text
stringlengths 89
2.49k
| category
stringclasses 19
values |
|---|---|
Stoner ferromagnetism in a thermal pseudospin-1/2 Bose gas: We compute the finite-temperature phase diagram of a pseudospin-$1/2$ Bose
gas with contact interactions, using two complementary methods: the random
phase approximation (RPA) and self-consistent Hartree-Fock theory. We show that
the inter-spin interactions, which break the (pseudo) spin-rotational symmetry
of the Hamiltonian, generally lead to the appearance of a magnetically ordered
phase at temperatures above the superfluid transition. In three dimensions, we
predict a normal easy-axis/easy-plane ferromagnet for sufficiently strong
repulsive/attractive inter-species interactions respectively. The normal
easy-axis ferromagnet is the bosonic analog of Stoner ferromagnetism known in
electronic systems. For the case of inter-spin attraction, we also discuss the
possibility of a \textit{bosonic} analog of the Cooper paired phase. This state
is shown to significantly lose in energy to the transverse ferromagnet in three
dimensions, but is more energetically competitive in lower dimensions.
Extending our calculations to a spin-orbit-coupled Bose gas with equal Rashba
and Dresselhaus-type couplings (as recently realized in experiment), we
investigate the possibility of stripe ordering in the normal phase. Within our
approximations however, we do not find an instability towards stripe formation,
suggesting that the stripe order melts below the condensation temperature,
which is consistent with the experimental observations of Ji \textit{et al.}
[Ji \textit{et al.}, Nature Physics \textbf{10}, 314 (2014)]. | cond-mat_quant-gas |
The one-dimensional Bose gas with strong two-body losses: the effect of
the harmonic confinement: We study the dynamics of a one-dimensional Bose gas in presence of strong
two-body losses. In this dissipative quantum Zeno regime, the gas fermionises
and its dynamics can be described with a simple set of rate equations.
Employing the local density approximation and a Boltzmann-like dynamical
equation, the description is easily extended to take into account an external
potential. We show that in the absence of confinement the population is
depleted in an anomalous way and that the gas behaves as a low-temperature
classical gas. The harmonic confinement accelerates the depopulation of the gas
and introduces a novel decay regime, which we thoroughly characterise. | cond-mat_quant-gas |
Observation of microscopic confinement dynamics by a tunable topological
$θ$-angle: The topological $\theta$-angle is central to the understanding of a plethora
of phenomena in condensed matter and high-energy physics such as the strong CP
problem, dynamical quantum topological phase transitions, and the
confinement--deconfinement transition. Difficulties arise when probing the
effects of the topological $\theta$-angle using classical methods, in
particular through the appearance of a sign problem in numerical simulations.
Quantum simulators offer a powerful alternate venue for realizing the
$\theta$-angle, which has hitherto remained an outstanding challenge due to the
difficulty of introducing a dynamical electric field in the experiment. Here,
we report on the experimental realization of a tunable topological
$\theta$-angle in a Bose--Hubbard gauge-theory quantum simulator, implemented
through a tilted superlattice potential that induces an effective background
electric field. We demonstrate the rich physics due to this angle by the direct
observation of the confinement--deconfinement transition of $(1+1)$-dimensional
quantum electrodynamics. Using an atomic-precision quantum gas microscope, we
distinguish between the confined and deconfined phases by monitoring the
real-time evolution of particle--antiparticle pairs, which exhibit constrained
(ballistic) propagation for a finite (vanishing) deviation of the
$\theta$-angle from $\pi$. Our work provides a major step forward in the
realization of topological terms on modern quantum simulators, and the
exploration of rich physics they have been theorized to entail. | cond-mat_quant-gas |
Dynamical spin-flip susceptibility for a strongly interacting ultracold
Fermi gas: The Stoner model predicts that a two-component Fermi gas at increasing
repulsive interactions undergoes a ferromagnetic transition. Using the
random-phase approximation we study the dynamical properties of the interacting
Fermi gas. For an atomic Fermi gas under harmonic confinement we show that the
transverse (spin-flip) dynamical susceptibility displays a clear signature of
the ferromagnetic phase in a magnon peak emerging from the Stoner particle-hole
continuum. The dynamical spin susceptibilities could be experimentally explored
via spin-dependent Bragg spectroscopy. | cond-mat_quant-gas |
Snell's Law for a vortex dipole in a Bose-Einstein condensate: A quantum vortex dipole, comprised of a closely bound pair of vortices of
equal strength with opposite circulation, is a spatially localized travelling
excitation of a planar superfluid that carries linear momentum, suggesting a
possible analogy with ray optics. We investigate numerically and analytically
the motion of a quantum vortex dipole incident upon a step-change in the
background superfluid density of an otherwise uniform two-dimensional
Bose-Einstein condensate. Due to the conservation of fluid momentum and energy,
the incident and refracted angles of the dipole satisfy a relation analogous to
Snell's law, when crossing the interface between regions of different density.
The predictions of the analogue Snell's law relation are confirmed for a wide
range of incident angles by systematic numerical simulations of the
Gross-Piteavskii equation. Near the critical angle for total internal
reflection, we identify a regime of anomalous Snell's law behaviour where the
finite size of the dipole causes transient capture by the interface.
Remarkably, despite the extra complexity of the surface interaction, the
incoming and outgoing dipole paths obey Snell's law. | cond-mat_quant-gas |
Many-body dynamical localization in the kicked Bose-Hubbard chain: We provide evidence that a clean kicked Bose-Hubbard model exhibits a
many-body dynamically localized phase. This phase shows ergodicity breaking up
to the largest sizes we were able to consider. We argue that this property
persists in the limit of large size. The Floquet states violate eigenstate
thermalization and then the asymptotic value of local observables depends on
the initial state and is not thermal. This implies that the system does not
generically heat up to infinite temperature, for almost all the initial states.
Differently from many-body localization here the entanglement entropy linearly
increases in time. This increase corresponds to space-delocalized Floquet
states which are nevertheless localized across specific subsectors of the
Hilbert space: In this way the system is prevented from randomly exploring all
the Hilbert space and does not thermalize. | cond-mat_quant-gas |
Correlated Quantum Dynamics of a Single Atom Collisionally Coupled to an
Ultracold Finite Bosonic Ensemble: We explore the correlated quantum dynamics of a single atom, regarded as an
open system, with a spatio-temporally localized coupling to a finite bosonic
environment. The single atom, initially prepared in a coherent state of low
energy, oscillates in a one-dimensional harmonic trap and thereby periodically
penetrates an interacting ensemble of $N_A$ bosons, held in a displaced trap.
We show that the inter-species energy transfer accelerates with increasing
$N_A$ and becomes less complete at the same time. System-environment
correlations prove to be significant except for times when the excess energy
distribution among the subsystems is highly imbalanced. These correlations
result in incoherent energy transfer processes, which accelerate the early
energy donation of the single atom and stochastically favour certain energy
transfer channels depending on the instantaneous direction of transfer.
Concerning the subsystem states, the energy transfer is mediated by
non-coherent states of the single atom and manifests itself in singlet and
doublet excitations in the finite bosonic environment. These comprehensive
insights into the non-equilibrium quantum dynamics of an open system are gained
by ab-initio simulations of the total system with the recently developed
Multi-Layer Multi-Configuration Time-Dependent Hartree Method for Bosons. | cond-mat_quant-gas |
Broad universal Feshbach resonances in the chaotic spectrum of
Dysprosium atoms: We report on the observation of weakly-bound dimers of bosonic Dysprosium
with a strong universal s-wave halo character, associated with broad magnetic
Feshbach resonances. These states surprisingly decouple from the chaotic
backgound of narrow resonances, persisting across many such narrow resonances.
In addition they show the highest reported magnetic moment
$\mu\simeq20\,\mu_{\rm B}$ of any ultracold molecule. We analyze our findings
using a coupled-channel theory taking into account the short range van der
Waals interaction and a correction due to the strong dipole moment of
Dysprosium. We are able to extract the scattering length as a function of
magnetic field associated with these resonances and obtain a background
scattering length $a_{\rm bg}=91(16)\,a_0$. These results offer prospects of a
tunability of the interactions in Dysprosium, which we illustrate by observing
the saturation of three-body losses. | cond-mat_quant-gas |
Anderson localization of matter waves in quantum-chaos theory: We study the Anderson localization of atomic gases exposed to
three-dimensional optical speckles by analyzing the statistics of the
energy-level spacings. This method allows us to consider realistic models of
the speckle patterns, taking into account the strongly anisotropic correlations
which are realized in concrete experimental configurations. We first compute
the mobility edge $E_c$ of a speckle pattern created using a single laser beam.
We find that $E_c$ drifts when we vary the anisotropy of the speckle grains,
going from higher values when the speckles are squeezed along the beam
propagation axis, to lower values when they are elongated. We also consider the
case where two speckle patterns are superimposed forming interference fringes,
and we find that $E_c$ is increased compared to the case of idealized isotropic
disorder. We discuss the important implications of our findings for cold-atoms
experiments. | cond-mat_quant-gas |
The Gross-Pitaevskii Soliton: Relating Weakly and Strongly Repulsive
Bosonic condensates and the magnetic soliton: We show that the dark soliton of the Gross-Pitaevskii equation (GPE) that
describes the Bose-Einstein condensate (BEC) density of a system of weakly
repulsive bosons, also describes that of a system of strongly repulsive hard
core bosons at half filling. This connection establishes a relationship between
the GPE soliton and the magnetic soliton of an easy-plane ferromagnet, where
the BEC density relates to the square of the in-plane magnetization of the
system. This mapping between well known solitons in two distinct physical
systems provides an intuitive understanding of various characteristics of the
solitons. | cond-mat_quant-gas |
Twisted behavior of dipolar BECs: Dipole-dipole interaction beyond the
self-consistent field approximation and exchange electric dipole interaction: Dipole-dipole interaction is a long-range interaction, hence we could expect
that the self-consistent field approximation might be applied. In most cases it
is correct, but dipolar BECs reveal a surprise. Structure of the
self-consistent field term requires that interacting particles are in different
quantum states, while in BECs all particles in a single quantum state. This
fact requires to consider the two-particle polarisation, which describes
dipole-dipole interaction, in more details. We present this consideration and
show an astonishing result that the two-particle quantum correlation in dipolar
BECs reveals in the same form as the self-consistent field term. | cond-mat_quant-gas |
Controllable half-vortex lattices in an incoherently pumped polariton
condensate: We show how the transition between synchronized and desynchronized states of
a spinor polariton condensate can be used to drive a transition between
stationary vortex lattices and half-vortex lattices. This provides a way to
control polariton spin textures by a combination of pump spot profile and
applied magnetic fields. To do this, we extend the model of non-equilibrium
spinor condensates to include relaxation, and study how this affects the
desynchronization transition. We discuss how the pattern formation can be
explained by behavior of the homogeneous system. | cond-mat_quant-gas |
Polarization angle dependence of the breathing modes in confined
one-dimensional dipolar bosons: Probing the radial collective oscillation of a trapped quantum system is an
accurate experimental tool to investigate interactions and dimensionality
effects. We consider a fully polarized quasi-one dimensional dipolar quantum
gas of bosonic dysprosium atoms in a parabolic trap at zero temperature. We
model the dipolar gas with an effective quasi-one dimensional Hamiltonian in
the single-mode approximation, and derive the equation of state using a
variational approximation based on the Lieb-Liniger gas Bethe Ansatz
wavefunction or perturbation theory. We calculate the breathing mode
frequencies while varying polarization angles by a sum-rule approach, and find
them in good agreement with recent experimental findings. | cond-mat_quant-gas |
Generation of spin currents by a temperature gradient in a two-terminal
device: Theoretical and experimental studies of the interaction between spins and
temperature are vital for the development of spin caloritronics, as they
dictate the design of future devices. In this work, we propose a two-terminal
cold-atom simulator to study that interaction. The proposed quantum simulator
consists of strongly interacting atoms that occupy two temperature reservoirs
connected by a one-dimensional link. First, we argue that the dynamics in the
link can be described using an inhomogeneous Heisenberg spin chain whose
couplings are defined by the local temperature. Second, we show the existence
of a spin current in a system with a temperature difference by studying the
dynamics that follows the spin-flip of an atom in the link. A temperature
gradient accelerates the impurity in one direction more than in the other,
leading to an overall spin current similar to the spin Seebeck effect. | cond-mat_quant-gas |
Superglass phase of interaction-blockaded gases on a triangular lattice: We investigate the quantum phases of monodispersed bosonic gases confined to
a triangular lattice and interacting via a class of soft-shoulder potentials.
The latter correspond to soft-core potentials with an additional hard-core
onsite interaction. Using exact quantum Monte Carlo simulations, we show that
the low temperature phases for weak and strong interactions following a
temperature quench are a homogeneous superfluid and a glass, respectively. The
latter is an insulating phase characterized by inhomogeneity in the density
distribution and structural disorder. Remarkably, we find that for intermediate
interaction strengths a {\it superglass} occurs in an extended region of the
phase diagram, where glassy behavior coexists with a sizable finite superfluid
fraction. This glass phase is obtained in the absence of geometrical
frustration or external disorder and is a result of the competition of quantum
fluctuations and cluster formation in the corresponding classical ground state.
For high enough temperature, the glass and superglass turn into a floating
stripe solid and a supersolid, respectively. Given the simplicity and
generality of the model, these phases should be directly relevant for
state-of-the-art experiments with Rydberg-dressed atoms in optical lattices. | cond-mat_quant-gas |
Anyonic Haldane insulator in one dimension: We demonstrate numerically the existence of a nontrivial topological Haldane
phase for the one-dimensional extended ($U$-$V$) Hubbard model with a mean
density of one particle per site, not only for bosons but also for anyons,
despite a broken reflection parity symmetry. The Haldane insulator, surrounded
by superfluid, Mott insulator and density-wave phases in the $V$-$U$ parameter
plane, is protected by combined (modified) spatial-inversion and time-reversal
symmetries, which is verified within our matrix-product-state based infinite
density-matrix renormalization group scheme by analyzing generalized transfer
matrices. With regard to an experimental verification of the anyonic Haldane
insulator state the calculated asymmetry of the dynamical density structure
factor should be of particular importance. | cond-mat_quant-gas |
Quantum fluids of light: This article reviews recent theoretical and experimental advances in the
fundamental understanding and active control of quantum fluids of light in
nonlinear optical systems. In presence of effective photon-photon interactions
induced by the optical nonlinearity of the medium, a many-photon system can
behave collectively as a quantum fluid with a number of novel features stemming
from its intrinsically non-equilibrium nature. We present a rich variety of
photon hydrodynamical effects that have been recently observed, from the
superfluid flow around a defect at low speeds, to the appearance of a
Mach-Cherenkov cone in a supersonic flow, to the hydrodynamic formation of
topological excitations such as quantized vortices and dark solitons at the
surface of large impenetrable obstacles. While our review is mostly focused on
a class of semiconductor systems that have been extensively studied in recent
years (namely planar semiconductor microcavities in the strong light-matter
coupling regime having cavity polaritons as elementary excitations), the very
concept of quantum fluids of light applies to a broad spectrum of systems,
ranging from bulk nonlinear crystals, to atomic clouds embedded in optical
fibers and cavities, to photonic crystal cavities, to superconducting quantum
circuits based on Josephson junctions. The conclusive part of our article is
devoted to a review of the exciting perspectives to achieve strongly correlated
photon gases. In particular, we present different mechanisms to obtain
efficient photon blockade, we discuss the novel quantum phases that are
expected to appear in arrays of strongly nonlinear cavities, and we point out
the rich phenomenology offered by the implementation of artificial gauge fields
for photons. | cond-mat_quant-gas |
BEC-BCS crossover and the mobility edge: superfluid-insulator
transitions and reentrant superfluidity in disordered Fermi gases: A superfluid-insulator transition is known to occur in strongly disordered
Fermi gases, in both the BCS and BEC regimes; here, we address the properties
of this transition across the BEC-BCS crossover. We argue that the critical
disorder strength at which superfluidity is lost changes non-monotonically with
detuning from Feshbach resonance, and that a reentrant superfluid phase arises
for detunings near the fermionic mobility edge. Our analysis of the
intermediate regime is quantitatively valid for narrow resonances and near four
dimensions, and provides a simple physical picture of this regime, in terms of
two distinct but coexisting insulators. | cond-mat_quant-gas |
Interaction dependent heating and atom loss in a periodically driven
optical lattice: Periodic driving of optical lattices has enabled the creation of novel
bandstructures not realizable in static lattice systems, such as topological
bands for neutral particles. However, especially driven systems of interacting
bosonic particles often suffer from strong heating. We have systematically
studied heating in an interacting Bose-Einstein condensate in a driven
one-dimensional optical lattice. We find interaction-dependent heating rates
that depend both on the scattering length and the driving strength and identify
the underlying resonant intra- and interband scattering processes. By comparing
experimental data and theory, we find that for driving frequencies well above
the trap depth, the heating rate is dramatically reduced by the fact that
resonantly scattered atoms leave the trap before dissipating their energy into
the system. This mechanism of Floquet evaporative cooling offers a powerful
strategy to minimize heating in Floquet engineered quantum gases. | cond-mat_quant-gas |
Coupled Ferromagnetic and Nematic Ordering of Fermions in an Optical
Flux Lattice: Ultracold atoms in Raman-dressed optical lattices allow for effective
momentum-dependent interactions among single-species fermions originating from
short-range s-wave interactions. These dressed-state interactions combined with
very flat bands encountered in the recently introduced optical flux lattices
push the Stoner instability towards weaker repulsive interactions, making it
accessible with current experiments. As a consequence of the coupling between
spin and orbital degrees of freedom, the magnetic phase features Ising nematic
order. | cond-mat_quant-gas |
Dirty bosons in a three-dimensional harmonic trap: We study a three-dimensional Bose-Einstein condensate in an isotropic
harmonic trapping potential with an additional delta-correlated disorder
potential at both zero and finite temperature and investigate the emergence of
a Bose-glass phase for increasing disorder strength. To this end, we revisit a
quite recent non-perturbative approach towards the dirty boson problem, which
relies on the Hartree-Fock mean-field theory and is worked out on the basis of
the replica method, and extend it from the homogeneous case to a harmonic
confinement. At first, we solve the zero-temperature self-consistency equations
for the respective density contributions, which are obtained via the
Hartree-Fock theory within the Thomas-Fermi approximation. Additionally we use
a variational ansatz, whose results turn out to coincide qualitatively with
those obtained from the Thomas-Fermi approximation. In particular, a
first-order quantum phase transition from the superfluid phase to the
Bose-glass phase is detected at a critical disorder strength, which agrees with
findings in the literature. Afterwards, we consider the three-dimensional dirty
boson problem at finite temperature. This allows us to study the impact of both
temperature and disorder fluctuations on the respective components of the
density as well as their Thomas-Fermi radii. In particular, we find that a
superfluid region, a Bose-glass region, and a thermal region coexist for
smaller disorder strengths. Furthermore, depending on the respective system
parameters, three phase transitions are detected, namely, one from the
superfluid to the Bose-glass phase, another one from the Bose-glass to the
thermal phase, and finally one from the superfluid to the thermal phase. | cond-mat_quant-gas |
The in-plane gradient magnetic field induced vortex lattices in
spin-orbit coupled Bose-Einstein condensations: We consider the ground-state properties of the two-component spin-orbit
coupled ultracold bosons subject to a rotationally symmetric in-plane gradient
magnetic field. In the non-interacting case, the ground state supports
giant-vortices carrying large angular momenta without rotating the trap. The
vorticity is highly tunable by varying the amplitudes and orientations of the
magnetic field. Interactions drive the system from a giant-vortex state to
various configurations of vortex lattice states along a ring. Vortices exhibit
ellipse-shaped envelops with the major and minor axes determined by the
spin-orbit coupling and healing lengths, respectively. Phase diagrams of vortex
lattice configurations are constructed and their stabilities are analyzed. | cond-mat_quant-gas |
Localization transition in weakly-interacting Bose superfluids in
one-dimensional quasiperdiodic lattices: We study the localization of collective pair excitations in
weakly-interacting Bose superfluids in one-dimensional quasiperiodic lattices.
The localization diagram is first determined numerically. For intermediate
interaction and quasiperiodic amplitude we find a sharp localization
transition, with extended low-energy states and localized high-energy states.
We then develop an analytical treatment, which allows us to quantitatively map
the localization transition into that of an effective multiharmonic
quasiperiodic system. | cond-mat_quant-gas |
Quantum-tunneling dynamics of a spin-polarized Fermi gas in a
double-well potential: We study the exact dynamics of a one-dimensional spin-polarized gas of
fermions in a double-well potential at zero and finite temperature. Despite the
system is made of non-interacting fermions, its dynamics can be quite complex,
showing strongly aperiodic spatio-temporal patterns during the tunneling. The
extension of these results to the case of mixtures of spin-polarized fermions
in interaction with self-trapped Bose-Einstein condensates (BECs) at zero
temperature is considered as well. In this case we show that the fermionic
dynamics remains qualitatively similar to the one observed in absence of BEC
but with the Rabi frequencies of fermionic excited states explicitly depending
on the number of bosons and on the boson-fermion interaction strength. From
this, the possibility to control quantum fermionic dynamics by means of
Feshbach resonances is suggested. | cond-mat_quant-gas |
Lattice Polaron in a Bose-Einstein Condensate of Hard-Core Bosons: Lattice polarons, quasiparticles arising from the interaction between an
impurity and its surrounding bosonic environment confined to a lattice system,
have emerged as a platform for generating complex few-body states, probing
many-body phenomena, and addressing long-standing problems in physics. In this
study, we employ a variational ansatz to investigate the quasiparticle and
spectral properties of an impurity coupled to a condensate gas of hard-core
bosons in a two-dimensional optical lattice. Our findings demonstrate that the
polaron features can be tuned by adjusting the filling factor of the bath,
revealing intriguing polaron characteristics in the strongly interacting
regime. These results offer valuable insights for lattice polaron experiments
with ultracold gases and can serve as a guide for new experiments in emergent
quantum devices, such as moir\'e materials, where optical excitations can be
described in terms of hard-core bosons. | cond-mat_quant-gas |
Dispersions, weights, and widths of the single-particle spectral
function in the normal phase of a Fermi gas: The dispersions, weights, and widths of the peaks of the single-particle
spectral function in the presence of pair correlations, for a Fermi gas with
either attractive or repulsive short-range inter-particle interaction, are
determined in the normal phase over a wide range of wave vectors, with a
twofold purpose. The first one is to determine how these dispersions identify
both an energy scale known as the pseudo-gap near the Fermi wave vector, as
well as an additional energy scale related to the contact C at large wave
vectors. The second one is to differentiate the behaviors of the repulsive gas
from the attractive one in terms of crossing versus avoided crossing of the
dispersions near the Fermi wave vector. An analogy will also be drawn between
the occurrence of the pseudo-gap physics in a Fermi gas subject to pair
fluctuations and the persistence of local spin waves in the normal phase of
magnetic materials. | cond-mat_quant-gas |
On the finite-size effects in two segregated Bose-Einstein condensates
restricted by a hard wall: The finite-size effects in two segregated Bose-Einstein condensates (BECs)
restricted by a hard wall is studied by means of the Gross-Pitaevskii equations
in the double-parabola approximation (DPA). Starting from the consistency
between the boundary conditions (BCs) imposed on condensates in confined
geometry and in the full space, we find all possible BCs together with the
corresponding condensate profiles and interface tensions. We discover two
finite-size effects: a) The ground state derived from the Neumann BC is stable
whereas the ground states derived from the Robin and Dirichlet BCs are
unstable. b) Thereby, there equally manifest two possible wetting phase
transitions originating from two unstable states. However, the one associated
with the Robin BC is more favourable because it corresponds to a smaller
interface tension. | cond-mat_quant-gas |
Long-range s-wave interactions in Bose-Einstein Condensates: An exact
correspondence between truncated free energy and dynamics: We consider the Gross-Pitaevskii(GP) model of a Bose-Einstein Condensate(BEC)
with non-local s-wave interactions. The non-locality is represented by
corrections to the local GP equation. Due to such corrections to the GP
equation, there arise corrections to the free energy functional as well. We
present here a proof of the exact correspondence between the free energy and
the dynamics for typical terms appearing while considering corrections to the
GP equation at any order. This non-trivial correspondence can be used to study
BECs perturbatively while going beyond the Fermi pseudopotential. | cond-mat_quant-gas |
Realization of a sonic black hole analogue in a Bose-Einstein condensate: We have created an analogue of a black hole in a Bose-Einstein condensate. In
this sonic black hole, sound waves, rather than light waves, cannot escape the
event horizon. A step-like potential accelerates the flow of the condensate to
velocities which cross and exceed the speed of sound by an order of magnitude.
The Landau critical velocity is therefore surpassed. The point where the flow
velocity equals the speed of sound is the sonic event horizon. The effective
gravity is determined from the profiles of the velocity and speed of sound. A
simulation finds negative energy excitations, by means of Bragg spectroscopy. | 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 |
Quantum fluctuations inhibit symmetry breaking in the HMF model: It is widely believed that mean-field theory is exact for a wide-range of
classical long-range interacting systems. Is this also true once quantum
fluctuations have been accounted for? As a test case we study the Hamiltonian
Mean Field (HMF) model for a system of indistinguishable bosons which is
predicted (according to mean-field theory) to undergo a second-order quantum
phase transition at zero temperature. The ordered phase is characterized by a
spontaneously broken $O(2)$ symmetry, which, despite occurring in a
one-dimensional model, is not ruled out by the Mermin-Wagner theorem due to the
presence of long-range interactions. Nevertheless, a spontaneously broken
symmetry implies gapless Goldstone modes whose large fluctuations can restore
broken symmetries. In this work, we study the influence of quantum fluctuations
by projecting the Hamiltonian onto the continuous subspace of symmetry breaking
mean-field states. We find that the energetic cost of gradients in the center
of mass wavefunction inhibit the breaking of the $O(2)$ symmetry, but that the
energetic cost is very small --- scaling as $\mathcal{O}(1/N^2)$. Nevertheless,
for any finite $N$, no matter how large, this implies that the ground state has
a restored $O(2)$ symmetry. Implications for the finite temperature phases, and
classical limit, of the HMF model are discussed. | cond-mat_quant-gas |
Three-body correlations in a two-dimensional SU(3) Fermi gas: We consider a three-component Fermi gas that has SU(3) symmetry and is
confined to two dimensions (2D). For realistic cold atomic gas experiments, we
show that the phase diagram of the quasi-2D system can be characterized using
two 2D scattering parameters: the scattering length and the effective range.
Unlike the case in 3D, we argue that three-body bound states (trimers) in the
quasi-2D system can be stable against three-body losses. Using a low-density
expansion coupled with a variational approach, we investigate the fate of such
trimers in the many-body system as the attractive interactions are decreased
(or, conversely, as the density of particles is increased). We find that
remnants of trimers can persist in the form of strong three-body correlations
in the weak-coupling (high-density) limit. | cond-mat_quant-gas |
Extended Bose-Hubbard models with Rydberg macrodimer dressing: Extended Hubbard models have proven to bear novel quantum states, but their
experimental realization remains challenging. In this work we propose to use
bosonic quantum gases dressed with molecular bound states in Rydberg
interaction potentials for the observation of these quantum states. We study
the molecular Rabi coupling with respect to principal quantum number and
trapping frequency of the ground state atoms for various molecular potentials
of Rubidium and Potassium, and the hereby resulting dressed interaction
strength. Additionally, we propose a two-color excitation scheme which
significantly increases the dressed interaction and cancels AC Stark shifts
limiting the atomic motion in the itinerant regime. We study the various
equilibrium phases of the corresponding extended Bose-Hubbard model by means of
the Cluster Gutzwiller approach and perform time evolution simulations via the
Lindblad master equation. We find a supersolid phase by slowly ramping the
molecular Rabi coupling of an initially prepared superfluid and discuss the
role of dissipation. | cond-mat_quant-gas |
Diagrammatic Monte Carlo algorithm for the resonant Fermi gas: We provide a description of a diagrammatic Monte Carlo algorithm for the
resonant Fermi gas in the normal phase. Details are given on diagrammatic
framework, Monte Carlo moves, and incorporation of ultraviolet asymptotics.
Apart from the self-consistent bold scheme, we also describe a
non-self-consistent scheme, for which the ultraviolet treatment is more
involved. | cond-mat_quant-gas |
Many-body excitations and de-excitations in trapped ultracold bosonic
clouds: We employ the MultiConfiguraional Time-Dependent Hartree for Bosons (MCTDHB)
method to study excited states of interacting Bose-Einstein condensates
confined by harmonic and double-well trap potentials. Two approaches to access
excitations, a static and a dynamic one, have been studied and contrasted. In
static simulations the low-lying excitations have been computed by utilizing
the LR-MCTDHB method - a linear response theory constructed on-top of a static
MCTDHB solution. Complimentary, we propose two dynamic protocols that address
excitations by propagating the MCTDHB wave-function. In particular, we
investigate dipole-like oscillations induced by shifting the origin of the
confining potential and breathing-like excitations by quenching frequency of a
parabolic part of the trap. To contrast static predictions and dynamic results
we have computed time-evolutions and their Fourier transforms of several local
and non-local observables. Namely, we study evolution of the $\left< x(t)
\right>$, its variance $\operatorname{Var}(x(t))$, and of a local density
computed at a selected position. We found out that the variance is the most
sensitive and informative quantity - along with excitations it contains
information about the de-excitations even in a linear regime of the induced
dynamics. The dynamic protocols are found to access the many-body excitations
predicted by the static LR-MCTDHB approach. | cond-mat_quant-gas |
Exotic Superfluid Phases in Spin Polarized Systems on Optical Lattices: Leveraging cutting-edge numerical methodologies, we study the ground state of
the two-dimensional spin-polarized Fermi gas in an optical lattice. We focus on
systems at high density and small spin polarization, corresponding to the
parameter regime believed to be most favorable to the formation of the elusive
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid phase. Our systematic study
of large lattice sizes, hosting nearly $500$ atoms, provides strong evidence of
the stability of the FFLO state in this regime, as well as a high-accuracy
characterization of its properties. Our results for the density correlation
function reveal the existence of density order in the system, suggesting the
possibility of an intricate coexistence of long-range orders in the ground
state. The ground-state properties are seen to differ significantly from the
standard mean-field description, providing a compelling avenue for future
theoretical and experimental explorations of the interplay between interaction
and superfluidity in an exotic phase of matter. | cond-mat_quant-gas |
Non-integrable dynamics of matter-wave solitons in a density-dependent
gauge theory: We study interactions between bright matter-wave solitons which acquire
chiral transport dynamics due to an optically-induced density-dependent gauge
potential. Through numerical simulations, we find that the collision dynamics
feature several non-integrable phenomena, from inelastic collisions including
population transfer and radiation losses to short-lived bound states and
soliton fission. An effective quasi-particle model for the interaction between
the solitons is derived by means of a variational approximation, which
demonstrates that the inelastic nature of the collision arises from a coupling
of the gauge field to velocities of the solitons. In addition, we derive a set
of interaction potentials which show that the influence of the gauge field
appears as a short-range potential, that can give rise to both attractive and
repulsive interactions. | cond-mat_quant-gas |
Verifying the observer dependence of quasiparticle counts in the
analogue gravity of dilute ultracold quantum gases: The quasiparticle content of a quantum field depends on the observer, in
particular on its motional state, on the way the observer's detector couples to
the quantum field, and on the frequency standard in which the detector carried
by the observer measures the quanta to be detected. I review a procedure of
making this fundamental property of quantum field theory experimentally
manifest using quantum-optical means in Bose-Einstein condensates. | cond-mat_quant-gas |
2DEG on a cylindrical shell with a screw dislocation: A two dimensional electron gas on a cylindrical surface with a screw
dislocation is considered. More precisely, we investigate how both the geometry
and the deformed potential due to a lattice distortion affect the Landau levels
of such system. The case showing the deformed potential can be thought in the
context of 3D common semiconductors where the electrons are confined on a
cylindrical shell. We will show that important quantitative differences exist
due to this lattice distortion. For instance, the effective cyclotron frequency
is diminished by the deformed potential, which in turn enhances the Hall
conductivity. | cond-mat_quant-gas |
Rectification in Nonequilibrium Steady States of Open Many-Body Systems: We study how translationally invariant couplings of many-particle systems and
nonequilibrium baths can be used to rectify particle currents, for which we
consider minimal setups to realize bath-induced currents in nonequilibrium
steady states of one-dimensional open fermionic systems. We first analyze
dissipative dynamics associated with a nonreciprocal Lindblad operator and
identify a class of Lindblad operators that are sufficient to acquire a
unidirectional current. We show that unidirectional particle transport can in
general occur when a Lindblad operator is reciprocal provided that the
inversion symmetry and the time-reversal symmetry of the microscopic
Hamiltonian are broken. We demonstrate this mechanism on the basis of both
analytical and numerical approaches including the Rashba spin-orbit coupling
and the Zeeman magnetic field. | cond-mat_quant-gas |
Role of spatial inhomogeneity in dissociation of trapped molecular
condensates: We theoretically analyze dissociation of a harmonically trapped Bose-Einstein
condensate of molecular dimers and examine how the spatial inhomogeneity of the
molecular condensate affects the conversion dynamics and the atom-atom pair
correlations in the short-time limit. Both fermionic and bosonic statistics of
the constituent atoms are considered. Using the undepleted molecular-field
approximation, we obtain explicit analytic results for the asymptotic behavior
of the second-order correlation functions and for the relative number squeezing
between the dissociated atoms in one, two and three spatial dimensions.
Comparison with the numerical results shows that the analytic approach employed
here captures the main underlying physics and provides useful insights into the
dynamics of dissociation for conversion efficiencies up to 10%. The results
show explicitly how the strength of atom-atom correlations and relative number
squeezing degrade with the reduction of the size of the molecular condensate. | cond-mat_quant-gas |
Time-resolved density correlations as probe of squeezing in toroidal
Bose-Einstein condensates: I study the evolution of mean field and linear quantum fluctuations in a
toroidal Bose-Einstein condensate, whose interaction strength is quenched from
a finite (repulsive) value to zero. The azimuthal equal-time density-density
correlation function is calculated and shows temporal oscillations with twice
the (final) excitation frequencies after the transition. These oscillations are
a direct consequence of positive and negative frequency mixing during
non-adiabatic evolution. I will argue that a time-resolved measurement of the
equal-time density correlator might be used to calculate the moduli of the
Bogoliubov coefficients and thus the amount of squeezing imposed on a mode,
i.e., the number of atoms excited out of the condensate. | cond-mat_quant-gas |
Mixed triplet and singlet pairing in multicomponent ultracold fermion
systems with dipolar interactions: The symmetry properties of the Cooper pairing problem for multi-component
ultra-cold dipolar molecular systems are investigated. The dipolar anisotropy
provides a natural and robust mechanism for both triplet and singlet Cooper
pairing to first order in the interaction strength. With a purely dipolar
interaction, the triplet $p_z$-like polar pairing is the most dominant. A
short-range attractive interaction can enhance the singlet pairing to be nearly
degenerate with the triplet pairing. We point out that these two pairing
channels can mix by developing a relative phase of $\pm\frac{\pi}{2}$, thus
spontaneously breaking time-reversal symmetry. We also suggest the possibility
of such mixing of triplet and singlet pairing in other systems. | cond-mat_quant-gas |
Design of laser-coupled honeycomb optical lattices supporting Chern
insulators: We introduce an explicit scheme to realize Chern insulating phases employing
cold atoms trapped in a state-dependent optical lattice and laser-induced
tunneling processes. The scheme uses two internal states, a ground state and a
long-lived excited state, respectively trapped in separate triangular and
honeycomb optical lattices. A resonant laser coherently coupling the two
internal states enables hopping between the two sublattices with a Peierls-like
phase factor. Although laser-induced hopping by itself does not lead to
topological bands with non-zero Chern numbers, we find that such bands emerge
when adding an auxiliary lattice that perturbs the lattice structure,
effectively turning it at low energies into a realization of the Haldane model:
A two-dimensional honeycomb lattice breaking time-reversal symmetry. We
investigate the parameters of the resulting tight-binding model using
first-principles band structure calculations to estimate the relevant regimes
for experimental implementation. | cond-mat_quant-gas |
Beyond-mean-field stochastic corrections to the blueshift of a
driven-dissipative exciton-polariton condensate: In the absence of vortices or phase slips, the phase dynamics of
exciton-polariton condensates was shown to map onto the Kardar-Parisi-Zhang
(KPZ) equation, which describes the stochastic growth of a classical interface.
This implies that the coherence of such non-equilibrium quasi-condensates
decays in space and time following stretched exponentials, characterized by KPZ
universal critical exponents. In this work, we focus on the time evolution of
the average phase of a one-dimensional exciton-polariton condensate in the KPZ
regime and determine the frequency of its evolution, which is given by the
blueshift, i.e. the non-equilibrium analog of the chemical potential. We
determine the stochastic corrections to the blueshift within Bogoliubov
linearized theory and find that while this correction physically originates
from short scale effects, and depends both on density and phase fluctuations,
it can still be related to the effective large-scale KPZ parameters. Using
numerical simulations of the full dynamics, we investigate the dependence of
these blueshift corrections on both noise and interaction strength, and compare
the results to the Bogoliubov prediction. Our finding contributes both to the
close comparison between equilibrium and non-equilibrium condensates, and to
the theoretical understanding of the KPZ mapping. | cond-mat_quant-gas |
Analytical results for Josephson dynamics of ultracold Bosons: We study the dynamics of ultracold Bosons in a double-well potential within
the two-mode Bose-Hubbard model by means of semiclassical methods. By applying
a WKB quantization we find analytical results for the energy spectrum, which
are in excellent agreement with numerical exact results. They are valid in the
energy range of plasma oscillations, both in the Rabi and the Josephson regime.
Adopting the reflection principle and the Poisson summation formula we derive
an analytical expression for the dynamics of the population imbalance depending
on the few relevant parameters of the system only. This allows us to discuss
its characteristic dynamics, especially the oscillation frequency, and the
collapse- and revival time, as a function of the model parameters, leading to a
deeper understanding of Josephson physics. We find that our fomulae match
previous experimental observations. | cond-mat_quant-gas |
Oblique Half-Solitons and their Generation in Exciton-Polariton
Condensates: We describe oblique half-solitons, a new type of topological defects in a two
dimensional spinor Bose Einstein condensate. A realistic protocol based on the
optical spin Hall effect is proposed toward their generation within an
exciton-polariton system. | cond-mat_quant-gas |
Bose-Einstein Condensation in a Plasmonic Lattice: Bose-Einstein condensation is a remarkable manifestation of quantum
statistics and macroscopic quantum coherence. Superconductivity and
superfluidity have their origin in Bose-Einstein condensation. Ultracold
quantum gases have provided condensates close to the original ideas of Bose and
Einstein, while condensation of polaritons and magnons have introduced novel
concepts of non-equilibrium condensation. Here, we demonstrate a Bose-Einstein
condensate (BEC) of surface plasmon polaritons in lattice modes of a metal
nanoparticle array. Interaction of the nanoscale-confined surface plasmons with
a room-temperature bath of dye molecules enables thermalization and
condensation in picoseconds. The ultrafast thermalization and condensation
dynamics are revealed by an experiment that exploits thermalization under
propagation and the open cavity character of the system. A crossover from BEC
to usual lasing is realized by tailoring the band structure. This new
condensate of surface plasmon lattice excitations has promise for future
technologies due to its ultrafast, room-temperature and on-chip nature. | cond-mat_quant-gas |
Laser cooling to quantum degeneracy: We report on Bose-Einstein condensation (BEC) in a gas of strontium atoms,
using laser cooling as the only cooling mechanism. The condensate is formed
within a sample that is continuously Doppler cooled to below 1\muK on a
narrow-linewidth transition. The critical phase-space density for BEC is
reached in a central region of the sample, in which atoms are rendered
transparent for laser cooling photons. The density in this region is enhanced
by an additional dipole trap potential. Thermal equilibrium between the gas in
this central region and the surrounding laser cooled part of the cloud is
established by elastic collisions. Condensates of up to 10^5 atoms can be
repeatedly formed on a timescale of 100ms, with prospects for the generation of
a continuous atom laser. | cond-mat_quant-gas |
Incommensurability effects on dipolar bosons in optical lattices: We present a study that investigated a quantum dipolar gas in continuous
space where a potential lattice was imposed. Employing exact quantum Monte
Carlo techniques, we analysed the ground state properties of the scrutinised
system, varying the lattice depth and the dipolar interaction. For system
densities corresponding to a commensurate filling with respect to the optical
lattice, we observed a simple crystal-to-superfluid quantum phase transition,
being consistent with the physics of dipolar bosons in continuous space. In
contrast, an incommensurate density showed the presence of a supersolid phase.
Indeed, such a result opens up the tempting opportunity to observe a
defect-induced supersolidity with dipolar gases in combination with a tunable
optical lattice. Finally, the stability of the condensate was analysed at
finite temperature. | cond-mat_quant-gas |
Vortex dynamics and skyrmions in four to six dimensions: Coherence
vortices in Bose-Einstein condensates: I point out how coherence vortices, i.e., topological defects in a
correlation function, could help explore new physics if they are created in
matter waves. Vortex dynamics could be studied in up to six dimensions, and
spin topological defects unseen in lower dimensions could be created. A
rudimentary proof-of-principle experiment is sketched and simulated, in which
three Bose-Einstein condensates are used to create and detect coherence
vortices. | cond-mat_quant-gas |
Highly polarized Fermi gases in two dimensions: We investigate the highly polarized limit of a two-dimensional (2D) Fermi
gas, where we effectively have a single spin-down impurity atom immersed in a
spin-up Fermi sea. By constructing variational wave functions for the impurity,
we map out the ground state phase diagram as a function of mass ratio M/m and
interaction strength. In particular, we determine when it is favorable for the
dressed impurity (polaron) to bind particles from the Fermi sea to form a
dimer, trimer or even larger clusters. Similarly to 3D, we find that the Fermi
sea favors the trimer state so that it exists for M/m less than the critical
mass ratio for trimer formation in the vacuum. We also find a region where
dimers have finite momentum in the ground state, a scenario which corresponds
to the Fulde-Ferrell-Larkin-Ovchinnikov superfluid state in the limit of large
spin imbalance. For equal masses (M=m), we compute rigorous bounds on the
polaron-dimer transition, and we show that the polaron energy and residue is
well captured by the variational approach, with the former quantity being in
good agreement with experiment. When there is a finite density of impurities,
we find that this polaron-dimer transition is preempted by a first-order
superfluid-normal transition at zero temperature, but it remains an open
question what happens at finite temperature. | cond-mat_quant-gas |
Low-energy prethermal phase and crossover to thermalization in nonlinear
kicked rotors: In the presence of interactions, periodically-driven quantum systems
generically thermalize to an infinite-temperature state. Recently, however, it
was shown that in random kicked rotors with local interactions, this long-time
equilibrium could be strongly delayed by operating in a regime of weakly
fluctuating random phases, leading to the emergence of a metastable thermal
ensemble. Here we show that when the random kinetic energy is smaller than the
interaction energy, this system in fact exhibits a much richer dynamical phase
diagram, which includes a low-energy pre-thermal phase characterized by a
light-cone spreading of correlations in momentum space. We develop a
hydrodynamic theory of this phase and find a very good agreement with exact
numerical simulations. We finally explore the full dynamical phase diagram of
the system and find that the transition toward full thermalization is
characterized by relatively sharp crossovers. | cond-mat_quant-gas |
A mobile ion in a Fermi sea: The remarkable single particle control of individual ions combined with the
versatility of ultracold atomic gases makes hybrid ion-atom system an exciting
new platform for quantum simulation of few- and many-body quantum physics.
Here, we study theoretically the properties of a mobile ion immersed in a
quantum degenerate gas of fermionic atoms. Using an effective low-energy
atom-ion interaction together with a well established approach that includes
exactly two-body correlations, we calculate the full spectral response of the
ion and demonstrate the existence of several quasiparticle branches, which are
charged analogues of the Fermi polaron observed in neutral atomic gases. Due to
the long-range nature of the atom-ion interaction, these ionic Fermi polarons
have several properties distinct from their neutral counterparts such as the
simultaneous presence of several stable states and smooth transitions from
repulsive to attractive polarons with increasing interaction strength.
Surprisingly, the residue of the ionic polaron is shown to increase with the
Fermi density for fixed interaction strength, which is in marked contrast to
the neutral polaron. The properties of the ionic polaron approach that of the
neutral polaron only in the low density limit where the average interparticle
spacing is larger than the characteristic length of the atom-ion interaction.
We finally analyse the effects of the Fermi gas on the molecular ions, which
are bound atom-dimer states. | cond-mat_quant-gas |
Self-consistent Description of Bose-Bose Droplets: Modified Gapless
Hartree-Fock-Bogoliubov Method: We define a formalism of a self-consistent description of the ground state of
a weakly interacting Bose system, accounting for higher order terms in
expansion of energy in the diluteness parameter. The approach is designed to be
applied to a Bose-Bose mixture in a regime of weak collapse where quantum
fluctuations lead to stabilization of the system and formation of quantum
liquid droplets. The approach is based on the Generalized Gross -- Pitaevskii
equation accounting for quantum depletion and anomalous density terms. The
equation is self-consistently coupled to modified Bogoliubov equations.
The modification we introduce resolves the longstanding issue of missing
phonon-branch excitations when higher order terms are included. Our method
ensures a gapless phononic low-energy excitation spectrum, crucial to correctly
account for quantum fluctuations. We pay particular attention to the case of
droplets harmonically confined in some directions. The method allows to
determine the Lee-Huang-Yang-type contribution to the chemical potential of
inhomogeneous droplets when the local density approximation fails. | cond-mat_quant-gas |
Scattering of matter wave solitons on localized potentials: We present numerical and analytical results for the reflection and
transmission properties of matter wave solitons impinging on localized
scattering potentials in one spatial dimension. Our mean field analysis
identifies regimes where the solitons behave more like waves or more like
particles as a result of the interplay between the dispersive wave propagation
and the attractive interactions between the atoms. For a bright soliton
propagating together with a dark soliton void in a two-species Bose-Einstein
condensate of atoms with repulsive interactions, we find different reflection
and transmission properties of the dark and the bright components. | cond-mat_quant-gas |
Topological charge pumping of bound bosonic pairs: Experiments with bosonic atoms in optical superlattices allow for the
interesting possibility to study the adiabatic quantized pumping of bosonic
atoms in the presence of interactions. We investigate this exotic phenomenon
for bound bosonic pairs in the paradigmatic Su-Schrieffer-Heeger model where
the ground state exhibits topological phase transitions due to dimerized
hoppings. At unit filling we show that there exist crossovers and phase
transitions to bond-order phases of paired bosons known as pair-bond-order
phase as a function of attractive interactions. The pair bond order phase is
found to exhibit effective topological properties such as the presence of
polarized paired edge states. This is further analyzed by studying the
emergence and breakdown of the Thouless charge pumping of this bound bosonic
pairs by a parametric extension to the famous Rice-Mele model. Finally we
discuss how the pumping of paired bosons or different regimes of breakdown of
charge pumping can be probed by state-of-the art experiments with repulsively
bound bosons. | cond-mat_quant-gas |
Spontaneous inhomogeneous phases in ultracold dipolar Fermi gases: We study the collapse of ultracold fermionic gases into inhomogeneous states
due to strong dipolar interaction in both 2D and 3D. Depending on the
dimensionality, we find that two different types of inhomogeneous states are
stabilized once the dipole moment reaches a critical value $d>d_c$: the {\it
stripe phase} and {\it phase separation} between high and low densities. In 2D,
we prove that the stripe phase is always favored for $d\gtrsim d_c$, regardless
of the microscopic details of the system. In 3D, the one-loop perturbative
calculation suggests that the same type of instability leads to phase
separation. Experimental detection and finite-temperature effects are
discussed. | cond-mat_quant-gas |
Stabilization of a nonlinear bullet coexisting with a Bose-Einstein
condensate in a rapidly cooled magnonic system driven by a spin-orbit torque: We have recently shown that injection of magnons into a magnetic dielectric
via the spin-orbit torque (SOT) effect in the adjacent layer of a heavy metal
subjected to the action of short (0.1 $\mu$s) current pulses allows for control
of a magnon Bose-Einstein Condensate (BEC). Here, the BEC was formed in the
process of rapid cooling (RC), when the electric current heating the sample is
abruptly terminated. In the present study, we show that the application of a
longer (1.0 $\mu$s) electric current pulse triggers the formation of a
nonlinear localized magnonic bullet below the linear magnon spectrum. After
pulse termination, the magnon BEC, as before, is formed at the bottom of the
linear spectrum, but the nonlinear bullet continues to exist, stabilized for
additional 30 ns by the same process of RC-induced magnon condensation. Our
results suggest that a stimulated condensation of excess magnons to all highly
populated magnonic states occurs. | cond-mat_quant-gas |
Quantum criticality of a Bose gas in an optical lattice near the Mott
transition: We derive the equation of state of bosons in an optical lattice in the
framework of the Bose-Hubbard model. Near the density-driven Mott transition,
the expression of the pressure P({\mu},T) versus chemical potential and
temperature is similar to that of a dilute Bose gas but with renormalized mass
m^* and scattering length a^*. m^* is the mass of the elementary excitations at
the quantum critical point governing the transition from the superfluid phase
to the Mott insulating phase, while a^* is related to their effective
interaction at low energy. We use a nonperturbative renormalization-group
approach to compute these parameters as a function of the ratio t/U between
hopping amplitude and on-site repulsion. | cond-mat_quant-gas |
Origin and evolution of the multiply-quantised vortex instability: We show that the dynamical instability of quantum vortices with more than a
single quantum of angular momentum results from a superradiant bound state
inside the vortex core. Our conclusion is supported by an analytic WKB
calculation and numerical simulations of both linearised and fully non-linear
equations of motion for a doubly-quantised vortex at the centre of a circular
bucket trap. In the late stage of the instability, we reveal a striking novel
behaviour of the system in the non-linear regime. Contrary to expectation, in
the absence of dissipation the system never enters the regime of two
well-separated phase defects described by Hamiltonian vortex dynamics. Instead,
the separation between the two defects undergoes modulations which never exceed
a few healing lengths, in which compressible kinetic energy and incompressible
kinetic energy are exchanged. This suggests that, under the right conditions,
pairs of vortices may be able to form meta-stable bound states. | cond-mat_quant-gas |
Exciton-polariton X-waves in a microcavity: We investigate the possibility of creating X-waves, or localized wave
packets, in resonantly excited exciton-polariton superfluids. We demonstrate
the existence of X-wave traveling solutions in the coupled exciton-photon
system past the inflection point, where the effective mass of lower polaritons
is negative in the direction perpendicular to the wavevector of the pumping
beam. Contrary to the case of bright solitons, X-waves do not require
nonlinearity for sustaining their shape. Nevertheless, we show that
nonlinearity is important for their dynamics, as it allows for their
spontaneous formation from an initial Gaussian wave packet. Unique properties
of exciton-polaritons may lead to applications of their X-waves in
long-distance signal propagation inside novel integrated optoelectronic
circuits based on excitons. | cond-mat_quant-gas |
Gapped spectrum in pair-superfluid bosons: We study the ground state of a bilayer system of dipolar bosons with dipoles
oriented by an external field perpendicularly to the two parallel planes. By
decreasing the interlayer distance, for a fixed value of the strength of the
dipolar interaction, the system undergoes a quantum phase transition from an
atomic to a pair superfluid. We investigate the excitation spectrum across this
transition by using microscopic approaches. Quantum Monte Carlo methods are
employed to obtain the static structure factors and intermediate scattering
functions in imaginary time. The dynamic response is calculated using both the
correlated basis functions (CBF) method and the approximate inversion of the
Laplace transform of the quantum Monte Carlo imaginary time data. In the atomic
phase, both density and spin excitations are gapless. However, in the
pair-superfluid phase a gap opens in the excitation energy of the spin mode.
For small separation between layers, the minimal spin excitation energy equals
the binding energy of a dimer and is twice the gap value. | cond-mat_quant-gas |
Effective dynamics of a tracer particle in a dense homogeneous quantum
gas: We investigate the mean field regime of the dynamics of a tracer particle in
a homogenous quantum gas. For a bosonic gas, we show that this regime is
constrained by the well known requirement of an appropriate mean field scaling
of the interaction. For fermions, however, we find an important qualitative
difference. Not only are fermions much more homogeneously distributed than
bosons but also deviations from the mean are due only to fast degrees of
freedom in the gas. This observation leads to an explanation of why a tracer
particle behaves freely in the dense homogeneous fermion gas despite of a
non-scaled interaction, i.e., despite of non-vanishing statistical
fluctuations. Finally, we indicate how the gained insight can be rigorously
justified. | cond-mat_quant-gas |
Role of conservation laws in the Density Matrix Renormalization Group: We explore matrix product state approximations to wavefunctions which have
spontaneously broken symmetries or are critical. We are motivated by the fact
that symmetries, and their associated conservation laws, lead to block-sparse
matrix product states. Numerical calculations which take advantage of these
symmetries run faster and require less memory. However, in symmetry-broken and
critical phases the block sparse ansatz yields less accurate energies. We
characterize the role of conservation laws in matrix product states and
determine when it is beneficial to make use of them. | cond-mat_quant-gas |
Quantum anomaly and 2D-3D crossover in strongly interacting Fermi gases: We present an experimental investigation of collective oscillations in
harmonically trapped Fermi gases through the crossover from two to three
dimensions. Specifically, we measure the frequency of the radial monopole or
breathing mode as a function of dimensionality in Fermi gases with tunable
interactions. The frequency of this mode is set by the adiabatic
compressibility and probes the thermodynamic equation of state. In 2D, a
dynamical scaling symmetry for atoms interacting via a {\delta}-potential
predicts the breathing mode to occur at exactly twice the harmonic confinement
frequency. However, a renormalized quantum treatment introduces a new length
scale which breaks this classical scale invariance resulting in a so-called
quantum anomaly. Our measurements deep in the 2D regime lie above the
scale-invariant prediction for a range of interaction strengths indicating the
breakdown of a {\delta}-potential model for atomic interactions. As the
dimensionality is tuned from 2D to 3D we see the breathing oscillation
frequency evolve smoothly towards the 3D limit. | cond-mat_quant-gas |
Atom chips with two-dimensional electron gases: theory of near surface
trapping and ultracold-atom microscopy of quantum electronic systems: We show that current in a two-dimensional electron gas (2DEG) can trap
ultracold atoms $<1 \mu$m away with orders of magnitude less spatial noise than
a metal trapping wire. This enables the creation of hybrid systems, which
integrate ultracold atoms with quantum electronic devices to give extreme
sensitivity and control: for example, activating a single quantized conductance
channel in the 2DEG can split a Bose-Einstein condensate (BEC) for atom
interferometry. In turn, the BEC offers unique structural and functional
imaging of quantum devices and transport in heterostructures and graphene. | cond-mat_quant-gas |
Magnon Bose-Einstein condensates: from time crystals and quantum
chromodynamics to vortex sensing and cosmology: Under suitable experimental conditions collective spin-wave excitations,
magnons, form a Bose-Einstein condensate (BEC) where the spins precess with a
globally coherent phase. Bose-Einstein condensation of magnons has been
reported in a few systems, including superfluid phases of $^3$He, solid state
systems such as Yttrium-iron-garnet (YIG) films, and cold atomic gases. Among
these systems, the superfluid phases of $^3$He provide a nearly ideal test
bench for coherent magnon physics owing to experimentally proven spin
superfluidity, the long lifetime of the magnon condensate, and the versatility
of the accessible phenomena. We first briefly recap the properties of the
different magnon BEC systems, with focus on superfluid $^3$He. The main body of
this review summarizes recent advances in application of magnon BEC as a
laboratory to study basic physical phenomena connecting to diverse areas from
particle physics and cosmology to new phases of condensed matter. This line of
research complements the ongoing efforts to utilize magnon BECs as probes and
components for potentially room-temperature quantum devices. In conclusion, we
provide a roadmap for future directions in the field of applications of magnon
BEC to fundamental research. | cond-mat_quant-gas |
Reactive collisions in confined geometries: We consider low energy threshold reactive collisions of particles interacting
via a van der Waals potential at long range in the presence of external
confinement and give analytic formulas for the confinement modified scattering
in such circumstances. The reaction process is described in terms of the short
range reaction probability. Quantum defect theory is used to express elastic
and inelastic or reaction collision rates analytically in terms of two
dimensionless parameters representing phase and reactivity. We discuss the
modifications to Wigner threshold laws for quasi-one-dimensional and
quasi-two-dimensional geometries. Confinement-induced resonances are suppressed
due to reactions and are completely absent in the universal limit where the
short-range loss probability approaches unity. | cond-mat_quant-gas |
Exact quantum dynamics of yrast states in the finite 1D Bose gas: We demonstrate that the quantum dynamics of yrast states in the
one-dimensional (1D) Bose gas gives an illustrative example to equilibration of
an isolated quantum many-body system. We first formulate the energy spectrum of
yrast states in terms of the dressed energy by applying the method of
finite-size corrections. We then review the exact time evolution of quantum
states constructed from yrast states shown by the Bethe ansatz. In time
evolution the density profile of an initially localized quantum state
constructed from yrast states collapses into a flat profile in the case of a
large particle number such as N=1000, while recurrence of the localized state
occurs in the case of a small particle number such as N=20. We suggest that the
dynamical relaxation behavior for the large N case is consistent with the
viewpoint of typicality for generic quantum states: the expectation values of
local operators valuated in most of quantum states are very close to those of
the micro-canonical ensemble. | cond-mat_quant-gas |
Mean-field dynamics of a Bose-Hubbard chain coupled to a non-Markovian
environment: We study the dynamics of an interacting Bose-Hubbard chain coupled to a
non-Markovian environment. Our basic tool is the reduced generating functional
expressed as a path integral over spin-coherent states. We calculate the
leading contribution to the corresponding effective action, and by minimizing
it, we derive mean-field equations that can be numerically solved. With this
tool at hand, we examine the influence of the system's initial conditions and
interparticle interactions on the dissipative dynamics. Moreover, we
investigate the presence of memory effects due to the non-Markovian
environment. | cond-mat_quant-gas |
The decay and collisions of dark solitons in superfluid Fermi gases: We study soliton collisions and the decay of solitons into sound in
superfluid Fermi gases across the Bose-Einstein condensate to
Bardeen-Cooper-Schrieffer (BEC-BCS) crossover by performing numerical
simulations of the time-dependent Bogoliubov-de Gennes equations. This decay
process occurs when the solitons are accelerated to the bulk pair-breaking
speed by an external potential. A similar decay process may occur when solitons
are accelerated by an inelastic collision with another soliton. We find that
soliton collisions become increasingly inelastic as we move from the BEC to BCS
regimes, and the excess energy is converted into sound. We interpret this
effect as being due to evolution of Andreev bound states localized within the
soliton. | cond-mat_quant-gas |
Truncation effects in the charge representation of the O(2) model: The O(2) model in Euclidean space-time is the zero-gauge-coupling limit of
the compact scalar quantum electrodynamics. We obtain a dual representation of
it called the charge representation. We study the quantum phase transition in
the charge representation with a truncation to ``spin $S$," where the quantum
numbers have an absolute value less than or equal to $S$. The charge
representation preserves the gapless-to-gapped phase transition even for the
smallest spin truncation $S = 1$. The phase transition for $S = 1$ is an
infinite-order Gaussian transition with the same critical exponents $\delta$
and $\eta$ as the Berezinskii-Kosterlitz-Thouless (BKT) transition, while there
are true BKT transitions for $S \ge 2$. The essential singularity in the
correlation length for $S = 1$ is different from that for $S \ge 2$. The
exponential convergence of the phase-transition point is studied in both
Lagrangian and Hamiltonian formulations. We discuss the effects of replacing
the truncated $\hat{U}^{\pm} = \exp(\pm i \hat{\theta})$ operators by the spin
ladder operators $\hat{S}^{\pm}$ in the Hamiltonian. The marginal operators
vanish at the Gaussian transition point for $S = 1$, which allows us to extract
the $\eta$ exponent with high accuracy. | cond-mat_quant-gas |
Three-body interaction effects on the ground state of one-dimensional
anyons: A quantum phase transition driven by the statistics was observed in an
anyon-Hubbard model with local three-body interactions. Using a fractional
Jordan-Wigner transformation, we arrived at a modified Bose-Hubbard model,
which exhibits Mott insulator and superfluid phases. The absence of a Mott
insulator state with one particle per site depends on the anyonic angle, and a
quantum phase transition from a superfluid to a Mott insulator state is
obtained for a fixed value of the hopping. The critical points were estimated
with the von Neumann block entropy and increase as the hopping grows. The
statistics modify the ground state, and three different superfluid regions were
observed for larger values of the anyonic angle. | cond-mat_quant-gas |
Non equilibrium phase transitions and Floquet Kibble-Zurek scaling: We study the slow crossing of non-equilibrium quantum phase transitions in
periodically-driven systems. We explicitly consider a spin chain with a uniform
time-dependent magnetic field and focus on the Floquet state that is
adiabatically connected to the ground state of the static model. We find that
this {\it Floquet ground state} undergoes a series of quantum phase transitions
characterized by a non-trivial topology. To dinamically probe these
transitions, we propose to start with a large driving frequency and slowly
decrease it as a function of time. Combining analytical and numerical methods,
we uncover a Kibble-Zurek scaling that persists in the presence of moderate
interactions. This scaling can be used to experimentally demonstrate
non-equilibrium transitions that cannot be otherwise observed. | cond-mat_quant-gas |
Thermodynamics of rotating Bose gases in a trap: Novel ground state properties of rotating Bose gases have been intensively
studied in the context of neutral cold atoms. We investigate the rotating Bose
gas in a trap from a thermodynamic perspective, taking the charged ideal Bose
gas in magnetic field (which is equivalent to a neutral gas in a synthetic
magnetic field) as an example. It is indicated that the Bose-Einstein
condensation temperature is irrelevant to the magnetic field, conflicting with
established intuition that the critical temperature decreases with the field
increasing. The specific heat and Landau diamagnetization also exhibit
intriguing behaviors. In contrast, we demonstrate that the condensation
temperature for neutral Bose gases in a rotating frame drops to zero in the
fast rotation limit, signaling a non-condensed quantum phase in the ground
state. | cond-mat_quant-gas |
Emergent spacetimes from Hermitian and non-Hermitian quantum dynamics: We show that quantum dynamics of any systems with $SU(1,1)$ symmetry give
rise to emergent Anti-de Sitter spacetimes in 2+1 dimensions (AdS$_{2+1}$).
Using the continuous circuit depth, a quantum evolution is mapped to a
trajectory in AdS$_{2+1}$. Whereas the time measured in laboratories becomes
either the proper time or the proper distance, quench dynamics follow geodesics
of AdS$_{2+1}$. Such a geometric approach provides a unified interpretation of
a wide range of prototypical phenomena that appear disconnected. For instance,
the light cone of AdS$_{2+1}$ underlies expansions of unitary fermions released
from harmonic traps, the onsite of parametric amplifications, and the
exceptional points that represent the $PT$ symmetry breaking in non-Hermitian
systems. Our work provides a transparent means to optimize quantum controls by
exploiting shortest paths in the emergent spacetimes. It also allows
experimentalists to engineer emergent spacetimes and induce tunnelings between
different AdS$_{2+1}$. | cond-mat_quant-gas |
Active Learning Approach to Optimization of Experimental Control: In this work we present a general machine learning based scheme to optimize
experimental control. The method utilizes the neural network to learn the
relation between the control parameters and the control goal, with which the
optimal control parameters can be obtained. The main challenge of this approach
is that the labeled data obtained from experiments are not abundant. The
central idea of our scheme is to use the active learning to overcome this
difficulty. As a demonstration example, we apply our method to control
evaporative cooling experiments in cold atoms. We have first tested our method
with simulated data and then applied our method to real experiments. We
demonstrate that our method can successfully reach the best performance within
hundreds of experimental runs. Our method does not require knowledge of the
experimental system as a prior and is universal for experimental control in
different systems. | cond-mat_quant-gas |
Interaction-induced dynamical phase locking of Bose-Einstein condensates: We show that interactions result in the emergence of a {\it definite}
relative-phase between two initially incoherent Bose-Einstein condensates. The
many-realization interference fringe visibility is universal at
$g_{12}^{(1)}\sim1/3$ throughout the Josephson interaction regime, as evident
from a semiclassical picture. Other types of incoherent preparation yield
qualitatively different coherence dynamics. | cond-mat_quant-gas |
Stable p-wave resonant two-dimensional Fermi-Bose dimers: We consider two-dimensional weakly-bound heterospecies molecules formed in a
Fermi-Bose mixture with attractive Fermi-Bose and repulsive Bose-Bose
interactions. Bosonic exchanges lead to an intermolecular attraction, which can
be controlled and tuned to a p-wave resonance. Such attractive fermionic
molecules can be realized in quasi-two-dimensional ultracold isotopic or
heteronuclear mixtures. We show that they are stable with respect to the
recombination to deeply-bound molecular states and with respect to the
formation of higher-order clusters (trimers, tetramers, etc.) | cond-mat_quant-gas |
Dynamical emergence of a Kosterlitz-Thouless transition in a disordered
Bose gas following a quench: We study the dynamical evolution of a two-dimensional Bose gas after a
disorder potential quench. Depending on the initial conditions, the system
evolves either to a thermal or a superfluid state. Using extensive quasi-exact
numerical simulations, we show that the two phases are separated by a
Kosterlitz-Thouless transition. The thermalization time is shown to be longer
in the superfluid phase, but no critical slowing down is observed at the
transition. The long-time phase diagram is well reproduced by a simple
theoretical model. The spontaneous emergence of Kosterlitz-Thouless transitions
following a quench is a generic phenomenon that should arise both in the
context of non-equilibrium quantum gases and nonlinear, classical wave systems. | cond-mat_quant-gas |
Beyond Gross-Pitaevskii equation for 1D gas: quasiparticles and solitons: Describing properties of a strongly interacting quantum many-body system
poses a serious challenge both for theory and experiment. In this work, we
study excitations of one-dimensional repulsive Bose gas for arbitrary
interaction strength using a hydrodynamic approach. We use linearization to
study particle (type-I) excitations and numerical minimization to study hole
(type-II) excitations. We observe a good agreement between our approach and
exact solutions of the Lieb-Liniger model for the particle modes and
discrepancies for the hole modes. Therefore, the hydrodynamical equations find
to be useful for long-wave structures like phonons and of a limited range of
applicability for short-wave ones like narrow solitons. We discuss potential
further applications of the method. | cond-mat_quant-gas |
Eigenmodal Analysis of Anderson Localization: Applications to Photonic
Lattices and Bose-Einstein Condensates: We present the eigenmodal analysis techniques enhanced towards calculations
of optical and non-interacting Bose-Einstein condensate (BEC) modes formed by
random potentials and localized by Anderson effect. The results are compared
with the published measurements and verified additionally by the convergence
criterion. In 2-D BECs captured in circular areas, the randomness shows edge
localization of the high-order Tamm-modes. To avoid strong diffusive effect,
which is typical for BECs trapped by speckle potentials, a 3-D-lattice
potential with increased step magnitudes is proposed, and the BECs in these
lattices are simulated and plotted. | cond-mat_quant-gas |
Properties of 2D and Quasi-2D Dipolar Bosons with Non-zero Tilt Angles
at T=0: Recent experimental advances in creating stable dipolar bosonic systems,
including polar molecules with large electric dipole moments, have led to
vigorous theoretical activities. Recent reporting of observation of roton
feature in dipolar erbium has provided added impetus to theoretical and
experimental work. Here we discuss our mean-field theory work on 2D and
quasi-2D dipolar bosons with dipoles oriented at an angle to the direction
perpendicular to the confining 2D plane, i.e. for {\it non-zero tilt angles}.
Using Bogoliubov-de Gennes equations, we present results on a number of T=0
properties of both 2D and quasi-2D systems, such as excitation spectra,
structure functions, sound velocities, quantum depletion, etc. We explore
instabilities at varying tilt angle, density and dipolar coupling. We map out
phase diagrams as a function of tilt angle, dipole strength and density. We
find the development of maxon-roton behavior leading to roton instabilities at
large densities for small tilt angles, and at low densities for large tilt
angles. The behavior is anisotropic in k-space; accordingly the roton
instabilities occur first in the $k_y$ direction, suggestive of inhomogeneity
and stripe phase, with density mode becoming soft in the $y$-direction. Beyond
a critical tilt angle, at any density, the dipolar system collapses owing to a
phonon instability. We discuss similarities and differences between the
properties of 2D and quasi-2D dipolar systems at non-zero tilt angles. | cond-mat_quant-gas |
Tunnel-coupled optical microtraps for ultracold atoms: Arrays of individual atoms trapped in optical microtraps with
micrometer-scale sizes have emerged as a fundamental, versatile, and powerful
platform for quantum sciences and technologies. This platform enables the
bottom-up engineering of quantum systems, offering the capability of
low-entropy preparation of quantum states with flexible geometry, as well as
manipulation and detection at the single-site level. The utilization of
ultracold itinerant atoms with tunnel coupling in optical microtraps provides
new opportunities for quantum simulation, enabling the exploration of exotic
quantum states, phases, and dynamics, which would otherwise be challenging to
achieve in conventional optical lattices due to high entropy and limited
geometric flexibility. Here the development of tunnel-coupled optical
microtraps for the manipulation of ultracold atomic quantum systems and its
recent advances are briefly reviewed. | cond-mat_quant-gas |
Quantum spiral spin-tensor magnetism: The characterization of quantum magnetism in a large spin ($\geq 1$) system
naturally involves both spin-vectors and -tensors. While certain types of
spin-vector (e.g., ferromagnetic, spiral) and spin-tensor (e.g., nematic in
frustrated lattices) orders have been investigated separately, the coexistence
and correlation between them have not been well explored. Here we propose a
novel quantum spiral spin-tensor order on a spin-1 Heisenberg chain subject to
a spiral spin-tensor Zeeman field, which can be experimentally realized using a
Raman-dressed cold atom optical lattice. We develop a method to fully
characterize quantum phases of such spiral tensor magnetism with the
coexistence of spin-vector and spin-tensor orders as well as their correlations
using eight geometric parameters. Our method provides a powerful tool for
characterizing spin-1 quantum magnetism and opens an avenue for exploring novel
magnetic orders and spin-tensor electronics/atomtronics in large-spin systems. | cond-mat_quant-gas |
Microscopic picture of superfluid $^4$He: We elucidate the microscopic quantum mechanism of superfluid $^4$He by
uncovering a novel characteristic of its many-body energy levels. At
temperature below the transition point, the system's low-lying levels exhibit a
fundamental grouping behavior, wherein each level belongs exclusively to a
single group. In a superflow state, the system establishes thermal equilibrium
with its surroundings on a group-specific basis. Specifically, the levels of a
selected group, initially occupied, become thermally populated, while the
remaining groups of levels stay vacant due to absence of transitions between
groups. The macroscopic properties of the system, such as its superflow
velocity and thermal energy density, are statistically determined by the
thermal distribution of the occupied group. Additionally, we infer that the
thermal energy of a superflow has an unusual relationship with flow velocity,
such that the larger the flow velocity, the smaller the thermal energy. This
relationship is responsible for a range of intriguing phenomena, including the
mechano-caloric effect and the fountain effect, which highlight a fundamental
coupling between the thermal motion and hydrodynamic motion of the
system.Furthermore, we present experimental evidence of a counterintuitive
self-heating effect in $^4$He superflows, confirming that a $^4$He superflow
carries significant thermal energy related to its velocity. | cond-mat_quant-gas |
Role of higher-order interactions on the modulational instability of
Bose-Einstein condensate trapped in a periodic optical lattice: In this paper, we investigate the impact of higher-order interactions on the
modulational instability (MI) of Bose-Einstein Condensates (BECs) immersed in
an optical lattice potential. We derive the new variational equations for the
time evolution of amplitude, phase of modulational perturbation, and effective
potential for the system. Through effective potential techniques, we find that
high density attractive and repulsive BECs exhibit new character with direct
impact over the MI phenomenon. Results of intensive numerical investigations
are presented and their convergence with the above semi analytical approach is
brought out. | cond-mat_quant-gas |
A Proposal for measuring Anisotropic Shear Viscosity in Unitary Fermi
Gases: We present a proposal to measure anisotropic shear viscosity in a strongly
interacting, ultra-cold, unitary Fermi gas confined in a harmonic trap. We
introduce anisotropy in this setup by strongly confining the gas in one of the
directions with relatively weak confinement in the remaining directions. This
system has a close resemblance to anisotropic strongly coupled field theories
studied recently in the context of gauge-gravity duality. Computations in such
theories (which have gravity duals) revealed that some of the viscosity
components of the anisotropic shear viscosity tensor can be made much smaller
than the entropy density, thus parametrically violating the bound proposed by
Kovtun, Son and Starinets (KSS): $\frac {\eta} {s} \geq \frac{1}{4 \pi}$. A
Boltzmann analysis performed in a system of weakly interacting particles in a
linear potential also shows that components of the viscosity tensor can be
reduced. Motivated by these exciting results, we propose two hydrodynamic modes
in the unitary Fermi gas whose damping is governed by the component of shear
viscosity expected to violate the KSS bound. One of these modes is the well
known scissor mode. We estimate trap parameters for which the reduction in the
shear viscosity is of order unity and find that the trap geometry, the damping
timescales, and mode amplitudes are within the range of existing experimental
setups on ultra-cold Fermi gases. | cond-mat_quant-gas |
Dimensional Effects on the Momentum distribution of Bosonic Trimer
States: The momentum distribution is a powerful probe of strongly-interacting systems
that are expected to display universal behavior. This is contained in the
contact parameters which relate few- and many-body properties. Here we consider
a Bose gas in two dimensions and explicitly show that the two-body contact
parameter is universal and then demonstrate that the momentum distribution at
next-to-leading order has a logarithmic dependence on momentum which is vastly
different from the three-dimensional case. Based on this, we propose a scheme
for measuring the effective dimensionality of a quantum many-body system by
exploiting the functional form of the momentum distribution. | cond-mat_quant-gas |
Spin-incoherent Luttinger liquid of one-dimensional spin-1
Tonks-Girardeau Bose gas: Spin-dependent properties: Spin-incoherent Luttinger liquid (SILL) is a different universal class from
the Luttinger liquid.\ This difference results from the spin incoherence of the
system when the thermal energy of the system is higher than the spin excitation
energy.\ We consider one-dimensional spin-$1$ Bose gas in the SILL regime and
investigate its spin-dependent many-body properties.\ In Tonks-Girardeau limit,
we are able to write down the general wave functions in a harmonic trap.\ We
numerically calculate the spin-dependent (spin-plus, minus, and $0$) momentum
distributions in the sector of zero magnetization which allows to demonstrate
the most significant spin-incoherent feature compared to the spinless or
spin-polarized case.\ In contrast to the spinless Bose gas, the momentum
distributions are broadened and in the large momentum limit follow the same
asymptotic $1/p^4$ dependence but with reduced coefficients.\ While the density
matrices and momentum distributions differ between different spin components
for small $N$, at large $N$ they approach each other.\ We show these by
analytic arguments and numerical calculations up to $N$ $=$ $16$. | cond-mat_quant-gas |
The self-energy of an impurity in an ideal Fermi gas to second order in
the interaction strength: We study in three dimensions the problem of a spatially homogeneous
zero-temperature ideal Fermi gas of spin-polarized particles of mass $m$
perturbed by the presence of a single distinguishable impurity of mass $M$. The
interaction between the impurity and the fermions involves only the partial
$s$-wave through the scattering length $a$, and has negligible range $b$
compared to the inverse Fermi wave number $1/\kf$ of the gas. Through the
interactions with the Fermi gas the impurity gives birth to a quasi-particle,
which will be here a Fermi polaron (or more precisely a {\sl monomeron}). We
consider the general case of an impurity moving with wave vector $\KK\neq\OO$:
Then the quasi-particle acquires a finite lifetime in its initial momentum
channel because it can radiate particle-hole pairs in the Fermi sea. A
description of the system using a variational approach, based on a finite
number of particle-hole excitations of the Fermi sea, then becomes
inappropriate around $\KK=\mathbf{0}$. We rely thus upon perturbation theory,
where the small and negative parameter $\kf a\to0^-$ excludes any branches
other than the monomeronic one in the ground state (as e.g.\ the dimeronic
one), and allows us a systematic study of the system. We calculate the impurity
self-energy $\Sigma^{(2)}(\KK,\omega)$ up to second order included in $a$.
Remarkably, we obtain an analytical explicit expression for
$\Sigma^{(2)}(\KK,\omega)$ allowing us to study its derivatives in the plane
$(K,\omega)$. These present interesting singularities, which in general appear
in the third order derivatives $\partial^3 \Sigma^{(2)}(\KK,\omega)$. In the
special case of equal masses, $M=m$, singularities appear already in the
physically more accessible second order derivatives $\partial^2
\Sigma^{(2)}(\KK,\omega)$; using a self-consistent heuristic approach based on
$\Sigma^{(2)}$ we then regularise the divergence of the second order derivative
$\partial\_K^2 \Delta E(\KK)$ of the complex energy of the quasi-particle found
in reference [C. Trefzger, Y. Castin, Europhys. Lett. {\bf 104}, 50005 (2013)]
at $K=\kf$, and we predict an interesting scaling law in the neighborhood of
$K=\kf$. As a by product of our theory we have access to all moments of the
momentum of the particle-hole pair emitted by the impurity while damping its
motion in the Fermi sea, at the level of Fermi's golden rule. | cond-mat_quant-gas |
Slow quench dynamics of periodically driven quantum gases: We study the evolution of bosons in a periodically driven optical lattice
during a slow change of the driving amplitude. Both the regime of high
frequency and low frequency driving are investigated. In the low frequency
regime, resonant absorption of energy is observed. In the high frequency
regime, the dynamics is compared to a system with an effective Hamiltonian in
which the atoms are `dressed' by the driving field. This `dressing' can
dramatically change the amplitude and sign of the effective tunneling. A
particular focus of this study is the investigation of the time-scales
necessary for the evolving quantum state to follow almost adiabatically to the
ground-state of the effective many body system. | cond-mat_quant-gas |
Dynamical self-stabilization of the Mott insulator: Time evolution of
the density and entanglement entropy of out-of-equilibrium cold fermion gases: The time evolution of the out-of-equilibrium Mott insulator is investigated
numerically through calculations of space-time resolved density and entropy
profiles resulting from the release of a gas of ultracold fermionic atoms from
an optical trap. For adiabatic, moderate and sudden switching-off of the
trapping potential, the out-of-equilibrium dynamics of the Mott insulator is
found to differ profoundly from that of the band insulator and the metallic
phase, displaying a self-induced stability that is robust within a wide range
of densities, system sizes and interaction strengths. The connection between
the entanglement entropy and changes of phase, known for equilibrium
situations, is found to extend to the out-of-equilibrium regime. Finally, the
relation between the system's long time behavior and the thermalization limit
is analyzed. | cond-mat_quant-gas |
Observation of quasiparticle pair-production and quantum entanglement in
atomic quantum gases quenched to an attractive interaction: We report observation of quasiparticle pair-production and characterize
quantum entanglement created by a modulational instability in an atomic
superfluid. By quenching the atomic interaction to attractive and then back to
weakly repulsive, we produce correlated quasiparticles and monitor their
evolution in a superfluid through evaluating the in situ density noise power
spectrum, which essentially measures a 'homodyne' interference between ground
state atoms and quasiparticles of opposite momenta. We observe large amplitude
growth in the power spectrum and subsequent coherent oscillations in a wide
spatial frequency band within our resolution limit, demonstrating coherent
quasiparticle generation and evolution. The spectrum is observed to oscillate
below a quantum limit set by the Peres-Horodecki separability criterion of
continuous-variable states, thereby confirming quantum entanglement between
interaction quench-induced quasiparticles. | cond-mat_quant-gas |
Non-local correlation and entanglement of ultracold bosons in the
two-dimensional Bose-Hubbard lattice at finite temperature: We investigate the temperature-dependent behavior emerging in the vicinity of
the superfluid (SF) to Mott-insulator (MI) transition of interacting bosons in
a two-dimensional optical lattice, described by the Bose-Hubbard model. The
equilibrium phase diagram at finite temperature is computed using the cluster
mean-field (CMF) theory including a finite cluster-size scaling. The SF, MI,
and normal fluid (NF) phases are characterized as well as the transition or
crossover temperatures between them are estimated by computing physical
quantities such as the superfluid fraction, compressibility and sound velocity
using the CMF method. We find that the non-local correlations included in a
finite cluster, when extrapolated to infinite size, leads to quantitative
agreement of the phase boundaries with quantum Monte Carlo (QMC) results as
well as with experiments. Moreover, we show that the von Neumann entanglement
entropy within a cluster corresponds to the system's entropy density and that
it is enhanced near the SF-MI quantum critical point (QCP) and at the SF- NF
boundary. The behavior of the transition lines near this QCP, at and away from
the particle-hole (p-h) symmetric point located at the Mott-tip, is also
discussed. Our results obtained by using the CMF theory can be tested
experimentally using the quantum gas microscopy method. | cond-mat_quant-gas |
Band-gap structure and chiral discrete solitons in optical lattices with
artificial gauge fields: We study three-leg-ladder optical lattices loaded with repulsive atomic
Bose-Einstein condensates and subjected to artificial gauge fields. By
employing the plane-wave analysis and variational approach, we analyze the
band-gap structure of the energy spectrum and reveal the exotic swallow-tail
loop structures in the energy-level anti-crossing regions due to an interplay
between the atom-atom interaction and artificial gauge field. Also, we discover
stable discrete solitons residing in a semi-infinite gap above the highest
band, these discrete solitons are associated with the chiral edge currents. | cond-mat_quant-gas |
Phases, transitions, and boundary conditions in a model of interacting
bosons: We carry out an extensive study of the phase diagrams of the extended Bose
Hubbard model, with a mean filling of one boson per site, in one dimension by
using the density matrix renormalization group and show that it contains
Superfluid (SF), Mott-insulator (MI), density-wave (DW) and Haldane-insulator
(HI) phases. We show that the critical exponents and central charges for the
HI-DW, MI-HI and SF-MI transitions are consistent with those for models in the
two-dimensional Ising, Gaussian, and Berezinskii-Kosterlitz-Thouless (BKT)
universality classes, respectively; and we suggest that the SF-HI transition
may be more exotic than a simple BKT transition. We show explicitly that
different boundary conditions lead to different phase diagrams. | cond-mat_quant-gas |
Experimental realization of ultracold Yb-$^{7}{\rm Li}$ mixtures in
mixed dimensions: We report on the experimental realization of ultracold $^{174}{\rm
Yb}$-$^{7}{\rm Li}$ (Boson-Boson) and $^{173}{\rm Yb}$-$^{7}{\rm Li}$
(Fermion-Boson) mixtures. They are loaded into three dimensional (3D) or one
dimensional (1D) optical lattices that are species-selectively deep for the
heavy Ytterbium (Yb) and shallow for the light bosonic Lithium (Li) component,
realizing novel mixed dimensional systems. In the 1D optical lattice the band
structure of $^{173}{\rm Yb}$ is reconstructed in the presence of $^{7}{\rm
Li}$. Spectroscopic measurements of the $^{174}{\rm Yb}$-$^{7}{\rm Li}$ mixture
in the 3D lattice give access to the $^{174}{\rm Yb}$ Mott-insulator structure.
Ground state inter-species scattering lengths are determined to be $|a_{\rm
bg}(^{174}{\rm Yb}$-$^{7}{\rm Li})|=(1.11 \pm 0.17)~{\rm nm}$ and $|a_{\rm
bg}(^{173}{\rm Yb}$-$^{7}{\rm Li})|=(1.16 \pm 0.18)~{\rm nm}$. The formation
and characterization of an ultracold $^{173}{\rm Yb}$-$^{7}{\rm Li}$ mixture is
a first step towards a possible realization of a topological $p_x + i\,p_y$
superfluid in this system. | cond-mat_quant-gas |
Second-order response theory of radio-frequency spectroscopy for cold
atoms: We present a theoretical description of the radio-frequency (rf) spectroscopy
of fermionic atomic gases, based on the second-order response theory at finite
temperature. This approach takes into account the energy resolution due to the
envelope of the rf pulse. For a noninteracting final state, the momentum- and
energy-resolved rf intensity depends on the fermion spectral function and pulse
envelope. The contributions due to interactions in the final state can be
classified by means of diagrams. Using this formalism, as well as the local
density approximation in two and three dimensions, we study the interplay of
inhomogeneities and Hartree energy in forming the line shape of the rf signal.
We show that the effects of inhomogeneities can be minimized by taking
advantage of interactions in the final state, and we discuss the most relevant
final-state effects at low temperature and density, in particular the effect of
a finite lifetime. | cond-mat_quant-gas |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.