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Driven dissipative preparation of few-body Laughlin states of Rydberg
polaritons in twisted cavities: We present a driven dissipative protocol for creating an optical analog of
the Laughlin state in a system of Rydberg polaritons in a twisted optical
cavity. We envision resonantly driving the system into a 4-polariton state by
injecting photons in carefully selected modes. The dissipative nature of the
polariton-polariton interactions leads to a decay into a two-polariton analog
of the Laughlin state. Generalizations of this technique could be used to
explore fractional statistics and anyon based quantum information processing.
We also model recent experiments that attempt to coherently drive into this
same state. | cond-mat_quant-gas |
Mobility edge of the two dimensional Bose-Hubbard model: We analyze the disorder driven localization of the two dimensional
Bose-Hubbard model by evaluating the full low energy quasiparticle spectrum via
a recently developed fluctuation operator expansion method. For any strength of
the local interaction we find a mobility edge that displays an approximately
exponential decay with increasing disorder strength. We determine the
finite-size scaling collapse and exponents at this critical line finding that
the localization of excitations is characterized by weak multi-fractality and a
thermal-like critical gap ratio. A direct comparison to a recent experiment
yields an excellent match of the predicted finite-size transition point and
scaling of single particle correlations. | cond-mat_quant-gas |
Thermodynamics of the Hubbard model on stacked honeycomb and square
lattices: We present a numerical study of the Hubbard model on simply stacked honeycomb
and square lattices, motivated by a recent experimental realization of such
models with ultracold atoms in optical lattices. We perform simulations with
different interlayer coupling and interaction strengths and obtain N\'eel
transition temperatures and entropies. We provide data for the equation of
state to enable comparisons of experiments and theory. We find an enhancement
of the short-range correlations in the anisotropic lattices compared to the
isotropic cubic lattice, in parameter regimes suitable for the interaction
driven adiabatic cooling. | cond-mat_quant-gas |
Confinement in 1+1D $\mathbb{Z}_2$ Lattice Gauge Theories at Finite
Temperature: Confinement is a paradigmatic phenomenon of gauge theories, and its
understanding lies at the forefront of high-energy physics. Here, we study
confinement in a simple one-dimensional \Zt lattice gauge theory at finite
temperature and filling, which is within the reach of current cold-atom and
superconducting-qubit platforms. By employing matrix product states (MPS)
calculations, we investigate the decay of the finite-temperature Green's
function and uncover a smooth crossover between the confined and deconfined
regimes. This is furthermore confirmed by considering the Friedel oscillations
and string length distributions obtained from snapshots sampled from MPS, both
of which are experimentally readily available. Finally, we verify that confined
mesons remain well-defined at finite temperature by probing their quench
dynamics with exact diagonalization. Our results shed new light on confinement
at finite temperature from an experimentally relevant standpoint. | cond-mat_quant-gas |
Momentum-Space Josephson Effects: The Josephson effect is a prominent phenomenon of quantum supercurrents that
has been widely studied in superconductors and superfluids. Typical Josephson
junctions consist of two real-space superconductors (superfluids) coupled
through a weak tunneling barrier. Here we propose a momentum-space Josephson
junction in a spin-orbit coupled Bose-Einstein condensate, where states with
two diffferent momenta are coupled through Raman-assisted tunneling. We show
that Josephson currents can be induced not only by applying the equivalent of
"voltages", but also by tuning tunneling phases. Such tunneling-phase-driven
Josephson junctions in momentum space are characterized through both full mean
field analysis and a concise two-level model, demonstrating the important role
of interactions between atoms. Our scheme provides a platform for
experimentally realizing momentum-space Josephson junctions and exploring their
applications in quantum-mechanical circuits. | cond-mat_quant-gas |
Thermalization measurements on an ultracold mixture of metastable $^4$He
and $^{87}$Rb atoms in a quadrupole magnetic trap: Recently we have reported (Knoop et al. [arXiv:1404.4826]) on an experimental
determination of metastable triplet $^4$He+$^{87}$Rb scattering length by
performing thermalization measurements for an ultracold mixture in a quadrupole
magnetic trap. Here we present our experimental apparatus and elaborate on
these thermalization measurements. In particular we give a theoretical
description of interspecies thermalization rate for a quadrupole magnetic trap,
i. e. in the presence of Majorana heating, and a general procedure to extract
the scattering length from the elastic cross section at finite temperature
based on knowledge of the $C_6$ coefficient alone. In addition, from our
thermalization data we obtain an upper limit of the total interspecies two-body
loss rate coefficient of $1.5\times 10^{-12}$ cm$^3$s$^{-1}$. | cond-mat_quant-gas |
Temperature Dependent Density Profiles of Dipolar Droplets: Recently, trapped dipolar gases were observed to form high density droplets
in a regime where mean field theory predicts collapse. These droplets present a
novel form of equilibrium where quantum fluctuations are critical for
stability. So far, the effect of quantum fluctuations have only been considered
at zero temperature through the local chemical potential arising from the
Lee--Huang--Yang correction. Here, we extend the theory of dipolar droplets to
non-zero temperatures using Hartree--Fock--Bogoliubov theory (HFBT), and show
that the equilibrium is strongly affected by temperature fluctuations. HFBT,
together with local density approximation for excitations, reproduces the zero
temperature results, and predict that the condensate density can change
dramatically even at low temperatures where the total depletion is small.
Particularly, we find that typical experimental temperatures ($T \sim $ 100 nK)
can significantly modify the transition between low density and droplet phases. | cond-mat_quant-gas |
Quasi-one-dimensional Bose-Einstein condensates in nonlinear lattices: We consider the three-dimensional (3D) mean-field model for the Bose-Einstein
condensate (BEC), with a 1D nonlinear lattice (NL), which periodically changes
the sign of the nonlinearity along the axial direction, and the
harmonic-oscillator trapping potential applied in the transverse plane. The
lattice can be created as an optical or magnetic one, by means of available
experimental techniques. The objective is to identify stable 3D solitons
supported by the setting. Two methods are developed for this purpose: The
variational approximation, formulated in the framework of the 3D
Gross-Pitaevskii equation, and the 1D nonpolynomial Schr\"{o}dinger equation
(NPSE) in the axial direction, which allows one to predict the collapse in the
framework of the 1D description. Results are summarized in the form of a
stability region for the solitons in the plane of the NL strength and
wavenumber. Both methods produce a similar form of the stability region. Unlike
their counterparts supported by the NL in the 1D model with the cubic
nonlinearity, kicked solitons of the NPSE cannot be set in motion, but the kick
may help to stabilize them against the collapse, by causing the solitons to
shed excess norm. A dynamical effect specific to the NL is found in the form of
freely propagating small-amplitude wave packets emitted by perturbed solitons. | cond-mat_quant-gas |
Quenched Magneto-association of Ultracold Feshbach Molecules: We study enhanced magneto-association of atoms into weakly-bound molecules
near a Feshbach resonance using a quench preparatory stage. In anticipation of
experiments with NASA's Cold Atom Laboratory aboard the International Space
Station, we assume as a baseline a dual-species ($^{87}$Rb and $^{41}$K) gas in
a parameter regime enabled by a microgravity environment. This includes
subnanokelvin temperatures and dual-species gases at densities as low as
10$^8$/cm$^3$. Our studies indicate that, in such a regime, traditional
magneto-association schemes are inefficient due to the weak coupling between
atomic and molecular states at low-densities, thus requiring extremely long
magnetic field sweeps. To address this issue we propose a modified scheme where
atoms are quenched to unitarity before proceeding with magneto-association.
This substantially improves molecular formation, allowing for up to $80\%$
efficiency, and within time-scales much shorter than those associated to atomic
and molecular losses. We show that this scheme also applies at higher
densities, therefore proving to be of interest to ground-based experiments as
well. | cond-mat_quant-gas |
Dynamical mean-field driven spinor condensate physics beyond the
single-mode approximation: $^{23}$Na spin-1 Bose-Einstein condensates are used to experimentally
demonstrate that mean-field physics beyond the single-mode approximation can be
relevant during the non-equilibrium dynamics. The experimentally observed spin
oscillation dynamics and associated dynamical spatial structure formation
confirm theoretical predictions that are derived by solving a set of coupled
mean-field Gross-Pitaevskii equations [J. Jie et al., Phys. Rev. A 102, 023324
(2020)]. The experiments rely on microwave dressing of the $f=1$ hyperfine
states, where $f$ denotes the total angular momentum of the $^{23}$Na atom. The
fact that beyond single-mode approximation physics at the mean-field level,
i.e., spatial mean-field dynamics that distinguishes the spatial density
profiles associated with different Zeeman levels, can -- in certain parameter
regimes -- have a pronounced effect on the dynamics when the spin healing
length is comparable to or larger than the size of the Bose-Einstein condensate
has implications for using Bose-Einstein condensates as models for quantum
phase transitions and spin squeezing studies as well as for non-linear SU(1,1)
interferometers. | cond-mat_quant-gas |
Lattice supersolid phase of strongly correlated bosons in an optical
cavity: We numerically simulate strongly correlated ultracold bosons coupled to a
high-finesse cavity field, pumped by a laser beam in the transverse direction.
Assuming a weak classical optical lattice added in the cavity direction, we
model this system by a generalized Bose-Hubbard model, which is solved by means
of bosonic dynamical mean-field theory. The complete phase diagram is
established, which contains two novel self-organized quantum phases, lattice
supersolid and checkerboard solid, in addition to conventional phases such as
superfluid and Mott insulator. At finite but low temperature, thermal
fluctuations are found to enhance the buildup of the self-organized phases. We
demonstrate that cavity-mediated long-range interactions can give rise to
stable lattice supersolid and checkerboard solid phases even in the regime of
strong s-wave scattering. In the presence of a harmonic trap, we discuss
coexistence of these self-organized phases, as relevant to experiments. | cond-mat_quant-gas |
Scaling solutions of the two fluid hydrodynamic equations in a
harmonically trapped gas at unitarity: We prove that the two fluid Landau hydrodynamic equations, when applied to a
gas interacting with infinite scattering length (unitary gas) in the presence
of harmonic trapping, admit exact scaling solutions of mixed compressional and
surface nature. These solutions are characterized by a linear dependence of the
velocity field on the spatial coordinates and a temperature independent
frequency which is calculated in terms of the parameters of the trap. Our
results are derived in the regime of small amplitude oscillations and hold both
below and above the superfluid phase transition. They apply to isotropic as
well as to deformed configurations, thereby providing a generalization of
Castin's theorem (Y. Castin, C. R. Phys. \textbf{5}, 407 (2004)) holding for
isotropic trapping. Our predictions agree with the experimental findings in
resonantly interacting atomic Fermi gases. The breathing scaling solution, in
the presence of isotropic trapping, is also used to prove the vanishing of two
bulk viscosity coefficients in the superfluid phase. | cond-mat_quant-gas |
Confinement-induced Resonance of Alkaline-earth-metal-like Atoms in
Anisotropic Quasi-one-dimensional Traps: We study the confinement-induced resonance (CIR) of $^{173}$Yb atoms near an
orbital Feshbach resonance in a quasi-one-dimensional tube with transversal
anisotropy. By solving the two-body scattering problem, we obtain the location
of CIR for various anisotropy ratio and magnetic field. Our results show that
the anisotropy of the trapping potential can serve as an additional knob to
tune the location of CIR. In particular, one can shift the location of CIR to
the region attainable in current experiment. We also study the energy spectrum
of the system and analyze the properties of CIR from the perspective of bound
states. We find that as the orbital Feshbach resonance acquires two nearly
degenerate scattering channels, which in general have different threshold
energies, CIR takes place when the closed channel bound state energy becomes
degenerate with one of the thresholds. | cond-mat_quant-gas |
Supersolid phases of bosonic particles in a bubble trap: Confinement can have a considerable effect on the behavior of particle
systems, and is therefore an effective way to discover new phenomena. A notable
example is a system of identical bosons at low temperature under an external
field mimicking an isotropic bubble trap, which constrains the particles to a
portion of space close to a spherical surface. Using Path Integral Monte Carlo
simulations, we examine the spatial structure and superfluid fraction in two
emblematic cases. First, we look at soft-core bosons, finding the existence of
supersolid cluster arrangements with polyhedral symmetry; we show how different
numbers of clusters are stabilized depending on the trap radius and the
particle mass, and we characterize the temperature behavior of the cluster
phases. A detailed comparison with the behavior of classical soft-core
particles is provided too. Then, we examine the case, of more immediate
experimental interest, of a dipolar condensate on the sphere, demonstrating how
a quasi-one-dimensional supersolid of clusters is formed on a great circle for
realistic values of density and interaction parameters. Crucially, this
supersolid phase is only slightly disturbed by gravity. We argue that the
predicted phases can be revealed in magnetic traps with spherical-shell
geometry, possibly even in a lab on Earth. Our results pave the way for future
simulation studies of correlated quantum systems in curved geometries. | cond-mat_quant-gas |
Type-II Weyl Points in Three-Dimensional Cold Atom Optical Lattices: Topological Lifshitz phase transition characterizes an abrupt change of the
topology of the Fermi surface through a continuous deformation of parameters.
Recently, Lifshitz transition has been predicted to separate two types of Weyl
points: type-I and type-II (or called structured Weyl points), which has
attracted considerable attention in various fields. Although recent
experimental investigation has seen a rapid progress on type-II Weyl points, it
still remains a significant challenge to observe their characteristic Lifshitz
transition. Here, we propose a scheme to realize both type-I and type-II Weyl
points in three-dimensional ultracold atomic gases by introducing an
experimentally feasible configuration based on current spin-orbit coupling
technology. In the resultant Hamiltonian, we find three degenerate points: two
Weyl points carrying a Chern number $-1$ and a four-fold degenerate point
carrying a Chern number $2$. Remarkably, by continuous tuning of a convenient
experimental knob, all these degenerate points can transition from type-I to
type-II, thereby providing an ideal platform to study different types of Weyl
points and directly probe their Lifshitz phase transition. | cond-mat_quant-gas |
Sound propagation in a Bose-Fermi mixture: from weak to strong
interactions: Particle-like excitations, or quasi-particles, emerging from interacting
fermionic and bosonic quantum fields underlie many intriguing quantum phenomena
in high energy and condensed matter systems. Computation of the properties of
these excitations is frequently intractable in the strong interaction regime.
Quantum degenerate Bose-Fermi mixtures offer promising prospects to elucidate
the physics of such quasi-particles. In this work, we investigate phonon
propagation in an atomic Bose-Einstein condensate immersed in a degenerate
Fermi gas with interspecies scattering length $a_\text{BF}$ tuned by a Feshbach
resonance. We observe sound mode softening with moderate attractive
interactions. For even greater attraction, surprisingly, stable sound
propagation re-emerges and persists across the resonance. The stability of
phonons with resonant interactions opens up opportunities to investigate novel
Bose-Fermi liquids and fermionic pairing in the strong interaction regime. | cond-mat_quant-gas |
Manifold approach for a many-body Wannier-Stark system: localization and
chaos in energy space: We study the resonant tunneling effect in a many-body Wannier-Stark system,
realized by ultracold bosonic atoms in an optical lattice subjected to an
external Stark force. The properties of the many-body system are effectively
described in terms of upper-band excitation manifolds, which allow for the
study of the transition between regular and quantum chaotic spectral
statistics. We show that our system makes it possible to control the spectral
statistics locally in energy space by the competition of the force and the
interparticle interaction. By a time-dependent sweep of the Stark force the
dynamics is reduced to a Landau-Zener problem in the single-particle setting. | cond-mat_quant-gas |
Fractional quantum anomalous Hall phase for Raman superarray of Rydberg
atoms: Rydberg atom arrays offer promising platforms for quantum simulation of
correlated quantum matter and raise great interests. This work proposes a novel
stripe-lattice model with Raman superarray of Rydberg atoms to realize bosonic
fractional quantum anomalous Hall (FQAH) phase. Two types of Rydberg states,
arranged in a supperarray configuration and with Raman-assisted dipole-exchange
couplings, are implemented to realize a minimal QAH model for hard-core bosons
populated into a topological flat band with large bulk gap under proper tunable
experimental condition. With this the bosonic FQAH phase can be further
achieved and probed feasibly. In particular, a novel quench protocol is
proposed to probe the fractionalized excitations by measuring the correlated
quench dynamics featured by fractional charge tunneling between bulk and chiral
edge modes in the open boundary. | cond-mat_quant-gas |
Thermometry by correlated dephasing of impurities in a 1D Fermi gas: We theoretically investigate the pure dephasing dynamics of two static
impurity qubits embedded within a common environment of ultracold fermionic
atoms, which are confined to one spatial dimension. Our goal is to understand
how bath-mediated interactions between impurities affect their performance as
nonequilibrium quantum thermometers. By solving the dynamics exactly using a
functional determinant approach, we show that the impurities become correlated
via retarded interactions of the Ruderman-Kittel-Kasuya-Yosida type. Moreover,
we demonstrate that these correlations can provide a metrological advantage,
enhancing the sensitivity of the two-qubit thermometer beyond that of two
independent impurities. This enhancement is most prominent in the limit of low
temperature and weak collisional coupling between the impurities and the gas.
We show that this precision advantage can be exploited using standard Ramsey
interferometry, with no need to prepare correlated initial states nor to
individually manipulate or measure the impurities. We also quantitatively
assess the impact of ignoring these correlations when constructing a
temperature estimate, finding that acceptable precision can still be achieved
from a simplified model of independent impurities. Our results demonstrate the
rich nonequilibrium physics of impurities dephasing in a common Fermi gas, and
may help to provide better temperature estimates at ultralow temperatures. | cond-mat_quant-gas |
Density correlations from analogue Hawking radiation in the presence of
atom losses: The sonic analogue of Hawking radiation can now be experimentally recreated
in Bose-Einstein Condensates that contain an acoustic black hole. In these
experiments the signal strength and analogue Hawking temperature increase for
denser condensates, which however also suffer increased atom losses from
inelastic collisions. To determine how these affect analogue Hawking radiation,
we numerically simulate creation of the latter in a Bose-Einstein Condensate in
the presence of atomic losses. In particular we explore modifications of
density-density correlations through which the radiation has been analyzed so
far. We find that losses increase the contrast of the correlation signal, which
we attribute to heating that in turn leads to a component of stimulated
radiation in addition to the spontaneous one. Another indirect consequence is
the modification of the white hole instability pattern. | cond-mat_quant-gas |
Dynamics of Few Co-rotating Vortices in Bose-Einstein Condensates: We study the dynamics of small vortex clusters with few (2--4) co-rotating
vortices in Bose-Einstein condensates by means of experiments, numerical
computations, and theoretical analysis. All of these approaches corroborate the
counter-intuitive presence of a dynamical instability of symmetric vortex
configurations. The instability arises as a pitchfork bifurcation at
sufficiently large values of the angular momentum that induces the emergence
and stabilization of asymmetric rotating vortex configurations. The latter are
quantified in the theoretical model and observed in the experiments. The
dynamics is explored both for the integrable two-vortex system, where a
reduction of the phase space of the system provides valuable insight, as well
as for the non-integrable three- (or more) vortex case, which additionally
admits the possibility of chaotic trajectories. | cond-mat_quant-gas |
A continuum of compass spin models on the honeycomb lattice: Quantum spin models with spatially dependent interactions, known as compass
models, play an important role in the study of frustrated quantum magnetism.
One example is the Kitaev model on the honeycomb lattice with spin-liquid
ground states and anyonic excitations. Another example is the geometrically
frustrated quantum $120^\circ$ model on the same lattice whose ground state has
not been unambiguously established. To generalize the Kitaev model beyond the
exactly solvable limit and connect it with other compass models, we propose a
new model, dubbed "the tripod model", which contains a continuum of
compass-type models. It smoothly interpolates the Ising model, the Kitaev
model, and the quantum $120^\circ$ model by tuning a single parameter
$\theta'$, the angle between the three legs of a tripod in the spin space.
Hence it not only unifies three paradigmatic spin models, but also enables the
study of their quantum phase transitions. We obtain the phase diagram of the
tripod model numerically by tensor networks in the thermodynamic limit. We show
that the ground state of the quantum $120^\circ$ model has long-range dimer
order. Moreover, we find an extended spin-disordered (spin-liquid) phase
between the dimer phase and an antiferromagnetic phase. The unification and
solution of a continuum of frustrated spin models as outline here may be useful
to exploring new domains of other quantum spin or orbital models. | cond-mat_quant-gas |
Generalized parametric resonance in a spin-1 Bose-Einstein condensate: We propose a generalized Mathieu equation (GME) which describes well the
dynamics for two different models in spin-1 Bose-Einstein condensates. The
stability chart of this GME differs significantly from that of Mathieu's
equation and the unstable dynamics under this GME is called generalized
parametric resonance. A typical region of $\epsilon \gtrsim 1$ and $\delta
\approx 0.25$ can be used to distinguish these two equations. The GME we
propose not only explains the experimental results of Hoang et al. [Nat.
Commun. 7, 11233 (2016)] in nematic space with a small driving strength, but
predicts the behavior in the regime of large driving strength. In addition, the
model in spin space we propose, whose dynamics also obeys this GME, can be
well-tuned such that it is easily implemented in experiments. | cond-mat_quant-gas |
Hartree-Fock-Bogoliubov Model and Simulation of Attractive and Repulsive
Bose-Einstein Condensates: We describe a model of dynamic Bose-Einstein condensates near a Feshbach
resonance that is computationally feasible under assumptions of spherical or
cylindrical symmetry. Simulations in spherical symmetry approximate the
experimentally measured time to collapse of an unstably attractive condensate
only when the molecular binding energy in the model is correct, demonstrating
that the quantum fluctuations and atom-molecule pairing included in the model
are the dominant mechanisms during collapse. Simulations of condensates with
repulsive interactions find some quantitative disagreement, suggesting that
pairing and quantum fluctuations are not the only significant factors for
condensate loss or burst formation. Inclusion of three-body recombination was
found to be inconsequential in all of our simulations, though we do not
consider recent experiments [1] conducted at higher densities. | cond-mat_quant-gas |
Quench dynamics of a Bose-Einstein condensate under synthetic spin-orbit
coupling: We study the quench dynamics of a Bose-Einstein condensate under a
Raman-assisted synthetic spin-orbit coupling. To model the dynamical process,
we adopt a self-consistent Bogoliubov approach, which is equivalent to applying
the time-dependent Bogoliubov-de-Gennes equations. We investigate the dynamics
of the condensate fraction as well as the momentum distribution of the Bose gas
following a sudden change of system parameters. Typically, the system evolves
into a steady state in the long-time limit, which features an oscillating
momentum distribution and a stationary condensate fraction which is dependent
on the quench parameters. We investigate how different quench parameters such
as the inter- and intra-species interactions and the spin-orbit-coupling
parameters affect the condensate fraction in the steady state. Furthermore, we
find that the time average of the oscillatory momentum distribution in the
long-time limit can be described by a generalized Gibbs ensemble with two
branches of momentum-dependent Gibbs temperatures. Our study is relevant to the
experimental investigation of dynamical processes in a spin-orbit coupled
Bose-Einstein condensate. | cond-mat_quant-gas |
Symmetry breaking Rayleigh-Taylor instability in a two-component
Bose-Einstein condensate: The interfacial instability and subsequent dynamics in a phase-separated
two-component Bose-Einstein condensate with rotation symmetry are studied. When
the interatomic interaction or the trap frequency is changed, the
Rayleigh-Taylor instability breaks the rotation symmetry of the interface,
which is subsequently deformed into nonlinear patterns including mushroom
shapes. | cond-mat_quant-gas |
Low-Dimensional Fluctuations and Pseudogap in Gaudin-Yang Fermi Gases: Pseudogap is a ubiquitous phenomenon in strongly correlated systems such as
high-$T_{\rm c}$ superconductors, ultracold atoms and nuclear physics. While
pairing fluctuations inducing the pseudogap are known to be enhanced in
low-dimensional systems, such effects have not been explored well in one of the
most fundamental 1D models, that is, Gaudin-Yang model. In this work, we show
that the pseudogap effect can be visible in the single-particle excitation in
this system using a diagrammatic approach. Fermionic single-particle spectra
exhibit a unique crossover from the double-particle dispersion to pseudogap
state with increasing the attractive interaction and the number density at
finite temperature. Surprisingly, our results of thermodynamic quantities in
unpolarized and polarized gases show an excellent agreement with the recent
quantum Monte Carlo and complex Langevin results, even in the region where the
pseudogap appears. | cond-mat_quant-gas |
Quantum filaments in dipolar Bose-Einstein condensates: Collapse in dipolar Bose-Einstein condensates may be arrested by quantum
fluctuations. Due to the anisotropy of the dipole-dipole interactions, the
dipole-driven collapse induced by soft excitations is compensated by the
repulsive Lee-Huang-Yang contribution resulting from quantum fluctuations of
hard excitations, in a similar mechanism as that recently proposed for
Bose-Bose mixtures. The arrested collapse results in self-bound filament-like
droplets, providing an explanation to recent dysprosium experiments. Arrested
instability and droplet formation are novel general features directly linked to
the nature of the dipole-dipole interactions, and should hence play an
important role in all future experiments with strongly dipolar gases. | cond-mat_quant-gas |
Anisotropic superfluidity in a dipolar Bose gas: We study the superfluid character of a dipolar Bose-Einstein condensate
(DBEC) in a quasi-two dimensional (q2D) geometry. In particular, we allow for
the dipole polarization to have some non-zero projection into the plane of the
condensate so that the effective interaction is anisotropic in this plane,
yielding an anisotropic dispersion for propagation of quasiparticles. By
performing direct numerical simulations of a probe moving through the DBEC, we
observe the sudden onset of drag or creation of vortex-antivortex pairs at
critical velocities that depend strongly on the direction of the probe's
motion. This anisotropy emerges because of the anisotropic manifestation of a
roton-like mode in the system. | cond-mat_quant-gas |
Relaxation of superfluid turbulence in highly oblate Bose-Einstein
condensates: We investigate thermal relaxation of superfluid turbulence in a highly oblate
Bose-Einstein condensate. We generate turbulent flow in the condensate by
sweeping the center region of the condensate with a repulsive optical
potential. The turbulent condensate shows a spatially disordered distribution
of quantized vortices and the vortex number of the condensate exhibits
nonexponential decay behavior which we attribute to the vortex pair
annihilation. The vortex-antivortex collisions in the condensate are identified
with crescent-shaped, coalesced vortex cores. We observe that the
nonexponential decay of the vortex number is quantitatively well described by a
rate equation consisting of one-body and two-body decay terms. In our
measurement, we find that the local two-body decay rate is closely proportional
to $T^2/\mu$, where $T$ is the temperature and $\mu$ is the chemical potential. | cond-mat_quant-gas |
Observation of Zitterbewegung in a spin-orbit coupled Bose-Einstein
condensate: Spin-orbit coupled ultra-cold atoms provide an intriguing new avenue for the
study of rich spin dynamics in superfluids. In this Letter, we observe
Zitterbewegung, the simultaneous velocity (thus position) and spin
oscillations, of neutral atoms between two spin-orbit coupled bands in a
Bose-Einstein condensate (BEC) through sudden quantum quenches of the
Hamiltonian. The observed Zitterbewegung oscillations are perfect on a short
time scale but gradually damp out on a long time scale, followed by sudden and
strong heating of the BEC. As an application, we also demonstrate how
Zitterbewegung oscillations can be exploited to populate the upper spin-orbit
band, and observe a subsequent dipole motion. Our experimental results are
corroborated by a theoretical and numerical analysis and showcase the great
flexibility that ultra-cold atoms provide for investigating rich spin dynamics
in superfluids. | cond-mat_quant-gas |
Supersolid Gap Soliton in a Bose-Einstein Condensate and Optical Ring
Cavity coupling system: The system of a transversely pumped Bose-Einstein condensate (BEC) coupled to
a lossy ring cavity can favor a supersolid steady state. Here we find the
existence of supersolid gap soliton in such a driven-dissipative system. By
numerically solving the mean-field atom-cavity field coupling equations, gap
solitons of a few different families have been identified. Their dynamical
properties, including stability, propagation and soliton collision, are also
studied. Due to the feedback atom-intracavity field interaction, these
supersolid gap solitons show numerous new features compared with the usual BEC
gap solitons in static optical lattices. | cond-mat_quant-gas |
Effective theory for the propagation of a wave-packet in a disordered
and nonlinear medium: The propagation of a wave-packet in a nonlinear disordered medium exhibits
interesting dynamics. Here, we present an analysis based on the nonlinear
Schr\"odinger equation (Gross-Pitaevskii equation). This problem is directly
connected to experiments on expanding Bose gases and to studies of transverse
localization in nonlinear optical media. In a nonlinear medium the energy of
the wave-packet is stored both in the kinetic and potential parts, and details
of its propagation are to a large extent determined by the transfer from one
form of energy to the other. A theory describing the evolution of the
wave-packet has been formulated in [G. Schwiete and A. Finkelstein, Phys. Rev.
Lett. 104, 103904 (2010)] in terms of a nonlinear kinetic equation. In this
paper, we present details of the derivation of the kinetic equation and of its
analysis. As an important new ingredient we study interparticle-collisions
induced by the nonlinearity and derive the corresponding collision integral. We
restrict ourselves to the weakly nonlinear limit, for which disorder scattering
is the dominant scattering mechanism. We find that in the special case of a
white noise impurity potential the mean squared radius in a two-dimensional
system scales linearly with t. This result has previously been obtained in the
collisionless limit, but it also holds in the presence of collisions. Finally,
we mention different mechanisms through which the nonlinearity may influence
localization of the expanding wave-packet. | cond-mat_quant-gas |
Prethermalization and universal dynamics in near-integrable quantum
systems: We review the recent progress in the understanding of the relaxation of
isolated near-integrable quantum many-body systems. Focusing on
prethermalization and universal dynamics following a quench, we describe the
experiments with ultracold atomic gases that illustrate these phenomena and
summarize the essential theoretical concepts employed to interpret them. Our
discussion highlights the key topics that link the different approaches to this
interdisciplinary field, including the generalized Gibbs ensemble, non-thermal
fixed points, critical slowing and universal scaling. Finally, we point to new
experimental challenges demonstrating these fundamental features of many-body
quantum systems out of equilibrium. | cond-mat_quant-gas |
Two-mode dipolar bosonic junctions: We consider a two-mode atomic Josephson junction realized with dilute dipolar
bosons confined by a double-well. We employ the two-site extended Bose-Hubbard
Hamiltonian and characterize the ground-state of this system by the Fisher
information, coherence visibility, and entanglement entropy. These quantities
are studied as functions of the interaction between bosons in different wells.
The emergence of Schroedinger-cat like state with a loss of coherence is also
commented. | cond-mat_quant-gas |
An effective-field-theory analysis of Efimov physics in heteronuclear
mixtures of ultracold atomic gases: We use an effective-field-theory framework to analyze the Efimov effect in
heteronuclear three-body systems consisting of two species of atoms with a
large interspecies scattering length. In the leading-order description of this
theory, various three-body observables in heteronuclear mixtures can be
universally parameterized by one three-body parameter. We present the
next-to-leading corrections, which include the effects of the finite
interspecies effective range and the finite intraspecies scattering length, to
various three-body observables. We show that only one additional three-body
parameter is required to render the theory predictive at this order. By
including the effective range and intraspecies scattering length corrections,
we derive a set of universal relations that connect the different Efimov
features near the interspecies Feshbach resonance. Furthermore, we show that
these relations can be interpreted in terms of the running of the three-body
counterterms that naturally emerge from proper renormalization. Finally, we
make predictions for recombination observables of a number of atomic systems
that are of experimental interest. | cond-mat_quant-gas |
Visualising Berry phase and diabolical points in a quantum
exciton-polariton billiard: Diabolical points (degeneracies) can naturally occur in spectra of
two-dimensional quantum systems and classical wave resonators due to simple
symmetries. Geometric Berry phase is associated with these spectral
degeneracies. Here, we demonstrate a diabolical point and the corresponding
Berry phase in the spectrum of hybrid light-matter quasiparticles --
exciton-polaritons in semiconductor microcavities. It is well known that
sufficiently strong optical pumping can drive exciton-polaritons to quantum
degeneracy, whereby they form a macroscopically populated quantum coherent
state similar to a Bose-Einstein condensate. By pumping a microcavity with a
spatially structured light, we create a two-dimensional quantum billiard for
the exciton-polariton condensate and demonstrate a diabolical point in the
spectrum of the billiard eigenstates. The fully reconfigurable geometry of the
potential walls controlled by the optical pump enables a striking experimental
visualisation of the Berry phase associated with the diabolical point. The
Berry phase is observed and measured by direct imaging of the macroscopic
exciton-polariton wavefunctions. | cond-mat_quant-gas |
Realization of a fractional quantum Hall state with ultracold atoms: Strongly interacting topological matter exhibits fundamentally new phenomena
with potential applications in quantum information technology. Emblematic
instances are fractional quantum Hall states, where the interplay of magnetic
fields and strong interactions gives rise to fractionally charged
quasi-particles, long-ranged entanglement, and anyonic exchange statistics.
Progress in engineering synthetic magnetic fields has raised the hope to create
these exotic states in controlled quantum systems. However, except for a recent
Laughlin state of light, preparing fractional quantum Hall states in engineered
systems remains elusive. Here, we realize a fractional quantum Hall (FQH) state
with ultracold atoms in an optical lattice. The state is a lattice version of a
bosonic $\nu=1/2$ Laughlin state with two particles on sixteen sites. This
minimal system already captures many hallmark features of Laughlin-type FQH
states: we observe a suppression of two-body interactions, we find a
distinctive vortex structure in the density correlations, and we measure a
fractional Hall conductivity of $\sigma_\text{H}/\sigma_0= 0.6(2)$ via the bulk
response to a magnetic perturbation. Furthermore, by tuning the magnetic field
we map out the transition point between the normal and the FQH regime through a
spectroscopic probe of the many-body gap. Our work provides a starting point
for exploring highly entangled topological matter with ultracold atoms. | cond-mat_quant-gas |
Monitoring squeezed collective modes of a one-dimensional Bose gas after
an interaction quench using density ripples analysis: We investigate the out-of-equilibrium dynamics following a sudden quench of
the interaction strength, in a one-dimensional quasi-condensate trapped at the
surface of an atom chip. Within a linearized approximation, the system is
described by independent collective modes and the quench squeezes the phase
space distribution of each mode, leading to a subsequent breathing of each
quadrature. We show that the collective modes are resolved by the power
spectrum of density ripples which appear after a short time of flight. This
allows us to experimentally probe the expected breathing phenomenon. Our
results are in good agreement with theoretical predictions which take the
longitudinal harmonic confinement into account. | cond-mat_quant-gas |
Breathing mode frequency of a strongly interacting Fermi gas across the
2D-3D dimensional crossover: We address the interplay between dimension and quantum anomaly on the
breathing mode frequency of a strongly interacting Fermi gas harmonically
trapped at zero temperature. Using a beyond mean-field, Gaussian pair
fluctuation theory, we employ periodic boundary conditions to simulate the
dimensionality of the system and impose a local density approximation, with two
different schemes, to model different trapping potentials in the
tightly-confined axial direction. By using a sum-rule approach, we compute the
breathing mode frequency associated with a small variation of the trapping
frequency along the weakly-confined transverse direction, and describe its
behavior as functions of the dimensionality, from two- to three-dimensions, and
of the interaction strength. We compare our predictions with previous
calculations on the two-dimensional breathing mode anomaly and discuss their
possible observation in ultracold Fermi gases of $^{6}$Li and $^{40}$K atoms. | cond-mat_quant-gas |
Comment on "On the relations between large-scale models of superfluid
helium-4" [Phys. Fluids 33, 127124 (2021]": We comment on the paper by M. S\'ykora, M. Pavelka, M. La Mantia, D. Jou, and
M. Grmela "On the relations between large-scale models of superfluid helium-4,"
Physics of Fluids, 33(12):127124(2021), where the authors have developed a
formalism for describing a coarse-grained flow of superfluid helium. This
formalism is greatly based on the Hall-Vinen-Bekarevich-Khalatnikov (HVBK)
model. We strongly disagree with the use of the HVBK equation approach for the
case of the three-dimensional quantum turbulence and expose our objections in
this comment. We discuss the HVBK method and also criticize the so-called
vortex bundles model, which serves as a basis for using the HVBK method in a
three-dimensional quantum turbulence. | cond-mat_quant-gas |
On quantum time crystals and interacting gauge theories in atomic
Bose-Einstein condensates: We study the dynamics of a Bose-Einstein condensate trapped circumferentially
on a ring, and which is governed by an interacting gauge theory. We show that
the associated density-dependent gauge potential and concomitant current
nonlinearity permits a ground state in the form of a rotating chiral bright
soliton. This chiral soliton is constrained to move in one direction by virtue
of the current nonlinearity, and represents a time crystal in the same vein as
Wilczek's original proposal. | cond-mat_quant-gas |
Spectroscopy of edge and bulk collective modes in fractional Chern
insulators: The exploration of atomic fractional quantum Hall (FQH) states is now within
reach in optical-lattice experiments. While ground-state signatures have been
observed in a system realizing the Hofstadter-Bose-Hubbard model in a box
[Leonard et al., Nature 2023], how to access hallmark low-energy collective
modes remains a central open question in this context. We introduce a
spectroscopic scheme based on two interfering Laguerre-Gaussian beams, which
transfer a controlled angular momentum and energy to the system. The edge and
bulk responses to the probe are detected through local density measurements, by
tracking the transfer of atoms between the bulk and the edge of the FQH
droplet. This detection scheme is shown to simultaneously reveal two specific
signatures of FQH states: their chiral edge branch and their bulk magneto-roton
mode. We numerically benchmark our method by considering few bosons in the
$\nu=1/2$ Laughlin ground state of the Hofstadter-Bose-Hubbard model, and
demonstrate that these signatures are already detectable in realistic systems
of two bosons, provided that the box potential is larger than the droplet. Our
work paves the way for the detection of fractional statistics in cold atoms
through edge signatures. | cond-mat_quant-gas |
Resistive flow in a weakly interacting Bose-Einstein condensate: We report the direct observation of resistive flow through a weak link in a
weakly interacting atomic Bose-Einstein condensate. Two weak links separate our
ring-shaped superfluid atomtronic circuit into two distinct regions, a source
and a drain. Motion of these weak links allows for creation of controlled flow
between the source and the drain. At a critical value of the weak link
velocity, we observe a transition from superfluid flow to superfluid plus
resistive flow. Working in the hydrodynamic limit, we observe a conductivity
that is 4 orders of magnitude larger than previously reported conductivities
for a Bose-Einstein condensate with a tunnel junction. Good agreement with
zero-temperature Gross-Pitaevskii simulations and a phenomenological model
based on phase slips indicate that the creation of excitations plays an
important role in the resulting conductivity. Our measurements of resistive
flow elucidate the microscopic origin of the dissipation and pave the way for
more complex atomtronic devices. | cond-mat_quant-gas |
Quantitative test of mean-field description of a trapped two-dimensional
Bose gas: We investigate the accuracy of two mean-field theories of the trapped
two-dimensional Bose gas at predicting transition region properties by
comparison to non-perturbative classical field calculations. To make these
comparisons we examine the density profiles and the predictions for the
Berezinskii-Kosterlitz-Thouless superfluid transition temperature over a
parameter range in which the degree of thermal activation in the tightly
trapped direction varies considerably. These results present an important test
of these mean-field theories, and provide a characterization of their typical
accuracy. | cond-mat_quant-gas |
Optomechanically-Based Probing of Spin-Charge Separation in Ultracold
Gases: We propose a new approach to investigate the spin-charge separation in 1D
quantum liquids via the optomechanical coupled atom-cavity system. We show
that, one can realize an effective two-modes optomechanical model with the
spin/charge modes playing the role of mechanical resonators. By tuning the weak
probe laser under a pump field, the signal of spin-charge separation could be
probed explicitly in the sideband regime via cavity transmissions. Moreover,
the spin/charge modes can be addressed separately by designing the probe field
configurations, which may be beneficial for future studies of the atom-cavity
systems and quantum many-body physics. | cond-mat_quant-gas |
Short range asymptotic behavior of the wave-functions of interacting
spin-half fermionic atoms with spin-orbit coupling: a model study: We consider spin-half fermionic atoms with isotropic Rashba spin-orbit
coupling in three directions. The interatomic potential is modeled by a square
well potential. We derive the analytic form of the asymptotic wave-functions at
short range of two fermions in the subspace of zero net momentum and zero total
angular momentum. We show that the spin-orbit coupling has perturbative effects
on the short range asymptotic behavior of the wave-functions away from
resonances. We argue that our conclusion should hold generally. | cond-mat_quant-gas |
Versatile electric fields for the manipulation of ultracold NaK
molecules: In this paper, we present an electrode geometry for the manipulation of
ultracold rovibrational ground state NaK molecules. The electrode system allows
to induce a dipole moment in trapped diatomic NaK molecules with a magnitude up
to $68 \%$ of their internal dipole moment along any direction in a given
two-dimensional plane. The strength, the sign and the direction of the induced
dipole moment is therefore fully tunable. Furthermore, the possibility to
create strong electric field gradients provides the opportunity to address
molecules in single layers of an optical lattice. The maximal relative
variation of the electric field over the trapping volume is below $10^{-6}$. At
the desired electric field value of 10 kV/cm this corresponds to a deviation of
0.01 V/cm. The electrode structure is made of transparent indium tin oxide and
combines large optical access for sophisticated optical dipole traps and
optical lattice configurations with the possibility to create versatile
electric field configurations. | cond-mat_quant-gas |
General memory kernels and further corrections to the variational path
integral approach for the Bogoliubov-Fröhlich Hamiltonian: The celebrated variational path integral approach to the polaron problem
shows remarkable discrepancies with diagrammatic Monte Carlo for the
Bogoliubov-Fr\"{o}hlich Hamiltonian which describes an impurity weakly coupled
to a Bose condensed atomic gas. It has been shown both via a renormalization
group approach and by the method of correlated Gaussian wavefunctions that the
model has a subtle UV divergence caused by quantum fluctuations, which are not
captured within Feynman's approach. In this work we address the issues with
Feynman's approach and show that by extending the model action to a more
general form, and by considering higher order corrections beyond the
Jensen-Feynman inequality, a good agreement with diagrammatic Monte Carlo can
be obtained. | cond-mat_quant-gas |
Measurement of the atom number distribution in an optical tweezer using
single photon counting: We demonstrate in this paper a method to reconstruct the atom number
distribution of a cloud containing a few tens of cold atoms. The atoms are
first loaded from a magneto-optical trap into a microscopic optical dipole trap
and then released in a resonant light probe where they undergo a Brownian
motion and scatter photons. We count the number of photon events detected on an
image intensifier. Using the response of our detection system to a single atom
as a calibration, we extract the atom number distribution when the trap is
loaded with more than one atom. The atom number distribution is found to be
compatible with a Poisson distribution. | cond-mat_quant-gas |
Geometrically induced complex tunnelings for ultracold atoms carrying
orbital angular momentum: We investigate the dynamics of angular momentum states for a single ultracold
atom trapped in two dimensional systems of sided coupled ring potentials. The
symmetries of the system show that tunneling amplitudes between different ring
states with variation of the winding number are complex. In particular, we
demonstrate that in a triangular ring configuration the complex nature of the
cross-couplings can be used to geometrically engineer spatial dark states to
manipulate the transport of orbital angular momentum states via quantum
interference. | cond-mat_quant-gas |
Two-mode dipolar bosonic junctions: We consider a two-mode atomic Josephson junction realized with dilute dipolar
bosons confined by a double-well. We employ the two-site extended Bose-Hubbard
Hamiltonian and characterize the ground-state of this system by the Fisher
information, coherence visibility, and entanglement entropy. These quantities
are studied as functions of the interaction between bosons in different wells.
The emergence of Schroedinger-cat like state with a loss of coherence is also
commented. | cond-mat_quant-gas |
Phase diagrams and Thomas-Fermi estimates for spin-orbit coupled
Bose-Einstein Condensates under rotation: We provide complete phase diagrams describing the ground state of a trapped
spinor BEC under the combined effects of rotation and a Rashba spin-orbit
coupling. The interplay between the different parameters (magnitude of
rotation, strength of the spin-orbit coupling and interaction) leads to a rich
ground state physics that we classify. We explain some features analytically in
the Thomas-Fermi approximation, writing the problem in terms of the total
density, total phase and spin. In particular, we analyze the giant skyrmion,
and find that it is of degree 1 in the strong segregation case. In some regions
of the phase diagrams, we relate the patterns to a ferromagnetic energy. | cond-mat_quant-gas |
Bose-Einstein condensates in toroidal traps: instabilities, swallow-tail
loops, and self-trapping: We study the stability and dynamics of an ultra-cold bosonic gas trapped in a
toroidal geometry and driven by rotation, in the absence of dissipation. We
first delineate, via the Bogoliubov mode expansion, the regions of stability
and the nature of instabilities of the system for both repulsive and attractive
interaction strengths. To study the response of the system to variations in the
rotation rate, we introduce a "disorder" potential, breaking the rotational
symmetry. We demonstrate the breakdown of adiabaticity as the rotation rate is
slowly varied and find forced tunneling between the system's eigenstates. The
non-adiabaticity is signaled by the appearance of a swallow-tail loop in the
lowest-energy level, a general sign of hysteresis. Then, we show that this
system is in one-to-one correspondence with a trapped gas in a double-well
potential and thus exhibits macroscopic quantum self-trapping. Finally, we show
that self-trapping is a direct manifestation of the behavior of the
lowest-energy level. | cond-mat_quant-gas |
Contour-time approach to the disordered Bose-Hubbard model in the strong
coupling regime: There has been considerable interest in the disordered Bose Hubbard model
(BHM) in recent years, particularly in the context of thermalization and
many-body localization. We develop a two-particle irreducible (2PI)
strong-coupling approach to the disordered BHM that allows us to treat both
equilibrium and out-of-equilibrium situations. We obtain equations of motion
for spatio-temporal correlations and explore their equilibrium solutions. We
study the equilibrium phase diagram as a function of disorder strength and
discuss applications of the formalism to out-of-equilibrium situations. We also
note that the disorder strengths where the emergence of non-ergodic dynamics
was observed in a recent experiment [Choi $et \,al.$, Science $\bf{352}$, 1547
(2016)] appear to correspond to the Mott insulator -- Bose glass phase
boundary. | cond-mat_quant-gas |
Spin-Orbit Coupled One-Dimensional Fermi Gases with Infinite Repulsion: The current efforts of studying many-body effects with spin-orbit coupling
(SOC) using alkali-metal atoms are impeded by the heating effects due to
spontaneous emission. Here, we show that even for SOCs too weak to cause any
heating, dramatic many-body effects can emerge in a one-dimensional(1D) spin
1/2 Fermi gas provided the interaction is sufficiently repulsive. For weak
repulsion, the effect of a weak SOC (with strength $\Omega$) is perturbative.
inducing a weak spin spiral (with magnitude proportional to $\Omega$). However,
as the repulsion $g$ increases beyond a critical value ($g_c\sim 1/\Omega$),
the magnitude of the spin spiral rises rapidly to a value of order 1
(independent of $\Omega$). Moreover, near $g=+\infty$, the spins of neighboring
fermions can interfere destructively due to quantum fluctuations of particle
motion, strongly distorting the spin spiral and pulling the spins substantially
away from the direction of the local field at various locations. These effects
are consequences of the spin-charge separation in the strongly repulsive limit.
They will also occur in other 1D quantum gases with higher spins. | cond-mat_quant-gas |
Quantum phase transitions of the spin-boson model within
multi-coherent-states: A variational approach based on the multi-coherent-state ansatz with
asymmetric parameters is employed to study the ground state of the spin-boson
model. Without any artificial approximations except for the finite number of
the coherent states, we find the robust Gaussian critical behavior in the whole
sub-Ohmic bath regime. The converged critical coupling strength can be
estimated with the $1/N$ scaling, where $N $ is the number of the coherent
states. It is strongly demonstrated the breakdown of the well-known
quantum-to-classical mapping for $1/2<s<1$. In addition, the entanglement
entropy displays more steep jump around the critical points for the Ohmic bath
than the sub-Ohmic bath. | cond-mat_quant-gas |
Observable Vortex Properties in Finite Temperature Bose Gases: We study the dynamics of vortices in finite temperature atomic Bose-Einstein
condensates, focussing on decay rates, precession frequencies and core
brightness, motivated by a recent experiment (Freilich et al. Science 329, 1182
(2010)) in which real-time dynamics of a single vortex was observed. Using the
ZNG formalism based on a dissipative Gross-Pitaevskii equation for the
condensate coupled to a semi-classical Boltzmann equation for the thermal
cloud, we find a rapid nonlinear increase of both the decay rate and precession
frequency with increasing temperatures. The increase, which is dominated by the
dynamical condensate-thermal coupling is also dependent on the intrinsic
thermal cloud collisional dynamics; the precession frequency also varies with
the initial radial coordinate. The integrated thermal cloud density in the
vortex core is for the most part independent of the position of the vortex
(except when it is near the condensate edge) with its value increasing with
temperature. This could potentially be used as a variant to the method of
Coddington et al. (Phys. Rev. A 70, 063607 (2004)) for experimentally
determining the temperature. | cond-mat_quant-gas |
Complex Langevin simulation of quantum vortices in a Bose-Einstein
condensate: The ab-initio simulation of quantum vortices in a Bose-Einstein condensate is
performed by adopting the complex Langevin techniques. We simulate the
nonrelativistic boson field theory at finite chemical potential under rotation.
In the superfluid phase, vortices are generated above a critical angular
velocity and the circulation is clearly quantized even in the presence of
quantum fluctuations. | cond-mat_quant-gas |
Materia cuántica y dinámica por mediciones: This article discusses some details of the course on "Quantum matter and
measurement induced dynamics" given in the Summer School of Physics XXIX at
UNAM in 2022. The notes describe useful concepts to study the dynamics induced
by photon losses, the method for simulation (quantum trajectories) is
summarized and details of models in optical lattices and high-Q cavities are
given. The notes are in Spanish.
En este art\'iculo se discuten algunos detalles del curso sobre "Materia
cu\'antica y din\'amica inducida por medici\'on" de la escuela de verano de
F\'isica XXIX (2022) en la UNAM. Las notas describen conceptos \'utiles para
estudiar la din\'amica emergente por efectos de medici\'on de fotones, se
resume el m\'etodo para simulaci\'on (trayectorias cu\'anticas) y se dan
detalles de modelos de materia cu\'antica y cavidades de alta reflectancia. | cond-mat_quant-gas |
Localization-delocalization transition of dipolar bosons in a four-well
potential: We study interacting dipolar atomic bosons in a four-well potential within a
ring geometry and outline how a four-site Bose-Hubbard (BH) model including
next-nearest-neighbor interaction terms can be derived for the above four-well
system. We analyze the ground state of dipolar bosons by varying the strength
of the interaction between particles in next-nearest-neighbor wells. We perform
this analysis both numerically and analytically by reformulating the
dipolar-boson model within the continuous variable picture applied in [Phys.
Rev. A {\bf 84}, 061601(R) (2011)]. By using this approach we obtain an
effective description of the transition mechanism and show that when the
next-nearest-neighbor interaction crosses a precise value of the on-site
interaction, the ground state exhibits a change from the uniform state
(delocalization regime) to a macroscopic two-pulse state, with strongly
localized bosons (localization regime). These predictions are confirmed by the
results obtained by diagonalizing numerically the four-site BH Hamiltonian. | cond-mat_quant-gas |
Fractionalization Waves in Two-dimensional Dirac Fermions: Quantum
Imprint from One Dimension: Particle fractionalization is believed to orchestrate the physics of many
strongly correlated systems, yet its direct experimental detection remains a
challenge. We propose a simple measurement for an ultracold matter system, in
which correlations in initially decoupled 1D chains are imprinted via quantum
quench upon two-dimensional Dirac fermions. Luttinger liquid correlations
launch relativistic "fractionalization waves" along the chains, while coupling
noninteracting chains induces perpendicular dispersion. These could be easily
distinguished in an ultracold gas experiment. | cond-mat_quant-gas |
Sagnac Interferometry Using Bright Matter-Wave Solitons: We use an effective one-dimensional Gross-Pitaevskii equation to study bright
matter-wave solitons held in a tightly confining toroidal trapping potential,
in a rotating frame of reference, as they are split and recombined on narrow
barrier potentials. In particular, we present an analytical and numerical
analysis of the phase evolution of the solitons and delimit a velocity regime
in which soliton Sagnac interferometry is possible, taking account of the
effect of quantum uncertainty. | cond-mat_quant-gas |
Thermodynamics of the two-component Fermi gas with unequal masses at
unitarity: We consider mass-imbalanced two-component Fermi gases for which the
unequal-mass atoms interact via a zero-range model potential with a diverging
s-wave scattering length $a_s$, i.e., with $1/a_s=0$. The high temperature
thermodynamics of the harmonically trapped and homogeneous systems are examined
using a virial expansion approach up to third order in the fugacity. We find
that the universal part of the third-order virial coefficient associated with
two light atoms and one heavy atom is negative, while that associated with two
heavy and one light atom changes sign from negative to positive as the mass
ratio $\kappa$ increases, and diverges when Efimov physics sets in at
$\kappa=13.61$. By examining the Helmholtz free energy, we find that the
equilibrium polarization of the trapped and homogeneous systems is 0 for
$\kappa=1$, but finite for $\kappa \ne 1$ (with a majority of heavy particles).
Compared to the equilibrium polarization of the non-interacting system, the
equilibrium polarization at unitarity is increased for the trapped system and
decreased for the homogeneous system. We find that unequal-mass Fermi gases are
stable for all polarizations. | cond-mat_quant-gas |
Energy bands and Landau levels of ultracold fermions in the bilayer
honeycomb optical lattice: We investigate the spectrum and eigenstates of ultracold fermionic atoms in
the bilayer honeycomb optical lattice. In the low energy approximation, the
dispersion relation has parabolic form and the quasiparticles are chiral. In
the presence of the effective magnetic field, which is created for the system
with optical means, the energy spectrum shows an unconventional Landau level
structure. Furthermore, the experimental detection of the spectrum is proposed
with the Bragg scattering techniques. | cond-mat_quant-gas |
Properties of a Bose Gas in the Presence of Disorder (Laurea thesis): The phenomenon of Bose-Einstein condensation and superfluidity in a Bose gas
with disorder is investigated. Diffusion Monte Carlo (DMC) method is used to
calculate superfluid and condensate fraction of the system as a function of
density and strength of disorder at zero temperature. The algorithm and
implementation of the Diffusion Monte Carlo method is explained in details.
Bogoliubov theory is developed for the analytical description of the problem.
Ground state energy, superfluid fraction and condensate fraction are
calculated. It is shown that same results for the superfluid fraction can be
obtained in a perturbative manner from Gross-Pitaevskii equation. Ground state
energy, obtained from DMC calculations, is compared to predictions of
Bogoliubov theory, which are found to be valid in the regime, when the strength
of disorder is small. It is shown that "unusual" situation, when the superfluid
fraction is smaller than the condensate fraction, can be realized in this
system. | cond-mat_quant-gas |
Thermodynamics and static response of anomalous 1D fermions via quantum
Monte Carlo in the worldline representation: A system of three-species fermions in one spatial dimension (1D) with a
contact three-body interaction is known to display a scale anomaly. This
anomaly is identical to that of a two-dimensional (2D) system of two-species
fermions. The exact relation between those two systems, however, is limited to
the two-particle sector of the 2D case and the three-particle sector of the 1D
case. Here, we implement a non-perturbative Monte Carlo approach, based on the
worldline representation, to calculate the thermodynamics and static response
of three-species fermions in 1D, thus tackling the many-body sector of the
problem. We determine the energy, density, and pressure equations of state, and
the compressibility and magnetic susceptibility for a wide range of
temperatures and coupling strengths. We compare our results with the
third-order virial expansion. | cond-mat_quant-gas |
Using off-diagonal confinement as a cooling method: In a recent letter [Phys. Rev. Lett. 104, 167201 (2010)] we proposed a new
confining method for ultracold atoms on optical lattices, based on off-diagonal
confinement (ODC). This method was shown to have distinct advantages over the
conventional diagonal confinement (DC) that makes use of a trapping potential,
including the existence of pure Mott phases and highly populated condensates.
In this paper we show that the ODC method can also lead to temperatures that
are smaller than with the conventional DC method, depending on the control
parameters. We determine these parameters using exact diagonalizations for the
hard-core case, then we extend our results to the soft-core case by performing
quantum Monte Carlo (QMC) simulations for both DC and ODC systems at fixed
temperatures, and analysing the corresponding entropies. We also propose a
method for measuring the entropy in QMC simulations. | cond-mat_quant-gas |
Realizing and optimizing an atomtronic SQUID: We demonstrate how a toroidal Bose-Einstein condensate with a movable barrier
can be used to realize an atomtronic SQUID. The magnitude of the barrier
height, which creates the analogue of an SNS junction, is of crucial
importance, as well as its ramp-up and -down protocol. For too low of a
barrier, the relaxation of the system is dynamically suppressed, due to the
small rate of phase slips at the barrier. For a higher barrier, the phase
coherence across the barrier is suppressed due to thermal fluctuations, which
are included in our Truncated Wigner approach. Furthermore, we show that the
ramp-up protocol of the barrier can be improved by ramping up its height first,
and its velocity after that. This protocol can be further improved by
optimizing the ramp-up and ramp-down time scales, which is of direct practical
relevance for on-going experimental realizations. | cond-mat_quant-gas |
Exact dynamics of two holes in two-leg antiferromagnetic ladders: We study the motion of holes in a mixed-dimensional setup of an
antiferromagnetic ladder, featuring nearest neighbor hopping $t$ along the
ladders and Ising-type spin interactions along, $J_\parallel$, and across,
$J_\perp$, the ladder. We determine exact solutions for the low-energy one- and
two-hole eigenstates. The presence of the trans-leg spin coupling, $J_\perp$,
leads to a linear confining potential between the holes. As a result, holes on
separate legs feature a super-linear binding energy scaling as $(J_\perp /
t)^{2/3}$ in the strongly correlated regime of $J_\perp,J_\parallel \leq t$.
This behavior is linked to an emergent length scale $\lambda \propto
(t/J_\perp)^{1/3}$, stemming from the linear confining potential, and which
describes how the size of the two-hole molecular state diverges for
$J_\perp,J_\parallel \ll t$. On the contrary, holes on the same leg unbind at
sufficiently low spin couplings. This is a consequence of the altered
short-range boundary condition for holes on the same leg, yielding an effective
Pauli repulsion between them, limiting their kinetic energy and making binding
unfavorable. Finally, we determine the exact nonequilibrium quench dynamics
following the sudden immersion of initially localized nearest neigbhor holes.
The dynamics is characterized by a crossover from an initial ballistic quantum
walk to an aperiodic oscillatory motion around a finite average distance
between the holes due to the confining potential between them. In the strongly
correlated regime of low spin couplings, $J_\perp, J_\parallel \leq t$, we find
this asymptotic distance to diverge as $t / J_\perp$, showing a much stronger
scaling than the eigenstates. The predicted results should be amenable to
state-of-the-art quantum simulation experiments using currently implemented
experimental techniques. | cond-mat_quant-gas |
Topological Phase Diagram of Optimally Shaken Honeycomb Lattices: A Dual
Perspective from Stroboscopic and Non-Stroboscopic Floquet Hamiltonians: We present a direct comparison between the stroboscopic and non-stroboscopic
effective approaches for ultracold atoms in shaken honeycomb lattices, focusing
specifically on the optimal driving introduced by A. Verdeny and F. Mintert
[Phys. Rev. A 92, 063615 (2015)]. In the fast-driving regime, we compare the
effective non-stroboscopic Hamiltonian derived through a perturbative expansion
with a non-perturbative calculation of the stroboscopic Floquet Hamiltonian,
obtained through a simple non-perturbative numerical approach. We show that
while some of the tunneling parameters are inherently model-dependent, the
topological properties of the system remains robust, as expected. Using the
same numerical approach we compute the topological phase diagram, arguing that
it is most effectively represented in terms of the physical parameters
characterizing the driving and the bare Hamiltonian -- parameters directly
accessible in experiments -- rather than the emergent tunneling parameters,
that depend on the model representation. | cond-mat_quant-gas |
Spatial correlation of two-dimensional Bosonic multi-mode condensates: We studied the spatial coherence of a Bosonic two-dimensional multi-mode
condensate both through measurements and simulations. It is shown that
condensates with a constant spatial density must be described as the
superposition of several quantized modes which reduces the overall coherence.
In this case, the spatial coherence can appear to decay faster than allowed by
the Berezinskii-Kosterlitz-Thouless (BKT) theory. However, we find through
spectroscopic measurements that the individual modes show a slower decay of the
spatial coherence than the overall system. | cond-mat_quant-gas |
Noise-induced transition from superfluid to vortex state in
two-dimensional nonequilibrium polariton condensates: We study the Berezinskii-Kosterlitz-Thouless mechanism for vortex-antivortex
pair formation in two-dimensional superfluids for nonequilibrium condensates.
Our numerical study is based on a classical field model for driven-dissipative
quantum fluids that is applicable to polariton condensates. We investigate the
critical noise needed to create vortex-antivortex pairs in the systems,
starting from a state with uniform phase. The dependence of the critical noise
on the nonequilibrium and energy relaxation parameters is analyzed in detail. | cond-mat_quant-gas |
Ion induced density bubble in a strongly correlated one dimensional gas: We consider a harmonically trapped Tonks-Girardeau gas of impenetrable bosons
in the presence of a single embedded ion, which is assumed to be tightly
confined in a RF trap. In an ultracold ion-atom collision the ion's charge
induces an electric dipole moment in the atoms which leads to an attractive
$r^{-4}$ potential asymptotically. We treat the ion as a static deformation of
the harmonic trap potential and model its short range interaction with the gas
in the framework of quantum defect theory. The molecular bound states of the
ionic potential are not populated due to the lack of any possible relaxation
process in the Tonks-Girardeau regime. Armed with this knowledge we calculate
the density profile of the gas in the presence of a central ionic impurity and
show that a density \textit{bubble} of the order of a micron occurs around the
ion for typical experimental parameters. From these exact results we show that
an ionic impurity in a Tonks gas can be described using a pseudopotential,
allowing for significantly easier treatment. | cond-mat_quant-gas |
Reaction kinetics of ultracold molecule-molecule collisions: Studying chemical reactions on a state-to-state level tests and improves our
fundamental understanding of chemical processes. For such investigations it is
convenient to make use of ultracold atomic and molecular reactants as they can
be prepared in well defined internal and external quantum states$^{1-4}$. In
general, even cold reactions have many possible final product states$^{5-15}$
and reaction channels are therefore hard to track individually$^{16}$. In
special cases, however, only a single reaction channel is essentially
participating, as observed e.g. in the recombination of two atoms forming a
Feshbach molecule$^{17-19}$ or in atom-Feshbach molecule exchange
reactions$^{20,21}$. Here, we investigate a single-channel reaction of two
Li$_2$-Feshbach molecules where one of the molecules dissociates into two atoms
$2\mathrm{AB}\Rightarrow \mathrm{AB}+\mathrm{A}+\mathrm{B}$. The process is a
prototype for a class of four-body collisions where two reactants produce three
product particles. We measure the collisional dissociation rate constant of
this process as a function of collision energy/ temperature and scattering
length. We confirm an Arrhenius-law dependence on the collision energy, an
$a^4$ power-law dependence on the scattering length $a$ and determine a
universal four body reaction constant. | cond-mat_quant-gas |
Super Efimov effect for mass-imbalanced systems: We study two species of particles in two dimensions interacting by isotropic
short-range potentials with the interspecies potential fine-tuned to a p-wave
resonance. Their universal low-energy physics can be extracted by analyzing a
properly constructed low-energy effective field theory with the renormalization
group method. Consequently, a three-body system consisting of two particles of
one species and one of the other is shown to exhibit the super Efimov effect,
the emergence of an infinite tower of three-body bound states with orbital
angular momentum $l=\pm1$ whose binding energies obey a doubly exponential
scaling, when the two particles are heavier than the other by a mass ratio
greater than 4.03404 for identical bosons and 2.41421 for identical fermions.
With increasing the mass ratio, the super Efimov spectrum becomes denser which
would make its experimental observation easier. We also point out that the
Born-Oppenheimer approximation is incapable of reproducing the super Efimov
effect, the universal low-energy asymptotic scaling of the spectrum. | cond-mat_quant-gas |
Quantized Adiabatic Transport in Momentum Space: Though topological aspects of energy bands are known to play a key role in
quantum transport in solid-state systems, the implications of Floquet band
topology for transport in momentum space (i.e., acceleration) are not explored
so far. Using a ratchet accelerator model inspired by existing cold-atom
experiments, here we characterize a class of extended Floquet bands of
one-dimensional driven quantum systems by Chern numbers, reveal topological
phase transitions therein, and theoretically predict the quantization of
adiabatic transport in momentum space. Numerical results confirm our theory and
indicate the feasibility of experimental studies. | cond-mat_quant-gas |
Many-body interferometry of a Rydberg-dressed spin lattice: Ultracold atoms are an ideal platform to study strongly correlated phases of
matter in and out of equilibrium. Much of the experimental progress in this
field crucially relies on the control of the contact interaction between two
atoms. Control of strong long-range interactions between distant ground state
atoms has remained a long standing goal, opening the path towards the study of
fundamentally new quantum many-body systems including frustrated or topological
magnets and supersolids. Optical dressing of ground state atoms by
near-resonant laser coupling to Rydberg states has been proposed as a versatile
method to engineer such interactions. However, up to now the great potential of
this approach for interaction control in a many-body setting has eluded
experimental confirmation. Here we report the realisation of coherent
Rydberg-dressing in an ultracold atomic lattice gas and directly probe the
induced interaction potential using an interferometric technique with single
atom sensitivity. We use this approach to implement a two-dimensional synthetic
spin lattice and demonstrate its versatility by tuning the range and anisotropy
of the effective spin interactions. Our measurements are in remarkable
agreement with exact solutions of the many-body dynamics, providing further
evidence for the high degree of accurate interaction control in these systems.
Finally, we identify a collective many-body decay process, and discuss possible
routes to overcome this current limitation of coherence times. Our work marks
the first step towards the use of laser-controlled Rydberg interactions for the
study of exotic quantum magnets in optical lattices. | cond-mat_quant-gas |
Quasiadiabatic dynamics of ultracold bosonic atoms in a one-dimensional
optical superlattice: We study the quasiadiabatic dynamics of a one-dimensional system of ultracold
bosonic atoms loaded in an optical superlattice. Focusing on a slow linear
variation in time of the superlattice potential, the system is driven from a
conventional Mott insulator phase to a superlattice-induced Mott insulator,
crossing in between a gapless critical superfluid region. Due to the presence
of a gapless region, a number of defects depending on the velocity of the
quench appear. Our findings suggest a power-law dependence similar to the
Kibble-Zurek mechanism for intermediate values of the quench rate. For the
temporal ranges of the quench dynamics that we considered, the scaling of
defects depends nontrivially on the width of the superfluid region. | cond-mat_quant-gas |
Probing ultracold Fermi gases with light-induced gauge potentials: We theoretically investigate the response of a two component Fermi gas to
vector potentials which couple separately to the two spin components. Such
vector potentials may be implemented in ultracold atomic gases using optically
dressed states. Our study indicates that light-induced gauge potentials may be
used to probe the properies of the interacting ultracold Fermi gas, providing.
amongst other things, ways to measure the superfluid density and the strength
of pairing. | cond-mat_quant-gas |
Using dark solitons from a Bose-Einstein condensate necklace to imprint
soliton states in the spectral memory of a free boson gas: A possible use of matter-wave dark-soliton crystal produced by a
Bose-Einstein condensate with ring geometry, to store soliton states in the
quantum memory of a free boson gas, is explored. A self-defocusing nonlinearity
combined with dispersion and the finite size of the Bose-Einstein condensate,
favor the creation of dark-soliton crystals that imprint quantum states with
Jacobi elliptic-type soliton wavefunctions in the spectrum of the free boson
gas. The problem is formulated by considering the Gross-Pitaevskii equation
with a positive scattering length, coupled to a linear Schr\"odinger equation
for the free boson gas. With the help of the matter-wave dark soliton-crystal
solution, the spectrum of bound states created in the free boson gas is shown
to be determined by the Lam\'e eigenvalue problem. This spectrum consists of
$\vert \nu, \mathcal{L} \rangle$ quantum states whose wave functions and energy
eigenvalues can be unambiguously identified. Among these eigenstates some have
their wave functions that are replicas of the generating dark soliton crystal. | cond-mat_quant-gas |
Molecular state in a spin-orbital-angular-momentum coupled Fermi gas: We study the two-body bound states in a spin-orbital-angular-momentum (SOAM)
coupled quantum gas of fermions. Two different configurations are considered:
an attractive $s$-wave interaction exists between two spin species that are
SOAM coupled; and an atom with SOAM coupled internal spins interacts
state-selectively with another atom. For both cases, we identify the condition
for the emergence of molecular states with finite total angular momenta.These
molecular states with quantized total angular momenta correspond to the
SOAM-coupling-induced vortices in the corresponding Fermi superfluid. We
propose to detect the molecules through Raman spectroscopy with
Laguerre-Gaussian lasers. As the molecular states can form above the superfluid
temperature, they offer an experimentally more accessible route toward the
study of the underlying pairing mechanism under SOAM coupling. | cond-mat_quant-gas |
Dimensional crossover and cold-atom realization of topological Mott
insulators: We propose a cold-atom setup which allows for a dimensional crossover from a
two-dimensional quantum spin Hall insulating phase to a three-dimensional
strong topological insulator by tuning the hopping between the layers. We
further show that additional Hubbard onsite interactions can give rise to spin
liquid-like phases: weak and strong topological Mott insulators. They represent
the celebrated paradigm of a quantum state of matter which merely exists
because of the interplay of the non-trivial topology of the band structure and
strong interactions. While the theoretical understanding of this phase has
remained elusive, our proposal shall help to shed some light on this exotic
state of matter by paving the way for a controlled experimental investigation
in optical lattices. | cond-mat_quant-gas |
Propagation of collective pair excitations in disordered Bose
superfluids: We study the effect of disorder on the propagation of collective excitations
in a disordered Bose superfluid. We incorporate local density depletion induced
by strong disorder at the meanfield level, and formulate the transport of the
excitations in terms of a screened scattering problem. We show that the
competition of disorder, screening, and density depletion induces a strongly
non-monotonic energy dependence of the disorder parameter. In three dimensions,
it results in a rich localization diagram with four different classes of
mobility spectra, characterized by either no or up to three mobility edges.
Implications on experiments with disordered ultracold atoms are discussed. | cond-mat_quant-gas |
Efimov three-body states on top of a Fermi sea: The stabilization of Cooper pairs of bound electrons in the background of a
Fermi sea is the origin of superconductivity and the paradigmatic example of
the striking influence of many-body physics on few-body properties. In the
quantum-mechanical three-body problem the famous Efimov effect yields
unexpected scaling relations among a tower of universal states. These seemingly
unrelated problems can now be studied in the same setup thanks to the success
of ultracold atomic gas experiments. In light of the tremendous effect of a
background Fermi sea on two-body properties, a natural question is whether a
background can modify or even destroy the Efimov effect. Here we demonstrate
how the generic problem of three interacting particles changes when one
particle is embedded in a background Fermi sea, and show that Efimov scaling
persists. It is found in a scaling that relates the three-body physics to the
background density of fermionic particles. | cond-mat_quant-gas |
The two-state Bose-Hubbard model in the hard-core boson limit:
Non-ergodicity and the Bose-Einstein condensation: The Bose-Einstein condensation in the hard-core boson limit (HCB) of the
Bose-Hubbard model with two local states and the particle hopping in the
excited band only is investigated. For the purpose of considering the
non-ergodicity, a single-particle spectral density is calculated in the random
phase approximation by means of the temperature boson Green functions. The
non-ergodic contribution to the momentum distribution function of particles
(connected with the static density fluctuations) increases significantly and
becomes comparable with the ergodic contribution in the superfluid phase near
the tricritical point. | cond-mat_quant-gas |
Correlation energy of a homogeneous dipolar Fermi gas: We study the normal state of a 3-$d$ homogeneous dipolar Fermi gas beyond the
Hartree-Fock approximation. The correlation energy is found of the same order
as the Fock energy, unusually strong for a Fermi-liquid system. As a result,
the critical density of mechanical collapse is smaller than that estimated in
the Hartree-Fock approximation. With the correlation energy included, a new
energy functional is proposed for the trapped system, and its property is
explored. | cond-mat_quant-gas |
Long time non-equilibrium dynamics of binary Bose condensates: We explore the out-of-equilibrium temporal dynamics of demixing and phase
separation in a two dimensional binary Bose fluid at zero temperature,
following a sudden quench across the miscible-immiscible phase boundary. On
short timescales, the system rapidly settles into a steady state characterized
by short-range correlations in the relative density. The subsequent dynamics is
extremely slow: domains of the relative density appear to grow with time,
however, the rate of growth is much slower than that predicted by conventional
theories of phase ordering kinetics. Moreover, we find that the growth dynamics
slows down with increasing time, and is consistent with logarithmic growth. Our
study sheds light on ongoing investigations of how isolated quantum systems
approach equilibrium, and indicates that studying the quantum phase diagram of
the binary Bose fluids following a quench, may be difficult due to
equilibration problems. | 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 |
Superfluidity in the 1D Bose-Hubbard Model: We study superfluidity in the 1D Bose-Hubbard model using a variational
matrix product state technique. We determine the superfluid density as a
function of the Hubbard parameters by calculating the energy cost of phase
twists in the thermodynamic limit. As the system is critical, correlation
functions decay as power laws and the entanglement entropy grows with the bond
dimension of our variational state. We relate the resulting scaling laws to the
superfluid density. We compare two different algorithms for optimizing the
infinite matrix product state and develop a physical explanation why one of
them (VUMPS) is more efficient than the other (iDMRG). Finally, we comment on
finite-temperature superfluidity in one dimension and how our results can be
realized in cold atom experiments. | cond-mat_quant-gas |
Continuous atom laser with Bose-Einstein condensates involving
three-body interactions: We demonstrate, through numerical simulations, the emission of a coherent
continuous matter wave of constant amplitude from a Bose-Einstein Condensate in
a shallow optical dipole trap. The process is achieved by spatial control of
the variations of the scattering length along the trapping axis, including
elastic three body interactions due to dipole interactions. In our approach,
the outcoupling mechanism are atomic interactions and thus, the trap remains
unaltered. We calculate analytically the parameters for the experimental
implementation of this CW atom laser. | cond-mat_quant-gas |
Hartree-Fock treatment of Fermi polarons using the Lee-Low-Pine
transformation: We consider the Fermi polaron problem at zero temperature, where a single
impurity interacts with non-interacting host fermions. We approach the problem
starting with a Frohlich-like Hamiltonian where the impurity is described with
canonical position and momentum operators. We apply the Lee-Low-Pine (LLP)
transformation to change the fermionic Frohlich Hamiltonian into the fermionic
LLP Hamiltonian which describes a many-body system containing host fermions
only. We adapt the self-consistent Hartree-Fock (HF) approach, first proposed
by Edwards, to the fermionic LLP Hamiltonian in which a pair of host fermions
with momenta $\mathbf{k}$ and $\mathbf{k}'$ interact with a potential
proportional to $\mathbf{k}\cdot\mathbf{k}'$. We apply the HF theory, which has
the advantage of not restricting the number of particle-hole pairs, to
repulsive Fermi polarons in one dimension. When the impurity and host fermion
masses are equal our variational ansatz, where HF orbitals are expanded in
terms of free-particle states, produces results in excellent agreement with
McGuire's exact analytical results based on the Bethe ansatz. This work raises
the prospect of using the HF ansatz and its time-dependent generalization as
building blocks for developing all-coupling theories for both equilibrium and
nonequilibrium Fermi polarons in higher dimensions | cond-mat_quant-gas |
Twisted complex superfluids in optical lattices: We show that correlated pair tunneling drives a phase transition to a twisted
superfluid with a complex order parameter. This unconventional superfluid phase
spontaneously breaks the time-reversal symmetry and is characterized by a
twisting of the complex phase angle between adjacent lattice sites. We discuss
the entire phase diagram of the extended Bose--Hubbard model for a honeycomb
optical lattice showing a multitude of quantum phases including twisted
superfluids, pair superfluids, supersolids and twisted supersolids.
Furthermore, we show that the nearest-neighbor interactions lead to a
spontaneous breaking of the inversion symmetry of the lattice and give rise to
dimerized density-wave insulators, where particles are delocalized on dimers.
For two components, we find twisted superfluid phases with strong correlations
between the species already for surprisingly small pair-tunneling amplitudes.
Interestingly, this ground state shows an infinite degeneracy ranging
continuously from a supersolid to a twisted superfluid. | cond-mat_quant-gas |
Squeezing in driven bimodal Bose-Einstein Condensates: Erratic driving
versus noise: We study the interplay of squeezing and phase randomization near the
hyperbolic instability of a two-site Bose-Hubbard model in the Josephson
interaction regime. We obtain results for the quantum Zeno suppression of
squeezing, far beyond the previously found short time behavior. More
importantly, we contrast the expected outcome with the case where randomization
is induced by erratic driving with the same fluctuations as the quantum noise
source, finding significant differences. These are related to the distribution
of the squeezing factor, which has log-normal characteristics: hence its
average is significantly different from its median due to the occurrence of
rare events. | cond-mat_quant-gas |
Self-trapped quantum balls in binary Bose-Einstein condensates: We study the formation of a stable self-trapped spherical quantum ball in a
binary Bose-Einstein condensate (BEC) with two-body inter-species attraction
and intra-species repulsion employing the beyond-mean-field Lee-Huang-Yang and
the three-body interactions. We find that either of these interactions or a
combination of these can stabilize the binary BEC quantum ball with very
similar stationary results, and for a complete description of the problem both
the terms should be considered. These interactions lead to higher-order
nonlinearities, e.g. quartic and quintic, respectively, in a nonlinear
dynamical equation compared to the cubic nonlinearity of the two-body contact
interaction in the mean-field Gross-Pitaevskii equation. The higher-order
nonlinearity makes the energy infinitely large at the center of the binary ball
and thus avoids its collapse. In addition to the formation of stationary binary
balls, we also study a collision between two such balls. At large velocities,
the collision is found to be elastic, which turns out to be inelastic as the
velocity is lowered. We consider the numerical solution of a beyond-mean-field
model for the binary ball as well as a single-mode variational approximation to
it in this study. | cond-mat_quant-gas |
$F$-wave pairing of cold atoms in optical lattices: The tremendous development of cold atom physics has opened up a whole new
opportunity to study novel states of matter which are not easily accessible in
solid state systems. Here we propose to realize the $f$-wave pairing
superfluidity of spinless fermions in the $p_{x,y}$-orbital bands of the two
dimensional honeycomb optical lattices. The non-trivial orbital band structure
rather than strong correlation effects gives rise to the unconventional pairing
with the nodal lines of the $f$-wave symmetry. With a confining harmonic trap,
zero energy Andreev bound states appear around the circular boundary with a
six-fold symmetry. The experimental realization and detection of this novel
pairing state are feasible. | cond-mat_quant-gas |
Interplay between temperature and trap effects in one-dimensional
lattice systems of bosonic particles: We investigate the interplay of temperature and trap effects in cold particle
systems at their quantum critical regime, such as cold bosonic atoms in optical
lattices at the transitions between Mott-insulator and superfluid phases. The
theoretical framework is provided by the one-dimensional Bose-Hubbard model in
the presence of an external trapping potential, and the trap-size scaling
theory describing the large trap-size behavior at a quantum critical point. We
present numerical results for the low-temperature behavior of the particle
density and the density-density correlation function at the Mott transitions,
and within the gapless superfluid phase. | cond-mat_quant-gas |
Supersolid Phase of Cold Fermionic Polar Molecules in 2D Optical
Lattices: We study a system of ultra-cold fermionic polar molecules in a
two-dimensional square lattice interacting via both the long-ranged
dipole-dipole interaction and a short-ranged on-site attractive interaction.
Singlet superfluid, charge density wave, and supersolid phases are found to
exist in the system. We map out the zero temperature phase diagram and find
that the supersolid phase is considerably stabilized by the dipole-dipole
interaction and thus can exist over a large region of filling factors. We study
the melting of the supersolid phase with increasing temperature, map out a
finite temperature phase diagram of the system at fixed filling, and determine
the parameter region where the supersolid phase can possibly be observed in
experiments. | cond-mat_quant-gas |
Many-body braiding phases in a rotating strongly correlated photon gas: We present a theoretical study of a rotating trapped photon gas where a
Laguerre-Gauss laser pump with a non-zero orbital angular momentum is used to
inject rotating photons into a cavity with strong optical nonlinearity. The
Laughlin-like few-photon eigenstates appear as sharp resonances in the
transmission spectra. Using additional localized repulsive potentials,
quasi-holes can be created in the quantum Hall liquid of photons and then
braided around in space: an unambiguous signature of the many-body Berry phase
under exchange of two quasi-holes is observed as a spectral shift of the
corresponding transmission resonance. | cond-mat_quant-gas |
Collective behaviour of large number of vortices in the plane: We investigate the dynamics of $N$ point vortices in the plane, in the limit
of large $N$. We consider {\em relative equilibria}, which are rigidly rotating
lattice-like configurations of vortices. These configurations were observed in
several recent experiments [Durkin and Fajans, Phys. Fluids (2000) 12, 289-293;
Grzybowski {\em et.al} PRE (2001)64, 011603]. We show that these solutions and
their stability are fully characterized via a related {\em aggregation model}
which was recently investigated in the context of biological swarms [Fetecau
{\em et.al.}, Nonlinearity (2011) 2681; Bertozzi {\em et.al.}, M3AS (2011)]. By
utilizing this connection, we give explicit analytic formulae for many of the
configurations that have been observed experimentally. These include
configurations of vortices of equal strength; the $N+1$ configurations of $N$
vortices of equal strength and one vortex of much higher strength; and more
generally, $N+K$ configurations. We also give examples of configurations that
have not been studied experimentally, including $N+2$ configurations where $N$
vortices aggregate inside an ellipse. Finally, we introduce an artificial
``damping'' to the vortex dynamics, in an attempt to explain the phenomenon of
crystalization that is often observed in real experiments. The diffusion breaks
the conservative structure of vortex dynamics so that any initial conditions
converge to the lattice-like relative equilibrium. | cond-mat_quant-gas |
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