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Observation of self-oscillating supersonic flow across an acoustic
horizon in two dimensions: Understanding the dynamics and stability of transonic flows in quantum
fluids, especially for those beyond one spatial dimension, is an outstanding
challenge, with applications ranging from nonlinear optics and condensed matter
to analogue gravity. One intriguing possibility is that a system with a
spatially bounded supersonic flow may evolve into a self-oscillating state that
periodically emits solitons, in a process originating from the well-known
Landau instability. Here, we report observation of self-oscillating supersonic
flows in a two-dimensional atomic superfluid. By imposing a local particle sink
with strong loss, we induce a convergent radial flow forming an acoustic
analogue of a black-hole horizon and an inner horizon around the sink. The
observed superflow appears to be modulated by quasi-periodic bursts of
superluminal signals. We measure their frequencies and find agreement with
numerical simulations of soliton oscillation frequencies within the black-hole
horizon. The presented experiment demonstrates a new method for creating
supersonic flows in atomic superfluids, which may find applications in quantum
simulations of curved spacetime, supersonic turbulence, and self-oscillating
dynamics in dissipative many-body systems. | cond-mat_quant-gas |
Quantum glass of interacting bosons with off-diagonal disorder: We study disordered interacting bosons described by the Bose-Hubbard model
with Gaussian-distributed random tunneling amplitudes. It is shown that the
off-diagonal disorder induces a spin-glass-like ground state, characterized by
randomly frozen quantum-mechanical U(1) phases of bosons. To access
criticality, we employ the "$n$-replica trick", as in the spin-glass theory,
and the Trotter-Suzuki method for decomposition of the statistical density
operator, along with numerical calculations. The interplay between disorder,
quantum and thermal fluctuations leads to phase diagrams exhibiting a glassy
state of bosons, which are studied as a function of model parameters. The
considered system may be relevant for quantum simulators of optical-lattice
bosons, where the randomness can be introduced in a controlled way. The latter
is supported by a proposition of experimental realization of the system in
question. | cond-mat_quant-gas |
From Nodal Ring Topological Superfluids to Spiral Majorana Modes in Cold
Atomic Systems: In this work, we consider a 3D cubic optical lattice composed of coupled 1D
wires with 1D spin-orbit coupling. When the s-wave pairing is induced through
Feshbach resonance, the system becomes a topological superfluid with ring
nodes, which are the ring nodal degeneracies in the bulk, and supports a large
number of surface Majorana zero energy modes. The large number of surface
Majorana modes remain at zero energy even in the presence of disorder due to
the protection from a chiral symmetry. When the chiral symmetry is broken, the
system becomes a Weyl topological superfluid with Majorana arcs. With 3D
spin-orbit coupling, the Weyl superfluid becomes a novel gapless phase with
spiral Majorana modes on the surface. The spatial resolved radio frequency
spectroscopy is suggested to detect this novel nodal ring topological
superfluid phase. | cond-mat_quant-gas |
OpenMP Fortran programs for solving the time-dependent dipolar
Gross-Pitaevskii equation: In this paper we present Open Multi-Processing (OpenMP) Fortran 90/95
versions of previously published numerical programs for solving the dipolar
Gross-Pitaevskii (GP) equation including the contact interaction in one, two
and three spatial dimensions. The atoms are considered to be polarized along
the z axis and we consider different cases, e.g., stationary and non-stationary
solutions of the GP equation for a dipolar Bose-Einstein condensate (BEC) in
one dimension (along x and z axes), two dimensions (in x-y and x-z planes), and
three dimensions. The algorithm used is the split-step semi-implicit
Crank-Nicolson scheme for imaginary- and real-time propagation to obtain
stationary states and BEC dynamics, respectively, as in the previous version
[R. Kishor Kumar et al., Comput. Phys. Commun. 195, 117 (2015)]. These OpenMP
versions have significantly reduced execution time in multicore processors. | cond-mat_quant-gas |
Universal van der Waals Force Between Heavy Polarons in Superfluids: We investigate the long-range behavior of the induced Casimir interaction
between two spinless heavy impurities, or polarons, in superfluid cold atomic
gases. With the help of effective field theory (EFT) of a Galilean invariant
superfluid, we show that the induced impurity-impurity potential at long
distance universally shows a relativistic van der Waals-like attraction ($\sim
1/r^7$) resulting from the exchange of two superfluid phonons. We also clarify
finite temperature effects from the same two-phonon exchange process. The
temperature $T$ introduces the additional length scale $c_s/T$ with the speed
of sound $c_s$. Leading corrections at finite temperature scale as $T^6/r$ for
distances $r \ll c_s/T$ smaller than the thermal length. For larger distances
the potential shows a nonrelativistic van der Waals behavior ($\sim T/r^6$)
instead of the relativistic one. Our EFT formulation applies not only to weakly
coupled Bose or Fermi superfluids but also to that composed of strongly-coupled
unitary fermions with a weakly coupled impurity. The sound velocity controls
the magnitude of the van der Waals potential, which we evaluate for the
fermionic superfluid in the BCS-BEC crossover. | cond-mat_quant-gas |
Symmetry analysis of crystalline spin textures in dipolar spinor
condensates: We study periodic crystalline spin textures in spinor condensates with
dipolar interactions via a systematic symmetry analysis of the low-energy
effective theory. By considering symmetry operations which combine real and
spin space operations, we classify symmetry groups consistent with non-trivial
experimental and theoretical constraints. Minimizing the energy within each
symmetry class allows us to explore possible ground states. | cond-mat_quant-gas |
Cavity-mediated unconventional pairing in ultracold fermionic atoms: We investigate long-range pairing interactions between ultracold fermionic
atoms confined in an optical lattice which are mediated by the coupling to a
cavity. In the absence of other perturbations, we find three degenerate pairing
symmetries for a two-dimensional square lattice. By tuning a weak local atomic
interaction via a Feshbach resonance or by tuning a weak magnetic field, the
superfluid system can be driven from a topologically trivial s-wave to
topologically ordered, chiral superfluids containing Majorana edge states. Our
work points out a novel path towards the creation of exotic superfluid states
by exploiting the competition between long-range and short-range interactions. | cond-mat_quant-gas |
Collective excitations of exciton-polariton condensates in a synthetic
gauge field: Collective (elementary) excitations of quantum bosonic condensates, including
condensates of exciton polaritons in semiconductor microcavities, are a
sensitive probe of interparticle interactions. In anisotropic microcavities
with momentum-dependent TE-TM splitting of the optical modes, the excitations
dispersions are predicted to be strongly anisotropic, which is a consequence of
the synthetic magnetic gauge field of the cavity, as well as the interplay
between different interaction strengths for polaritons in the singlet and
triplet spin configurations. Here, by directly measuring the dispersion of the
collective excitations in a high-density optically trapped exciton-polariton
condensate, we observe excellent agreement with the theoretical predictions for
spinor polariton excitations. We extract the inter- and intra-spin polariton
interaction constants and map out the characteristic spin textures in an
interacting spinor condensate of exciton polaritons. | cond-mat_quant-gas |
Sudden and slow quenches into the antiferromagnetic phase of ultracold
fermions: We propose a method to reach the antiferromagnetic state of two-dimensional
Fermi gases trapped in optical lattices: Independent subsystems are prepared in
suitable initial states and then connected by a sudden or slow quench of the
tunneling between the subsystems. Examples of suitable low-entropy subsystems
are double wells or plaquettes, which can be experimentally realised in Mott
insulating shells using optical super-lattices. We estimate the effective
temperature T* of the system after the quench by calculating the distribution
of excitations created using the spin wave approximation in a Heisenberg model.
We investigate the effect of an initial staggered magnetic field and find that
for an optimal polarisation of the initial state the effective temperature can
be significantly reduced from T*$\approx$1.7 Tc at zero polarisation to
T*<0.65Tc, where Tc is the crossover temperature to the antiferromagnetic
state. The temperature can be further reduced using a finite quench time. We
also show that T* decreases logarithmically with the linear size of the
subsystem. | cond-mat_quant-gas |
Quantum dynamics of a binary mixture of BECs in a double well potential:
an Holstein-Primakoff approach: We study the quantum dynamics of a binary mixture of Bose-Einstein
condensates (BEC) in a double-well potential starting from a two-mode
Bose-Hubbard Hamiltonian. Focussing on the regime where the number of atoms is
very large, a mapping onto a SU(2) spin problem together with a
Holstein-Primakoff transformation is performed. The quantum evolution of the
number difference of bosons between the two wells is investigated for different
initial conditions, which range from the case of a small imbalance between the
two wells to a coherent spin state. The results show an instability towards a
phase-separation above a critical positive value of the interspecies
interaction while the system evolves towards a coherent tunneling regime for
negative interspecies interactions. A comparison with a semiclassical approach
is discussed together with some implications on the experimental realization of
phase separation with cold atoms. | cond-mat_quant-gas |
Doping a lattice-trapped bosonic species with impurities: From ground
state properties to correlated tunneling dynamics: We investigate the ground state properties and the nonequilibrium dynamics of
a lattice trapped bosonic mixture consisting of an impurity species and a
finite-sized medium. For the case of one as well as two impurities we observe
that, depending on the lattice depth and the interspecies interaction strength,
a transition from a strongly delocalized to a localized impurity distribution
occurs. In the latter regime the two species phase separate, thereby forming a
particle-hole pair. For two impurities we find that below a critical lattice
depth they are delocalized among two neighboring outer lattice wells and are
two-body correlated. This transition is characterized by a crossover from
strong to a suppressed interspecies entanglement for increasing impurity-medium
repulsion. Turning to the dynamical response of the mixture, upon quenching the
interspecies repulsion to smaller values, we reveal that the predominant
tunneling process for a single impurity corresponds to that of a particle-hole
pair, whose dynamical stability depends strongly on the quench amplitude.
During the time-evolution a significant increase of the interspecies
entanglement is observed, caused by the build-up of a superposition of states
and thus possesses a many-body nature. In the case of two bosonic impurities
the particle-hole pair process becomes unstable in the course of the dynamics
with the impurities aggregating in adjacent lattice sites while being strongly
correlated. | cond-mat_quant-gas |
Parametric instabilities of interacting bosons in periodically-driven 1D
optical lattices: Periodically-driven quantum systems are currently explored in view of
realizing novel many-body phases of matter. This approach is particularly
promising in gases of ultracold atoms, where sophisticated shaking protocols
can be realized and inter-particle interactions are well controlled. The
combination of interactions and time-periodic driving, however, often leads to
uncontrollable heating and instabilities, potentially preventing practical
applications of Floquet-engineering in large many-body quantum systems. In this
work, we experimentally identify the existence of parametric instabilities in
weakly-interacting Bose-Einstein condensates in strongly-driven optical
lattices through momentum-resolved measurements. Parametric instabilities can
trigger the destruction of weakly-interacting Bose-Einstein condensates through
the rapid growth of collective excitations, in particular in systems with weak
harmonic confinement transverse to the lattice axis. | cond-mat_quant-gas |
Glass-like Behavior in a System of One Dimensional Fermions after a
Quantum Quench: We investigate the non-equilibrium relaxation dynamics of a one dimensional
system of interacting spinless fermions near the XXZ integrable point. We
observe two qualitatively different regimes: close to integrability and for low
energies the relaxation proceeds in two steps (prethermalization scenario),
while for large energies and/or away from integrability the dynamics develops
in a single step. When the integrability breaking parameter is below a certain
finite threshold and the energy of the system is sufficiently low the lifetime
of the metastable states increases abruptly by several orders of magnitude,
resembling the physics of glassy systems. This is reflected in a sudden jump in
the relaxation timescales. We present results for finite but large systems and
for large times compared to standard numerical methods. Our approach is based
on the construction of equations of motion for one- and two-particle
correlation functions using projection operator techniques. | cond-mat_quant-gas |
Fractional quantum Hall physics with ultracold Rydberg gases in
artificial gauge fields: We study ultracold Rydberg-dressed Bose gases subject to artificial gauge
fields in the fractional quantum Hall (FQH) regime. The characteristics of the
Rydberg interaction gives rise to interesting many-body ground states different
from standard FQH physics in the lowest Landau level (LLL). The non-local but
rapidly decreasing interaction potential favors crystalline ground states for
very dilute systems. While a simple Wigner crystal becomes energetically
favorable compared to the Laughlin liquid for filling fractions $\nu<1/12$, a
correlated crystal of composite particles emerges already for $\nu \leq 1/6$
with a large energy gap to the simple Wigner crystal. The presence of a new
length scale, the Rydberg blockade radius $a_B$, gives rise to a bubble crystal
phase for $\nu\lesssim 1/4$ when the average particle distance becomes less
than $a_B$, which describes the region of saturated, almost constant
interaction potential. For larger fillings indications for strongly correlated
cluster liquids are found. | cond-mat_quant-gas |
Anomalous quantum-reflection of Bose-Einstein condensates as a
self-screening effect: We discuss the effect of anomalous quantum-reflection of Bose-Einstein
condensates as a screening effect, that is created by the condensate itself. We
derive an effective, time-independent single-mode approach, that allows us to
define different paths of reflection. We compare our theory with experimental
results. | cond-mat_quant-gas |
From non equilibrium quantum Brownian motion to impurity dynamics in 1D
quantum liquids: Impurity motion in one dimensional ultra cold quantum liquids confined in an
optical trap has attracted much interest recently. As a step towards its full
understanding, we construct a generating functional from which we derive the
position non equilibrium correlation function of a quantum Brownian particle
with general Gaussian non-factorizing initial conditions. We investigate the
slow dynamics of a particle confined in a harmonic potential after a position
measurement; the rapid relaxation of a particle trapped in a harmonic potential
after a quantum quench realized as a sudden change in the potential parameters;
and the evolution of an impurity in contact with a one dimensional bosonic
quantum gas. We argue that such an impurity-Luttinger liquid system, that has
been recently realized experimentally, admits a simple modeling as quantum
Brownian motion in a super Ohmic bath. | cond-mat_quant-gas |
A Hubbard model for ultracold bosonic atoms interacting via
zero-point-energy induced three-body interactions: We show that for ultra-cold neutral bosonic atoms held in a three-dimensional
periodic potential or optical lattice, a Hubbard model with dominant,
attractive three-body interactions can be generated. In fact, we derive that
the effect of pair-wise interactions can be made small or zero starting from
the realization that collisions occur at the zero-point energy of an optical
lattice site and the strength of the interactions is energy dependent from
effective-range contributions. We determine the strength of the two- and
three-body interactions for scattering from van-der-Waals potentials and near
Fano-Feshbach resonances. For van-der-Waals potentials, which for example
describe scattering of alkaline-earth atoms, we find that the pair-wise
interaction can only be turned off for species with a small negative scattering
length, leaving the $^{88}$Sr isotope a possible candidate. Interestingly, for
collisional magnetic Feshbach resonances this restriction does not apply and
there often exist magnetic fields where the two-body interaction is small. We
illustrate this result for several known narrow resonances between alkali-metal
atoms as well as chromium atoms. Finally, we compare the size of the three-body
interaction with hopping rates and describe limits due to three-body
recombination. | cond-mat_quant-gas |
Thermodynamics of ideal Fermi gas under generic power law potential in
$d$-dimension: Thermodynamics of ideal Fermi gas trapped in an external generic power law
potential $U=\sum_{i=1} ^d c_i |\frac{x_i}{a_i}|^{n_i}$ are investigated
systematically from the grand thermodynamic potential in $d$ dimensional space.
These properties are explored deeply in the degenerate limit ($\mu>> K_BT$),
where the thermodynamic properties are greatly dominated by Pauli exclusion
principle. Pressure and energy along with the isothermal compressibilty is non
zero at $T=0K$, denoting trapped Fermi system is quite live even at absolute
zero temperature. The nonzero value of compressibilty denotes zero point
pressure is not just a constant but depends on volume. | cond-mat_quant-gas |
Dirac and Weyl Rings in Three Dimensional Cold Atom Optical Lattices: Recently three dimensional topological quantum materials with gapless energy
spectra have attracted considerable interests in many branches of physics.
Besides the celebrated example, Dirac and Weyl points which possess gapless
point structures in the underlying energy dispersion, the topologically
protected gapless spectrum can also occur along a ring, named Dirac and Weyl
nodal rings. Ultra-cold atomic gases provide an ideal platform for exploring
new topological materials with designed symmetries. However, whether Dirac and
Weyl rings can exist in the single-particle spectrum of cold atoms remains
elusive. Here we propose a realistic model for realizing Dirac and Weyl rings
in the single-particle band dispersion of a cold atom optical lattice. Our
scheme is based on previously experimentally already implemented Raman coupling
setup for realizing spin-orbit coupling. Without the Zeeman field, the model
preserves both pseudo-time-reversal and inversion symmetries, allowing Dirac
rings. The Dirac rings split into Weyl rings with a Zeeman field that breaks
the pseudo-time-reversal symmetry. We examine the superfluidity of attractive
Fermi gases in this model and also find Dirac and Weyl rings in the
quasiparticle spectrum. | cond-mat_quant-gas |
Many-body stabilization of a resonant p-wave Fermi gas in one dimension: Using the asymptotic Bethe Ansatz, we study the stabilization problem of the
one-dimensional spin-polarized Fermi gas confined in a hard-wall potential with
tunable p-wave scattering length and finite effective range. We find that the
interplay of two factors, i.e., the finite interaction range and the hard-wall
potential, will stabilize the system near resonance. The stabilization occurs
even in the positive scattering length side, where the system undergoes a
many-body collapse if any of the factors is absent. At p-wave resonance, the
fermion system is found to feature the "quasi-particle condensation" for any
value of effective range, which is stabilized if the range is above twice the
mean particle distance. Slightly away from resonance, the correction to the
stability condition linearly depends on the inverse scattering length. Finally,
a global picture is presented for the energetics and stability properties of
fermions from weakly attractive to deep bound state regime. Our results raise
the possibility for achieving stable p-wave superfluidity in quasi-1D atomic
systems, and meanwhile, shed light on the intriguing s- and p-wave physics in
1D that violate the Bose-Fermi duality. | cond-mat_quant-gas |
Control of the vortex lattice formation in coupled atom-molecular
Bose-Einstein condensate in a double well potential: Role of atom-molecule
coupling, trap rotation frequency and detuning: We study the vortex formation in coupled atomic and molecular condensates in
a rotating double well trap by numerically solving the coupled Gross-Pitaevskii
like equations. Starting with the atomic condensate in the double well
potential we considered two-photon Raman photoassociation for coherent
conversion of atoms to molecules. It is shown that the competition between
atom-molecule coupling strength and repulsive atom-molecule interaction
controls the spacings between atomic and molecular vortices and the rotation
frequency of the trap is the key player for controlling the number of visible
atomic and molecular vortices. Whereas the Raman detuning controls the spacing
between atomic and molecular vortices as well as the number of atomic and
molecular vortices in the trap. We have shown by considering the molecular
lattices the distance between two molecular vortices can be controlled by
varying the Raman detuning. In addition we have found that the Feynman rule
relating the total number of vortices and average angular momentum both for
atoms and molecules can be satisfied by considering the atomic and molecular
vortices those are hidden in density distribution and seen as singularities in
phase distribution of the coupled system except for the lattice structure where
molecular vortices are overlapped with each other. It is found that although
the number of visible/core vortices in atomic and molecular vortex lattices
depends significantly on the system parameters the number of atomic and
molecular hidden vortices remains constant in most of the cases. | cond-mat_quant-gas |
Rényi generalization of the operational entanglement entropy: Operationally accessible entanglement in bipartite systems of
indistinguishable particles could be reduced due to restrictions on the allowed
local operations as a result of particle number conservation. In order to
quantify this effect, Wiseman and Vaccaro [Phys. Rev. Lett. 91, 097902 (2003)]
introduced an operational measure of the von Neumann entanglement entropy.
Motivated by advances in measuring R\'enyi entropies in quantum many-body
systems subject to conservation laws, we derive a generalization of the
operational entanglement that is both computationally and experimentally
accessible. Using the Widom theorem, we investigate its scaling with the size
of a spatial subregion for free fermions and find a logarithmically violated
area law scaling, similar to the spatial entanglement entropy, with at most, a
double-log leading-order correction. A modification of the correlation matrix
method confirms our findings in systems of up to $10^5$ particles. | cond-mat_quant-gas |
Effect of finite range interactions on roton mode softening in a
multi-component BEC: We consider the Gross-Pitaevskii(GP) model of a Bose-Einstein Condensate(BEC)
for single-component and multi-component BEC. The pseudopotential for s-wave
scattering between atoms is taken to be of width of the order of the s-wave
scattering length. Such an interaction giving rise to a roton minimum in the
spectrum of elementary excitations of a single component BEC is well known.
However, softening roton modes takes us in the strongly interacting BEC regime
where three body losses occur. We study the roton mode softening for a
multi-component BEC. We show that by increasing the number of components of a
multi-component BEC, the roton mode can be softened at a progressively lower
value of the gas parameter ($a^{3}n$), thus reducing three body losses. | cond-mat_quant-gas |
Superfluid Bloch dynamics in an incommensurate lattice: We investigate the interplay of disorder and interactions in the accelerated
transport of a Bose-Einstein condensate through an incommensurate optical
lattice. We show that interactions can effectively cancel the damping of Bloch
oscillations due to the disordered potential and we provide a simple model to
qualitatively capture this screening effect. We find that the characteristic
interaction energy, above which interactions and disorder cooperate to enhance,
rather than reduce, the damping of Bloch oscillations, coincides with the
average disorder depth. This is consistent with results of a mean-field
simulation. | cond-mat_quant-gas |
Delayed response of a fermion-pair condensate to a modulation of the
interaction strength: The effect of a sinusoidal modulation of the interaction strength on a
fermion-pair condensate is analytically studied. The system is described by a
generalization of the coupled fermion-boson model that incorporates a
time-dependent intermode coupling induced via a magnetic Feshbach resonance.
Nontrivial effects are shown to emerge depending on the relative magnitude of
the modulation period and the relaxation time of the condensate. Specifically,
a nonadiabatic modulation drives the system out of thermal equilibrium: the
external field induces a variation of the quasiparticle energies, and, in turn,
a disequilibrium of the associated populations. The subsequent relaxation
process is studied and an analytical description of the gap dynamics is
obtained. Recent experimental findings are explained: the delay observed in the
response to the applied field is understood as a temperature effect linked to
the condensate relaxation time. | cond-mat_quant-gas |
Drag force on a moving impurity in a spin-orbit coupled Bose-Einstein
condensate: We investigate the drag force on a moving impurity in a spin-orbit coupled
Bose-Einstein condensate. We prove rigorously that the superfluid critical
velocity is zero when the impurity moves in all but one directions, in contrast
to the case of liquid helium and superconductor where it is finite in all
directions. We also find that when the impurity moves in all directions except
two special ones, the drag force has nonzero transverse component at small
velocity. When the velocity becomes large and the states of the upper band are
also excited, the transverse force becomes very small due to opposite
contributions of the two bands. The characteristics of the superfluid critical
velocity and the transverse force are results of the order by disorder
mechanism in spin-orbit coupled boson systems. | cond-mat_quant-gas |
Dynamics of a degenerate Cs-Yb mixture with attractive interspecies
interactions: We probe the collective dynamics of a quantum degenerate Bose-Bose mixture of
Cs and $^{174}$Yb with attractive interspecies interactions. Specifically, we
excite vertical center of mass oscillations of the Cs condensate, and observe
significant damping for the Cs dipole mode, due to the rapid transfer of energy
to the larger Yb component, and the ensuing acoustic dissipation. Numerical
simulations based on coupled Gross-Pitaevskii equations provide excellent
agreement, and additionally reveal the possibility of late-time revivals
(beating) which are found to be highly sensitive to the Cs and Yb atom number
combinations. By further tuning the interaction strength of Cs using a broad
Feshbach resonance, we explore the stability of the degenerate mixture, and
observe collapse of the Cs condensate mediated by the attractive Cs-Yb
interaction when $a_{\mathrm{Cs}}<50 \, a_0$, well above the single-species
collapse threshold, in good agreement with simulations. | cond-mat_quant-gas |
Atomic topological quantum matter using synthetic dimensions: The realization of topological states of matter in ultracold atomic gases is
currently the subject of intense experimental activity. Using a synthetic
dimension, encoded in a non-spatial degree of freedom, can greatly simplify the
simulation of gauge fields and give access to exotic topological states. We
review here recent advances in the field and discuss future perspectives for
interacting systems. | 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 |
Tensor networks for Lattice Gauge Theories and Atomic Quantum Simulation: We show that gauge invariant quantum link models, Abelian and non-Abelian,
can be exactly described in terms of tensor networks states. Quantum link
models represent an ideal bridge between high-energy to cold atom physics, as
they can be used in cold-atoms in optical lattices to study lattice gauge
theories. In this framework, we characterize the phase diagram of a (1+1)-d
quantum link version of the Schwinger model in an external classical background
electric field: the quantum phase transition from a charge and parity ordered
phase with non-zero electric flux to a disordered one with a net zero electric
flux configuration is described by the Ising universality class. | cond-mat_quant-gas |
Manipulation of a Bose-Einstein condensate by a time-averaged orbiting
potential using phase jumps of the rotating field: We report on the manipulation of the center-of-mass motion (`sloshing') of a
Bose Einstein condensate in a time-averaged orbiting potential (TOP) trap. We
start with a condensate at rest in the center of a static trapping potential.
When suddenly replacing the static trap with a TOP trap centered about the same
position, the condensate starts to slosh with an amplitude much larger than the
TOP micromotion. We show, both theoretically and experimentally, that the
direction of sloshing is related to the initial phase of the rotating magnetic
field of the TOP. We show further that the sloshing can be quenched by applying
a carefully timed and sized jump in the phase of the rotating field. | cond-mat_quant-gas |
Controllable splitting dynamics of a doubly quantized vortex in a
rotating ring-shaped condensate: We study the dynamics of a doubly quantized vortex (DQV), created by
releasing a ring-shaped Bose-Einstein condensate with quantized circulation
into harmonic potential traps. It is shown that a DQV can be generated and
exists stably in the middle of the ring-shaped condensate with the initial
circulation $s = 2$ after released into the rotationally symmetric trap
potential. For an asymmetric trap with a small degree of anisotropy the DQV
initially splits into two singly quantized vortices and revives again but
eventually evolves into two unit vortices due to the dynamic instability. For
the degree of anisotropy above a critical value, the DQV is extremely unstably
and decays rapidly into two singlet vortices. The geometry-dependent lifetime
of the DQV and vortex-induced excitations are also discussed intensively. | cond-mat_quant-gas |
Oscillating Quantum Droplets from the free expansion of Logarithmic
One-Dimensional Bose Gases: We analyze some issues related to the stability and free expansion of a
one-dimensional logarithmic Bose-Einstein condensate, particularly its eventual
relation to the formation of quantum droplet-type configurations. We prove that
the corresponding properties, such as the energy of the associated N-body
ground state, differ substantially with respect to its three-dimensional
counterpart. Consequently, the free velocity expansion also shows differences
with respect to the three-dimensional system when logarithmic interactions are
taken into account. The one-dimensional logarithmic condensate tends to form
quantum droplet-type configurations when the external trapping potential is
turned off, i.e., the self-sustainability or self-confinement appears as in
three-dimensions. However, we obtain that for some specific values of the
self-interaction parameters and the number of particles under consideration,
the cloud oscillates during the free expansion around to a specific equilibrium
size. These results show that we can get scenarios in which the one-dimensional
cloud reaches stable configurations, i.e., oscillating quantum droplets. | cond-mat_quant-gas |
Transport regimes of cold gases in a two-dimensional anisotropic
disorder: We numerically study the dynamics of cold atoms in a two-dimensional
disordered potential. We consider an anisotropic speckle potential and focus on
the classical regime, which is relevant to some recent experiments. First, we
study the behavior of particles with a fixed energy and identify different
transport regimes. For low energy, the particles are classically localized due
to the absence of a percolating cluster. For high energy, the particles undergo
normal diffusion and we show that the diffusion constants scale algebraically
with the particle energy, with an anisotropy factor which significantly differs
from that of the disordered potential. For intermediate energy, we find a
transient sub-diffusive regime, which is relevant to the time scale of typical
experiments. Second, we study the behavior of a cold-atomic gas with an
arbitrary energy distribution, using the above results as a groundwork. We show
that the density profile of the atomic cloud in the diffusion regime is
strongly peaked and, in particular, that it is not Gaussian. Its behavior at
large distances allows us to extract the energy-dependent diffusion constants
from experimental density distributions. For a thermal cloud released into the
disordered potential, we show that our numerical predictions are in agreement
with experimental findings. Not only does this work give insights to recent
experimental results, but it may also serve interpretation of future
experiments searching for deviation from classical diffusion and traces of
Anderson localization. | cond-mat_quant-gas |
Emergence of damped-localized excitations of the Mott state due to
disorder: A key aspect of ultracold bosonic quantum gases in deep optical lattice
potential wells is the realization of the strongly interacting Mott insulating
phase. Many characteristics of this phase are well understood, however little
is known about the effects of a random external potential on its gapped
quasiparticle and quasihole low-energy excitations. In the present study we
investigate the effect of disorder upon the excitations of the Mott insulating
state at zero temperature described by the Bose-Hubbard model. Using a
field-theoretical approach we obtain a resummed expression for the disorder
ensemble average of the spectral function. Its analysis shows that disorder
leads to an increase of the effective mass of both quasiparticle and quasihole
excitations. Furthermore, it yields the emergence of damped states, which
exponentially decay during propagation in space and dominate the whole band
when disorder becomes comparable to interactions. We argue that such
damped-localized states correspond to single-particle excitations of the
Bose-glass phase. | cond-mat_quant-gas |
Instability of the superfluid flow as black-hole lasing effect: We show that the instability leading to the decay of the one-dimensional
superfluid flow through a penetrable barrier are due to the black-hole lasing
effect. This dynamical instability is triggered by modes resonating in an
effective cavity formed by two horizons enclosing the barrier. The location of
the horizons is set by $v(x)=c(x)$, with $v(x),c(x)$ being the local fluid
velocity and sound speed, respectively. We compute the critical velocity
analytically and show that it is univocally determined by the horizons
configuration. In the limit of broad barriers, the continuous spectrum at the
origin of the Hawking-like radiation and of the Landau energetic instability is
recovered. | cond-mat_quant-gas |
Atomic matter-wave revivals with definite atom number in an optical
lattice: We study the collapse and revival of interference patterns in the momentum
distribution of atoms in optical lattices, using a projection technique to
properly account for the fixed total number of atoms in the system. We consider
the common experimental situation in which weakly interacting bosons are loaded
into a shallow lattice, which is suddenly made deep. The collapse and revival
of peaks in the momentum distribution is then driven by interactions in a
lattice with essentially no tunnelling. The projection technique allows to us
to treat inhomogeneous (trapped) systems exactly in the case that
non-interacting bosons are loaded into the system initially, and we use
time-dependent density matrix renormalization group techniques to study the
system in the case of finite tunnelling in the lattice and finite initial
interactions. For systems of more than a few sites and particles, we find good
agreement with results calculated via a naive approach, in which the state at
each lattice site is described by a coherent state in the particle occupation
number. However, for systems on the order of 10 lattice sites, we find
experimentally measurable discrepancies to the results predicted by this
standard approach. | cond-mat_quant-gas |
Partial Fermionization---Spectral Universality in 1D Repulsive Bose
Gases: Due to the vast growth of the many-body level density with excitation energy,
its smoothed form is of central relevance for spectral and thermodynamic
properties of interacting quantum systems. We compute the cumulative of this
level density for confined one-dimensional continuous systems with repulsive
short-range interactions. We show that the crossover from an ideal Bose gas to
the strongly correlated, fermionized gas, i.e., partial fermionization,
exhibits universal behavior: Systems with very few up to many particles share
the same underlying spectral features. In our derivation we supplement quantum
cluster expansions with short-time dynamical information. Our nonperturbative
analytical results are in excellent agreement with numerics for systems of
experimental relevance in cold atom physics, such as interacting bosons on a
ring (Lieb-Liniger model) or subject to harmonic confinement. Our method
provides predictions for excitation spectra that enable access to
finite-temperature thermodynamics in large parameter ranges. | cond-mat_quant-gas |
Anisotropic and long-range vortex interactions in two-dimensional
dipolar Bose gases: We perform a theoretical study into how dipole-dipole interactions modify the
properties of superfluid vortices within the context of a two-dimensional
atomic Bose gas of co-oriented dipoles. The reduced density at a vortex acts
like a giant anti-dipole, changing the density profile and generating an
effective dipolar potential centred at the vortex core whose most slowly
decaying terms go as $1/\rho^2$ and $\ln(\rho)/\rho^3$. These effects modify
the vortex-vortex interaction which, in particular, becomes anisotropic for
dipoles polarized in the plane. Striking modifications to vortex-vortex
dynamics are demonstrated, i.e. anisotropic co-rotation dynamics and the
suppression of vortex annihilation. | cond-mat_quant-gas |
Quantum and thermal fluctuations in two-component Bose gases: We study the effects of quantum and thermal fluctuations on Bose-Bose
mixtures at finite temperature employing the time-dependent
Hartree-Fock-Bogoliubov (TDHFB) theory. The theory governs selfconsistently the
motion of the condensates, the noncondensates and of the anomalous components
on an equal footing. The finite temperature criterion for the phase separation
is established. We numerically analyze the temperature dependence of different
densities for both miscible and immiscible mixtures. We show that the degree of
the overlap between the two condensates and the thermal clouds is lowered and
the relative motion of the centers-of-mass of the condensed and thermal
components is strongly damped due to the presence of the pair anomalous
fluctuations. Our results are compared with previous theoretical and
experimental findings. On the other hand, starting from our TDHFB equations, we
develop a random-phase theory for the elementary excitations in a homogeneous
mixture. We find that the normal and anomalous fluctuations may lead to enhance
the excitations and the thermodynamics of the system. | cond-mat_quant-gas |
Nambu-Goldstone modes in segregated Bose-Einstein condensates: Nambu-Goldstone modes in immiscible two-component Bose-Einstein condensates
are studied theoretically. In a uniform system, a flat domain wall is
stabilized and then the translational invariance normal to the wall is
spontaneously broken in addition to the breaking of two U(1) symmetries in the
presence of two complex order parameters. We clarify properties of the
low-energy excitations and identify that there exist two Nambu-Goldstone modes:
in-phase phonon with a linear dispersion and ripplon with a fractional
dispersion. The signature of the characteristic dispersion can be verified in
segregated condensates in a harmonic potential. | cond-mat_quant-gas |
Exact relaxation dynamics of a localized many-body state in the 1D bose
gas: Through an exact method we numerically solve the time evolution of the
density profile for an initially localized state in the one-dimensional bosons
with repulsive short-range interactions. We show that a localized state with a
density notch is constructed by superposing one-hole excitations. The initial
density profile overlaps the plot of the squared amplitude of a dark soliton in
the weak coupling regime. We observe the localized state collapsing into a flat
profile in equilibrium for a large number of particles such as N=1000. The
relaxation time increases as the coupling constant decreases, which suggests
the existence of off-diagonal long-range order. We show a recurrence phenomenon
for a small number of particles such as N=20. | cond-mat_quant-gas |
Vortex Thermometry for Turbulent Two-Dimensional Fluids: We introduce a new method of statistical analysis to characterise the
dynamics of turbulent fluids in two dimensions. We establish that, in
equilibrium, the vortex distributions can be uniquely connected to the
temperature of the vortex gas, and apply this vortex thermometry to
characterise simulations of decaying superfluid turbulence. We confirm the
hypothesis of vortex evaporative heating leading to Onsager vortices proposed
in Phys. Rev. Lett. 113, 165302 (2014), and find previously unidentified vortex
power-law distributions that emerge from the dynamics. | cond-mat_quant-gas |
Separation induced resonances in quasi-one-dimensional ultracold atomic
gases: We study the effective one-dimensional (1D) scattering of two distinguishable
atoms confined individually by {\em separated} transverse harmonic traps. With
equal trapping frequency for two s-wave interacting atoms, we find that by
tuning the trap separations, the system can undergo {\em double} 1D scattering
resonance, named as the separation induced resonance(SIR), when the ratio
between the confinement length and s-wave scattering length is within
$(0.791,1.46]$. Near SIR, the scattering property shows unique dependence on
the resonance position. The universality of a many-body system on scattering
branch near SIR is demonstrated by studying the interaction effect of a
localized impurity coupled with a Fermi sea of light atoms in a quasi-1D trap. | cond-mat_quant-gas |
Thermal fluctuations and quantum phase transition in antiferromagnetic
Bose-Einstein condensates: We develop a method for investigating nonequilibrium dynamics of an ultracold
system that is initially at thermal equilibrium. Our procedure is based on the
classical fields approximation with appropriately prepared initial state. As an
application of the method, we investigate the influence of thermal fluctuations
on the quantum phase transition from an antiferromagnetic to phase separated
ground state in a spin-1 Bose-Einstein condensate of ultracold atoms. We find
that at temperatures significantly lower than the critical condensation
temperature $T_c$ the scaling law for the number of created spin defects
remains intact. | cond-mat_quant-gas |
Anderson localization and Mott insulator phase in the time domain: Particles in space periodic potentials constitute standard models for
investigation of crystalline phenomena in solid state physics. Time periodicity
of periodically driven systems is a close analogue of space periodicity of
solid state crystals. There is an intriguing question if solid state phenomena
can be observed in the time domain. Here we show that wave-packets localized on
resonant classical trajectories of periodically driven systems are ideal
elements to realize Anderson localization or Mott insulator phase in the time
domain. Uniform superpositions of the wave-packets form stationary states of a
periodically driven particle. However, an additional perturbation that
fluctuates in time results in disorder in time and Anderson localization
effects emerge. Switching to many-particle systems we observe that depending on
how strong particle interactions are, stationary states can be Bose-Einstein
condensates or single Fock states where definite numbers of particles occupy
the periodically evolving wave-packets. Our study shows that non-trivial
crystal-like phenomena can be observed in the time domain. | cond-mat_quant-gas |
SU(3) truncated Wigner approximation for strongly interacting Bose gases: We develop and utilize the SU(3) truncated Wigner approximation (TWA) in
order to analyze far-from-equilibrium quantum dynamics of strongly interacting
Bose gases in an optical lattice. Specifically, we explicitly represent the
corresponding Bose--Hubbard model at an arbitrary filling factor with
restricted local Hilbert spaces in terms of SU(3) matrices. Moreover, we
introduce a discrete Wigner sampling technique for the SU(3) TWA and examine
its performance as well as that of the SU(3) TWA with the Gaussian
approximation for the continuous Wigner function. We directly compare outputs
of these two approaches with exact computations regarding dynamics of the
Bose--Hubbard model at unit filling with a small size and that of a
fully-connected spin-1 model with a large size. We show that both approaches
can quantitatively capture quantum dynamics on a timescale of $\hbar/(Jz)$,
where $J$ and $z$ denote the hopping energy and the coordination number. We
apply the two kinds of SU(3) TWA to dynamical spreading of a two-point
correlation function of the Bose--Hubbard model on a square lattice with a
large system size, which has been measured in recent experiments. Noticeable
deviations between the theories and experiments indicate that proper inclusion
of effects of the spatial inhomogeneity, which is not straightforward in our
formulation of the SU(3) TWA, may be necessary. | cond-mat_quant-gas |
Quench dynamics of a weakly interacting disordered Bose gas in momentum
space: We theoretically study the out-of-equilibrium dynamics in momentum space of a
weakly interacting disordered Bose gas launched with a finite velocity. In the
absence of interactions, coherent multiple scattering gives rise to a
background of diffusive particles, on top of which a coherent backscattering
interference emerges. We revisit this scenario in the presence of interactions,
using a diagrammatic quantum transport theory. We find that the dynamics is
governed by coupled kinetic equations describing the thermalization of the
diffusive and coherent components of the gas. This phenomenon leads to a
destruction of coherent backscattering, well described by an exponential
relaxation whose rate is controlled by the particle collision time. These
predictions are confirmed by numerical simulations. | cond-mat_quant-gas |
Realizing a 1D topological gauge theory in an optically dressed BEC: Topological gauge theories describe the low-energy properties of certain
strongly correlated quantum systems through effective weakly interacting
models. A prime example is the Chern-Simons theory of fractional quantum Hall
states, where anyonic excitations emerge from the coupling between weakly
interacting matter particles and a density-dependent gauge field. Although in
traditional solid-state platforms such gauge theories are only convenient
theoretical constructions, engineered quantum systems enable their direct
implementation and provide a fertile playground to investigate their
phenomenology without the need for strong interactions. Here, we report the
quantum simulation of a topological gauge theory by realizing a one-dimensional
reduction of the Chern-Simons theory (the chiral BF theory) in a Bose-Einstein
condensate. Using the local conservation laws of the theory, we eliminate the
gauge degrees of freedom in favour of chiral matter interactions, which we
engineer by synthesizing optically dressed atomic states with
momentum-dependent scattering properties. This allows us to reveal the key
properties of the chiral BF theory: the formation of chiral solitons and the
emergence of an electric field generated by the system itself. Our results
expand the scope of quantum simulation to topological gauge theories and open a
route to the implementation of analogous gauge theories in higher dimensions. | cond-mat_quant-gas |
Quantum criticality in disordered bosonic optical lattices: Using the exact Bose-Fermi mapping, we study universal properties of
ground-state density distributions and finite-temperature quantum critical
behavior of one-dimensional hard-core bosons in trapped incommensurate optical
lattices. Through the analysis of universal scaling relations in the quantum
critical regime, we demonstrate that the superfluid to Bose glass transition
and the general phase diagram of disordered hard-core bosons can be uniquely
determined from finite-temperature density distributions of the trapped
disordered system. | cond-mat_quant-gas |
Finite-temperature properties of interacting bosons on a two-leg flux
ladder: Quasi-one-dimensional lattice systems such as flux ladders with artificial
gauge fields host rich quantum-phase diagrams that have attracted great
interest. However, so far, most of the work on these systems has concentrated
on zero-temperature phases while the corresponding finite-temperature regime
remains largely unexplored. The question if and up to which temperature
characteristic features of the zero-temperature phases persist is relevant in
experimental realizations. We investigate a two-leg ladder lattice in a uniform
magnetic field and concentrate our study on chiral edge currents and
momentum-distribution functions, which are key observables in ultracold
quantum-gas experiments. These quantities are computed for hard-core bosons as
well as noninteracting bosons and spinless fermions at zero and finite
temperatures. We employ a matrix-product-state based purification approach for
the simulation of strongly interacting bosons at finite temperatures and
analyze finite-size effects. Our main results concern the
vortex-fluid-to-Meissner crossover of strongly interacting bosons. We
demonstrate that signatures of the vortex-fluid phase can still be detected at
elevated temperatures from characteristic finite-momentum maxima in the
momentum-distribution functions, while the vortex-fluid phase leaves weaker
fingerprints in the local rung currents and the chiral edge current. In order
to determine the range of temperatures over which these signatures can be
observed, we introduce a suitable measure for the contrast of these maxima. The
results are condensed into a finite-temperature crossover diagram for hard-core
bosons. | cond-mat_quant-gas |
Tunable dual-species Bose-Einstein condensates of $^{39}$K and $^{87}$Rb: We present the production of dual-species Bose-Einstein condensates of
$^{39}\mathrm{K}$ and $^{87}\mathrm{Rb}$. Preparation of both species in the
$\left| F=1,m_F=-1 \right\rangle$ state enabled us to exploit a total of three
Fesh\-bach resonances which allows for simultaneous Feshbach tuning of the
$^{39}\mathrm{K}$ intraspecies and the $^{39}\mathrm{K}$-$^{87}\mathrm{Rb}$
interspecies scattering length. Thus dual-species Bose-Einstein condensates
were produced by sympathetic cooling of $^{39}\mathrm{K}$ with
$^{87}\mathrm{Rb}$. A dark spontaneous force optical trap was used for
$^{87}\mathrm{Rb}$, to reduce the losses in $^{39}\mathrm{K}$ due to
light-assisted collisions in the optical trapping phase, which can be of
benefit for other dual-species experiments. The tunability of the scattering
length was used to perform precision spectroscopy of the interspecies Feshbach
resonance located at $117.56(2)\,\mathrm{G}$ and to determine the width of the
resonance to $1.21(5)\,\mathrm{G}$ by rethermalization measurements. The
transition region from miscible to immiscible dual-species condensates was
investigated and the interspecies background scattering length was determined
to $28.5\,a_\mathrm{0}$ using an empirical model. This paves the way for
dual-species experiments with $^{39}\mathrm{K}$ and $^{87}\mathrm{Rb}$ BECs
ranging from molecular physics to precision metrology. | cond-mat_quant-gas |
Mott Insulators of Ultracold Fermionic Alkaline Earth Atoms:
Underconstrained Magnetism and Chiral Spin Liquid: We study Mott insulators of fermionic alkaline earth atoms, described by
Heisenberg spin models with enhanced SU(N) symmetry. In dramatic contrast to
SU(2) magnetism, more than two spins are required to form a singlet. On the
square lattice, the classical ground state is highly degenerate and magnetic
order is thus unlikely. In a large-N limit, we find a chiral spin liquid ground
state with topological order and Abelian fractional statistics. We discuss its
experimental detection. Chiral spin liquids with non-Abelian anyons may also be
realizable with alkaline earth atoms. | cond-mat_quant-gas |
Quantum engineering of Majorana quasiparticles in one-dimensional
optical lattices: We propose a feasible way of engineering Majorana-type quasiparticles in
ultracold fermionic gases on a one-dimensional (1D) optical lattice. For this
purpose, imbalanced ultracold atoms interacting by the spin-orbit coupling
should be hybridized with a three-dimensional Bose-Einstein condensate (BEC)
molecular cloud. By constraining the profile of an internal defect potential we
show that the Majorana-type excitations can be created or annihilated. This
process is modelled within the Bogoliubov-de Gennes approach. This study is
relevant also to nanoscopic 1D superconductors where modification of the
internal defect potential can be obtained by electrostatic means. | cond-mat_quant-gas |
Roton-Maxon Excitation Spectrum of Bose Condensates in a Shaken Optical
Lattice: We present experimental evidence showing that an interacting Bose condensate
in a shaken optical lattice develops a roton-maxon excitation spectrum, a
feature normally associated with superfluid helium. The roton-maxon feature
originates from the double-well dispersion in the shaken lattice, and can be
controlled by both the atomic interaction and the lattice shaking amplitude. We
determine the excitation spectrum using Bragg spectroscopy and measure the
critical velocity by dragging a weak speckle potential through the condensate -
both techniques are based on a digital micromirror device. Our dispersion
measurements are in good agreement with a modified-Bogoliubov model. | cond-mat_quant-gas |
Competing Superconducting States for Ultracold Atoms in Optical Lattices
with Artificial Staggered Magnetic Field: We study superconductivity in an ultracold Bose-Fermi mixture loaded into a
square optical lattice subjected to a staggered flux. While the bosons form a
superfluid at very low temperature and weak interaction, the interacting
fermions experience an additional long-ranged attractive interaction mediated
by phonons in the bosonic superfluid. This leads us to consider a generalized
Hubbard model with on-site and nearest-neighbor attractive interactions, which
give rise to two competing superconducting channels. We use the
Bardeen-Cooper-Schrieffer theory to determine the regimes where distinct
superconducting ground states are stabilized, and find that the non-local
pairing channel favors a superconducting ground state which breaks both the
gauge and the lattice symmetries, thus realizing unconventional
superconductivity. Furthermore, the particular structure of the single-particle
spectrum leads to unexpected consequences, for example, a dome-shaped
superconducting region in the temperature versus filing fraction phase diagram,
with a normal phase that comprises much richer physics than a Fermi-liquid.
Notably, the relevant temperature regime and coupling strength is readily
accessible in state of the art experiments with ultracold trapped atoms. | cond-mat_quant-gas |
Evolution of the unitary Bose gas for broad to narrow Feshbach
resonances: We study the post-quench dynamics of unitary Bose gases using a two-channel
model, focusing on the effect of variations in the width of the Feshbach
resonance due to density changes. We generally find that increasing the density
leads to a corresponding increase in the production of closed channel
molecules, a decrease in the build up of quantum depletion and a transition
from linear to quadratic early-time growth of the two-body contact as well as
the condensed pair fraction. Motivated by the presence of closed-channel
molecules in the unitary regime, we study the embedded two-body problem finding
a transition from open to closed-channel dominated dimers due to many-body
effects. | cond-mat_quant-gas |
Landau Effective Interaction between Quasiparticles in a Bose-Einstein
Condensate: Landau's description of the excitations in a macroscopic system in terms of
quasiparticles stands out as one of the highlights in quantum physics. It
provides an accurate description of otherwise prohibitively complex many-body
systems, and has led to the development of several key technologies. In this
paper, we investigate theoretically the Landau effective interaction between
quasiparticles, so-called Bose polarons, formed by impurity particles immersed
in a Bose-Einstein condensate (BEC). In the limit of weak interactions between
the impurities and the BEC, we derive rigorous results for the effective
interaction. They show that it can be strong even for weak impurity-boson
interaction, if the transferred momentum/energy between the quasiparticles is
resonant with a sound mode in the BEC. We then develop a diagrammatic scheme to
calculate the effective interaction for arbitrary coupling strengths, which
recovers the correct weak coupling results. Using this, we show that the Landau
effective interaction in general is significantly stronger than that between
quasiparticles in a Fermi gas, mainly because a BEC is more compressible than a
Fermi gas. The interaction is particularly large near the unitarity limit of
the impurity-boson scattering, or when the quasiparticle momentum is close to
the threshold for momentum relaxation in the BEC. Finally, we show how the
Landau effective interaction leads to a sizeable shift of the quasiparticle
energy with increasing impurity concentration, which should be detectable with
present day experimental techniques. | cond-mat_quant-gas |
Quantum quench in an atomic one-dimensional Ising chain: We study non-equilibrium dynamics for an ensemble of tilted one-dimensional
atomic Bose-Hubbard chains after a sudden quench to the vicinity of the
transition point of the Ising paramagnetic to anti-ferromagnetic quantum phase
transition. The quench results in coherent oscillations for the orientation of
effective Ising spins, detected via oscillations in the number of
doubly-occupied lattice sites. We characterize the quench by varying the system
parameters. We report significant modification of the tunneling rate induced by
interactions and show clear evidence for collective effects in the oscillatory
response. | cond-mat_quant-gas |
Highly Polarized Fermi Gases across a Narrow Feshbach Resonance: We address the phase of a highly polarized Fermi gas across a narrow Feshbach
resonance starting from the problem of a single down spin fermion immersed in a
Fermi sea of up spins. Both polaron and pairing states are considered using the
variational wave function approach, and we find that the polaron to pairing
transition will take place at the BCS side of the resonance, strongly in
contrast to a wide resonance where the transition is located at the BEC side.
For pairing phase, we find out the critical strength of repulsive interaction
between pairs above which the mixture of pairs and fermions will not phase
separate. Therefore, nearby a narrow resonance, it is quite likely that
magnetism can coexist with s-wave BCS superfluidity at large Zeeman field,
which is a remarkable property absent in conventional BCS superconductors (or
fermion pair superfluids). | cond-mat_quant-gas |
A new Sobolev gradient method for direct minimization of the
Gross-Pitaevskii energy with rotation: In this paper we improve traditional steepest descent methods for the direct
minimization of the Gross-Pitaevskii (GP) energy with rotation at two levels.
We first define a new inner product to equip the Sobolev space $H^1$ and derive
the corresponding gradient. Secondly, for the treatment of the mass
conservation constraint, we use a projection method that avoids more
complicated approaches based on modified energy functionals or traditional
normalization methods. The descent method with these two new ingredients is
studied theoretically in a Hilbert space setting and we give a proof of the
global existence and convergence in the asymptotic limit to a minimizer of the
GP energy. The new method is implemented in both finite difference and finite
element two-dimensional settings and used to compute various complex
configurations with vortices of rotating Bose-Einstein condensates. The new
Sobolev gradient method shows better numerical performances compared to
classical $L^2$ or $H^1$ gradient methods, especially when high rotation rates
are considered. | cond-mat_quant-gas |
Modulational instability in binary spin-orbit-coupled Bose-Einstein
condensates: We study modulation instability (MI) of flat states in two-component
spin-orbit-coupled (SOC) Bose-Einstein condensates (BECs) in the framework of
coupled Gross-Pitaevskii equations for two components of the pseudospinor wave
function. The analysis is performed for equal densities of the components.
Effects of the interaction parameters, Rabi coupling, and SOC on the MI are
investigated. In particular, the results demonstrate that the SOC strongly
alters the commonly known MI (immiscibility) condition, $g_{12} > g_{1} g_{2}$,
for the binary superfluid with coefficients $g_{1,2}$ and $g_{12}$ of the
intra- and interspecies repulsive interactions. In fact, the binary BEC is
always subject to the MI under the action of the SOC, which implies that the
ground state of the system is plausibly represented by a striped phase. | cond-mat_quant-gas |
Resonant enhancement of the FFLO-state in 3D by a one-dimensional
optical potential: We describe an imbalanced superfluid Fermi gas in three dimensions within the
path-integral framework. To allow for the formation of the
Fulde-Ferell-Larkin-Ovchinnikov-state (FFLO-state), a suitable form of the
saddle-point is chosen, in which the pairs have a finite centre-of-mass
momentum. To test the correctness of this path-integral description, the
zero-temperature phase diagram for an imbalanced Fermi gas in three dimensions
is calculated, and compared to recent theoretical results. Subsequently, we
investigate two models that describe the effect of imposing a one-dimensional
optical potential on the 3D imbalanced Fermi gas. We show that this 1D optical
potential can greatly enlarge the stability region of the FFLO-state, relative
to the case of the 3D Fermi gas without 1D periodic modulation. Furthermore it
is show that there exists a direct connection between the centre-of-mass
momentum of the FFLO-pairs and the wavevector of the optical potential. We
propose that this concept can be used experimentally to resonantly enhance the
stability region of the FFLO-state. | cond-mat_quant-gas |
Evidence for a Bose-Einstein condensate of excitons: The demonstration of Bose-Einstein condensation in atomic gases at
micro-Kelvin temperatures is a striking landmark while its evidence for
semiconductor excitons still is a long-awaited milestone. This situation was
not foreseen because excitons are light-mass boson-like particles with a
condensation expected to occur around a few Kelvins. An explanation can be
found in the underlying fermionic nature of excitons which rules their
condensation. Precisely, it was recently predicted that, at accessible
experimental conditions, the exciton condensate shall be "gray" with a dominant
dark part coherently coupled to a weak bright component through fermion
exchanges. This counter-intuitive quantum condensation, since excitons are
mostly known for their optical activity, directly follows from the excitons
internal structure which has an optically inactive, i.e., dark, ground state.
Here, we report compelling evidence for such a "gray" condensate. We use an
all-optical approach in order to produce microscopic traps which confine a
dense exciton gas that yet exhibits an anomalously weak photo-emission at
sub-Kelvin temperatures. This first fingerprint for a "gray" condensate is then
confirmed by the macroscopic spatial coherence and the linear polarization of
the weak excitonic photoluminescence emitted from the trap, as theoretically
predicted. | cond-mat_quant-gas |
Quantum criticality in interacting bosonic Kitaev-Hubbard models: Motivated by recent work on the non-Hermitian skin effect in the bosonic
Kitaev-Majorana model, we study the quantum criticality of interacting bosonic
Kitaev-Hubbard models on a chain and a two-leg ladder. In the hard-core limit,
we show exactly that the non-Hermitian skin effect disappears via a
transformation from hard-core bosonic models to spin-1/2 models. We also show
that hard-core bosons can engineer the Kitaev interaction, the
Dzyaloshinskii-Moriya interaction and the compass interaction in the presence
of the complex hopping and pairing terms. Importantly, quantum criticalities of
the chain with a three-body constraint and unconstrained soft-core bosons are
investigated by the density matrix renormalization group method. This work
reveals the effect of many-body interactions on the non-Hermitian skin effect
and highlights the power of bosons with pairing terms as a probe for the
engineering of interesting models and quantum phase transitions. | cond-mat_quant-gas |
Bose-Einstein condensation into non-equilibrium states studied by
condensate focusing: We report the formation of Bose-Einstein condensates into non-equilibrium
states. Our condensates are much longer than equilibrium condensates with the
same number of atoms, show strong phase fluctuations, and have a dynamical
evolution similar to that of quadrupole shape oscillations of regular
condensates. The condensates emerge in elongated traps as the result of local
thermalization when the nucleation time is short compared to the axial
oscillation time. We introduce condensate focusing as a powerful method to
extract the phase-coherence length of Bose-Einstein condensates. | cond-mat_quant-gas |
First-order superfluid-Mott-insulator transition for quantum optical
switching in cavity QED arrays with two cavity modes: We theoretically investigated the ground states of coupled arrays of cavity
quantum electrodynamical (cavity QED) systems in presence of two photon modes.
Within the Gutzwiller-type variational approach, we found the first-order
quantum phase transition between Mott insulating and superfluid phases as well
as the conventional second-order one. The first-order phase transition was
found only for specific types of emitter models, and its physical origin is
clarified based on the analytic arguments which are allowed in the perturbative
and semiclassical limits. The first-order transition of the correlated photons
is accompanied with discontinuous change in the emitter states, not only with
the appearance of inter-cavity coherence in the superfluid phase. We also
discuss the condition for the first-order transition to occur, which can lead
to a strategy for future design of quantum optical switching devices with
cavity QED arrays. | cond-mat_quant-gas |
Direct evaporative cooling of 39K atoms to Bose-Einstein condensation: We report the realization of Bose-Einstein condensates of 39K atoms without
the aid of an additional atomic coolant. Our route to Bose-Einstein
condensation comprises Sub Doppler laser cooling of large atomic clouds with
more than 10^10 atoms and evaporative cooling in optical dipole traps where the
collisional cross section can be increased using magnetic Feshbach resonances.
Large condensates with almost 10^6 atoms can be produced in less than 15
seconds. Our achievements eliminate the need for sympathetic cooling with Rb
atoms which was the usual route implemented till date due to the unfavourable
collisional property of 39K. Our findings simplify the experimental set-up for
producing Bose-Einstein condensates of 39K atoms with tunable interactions,
which have a wide variety of promising applications including
atom-interferometry to studies on the interplay of disorder and interactions in
quantum gases. | cond-mat_quant-gas |
Precise characterization of ^6Li Feshbach resonances using trap-sideband
resolved RF spectroscopy of weakly bound molecules: We have performed radio-frequency dissociation spectroscopy of weakly bound
^6Li_2 Feshbach molecules using low-density samples of about 30 molecules in an
optical dipole trap. Combined with a high magnetic field stability this allows
us to resolve the discrete trap levels in the RF dissociation spectra. This
novel technique allows the binding energy of Feshbach molecules to be
determined with unprecedented precision. We use these measurements as an input
for a fit to the ^6Li scattering potential using coupled-channel calculations.
From this new potential, we determine the pole positions of the broad ^6Li
Feshbach resonances with an accuracy better than 7 \times 10^{-4} of the
resonance widths. This eliminates the dominant uncertainty for current
precision measurements of the equation of state of strongly interacting Fermi
gases. For example, our results imply a corrected value for the Bertsch
parameter \xi measured by Ku et al. [Science 335, 563 (2012)], which is \xi =
0.370(5)(8). | cond-mat_quant-gas |
Observation of Pauli Crystals: The Pauli exclusion principle is a fundamental law underpinning the structure
of matter. Due to their anti-symmetric wave function, no two fermions can
occupy the same quantum state. Here, we report on the direct observation of the
Pauli principle in a continuous system of up to six particles in the ground
state of a two-dimensional harmonic oscillator. To this end, we sample the full
many-body wavefunction by applying a single atom resolved imaging scheme in
momentum space. We find so-called Pauli crystals as a manifestation of higher
order correlations. In contrast to true crystalline phases, these unique
high-order density correlations emerge even without any interactions present.
Our work lays the foundation for future studies of correlations in strongly
interacting systems of many fermions. | cond-mat_quant-gas |
Circumnavigating an ocean of incompressible light: This is a popular science article to appear on the "Il Nuovo Saggiatore"
magazine of the Italian Physical Society. It aims at introducing a broad
audience of physicists to the most recent trends in many-body physics of
degenerate quantum gases with a special attention to quantum fluids of light
and the quest towards quantum Hall liquids of light. | cond-mat_quant-gas |
Pseudogap phenomenon and effects of population imbalance in the normal
state of a unitary Fermi gas: We investigate strong-coupling corrections to single-particle excitations in
the normal state of a spin-polarized unitary Fermi gas. Within the framework of
an extended T-matrix approximation, we calculate the single-particle density of
states, as well as the single-particle spectral weight, to show that the
so-called pseudogap phenomenon gradually disappears with increasing the
magnitude of an effective magnetic field. In the highly spin-polarized regime,
the calculated spin-polarization rate as a function of the effective magnetic
field agrees well with the recent experiment on a 6Li Fermi gas. Although this
experiment has been considered to be incompatible with the existence of the
pseudogap in an unpolarized Fermi gas, our result clarifies that the observed
spin-polarization rate in the highly spin-polarized regime and the pseudogap in
the unpolarized limit can be explained in a consistent manner, when one
correctly includes effects of population imbalance on single-particle
excitations. Since it is a crucial issue to clarify whether the pseudogap
exists or not in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein
condensation) crossover regime of an ultracold Fermi gas, our results would be
useful for the understanding of this strongly interacting fermion system. | cond-mat_quant-gas |
Time-Averaged Adiabatic Potentials: Versatile traps and waveguides for
ultracold quantum gases: We demonstrate a novel class of trapping potentials, time-averaged adiabatic
potentials (TAAP) which allows the generation of a large variety of traps and
waveguides for ultracold atoms. Multiple traps can be coupled through
controllable tunneling barriers or merged altogether. We present analytical
expressions for pancake-, cigar-, and ring- shaped traps. The ring-geometry is
of particular interest for guided matter-wave interferometry as it provides a
perfectly smooth waveguide of controllable diameter, and thus a tunable
sensitivity of the interferometer. | cond-mat_quant-gas |
Cold atoms in cavity-generated dynamical optical potentials: We review state-of-the-art theory and experiment of the motion of cold and
ultracold atoms coupled to the radiation field within a high-finesse optical
resonator in the dispersive regime of the atom-field interaction with small
internal excitation. The optical dipole force on the atoms together with the
back-action of atomic motion onto the light field gives rise to a complex
nonlinear coupled dynamics. As the resonator constitutes an open driven and
damped system, the dynamics is non-conservative and in general enables cooling
and confining the motion of polarizable particles. In addition, the emitted
cavity field allows for real-time monitoring of the particle's position with
minimal perturbation up to sub-wavelength accuracy. For many-body systems, the
resonator field mediates controllable long-range atom-atom interactions, which
set the stage for collective phenomena. Besides correlated motion of distant
particles, one finds critical behavior and non-equilibrium phase transitions
between states of different atomic order in conjunction with superradiant light
scattering. Quantum degenerate gases inside optical resonators can be used to
emulate opto-mechanics as well as novel quantum phases like supersolids and
spin glasses. Non-equilibrium quantum phase transitions, as predicted by e.g.
the Dicke Hamiltonian, can be controlled and explored in real-time via
monitoring the cavity field. In combination with optical lattices, the cavity
field can be utilized for non-destructive probing Hubbard physics and tailoring
long-range interactions for ultracold quantum systems. | cond-mat_quant-gas |
Towards strongly correlated photons in arrays of dissipative nonlinear
cavities under a frequency-dependent incoherent pumping: We report a theoretical study of a quantum optical model consisting of an
array of strongly nonlinear cavities incoherently pumped by an ensemble of
population-inverted two-level atoms. Projective methods are used to eliminate
the atomic dynamics and write a generalized master equation for the photonic
degrees of freedom only, where the frequency-dependence of gain introduces
non-Markovian features. In the simplest single cavity configuration, this
pumping scheme gives novel optical bistability effects and allows for the
selective generation of Fock states with a well-defined photon number. For many
cavities in a weakly non-Markovian limit, the non-equilibrium steady state
recovers a Grand-Canonical statistical ensemble at a temperature determined by
the effective atomic linewidth. For a two-cavity system in the strongly
nonlinear regime, signatures of a Mott state with one photon per cavity are
found. | cond-mat_quant-gas |
Realization of a stroboscopic optical lattice for cold atoms with
subwavelength spacing: Optical lattices are typically created via the ac-Stark shift, which are
limited by diffraction to periodicities $\ge\lambda/2$, where $\lambda$ is the
wavelength of light used to create them. Lattices with smaller periodicities
may be useful for many-body physics with cold atoms and can be generated by
stroboscopic application of a phase-shifted lattice with subwavelength
features. Here we demonstrate a $\lambda/4$-spaced lattice by stroboscopically
applying optical Kronig-Penney(KP)-like potentials which are generated using
spatially dependent dark states. We directly probe the periodicity of the
$\lambda/4$-spaced lattice by measuring the average probability density of the
atoms loaded into the ground band of the lattice. We measure lifetimes of atoms
in this lattice and discuss the mechanisms that limit the applicability of this
stroboscopic approach. | cond-mat_quant-gas |
Elementary excitations in dipolar spin-1 Bose-Einstein condensates: We have numerically solved the low-energy excitation spectra of ferromagnetic
Bose-Einstein condensates subject to dipolar interparticle interactions. The
system is assumed to be harmonically confined by purely optical means, thereby
maintaining the spin degree of freedom of the condensate order parameter. Using
a zero-temperature spin-1 model, we solve the Bogoliubov excitations for
different spin textures, including a spin-vortex state in the absence of
external magnetic fields and a rapidly rotating polarized spin texture in a
finite homogeneous field. In particular, we consider the effect of dipolar
interactions on excitations characteristic of ferromagnetic condensates. The
energies of spin waves and magnetic quadrupole modes are found to increase
rapidly with the dipolar coupling strength, whereas the energies of density
oscillations change only slightly. | cond-mat_quant-gas |
Controlling coherence via tuning of the population imbalance in a
bipartite optical lattice: The control of transport properties is a key tool at the basis of many
technologically relevant effects in condensed matter. The clean and precisely
controlled environment of ultracold atoms in optical lattices allows one to
prepare simplified but instructive models, which can help to better understand
the underlying physical mechanisms. Here we show that by tuning a structural
deformation of the unit cell in a bipartite optical lattice, one can induce a
phase transition from a superfluid into various Mott insulating phases forming
a shell structure in the superimposed harmonic trap. The Mott shells are
identified via characteristic features in the visibility of Bragg maxima in
momentum spectra. The experimental findings are explained by Gutzwiller
mean-field and quantum Monte Carlo calculations. Our system bears similarities
with the loss of coherence in cuprate superconductors, known to be associated
with the doping induced buckling of the oxygen octahedra surrounding the copper
sites. | cond-mat_quant-gas |
Stabilizing Gauge Theories in Quantum Simulators: A Brief Review: Quantum simulation is at the heart of the ongoing "second" quantum
revolution, with various synthetic quantum matter platforms realizing evermore
exotic condensed matter and particle physics phenomena at high levels of
precision and control. The implementation of gauge theories on modern quantum
simulators is especially appealing due to three main reasons: (i) it offers a
new probe of high-energy physics on low-energy tabletop devices, (ii) it allows
exploring condensed matter phenomena that are prominent in gauge theories even
without a direct connection to high-energy physics, and (iii) it serves as a
banner of experimental benchmarking given the plethora of local constraints
arising from the gauge symmetry that need to be programmed and controlled. In
order to faithfully model gauge-theory phenomena on a quantum simulator,
stabilizing the underlying gauge symmetry is essential. In this brief review,
we outline recently developed experimentally feasible methods introduced by us
that have shown, in numerical and experimental benchmarks, reliable
stabilization of quantum-simulator implementations of gauge theories. We
explain the mechanism behind these \textit{linear gauge protection} schemes,
and illustrate their power in protecting salient features such as gauge
invariance, disorder-free localization, quantum many-body scars, and other
phenomena of topical interest. We then discuss their application in experiments
based on Rydberg atoms, superconducting qubits, and in particular ultracold
neutral atoms in optical superlattices. We hope this review will illustrate
some facets of the exciting progress in stabilization of gauge symmetry and in
gauge-theory quantum simulation in general. | cond-mat_quant-gas |
Phase diagram of dipolar bosons in 2D with tilted polarization: We analyze the ground state of a system of dipolar bosons moving in the $XY$
plane and such that their dipolar moments are all aligned in a fixed direction
in space. We focus on the general case where the polarization field forms a
generic angle $\alpha$ with respect to the $Z$ axis. We use the Path Integral
Ground State method to analyze the static properties of the system as both
$\alpha$ and the density $n$ vary over a wide range were the system is stable.
We use the maximum of the static structure function as an order parameter to
characterize the different phases and the transition lines among them. We find
that aside of a superfluid gas and a solid phase, the system reaches a stripe
phase at large tilting angles that is entirely induced by the anisotropic
character of the interaction. We also show that the quantum phase transition
from the gas to the stripe phase is of second order, and report approximate
values for the critical exponents. | cond-mat_quant-gas |
Detuning control of Rabi vortex oscillations in light matter coupling: We study analytically the dynamics of vortices in strongly coupled
exciton--photon fields in the presence of energy detuning. We derive equations
for the vortex core velocity and mass, where they mainly depend on Rabi
coupling and the relative distance between the vortex cores in photon and
exciton fields, and as the result core positions oscillate in each field. We
use Magnus force balanced with a Rabi induced force to show that the core of
the vortex behaves as an inertial-like particle. Our analysis reveals that the
core is lighter at periphery of the beam and therefore it is faster at that
region. While detuning induces oscillations in population imbalance of
components through relative phase between coupled fields, in the presence of
topological charges detuning can control the orbital dynamics of the cores.
Namely, it brings the vortex core to move on larger or smaller orbits with
different velocities, and changes angular momentum and energy content of vortex
field. | cond-mat_quant-gas |
Exact three-body local correlations for excited states of the 1D Bose
gas: We derive an exact analytic expression for the three-body local correlations
in the Lieb-Liniger model of 1D Bose gas with contact repulsion. The local
three-body correlations control the thermalization and particle loss rates in
the presence of terms which break integrability, as is realized in the case of
1D ultracold bosons. Our result is valid not only at finite temperature but
also for a large class of non-thermal excited states in the thermodynamic
limit. We present finite temperature calculations in the presence of external
harmonic confinement within local density approximation, and for a highly
excited state that resembles an experimentally realized configuration. | cond-mat_quant-gas |
Second-order topological insulator in periodically driven lattice: The higher-order topological insulator (HOTI) is a new type of topological
system which has special bulkedge correspondence compared with conventional
topological insulators. In this work, we propose a scheme to realize Floquet
HOTI in ultracold atom systems. With the combination of periodically
spin-dependent driving of the superlattices and a next-next-nearest-neighbor
d-wave-like anisotropic coupling term between different spin components, a
Floquet second-order topological insulator with four zero-energy corner states
emerges, whose Wannier bands are gapless and exhibit interesting bulk topology.
Furthermore, the anisotropic coupling with nearest-neighbor form will also
induce some intriguing topological phenomena, e.g. non-topologically protected
corner states and topological semimetal for two different types of lattice
structures respectively. Our scheme may give insight into the construction of
different types of higher-order topological insulators in synthetic systems. It
also provides an experimentally feasible platform to research the relations
between different types of topological states and may have a wide range of
applications in future. | cond-mat_quant-gas |
Real-Time Dynamics of an Impurity in an Ideal Bose Gas in a Trap: We investigate the behavior of a harmonically trapped system consisting of an
impurity in a dilute ideal Bose gas after the boson-impurity interaction is
suddenly switched on. As theoretical framework, we use a field theory approach
in the space-time domain within the T-matrix approximation. We establish the
form of the corresponding T-matrix and address the dynamical properties of the
system. As a numerical application, we consider a simple system of a weakly
interacting impurity in one dimension where the interaction leads to
oscillations of the impurity density. Moreover, we show that the amplitude of
the oscillations can be driven by periodically switching the interaction on and
off. | cond-mat_quant-gas |
Nontrivial Haldane phase of an atomic two-component Fermi gas trapped in
a 1d optical lattice: We propose how to create a non-trivial Haldane phase in atomic two-component
Fermi-gas loaded on one-dimensional (1-D) optical lattice with trap potential.
The Haldane phase is naturally formed on $p$-band Mott core in a wide range of
the strong on-site repulsive interaction. The present proposal is composed of
two steps, one of which is theoretical derivation of an effective 1-D S=1
interacting-chain model from the original tight-binding Hamiltonian handling
the two $p$-orbitals, and the other of which is numerical demonstration
employing the density-matrix renormalization-group for the formation of the
Haldane phase on $p$-band Mott core and its associated features in the original
tight-binding model with the harmonic trap potential. | cond-mat_quant-gas |
Universal relations and normal phase of an ultracold Fermi gas with
coexisting $s$- and $p$-wave interactions: We study the universal relations and normal-phase thermodynamics of a
two-component ultracold Fermi gas with coexisting $s$- and $p$-wave
interactions. Due to the orthogonality of two-body wave functions of different
scattering channels, the universal thermodynamic relations of the system appear
to be direct summations of contributions from each partial-wave scattering
channels. These universal relations are dictated by a set of contacts, which
can be associated with either $s$- or $p$-wave interactions. Interestingly, due
to the interplay of $s$- and $p$-wave interactions on the many-body level, the
contacts, and hence all the relevant thermodynamic quantities, behave
differently from those with only $s$- or $p$-wave interactions. These are
manifest in our numerical calculations based on second-order virial expansions
for $^{40}$K atoms under typical experimental parameters. A particularly
interesting finding is that, due to the coexistence of $s$- and $p$-wave
scatterings, the interaction energy of the repulsive branch features abrupt
changes across the $p$-wave resonances. Our results can be readily checked
experimentally for $^{40}$K atoms near the $198$G $p$-wave Feshbach resonance,
where multiple partial-wave scatterings naturally coexist. | cond-mat_quant-gas |
Many-polaron description of impurities in a Bose-Einstein condensate in
the weak coupling regime: The weak coupling many-polaron formalism is applied to the case of the
polaronic system consisting of impurities in a Bose-Einstein condensate. This
allows to investigate the groundstate properties and the response of the system
to Bragg spectroscopy. This theory is then applied to the system of
spin-polarized fermionic lithium-6 impurities in a sodium condensate. The Bragg
spectrum reveals a peak which corresponds to the emission of Bogoliubov
excitations. Both ground state properties and the response spectrum show that
the polaronic effect vanishes at large densities. We also look at two
possibilities to define the polaronic effective mass and observe that this
results in a different quantitative behavior if multiple impurities are
involved. | cond-mat_quant-gas |
Quantum glass of interacting bosons with off-diagonal disorder: We study disordered interacting bosons described by the Bose-Hubbard model
with Gaussian-distributed random tunneling amplitudes. It is shown that the
off-diagonal disorder induces a spin-glass-like ground state, characterized by
randomly frozen quantum-mechanical U(1) phases of bosons. To access
criticality, we employ the "$n$-replica trick", as in the spin-glass theory,
and the Trotter-Suzuki method for decomposition of the statistical density
operator, along with numerical calculations. The interplay between disorder,
quantum and thermal fluctuations leads to phase diagrams exhibiting a glassy
state of bosons, which are studied as a function of model parameters. The
considered system may be relevant for quantum simulators of optical-lattice
bosons, where the randomness can be introduced in a controlled way. The latter
is supported by a proposition of experimental realization of the system in
question. | cond-mat_quant-gas |
Stable knots in the trapped Bose-Einstein condensates: The knot of spin texture is studied within the two-component Bose-Einstein
condensates which are described by the nonlinear Gross-Pitaevskii equations. We
start from the non-interacting equations including an axisymmetric harmonic
trap to obtain an exact solution, which exhibits a non-trivial topological
structure. The spin-texture is a knot with an integral Hopf invariant. The
stability of the knot is verified by numerically evolving the nonlinear
Gross-Pitaevskii equations along imaginary time. | cond-mat_quant-gas |
Illustration of universal relations for trapped four-fermion system with
arbitrary s-wave scattering length: A two-component four-fermion system with equal masses, interspecies s-wave
scattering length a and vanishing intraspecies interactions under external
spherically symmetric harmonic confinement is considered. Using a correlated
Gaussian basis set expansion approach, we determine the energies and various
structural properties of the energetically lowest-lying gas-like state
throughout the crossover for various ranges of the underlying two-body
potential. Extrapolating to the zero-range limit, our numerical results show
explicitly that the total energy, the trap energy as well as certain aspects of
the pair distribution function and of the momentum distribution are related
through the so-called integrated contact intensity I(a). Furthermore, it is
shown explicitly that the total energy and the trap energy are related through
a generalized virial theorem that accounts for a non-zero range. | 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 |
Spin-Energy Correlation in Degenerate Weakly-Interacting Fermi Gases: Weakly interacting Fermi gases exhibit rich collective dynamics in
spin-dependent potentials, arising from correlations between spin degrees of
freedom and conserved single atom energies, offering broad prospects for
simulating many-body quantum systems by engineering energy-space "lattices,"
with controlled energy landscapes and site to site interactions. Using quantum
degenerate clouds of $^6$Li, confined in a spin-dependent harmonic potential,
we measure complex, time-dependent spin-density profiles, varying on length
scales much smaller than the cloud size. We show that a one-dimensional mean
field model, without additional simplifying approximations, quantitatively
predicts the observed fine structure. We measure the magnetic fields where the
scattering lengths vanish for three different hyperfine state mixtures to
provide new constraints on the collisional (Feshbach) resonance parameters. | cond-mat_quant-gas |
The Raman dressed spin-1 spin-orbit coupled quantum gas: The recently realized spin-orbit coupled quantum gases (Y.-J Lin {\it et
al}., Nature 471, 83-86 (2011); P. Wang {\it et al}., PRL 109, 095301 (2012);
L. W. Cheuk {\it et al}., PRL 109, 095302 (2012)) mark a breakthrough in the
cold atom community. In these experiments, two hyperfine states are selected
from a hyperfine manifold to mimic a pseudospin-1/2 spin-orbit coupled system
by the method of Raman dressing, which is applicable to both bosonic and
fermionic gases. In this work, we show that the method used in these
experiments can be generalized to create any large pseudospin spin-orbit
coupled gas if more hyperfine states are coupled equally by the Raman lasers.
As an example, we study in detail a quantum gas with three hyperfine states
coupled by the Raman lasers, and show when the state-dependent energy shifts of
the three states are comparable, triple-degenerate minima will appear at the
bottom of the band dispersions, thus realizing a spin-1 spin-orbit coupled
quantum gas. A novel feature of this three minima regime is that there can be
two different kinds of stripe phases with different wavelengths, which has an
interesting connection to the ferromagnetic and polar phases of spin-1 spinor
BECs without spin-orbit coupling. | cond-mat_quant-gas |
Density-wave steady-state phase of dissipative ultracold fermions with
nearest-neighbor interactions: In this work we investigate the effect of local dissipation on the presence
of density-wave ordering in spinful fermions with both local and
nearest-neighbor interactions as described by the extended Hubbard model. We
find density-wave order to be robust against decoherence effects up to a
critical point where the system becomes homogeneous with no spatial ordering.
Our results will be relevant for future cold-atom experiments using fermions
with non-local interactions arising from the dressing by highly-excited Rydberg
states, which have finite lifetimes due to spontaneous emission processes. | cond-mat_quant-gas |
Excitation spectrum of the Lieb-Liniger model: We study the integrable model of one-dimensional bosons with contact
repulsion. In the limit of weak interaction, we use the microscopic
hydrodynamic theory to obtain the excitation spectrum. The statistics of
quasiparticles changes with the increase of momentum. At lowest momenta good
quasiparticles are fermions, while at higher momenta they are Bogoliubov
bosons, in accordance with recent studies. In the limit of strong interaction,
we analyze the exact solution and find exact results for the spectrum in terms
of the asymptotic series. Those results undoubtedly suggest that fermionic
quasiparticle excitations actually exist at all momenta for moderate and strong
interaction, and also at lowest momenta for arbitrary interaction. Moreover, at
strong interaction we find highly accurate analytical results for several
relevant quantities of the Lieb-Liniger model. | cond-mat_quant-gas |
Coherence and entanglement in the ground-state of a bosonic Josephson
junction:from macroscopic Schrödinger cats to separable Fock states: We consider a bosonic Josephson junction made of $N$ ultracold and dilute
atoms confined by a quasi one-dimensional double-well potential within the
two-site Bose-Hubbard model framework. The behaviour of the system is
investigated at zero temperature by varying the inter-atomic interaction from
the strongly attractive regime to the repulsive one. We show that the
ground-state exhibits a crossover from a macroscopic Schr\"odinger-cat state to
a separable Fock state through an atomic coherent regime. By diagonalizing the
Bose-Hubbard Hamiltonian we characterize the emergence of the mascroscopic cat
states by calculating the Fisher information $F$, the coherence by means of the
visibility $\alpha$ of the interference fringes in the momentum distribution,
and the quantum correlations by using the entanglement entropy $S$. Both Fisher
information and visibility are shown to be related to the ground state energy
by employing the Hellmann-Feynman theorem. This result, together with a
perturbative calculation of the ground-state energy, makes possible to obtain
simple analytical formulas for $F$ and $\alpha$ over a range of interactions,
in excellent agreement with the exact diagonalization of the Bose-Hubbard
Hamiltonian. In the attractive regime the entanglement entropy attains values
very close to its upper limit for a specific interaction strength lying in the
region where coherence is lost and self trapping sets in. | cond-mat_quant-gas |
Dark solitons in cigar-shaped Bose-Einstein condensates in double-well
potentials: We study the statics and dynamics of dark solitons in a cigar-shaped
Bose-Einstein condensate confined in a double-well potential. Using a
mean-field model with a non-cubic nonlinearity, appropriate to describe the
dimensionality crossover regime from one to three dimensional, we obtain
branches of solutions in the form of single- and multiple-dark soliton states,
and study their bifurcations and stability. It is demonstrated that there exist
dark soliton states which do not have a linear counterpart and we highlight the
role of anomalous modes in the excitation spectra. Particularly, we show that
anomalous mode eigenfrequencies are closely connected to the characteristic
soliton frequencies as found from the solitons' equations of motion, and how
anomalous modes are related to the emergence of instabilities. We also analyze
in detail the role of the height of the barrier in the double well setting,
which may lead to instabilities or decouple multiple dark soliton states. | cond-mat_quant-gas |
Comment on "Motion of an impurity particle in an ultracold
quasi-one-dimensional gas of hard-core bosons [Phys. Rev. A 79, 033610
(2009)]": Very recently Girardeau and Minguzzi [arXiv:0807.3366v2, Phys. Rev. A 79,
033610 (2009)] have studied an impurity in a one-dimensional gas of hard-core
bosons. In particular they deal with the general case where the mass of the
impurity is different from the mass of the bosons and the impurity-boson
interaction is not necessarily infinitely repulsive. We show that one of their
initial step is erroneous, contradicting both physical intuition and known
exact results. Their results in the general case apply only actually when the
mass of the impurity is infinite. | cond-mat_quant-gas |
Bosonic Kondo-Hubbard model: We study, using quantum Monte-Carlo simulations, the bosonic Kondo-Hubbard
model in a two dimensional square lattice. We explore the phase diagram and
analyse the mobility of particles and magnetic properties. At unit filling, the
transition from a paramagnetic Mott insulator to a ferromagnetic superfluid
appears continuous, contrary to what was predicted with mean field. For double
occupation per site, both the Mott insulating and superfluid phases are
ferromagnetic and the transition is still continuous. Multiband tight binding
Hamiltonians can be realized in optical lattice experiments, which offer not
only the possibility of tuning the different energy scales over wide ranges,
but also the option of loading the system with either fermionic or bosonic
atoms. | cond-mat_quant-gas |
Optical Control of Exchange Interaction and Kondo Temperature in cold
Atom Gas: The relevance of magnetic impurity problems in cold atom systems depends
crucially on the nature of exchange interaction between itinerant fermionic
atoms and a localized impurity atom. In particular, Kondo physics occurs only
if the exchange interaction is anti-ferromagnetic, and strong enough to yield
high enough Kondo temperature ($T_K/T_F \ge 0.1$). Focusing, as an example, on
the experimentally accessible system of ultra-cold $^{173}$Yb atoms, it is
shown that the sign and strength of an exchange interaction between an
itinerant Yb($^{1}$S$_{0}$) atom and a trapped Yb($^{3}$P$_{0}$) atom can be
optically controlled. Explicitly, as the light intensity increases (from zero),
the exchange interaction changes from ferromagnetic to anti-ferromagnetic. When
the light intensity is just below a singlet Feshbach resonance, the singlet
scattering length $a_S$ is large and negative, and the Kondo temperature
increases sharply. | cond-mat_quant-gas |
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