describing coupling to a reservoir that leads to dissipative preparation of
pairing states for fermions with various symmetries in the absence of direct
inter-particle interactions. We discuss the uniqueness of these states, and
demonstrate it with small-scale numerical simulations. In the late time
dissipative dynamics, we identify a "dissipative gap" that persists in the
thermodynamic limit. This gap implies exponential convergence of all many-body
observables to their steady state values. We then investigate how these pairing
states can be used as a starting point for the preparation of the ground state
of Fermi-Hubbard Hamiltonian via an adiabatic state preparation process also
involving the parent Hamiltonian of the pairing state. We also provide a
proof-of-principle example for implementing these dissipative processes and the
parent Hamiltonians of the pairing states, based on Yb171 atoms in optical
lattice potentials. | cond-mat_quant-gas |
AtomECS: Simulate laser cooling and magneto-optical traps: AtomECS is a software package that efficiently simulates the motion of
neutral atoms experiencing forces exerted by laser radiation, such as in
magneto-optical traps and Zeeman slowers. The program is implemented using the
Entity-Component-System pattern, which gives excellent performance, flexibility
and scalability to parallel computing resources. The simulation package has
been verified by comparison to analytic results, and extensively unit tested. | cond-mat_quant-gas |
Localization in spin chains with facilitation constraints and disordered
interactions: Quantum many-body systems with kinetic constraints exhibit intriguing
relaxation dynamics. Recent experimental progress in the field of cold atomic
gases offers a handle for probing collective behavior of such systems, in
particular for understanding the interplay between constraints and disorder.
Here we explore a spin chain with facilitation constraints --- a feature which
is often used to model classical glass formers --- together with disorder that
originates from spin-spin interactions. The specific model we study, which is
realized in a natural fashion in Rydberg quantum simulators, maps onto an
XX-chain with non-local disorder. Our study shows that the combination of
constraints and seemingly unconventional disorder may lead to interesting
non-equilibrium behaviour in experimentally relevant setups. | cond-mat_quant-gas |
Grüneisen Parameter for Gases: The Gr\"uneisen ratio ($\Gamma$), i.e.\,the ratio of the linear thermal
expansivity to the specific heat at constant pressure, quantifies the degree of
anharmonicity of the potential governing the physical properties of a system.
While $\Gamma$ has been intensively explored in solid state physics, very
little is known about its behavior for gases. This is most likely due to the
difficulties posed to carry out both thermal expansion and specific heat
measurements in gases with high accuracy as a function of pressure and
temperature. Furthermore, to the best of our knowledge a comprehensive
discussion about the peculiarities of the Gr\"uneisen ratio is still lacking in
the literature. Here we report on a detailed and comprehensive overview of the
Gr\"uneisen ratio. Particular emphasis is placed on the analysis of $\Gamma$
for gases. The main findings of this work are: \emph{i)} for the Van der Waals
gas $\Gamma$ depends only on the co-volume $b$ due to interaction effects, it
is smaller than that for the ideal gas ($\Gamma$ = 2/3) and diverges upon
approaching the critical volume; \emph{ii)} for the Bose-Einstein condensation
of an ideal boson gas, assuming the transition as first-order $\Gamma$ diverges
upon approaching a critical volume, similarly to the Van der Waals gas;
\emph{iii)} for $^4$He at the superfluid transition $\Gamma$ shows a singular
behavior. Our results reveal that $\Gamma$ can be used as an appropriate
experimental tool to explore pressure-induced critical points. | cond-mat_quant-gas |
Signatures of Fractional Exclusion Statistics in the Spectroscopy of
Quantum Hall Droplets: We show how spectroscopic experiments on a small Laughlin droplet of rotating
bosons can directly demonstrate Haldane fractional exclusion statistics of
quasihole excitations. The characteristic signatures appear in the
single-particle excitation spectrum. We show that the transitions are governed
by a "many-body selection rule" which allows one to relate the number of
allowed transitions to the number of quasihole states on a finite geometry. We
illustrate the theory with numerically exact simulations of small numbers of
particles. | cond-mat_quant-gas |
Rapidity distribution within the defocusing non-linear Schrödinger
equation model: We consider the classical field integrable system whose evolution equation is
the nonlinear Schr\"odinger equation with defocusing non-linearities, which is
the classical limit of the quantum Lieb-Liniger model. We propose a simple
derivation of the relation between two sets of conserved quantities: on the one
hand the trace of the monodromy matrix, parameterized by the spectral parameter
and introduced in the inverse-scattering framework, and on the other hand the
rapidity distribution, a concept imported from the Lieb-Liniger model. To do so
we use the definition of the rapidity distribution as the asymptotic momentum
distribution after an expansion. More precisely we use thought experiments
implementing an expansion and we present two different ways to derive our
result, based on different thought experiments which lead to different
calculations. | cond-mat_quant-gas |
Pairing and the spin susceptibility of the polarized unitary Fermi gas
in the normal phase: We theoretically study the pairing behavior of the unitary Fermi gas in the
normal phase. Our analysis is based on the static spin susceptibility, which
characterizes the response to an external magnetic field. We obtain this
quantity by means of the complex Langevin approach and compare our calculations
to available literature data in the spin-balanced case. Furthermore, we present
results for polarized systems, where we complement and expand our analysis at
high temperature with high-order virial expansion results. The implications of
our findings for the phase diagram of the spin-polarized unitary Fermi gas are
discussed, in the context of the state of the art. | cond-mat_quant-gas |
Non-perturbative method to compute thermal correlations in
one-dimensional systems: A brief overview: We develop a highly efficient method to numerically simulate thermal
fluctuations and correlations in non-relativistic continuous bosonic
one-dimensional systems. We start by noticing the equivalence of their
description through the transfer matrix formalism and a Fokker-Planck equation
for a distribution evolving in space. The corresponding stochastic differential
(It\={o}) equation is very suitable for computer simulations, allowing the
calculation of arbitrary correlation functions. As an illustration, we apply
our method to the case of two tunnel-coupled quasicondensates of bosonic atoms. | cond-mat_quant-gas |
Miscibility-Immiscibility transition of strongly interacting bosonic
mixtures in optical lattices: Interaction plays key role in the mixing properties of a multi-component
system. The miscibility-immiscibility transition (MIT) in a weakly interacting
mixture of Bose gases is predominantly determined by the strengths of the intra
and inter-component two-body contact interactions. On the other hand, in the
strongly interacting regime interaction induced processes become relevant.
Despite previous studies on bosonic mixtures in optical lattices, the effects
of the interaction induced processes on the MIT remains unexplored. In this
work, we investigate the MIT in the strongly interacting phases of
two-component bosonic mixture trapped in a homogeneous two-dimensional square
optical lattice. Particularly we examine the transition when both the
components are in superfluid (SF), one-body staggered superfluid (OSSF) or
supersolid (SS) phases. Our study prevails that, similar to the contact
interactions, the MIT can be influenced by competing intra and inter-component
density induced tunnelings and off-site interactions. To probe the MIT in the
strongly interacting regime, we study the extended version of the Bose-Hubbard
model with the density induced tunneling and nearest-neighbouring interaction
terms, and focus in the regime where the hopping processes are considerably
weaker than the on-site interaction. We solve this model through
site-decoupling mean-field theory with Gutzwiller ansatz and characterize the
miscibility through the site-wise co-existence of the two-component across the
lattice. Our study contributes to the better understanding of miscibility
properties of multi-component systems in the strongly interacting regime. | cond-mat_quant-gas |
Adiabatic spin cooling using high-spin Fermi gases: Spatial entropy redistribution plays a key role in adiabatic cooling of
ultra-cold lattice gases. We show that high-spin fermions with a spatially
variable quadratic Zeeman coupling may allow for the creation of an inner
spin-1/2 core surrounded by high-spin wings. The latter are always more
entropic than the core at high temperatures and, remarkably, at all
temperatures in the presence of frustration. Combining thermodynamic Bethe
Ansatz with local density approximation, we study the spatial entropy
distribution for the particular case of one-dimensional spin-3/2 lattice
fermions in the Mott phase. Interestingly, this spatially dependent entropy
opens a possible path for an adiabatic cooling technique that, in contrast to
previous proposals, would specifically target the spin degree of freedom. We
discuss a possible realization of this adiabatic cooling, which may allow for a
highly efficient entropy decrease in the spin-1/2 core and help access
antiferromagnetic order in experiments on ultracold spinor fermions. | cond-mat_quant-gas |
Two-Stage Melting in Systems of Strongly Interacting Rydberg Atoms: We analyze the ground state properties of a one-dimensional cold atomic
system in a lattice, where Rydberg excitations are created by an external laser
drive. In the classical limit, the ground state is characterized by a complete
devil's staircase for the commensurate solid structures of Rydberg excitations.
Using perturbation theory and a mapping onto an effective low energy
Hamiltonian, we find a transition of these commensurate solids into a floating
solid with algebraic correlations. For stronger quantum fluctuations the
floating solid eventually melts within a second quantum phase transition and
the ground state becomes paramagnetic. | cond-mat_quant-gas |
Probing quantum transport by engineering correlations in a speckle
potential: We develop a procedure to modify the correlations of a speckle potential.
This procedure, that is suitable for spatial light modulator devices, allows
one to increase the localization efficiency of the speckle in a narrow energy
region whose position can be easily tuned. This peculiar energy-dependent
localization behavior is explored by pulling the potential through a
cigar-shaped Bose-Einstein condensate. We show that the percentage of dragged
atoms as a function of the pulling velocity depends on the potential
correlations below a threshold of the disorder strength. Above this threshold,
interference effects are no longer clearly observable during the condensate
drag. | cond-mat_quant-gas |
Vortex gyroscope imaging of planar superfluids: We propose a robust imaging technique that makes it possible to distinguish
vortices from antivortices in quasi-two-dimensional Bose--Einstein condensates
from a single image of the density of the atoms. Tilting the planar condensate
prior to standard absorption imaging excites a generalized gyroscopic mode of
the condensate revealing the sign and location of each vortex. This technique
is anticipated to enable experimental measurement of the incompressible kinetic
energy spectrum of the condensate and the observation of a negative temperature
phase transition of the vortex gas, driven by two-dimensional superfluid
turbulence. | cond-mat_quant-gas |
Quantum interferometry at zero and finite temperature with two-mode
bosonic Josephson junctions: We analyze phase interferometry realized with a bosonic Josephson junction
made of trapped dilute and ultracold atoms. By using a suitable phase
sensitivity indicator we study the zero temperature junction states useful to
achieve sub shot-noise precisions. Sub shot-noise phase shift sensitivities can
be reached even at finite temperature under a suitable choice of the junction
state. We infer a scaling law in terms of the size system (that is, the number
of particles) for the temperature at which the shot-noise limit is not overcome
anymore | cond-mat_quant-gas |
Topological semimetal in a fermionic optical lattice: Optical lattices play a versatile role in advancing our understanding of
correlated quantum matter. The recent implementation of orbital degrees of
freedom in chequerboard and hexagonal optical lattices opens up a new thrust
towards discovering novel quantum states of matter, which have no prior analogs
in solid state electronic materials. Here, we demonstrate that an exotic
topological semimetal emerges as a parity-protected gapless state in the
orbital bands of a two-dimensional fermionic optical lattice. The new quantum
state is characterized by a parabolic band-degeneracy point with Berry flux
$2\pi$, in sharp contrast to the $\pi$ flux of Dirac points as in graphene. We
prove that the appearance of this topological liquid is universal for all
lattices with D$_4$ point group symmetry as long as orbitals with opposite
parities hybridize strongly with each other and the band degeneracy is
protected by odd parity. Turning on inter-particle repulsive interactions, the
system undergoes a phase transition to a topological insulator whose
experimental signature includes chiral gapless domain-wall modes, reminiscent
of quantum Hall edge states. | cond-mat_quant-gas |
Resonant dipolar collisions of ultracold molecules induced by microwave
dressing: We demonstrate microwave dressing on ultracold, fermionic
${}^{23}$Na${}^{40}$K ground-state molecules and observe resonant dipolar
collisions with cross sections exceeding three times the $s$-wave unitarity
limit. The origin of these collisions is the resonant alignment of the
approaching molecules' dipoles along the intermolecular axis, which leads to
strong attraction. We explain our observations with a conceptually simple
two-state picture based on the Condon approximation. Furthermore, we perform
coupled-channels calculations that agree well with the experimentally observed
collision rates. While collisions are observed here as laser-induced loss,
microwave dressing on chemically stable molecules trapped in box potentials may
enable the creation of strongly interacting dipolar gases of molecules. | cond-mat_quant-gas |
Effect of anisotropic spin-orbit coupling on condensation and
superfluidity of a two dimensional Fermi gases: We investigated the ground state properties of a two dimensional Fermi
superfluid with an anisotropic spin-orbit coupling (SOC) using path-integral
field theoretical method. Within the framework of mean-field theory, we
obtained the condensed fraction including contributions from both singlet and
triple pairing fields. We found that for small interaction parameters and large
anisotropic parameters, the total condensed fraction changes non-monotonically
when increasing the strength of SOC and has a global maximum. But this feature
disappears with decreasing the anisotropic parameter and increasing the
interaction parameter. However, condensed fraction always decrease with
increasing the anisotropic parameters. Because of the anisotropy of the SOC,
the superfluid fraction becomes a tensor. We obtained the superfluid fraction
tensor by deriving the effective action of the phase field of the order
parameter. Our numerical results show that for small interaction parameters and
large anisotropic parameters, superfluid fraction of the $x$ component
$\rho_{x}$ has a minimum as a function of the SOC strength. And this minimum of
$\rho_{x}$ disappears when decreasing the anisotropic parameters. In the strong
interaction regime, $\rho_{x}$ always decreases with increasing the strength of
SOC. While for the $y$ component of the superfluid fraction $\rho_{y}$, no
matter how large the interaction parameters and anisotropic parameters are, it
always has a minimum as a function of the SOC strength. As a function of the
anisotropic parameter, for strong SOC strength, $\rho_{x}<\rho_{y}$ with
$\rho_{x}$ having a minimum. For small SOC parameters $\rho_{x}>\rho_{y}$ with
$\rho_{y}$ developing a minimum only in the weak interaction limit. | cond-mat_quant-gas |
Imaginary Potential Induced Quantum Coherence for Bose-Einstein
Condensates: The role of complex potentials in single-body Schr\H{o}dinger equation has
been studied intensively. We study the quantum coherence for degenerate Bose
gases in complex potentials, when the exchange symmetry of identical bosons is
considered. For initially independent Bose-Einstein condensates, it is shown
that even very weak imaginary potential can induce perfect quantum coherence
between different condensates. The scheme to observe imaginary potential
induced quantum coherence is discussed. | cond-mat_quant-gas |
Two-body momentum correlations in a weakly interacting one-dimensional
Bose gas: We analyze the two-body momentum correlation function for a uniform weakly
interacting one-dimensional Bose gas. We show that the strong positive
correlation between opposite momenta, expected in a Bose-Einstein condensate
with a true long-range order, almost vanishes in a phase-fluctuating
quasicondensate where the long-range order is destroyed. Using the Luttinger
liquid approach, we derive an analytic expression for the momentum correlation
function in the quasicondensate regime, showing (i) the reduction and
broadening of the opposite-momentum correlations (compared to the singular
behavior in a true condensate) and (ii) an emergence of anticorrelations at
small momenta. We also numerically investigate the momentum correlations in the
crossover between the quasicondensate and the ideal Bose-gas regimes using a
classical field approach and show how the anticorrelations gradually disappear
in the ideal-gas limit. | cond-mat_quant-gas |
Chiral confinement in quasirelativistic Bose-Einstein condensates: In the presence of a laser-induced spin-orbit coupling an interacting ultra
cold spinor Bose-Einstein condensate may acquire a quasi-relativistic character
described by a non-linear Dirac-like equation. We show that as a result of the
spin-orbit coupling and the non-linearity the condensate may become
self-trapped, resembling the so-called chiral confinement, previously studied
in the context of the massive Thirring model. We first consider 1D geometries
where the self-confined condensates present an intriguing sinusoidal dependence
on the inter-particle interactions. We further show that multi-dimensional
chiral-confinement is also possible under appropriate feasible laser
arrangements, and discuss the properties of 2D and 3D condensates, which differ
significantly from the 1D case. | cond-mat_quant-gas |
Superfluid to Mott insulator transition in the one-dimensional
Bose-Hubbard model for arbitrary integer filling factors: We study the quantum phase transition between the superfluid and the Mott
insulator in the one-dimensional (1D) Bose-Hubbard model. Using the
time-evolving block decimation method, we numerically calculate the tunneling
splitting of two macroscopically distinct states with different winding
numbers. From the scaling of the tunneling splitting with respect to the system
size, we determine the critical point of the superfluid to Mott insulator
transition for arbitrary integer filling factors. We find that the critical
values versus the filling factor in 1D, 2D, and 3D are well approximated by a
simple analytical function. We also discuss the condition for determining the
transition point from a perspective of the instanton method. | cond-mat_quant-gas |
The spin evolution of spin-3 $^{52}$Cr Bose-Einstein condensate: The spin evolution of a Bose-Einstein condensate starting from a mixture of
two or three groups of $^{52}$Cr (spin-3) atoms in an optical trap has been
studied theoretically. The initial state is so chosen that the system does not
distinguish up and down. In this choice, the deviation caused by the
single-mode approximation is reduced. Moreover, since the particle number is
given very small (N=20), the deviation caused by the neglect of the long-range
dipole force is also reduced. Making use of these two simplifications, a
theoretical calculation beyond the mean field theory is performed. The
numerical results are help to evaluate the unknown strength $g_0$. | cond-mat_quant-gas |
Dynamics of first-order quantum phase transitions in extended
Bose-Hubbard model: From density wave to superfluid and vice-versa: In this paper, we study the nonequilibrium dynamics of the Bose-Hubbard model
with the nearest-neighbor repulsion by using time-dependent Gutzwiller (GW)
methods. In particular, we vary the hopping parameters in the Hamiltonian as a
function of time, and investigate the dynamics of the system from the density
wave (DW) to the superfluid (SF) crossing a first-order phase transition and
vice-versa. From the DW to SF, we find scaling laws for the correlation length
and vortex density with respect to the quench time. This is a reminiscence of
the Kibble-Zurek scaling for continuous phase transitions and contradicts the
common expectation. We give a possible explanation for this observation. On the
other hand from the SF to DW, the system evolution depends on the initial SF
state. When the initial state is the ground-state obtained by the static GW
methods, a coexisting state of the SF and DW domains forms after passing
through the critical point. Coherence of the SF order parameter is lost as the
system evolves. This is a phenomenon similar to the glass transition in
classical systems. When the state starts from the SF with small local phase
fluctuations, the system obtains a large-size DW-domain structure with thin
domain walls. | cond-mat_quant-gas |
Many-body exceptional points in colliding condensates: Exceptional points describe the coalescence of the eigenmodes of a
non-Hermitian matrix. When an exceptional point occurs in the unitary evolution
of a many-body system, it generically leads to a dynamical instability with a
finite wavevector [N. Bernier \etal, Phys. Rev. Lett. 113, 065303 (2014)].
Here, we study exceptional points in the context of the counterflow instability
of colliding Bose-Einstein condensates. We show that the instability of this
system is due to an exceptional point in the Bogoliubov spectrum. We further
clarify the connection of this effect to the Landau criterion of superfluidity
and to the scattering of classical particles. We propose an experimental set-up
to directly probe this exceptional point, and demonstrate its feasibility with
the aid of numerical calculations. Our work fosters the observation of
exceptional points in nonequilibrium many-body quantum systems. | cond-mat_quant-gas |
Three Identical Fermions with Resonant p-wave Interactions in Two
Dimensions: A new kind of "super-Efimov" states of binding energies scaling as
$\ln|E_n|\sim-e^{3n\pi/4}$ were predicted by a field theory calculation for
three fermions with resonant $p$-wave interactions in two dimensions [Phys.
Rev. Lett. \textbf{110}, 235301 (2013)]. However, the universality of these
"super-Efimov" states has not been proved independently. In this Letter, we
study the three fermion system through the hyperspherical formalism. Within the
adiabatic approximation, we find that at $p$-wave resonances, the low energy
physics of states of angular momentum $\ell=\pm1$ crucially depends on the
value of an emergent dimensionless parameter $Y$ determined by the detail of
the inter-particle potential. Only if $Y$ is exactly zero, the predicted
"super-Efimov" states exist. If $Y>0$, the scaling of the bound states changes
to $\ln|E_n|\sim-(n\pi)^2/2Y$, while there are no shallow bound states if
$Y<0$. | cond-mat_quant-gas |
Relation between the noise correlations and the spin structure factor
for Mott-insulating states in SU$(N)$ Hubbard models: It is well established that the noise correlations measured by time-of-flight
imaging in cold-atom experiments, which correspond to the density-density
correlations in the momentum space of trapped atomic gases, can probe the spin
structure factor deep in the Mott-insulating regime of SU(2) Hubbard models. We
explicitly derive the mathematical relation between the noise correlations and
the spin structure factor in the strong-interaction limit of SU$(N)$ Hubbard
models at any integer filling $\rho$. By calculating the ground states of
one-dimensional SU$(N)$ Fermi-Hubbard models for $2\leq N\leq 6$ with use of
the density-matrix renormalization-group method, we confirm the relation
numerically in the regime of strong interactions $U \gg t$, where $U$ and $t$
denote the onsite interaction and the hopping energy. We show that the
deviation between the actual noise correlations and those obtained from the
spin structure factor scales as approximately $(t/U)^2$ for $\rho=1$ at
intermediate and large lattice sizes on the basis of numeric and semi-analytic
arguments. | cond-mat_quant-gas |
Thermodynamics of a spin-1 Bose gas with fixed magnetization: We investigate the thermodynamics of a spin-1 Bose gas with fixed
magnetization including the quadratic Zeeman energy shift. Our calculations are
based on the grand canonical description for the ideal gas and the classical
fields approximation for atoms with ferromagnetic and antiferromagnetic
interactions. We confirm the occurence of a double phase transition in the
system that takes place due to two global constraints. We show analytically for
the ideal gas how critical temperatures and condensed fractions are changed by
a non-zero magnetic field. The interaction strongly affects the condensate
scenario below the second critical temperature. The effect imposed by
interaction energies becomes diminished in high magnetic fields where
condensation, of both ferromagnetic and antiferromagnetic atoms, agree with the
ideal gas results. | cond-mat_quant-gas |
Dynamical Cluster Quantum Monte Carlo Study of the Single Particle
Spectra of Strongly Interacting Fermion Gases: We study the single-particle spectral function of resonantly-interacting
fermions in the unitary regime, as described by the three-dimensional
attractive Hubbard model in the dilute limit. Our approach, based on the
Dynamical Cluster Approximation and the Maximum Entropy Method, shows the
emergence of a gap with decreasing temperature, as reported in recent cold-atom
photoemission experiments, for coupling values that span the BEC-BCS crossover.
By comparing the behavior of the spectral function to that of the imaginary
time dynamical pairing susceptibility, we attribute the development of the gap
to the formation of local bound atom pairs. | cond-mat_quant-gas |
Pseudopotential for the 2D contact interaction: We propose a smooth pseudopotential for the contact interaction acting
between ultracold atoms confined to two dimensions. The pseudopotential
reproduces the scattering properties of the repulsive contact interaction up to
200 times more accurately than a hard disk potential, and in the attractive
branch gives a 10-fold improvement in accuracy over the square well potential.
Furthermore, the new potential enables diffusion Monte Carlo simulations of the
ultracold gas to be run 15 times quicker than was previously possible. | cond-mat_quant-gas |
Particle Fluctuations in Mesoscopic Bose Systems: Particle fluctuations in mesoscopic Bose systems of arbitrary spatial
dimensionality are considered. Both ideal Bose gases and interacting Bose
systems are studied in the regions above the Bose-Einstein condensation
temperature $T_c$ as well as below this temperature. The strength of particle
fluctuations defines whether the system is stable or not. Stability conditions
depend on the spatial dimensionality $d$ and on the confining dimension $D$ of
the system. The consideration shows that mesoscopic systems, experiencing
Bose-Einstein condensation, are stable when: (i) ideal Bose gas is confined in
a rectangular box of spatial dimension $d>2$ above $T_c$ and in a box of $d>4$
below $T_c$; (ii) ideal Bose gas is confined in a power-law trap of a confining
dimension $D>2$ above $T_c$ and of a confining dimension $D>4$ below $T_c$;
(iii) interacting Bose system is confined in a rectangular box of dimension
$d>2$ above $T_c$, while below $T_c$ particle interactions stabilize the
Bose-condensed system making it stable for $d=3$; (iv) nonlocal interactions
diminish the condensation temperature, as compared with the fluctuations in a
system with contact interactions. | cond-mat_quant-gas |
How creating one additional well can generate Bose-Einstein condensation: The realization of Bose-Einstein condensation in ultracold trapped gases has
led to a revival of interest in that fascinating quantum phenomenon. This
experimental achievement necessitated both extremely low temperatures and
sufficiently weak interactions. Particularly in reduced spatial dimensionality
even an infinitesimal interaction immediately leads to a departure to
quasi-condensation. We propose a system of strongly interacting bosons which
overcomes those obstacles by exhibiting a number of intriguing related
features: (i) The tuning of just a single control parameter drives a transition
from quasi-condensation to complete condensation, (ii) the destructive
influence of strong interactions is compensated by the respective increased
mobility, (iii) topology plays a crucial role since a crossover from one- to
`infinite'-dimensionality is simulated, (iv) a ground state gap opens which
makes the condensation robust to thermal noise. Remarkably, all these features
can be derived by analytical and exact numerical means despite the
non-perturbative character of the system. | cond-mat_quant-gas |
Critical velocity, vortex shedding and drag in a unitary Fermi
superfluid: We study the real-time motion of a microscopic object in a cold Fermi gas at
unitary conditions by using an extended Thomas-Fermi density functional
approach. We find that spontaneous creation of singly quantized
vortex-antivortex pairs occurs as a critical velocity is exceeded, which leads
to a drag between the moving object and the Fermi gas. The resulting force is
linear in the velocity for subsonic motion and becomes quadratic for supersonic
motion. | cond-mat_quant-gas |
Direct Observation of Fragmentation in a Disordered, Strongly
Interacting Fermi Gas: Describing the behaviour of strongly interacting particles in the presence of
disorder is among the most challenging problems in quantum many-body physics.
The controlled setting of cold atom experiments provides a new avenue to
address these challenges [1], complementing studies in solid state physics,
where a number of puzzling findings have emerged in experiments using
superconducting thin films [2,3]. Here we investigate a strongly interacting
thin film of an atomic Fermi gas subject to a random potential. We use
high-resolution in-situ imaging [4-7] to resolve the atomic density at the
length scale of a single impurity, which would require scanning probe
techniques in solid state physics [8]. This allows us to directly observe the
fragmentation of the density profile and to extract its percolation properties.
Transport measurements in a two-terminal configuration indicate that the
fragmentation process is accompanied by a breakdown of superfluidity. Our
results suggest that percolation of paired atoms is responsible for the loss of
superfluidity, and that disorder is able to increase the binding energy of
pairs. | cond-mat_quant-gas |
Correlated quantum dynamics of graphene: Phase-space representations are a family of methods for dynamics of both
bosonic and fermionic systems, that work by mapping the system's density matrix
to a quasi-probability density and the Liouville-von Neumann equation of the
Hamiltonian to a corresponding density differential equation for the
probability. We investigate here the accuracy and the computational efficiency
of one approximate phase-space representation, called the fermionic Truncated
Wigner Approximation (fTWA), applied to the Fermi-Hubbard model. On a many-body
2D system, with hopping strength and Coulomb $U$ tuned to represent the
electronic structure of graphene, the method is found to be able to capture the
time evolution of first-order (site occupation) and second-order (correlation
functions) moments significantly better than the mean-field, Hartree-Fock
method. The fTWA was also compared to results from the exact diagonalization
method for smaller systems, and in general the agreement was found to be good.
The fully parallel computational requirement of fTWA scales in the same order
as the Hartree-Fock method, and the largest system considered here contained
198 lattice sites. | cond-mat_quant-gas |
Strong Boundary and Trap Potential Effects on Emergent Physics in
Ultra-Cold Fermionic Gases: The field of quantum simulations in ultra-cold atomic gases has been
remarkably successful. In principle it allows for an exact treatment of a
variety of highly relevant lattice models and their emergent phases of matter.
But so far there is a lack in the theoretical literature concerning the
systematic study of the effects of the trap potential as well as the finite
size of the systems, as numerical studies of such non periodic, correlated
fermionic lattices models are numerically demanding beyond one dimension. We
use the recently introduced real-space truncated unity functional
renormalization group to study these boundary and trap effects with a focus on
their impact on the superconducting phase of the $2$D Hubbard model. We find
that in the experiments not only lower temperatures need to be reached compared
to current capabilities, but also system size and trap potential shape play a
crucial role to simulate emergent phases of matter. | cond-mat_quant-gas |
The response to dynamical modulation of the optical lattice for fermions
in the Hubbard model: Fermionic atoms in a periodic optical lattice provide a realization of the
single-band Hubbard model. Using Quantum Monte Carlo simulations along with the
Maximum Entropy Method, we evaluate the effect of a time-dependent perturbative
modulation of the optical lattice amplitude on atomic correlations, revealed in
the fraction of doubly-occupied sites. Our treatment extends previous
approaches which neglected the time dependence of the on-site interaction, and
shows that this term changes the results in a quantitatively significant way.
The effect of modulation depends strongly on the filling-- the response of the
double occupation is significantly different in the half-filled Mott insulator
from the doped Fermi liquid region. | cond-mat_quant-gas |
Decoherence of an impurity in a one-dimensional fermionic bath with mass
imbalance: We study the transport, decoherence and dissipation of an impurity
interacting with a bath of free fermions in a one-dimensional lattice.
Numerical simulations are made with the time-evolving block decimation method.
We introduce a mass imbalance between the impurity and bath particles and find
that the fastest decoherence occurs for a light impurity in a bath of heavy
particles. By contrast, the fastest dissipation of energy occurs when the
masses are equal. We present a simple model for decoherence in the heavy bath
limit, and a linear density response description of the interaction which
predicts maximum dissipation for equal masses. | cond-mat_quant-gas |
Rotating Fulde-Ferrell-Larkin-Ovchinnikov state in cold Fermi gases: We study an effect of rotation on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)
state of two component Fermi superfluid gases in a toroidal trap. We
investigate a stability of the FFLO states in the quasi-one-dimensional regime
on the basis of the Bogoliubov-de Gennes equation. We find that two novel FFLO
phases, i.e., the half quantum vortex state and the intermediate state of
Fulde-Ferrell (FF) state and Larkin-Ovchinnikov (LO) state, are stabilized by
the rotation. The phase diagram for the FF state, LO state, intermediate state,
and half quantum vortex state is shown in both T-P plane and T-h plane. We
demonstrate characteristic features of these states, such as the order
parameter, flux quantization, and local polarization. Several related works are
discussed, and the advantages of cold Fermi gases are indicated. | cond-mat_quant-gas |
Bloch oscillations of spin-orbit-coupled cold atoms in an optical
lattice and spin current generation: We study the Bloch oscillation dynamics of a spin-orbit-coupled cold atomic
gas trapped inside a one-dimensioanl optical lattice. The eigenspectra of the
system is identified as two interpenetrating Wannier-Stark ladder. Based on
that, we carefully analyzed the Bloch oscillation dynamics and found out that
intraladder coupling between neighboring rungs of Wannier-Stark ladder give
rise to ordinary Bloch oscillation while interladder coupling lead to small
amplitude high frequency oscillation superimposed on it. Specifically
spin-orbit interaction breaks Galilean invariance, which can be reflected by
out-of-phase oscillation of the two spin components in the accelerated frame.
The possibility of generating spin current in this system are also explored. | cond-mat_quant-gas |
Mixtures of ultra-cold atoms in 1D disordered potentials: We study interacting 1D two-component mixtures of cold atoms in a random
potential, and extend the results reported earlier [{\it Phys. Rev. Lett.} {\bf
105}, 115301 (2010)]. We construct the phase diagram of a disordered Bose-Fermi
mixture as a function of the strength of the Bose-Bose and Bose-Fermi
interactions, and the ratio of the bosonic sound velocity and the Fermi
velocity. Performing renormalization group and variational calculations, three
phases are identified: (i) a fully delocalized two-component Luttinger liquid
with superfluid bosons and fermions (ii) a fully localized phase with both
components pinned by disorder, and (iii) an intermediate phase where fermions
are localized but bosons are superfluid. Within the variational approach, each
phase corresponds to a different level of replica symmetry breaking. In the
fully localized phase we find that the bosonic and fermionic localization
lengths can largely differ. We also compute the momentum distribution as well
as the structure factor of the atoms (both experimentally accessible), and
discuss how the three phases can be experimentally distinguished. | cond-mat_quant-gas |
Lattice bosons with infinite range checkerboard interactions: Motivated by experiments performed by Landig et al. [Nature 532, 476-479], we
consider a two dimensional Bose gas in an optical lattice, trapped inside a
single mode superradiant Fabry Perot cavity. The cavity mediates infinite range
checkerboard interactions between the atoms, which produces competition between
Mott insulator, charge density wave, superfluid and supersolid phases. We
calculate the phase diagram of this Bose gas in a homogeneous system and in the
presence of a harmonic trap. | cond-mat_quant-gas |
Theory of Non-Hermitian Fermionic Superfluidity with a Complex-Valued
Interaction: Motivated by recent experimental advances in ultracold atoms, we analyze a
non-Hermitian (NH) BCS Hamiltonian with a complex-valued interaction arising
from inelastic scattering between fermions. We develop a mean-field theory to
obtain a NH gap equation for order parameters, which are different from the
standard BCS ones due to the inequivalence of left and right eigenstates in the
NH physics. We find unconventional phase transitions unique to NH systems:
superfluidity shows reentrant behavior with increasing dissipation, as a
consequence of non-diagonalizable exceptional points, lines, and surfaces in
the quasiparticle Hamiltonian for weak attractive interactions. For strong
attractive interactions, the superfluid gap never collapses but is enhanced by
dissipation due to an interplay between the BCS-BEC crossover and the quantum
Zeno effect. Our results lay the groundwork for studies of fermionic
superfluidity subject to inelastic collisions. | cond-mat_quant-gas |
Quantum phases in spin-orbit-coupled Floquet spinor Bose gases: We propose a spin-orbit-coupled Floquet spinor Bose-Einstein condensate (BEC)
which can be implemented by Floquet engineering of a quadratic Zeeman field.
The Floquet spinor BEC has a Bessel-function-modulated Rabi frequency and a
Floquet-induced spin-exchange interaction. The quantum phase diagram of the
spin-orbit-coupled Floquet spinor BEC is investigated by considering
antiferromagnetic or ferromagnetic spin-spin interactions. In comparison with
the usual spin-orbit-coupled spin-1 BEC, we find that a stripe phase for
antiferromagnetic interactions can exist in a large quadratic Zeeman field
regime, and a different stripe phase with an experimentally favorable contrast
for ferromagnetic interactions is uncovered. | cond-mat_quant-gas |
First-order superfluid to Mott-insulator phase transitions in spinor
condensates: We observe evidence of first-order superfluid to Mott-insulator quantum phase
transitions in a lattice-confined antiferromagnetic spinor Bose-Einstein
condensate. The observed signatures include hysteresis effect and significant
heatings across the phase transitions. The nature of the phase transitions is
found to strongly depend on the ratio of the quadratic Zeeman energy to the
spin-dependent interaction. Our observations are qualitatively understood by
the mean field theory, and in addition suggest tuning the quadratic Zeeman
energy is a new approach to realize superfluid to Mott-insulator phase
transitions. | cond-mat_quant-gas |
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