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Anderson-Bogoliubov collective excitations in superfluid Fermi gases at
nonzero temperatures: The Anderson-Bogoliubov branch of collective excitations in a condensed Fermi
gas is treated using the effective bosonic action of Gaussian pair
fluctuations. The spectra of collective excitations are treated for finite
temperature and momentum throughout the BCS-BEC crossover. The obtained spectra
explain, both qualitatively and quantitatively, recent experimental results on
Goldstone modes in atomic Fermi superfluids. | cond-mat_quant-gas |
Observation of Fermi surface deformation in a dipolar quantum gas: The deformation of a Fermi surface is a fundamental phenomenon leading to a
plethora of exotic quantum phases. Understanding these phases, which play
crucial roles in a wealth of systems, is a major challenge in atomic and
condensed-matter physics. Here, we report on the observation of a Fermi surface
deformation in a degenerate dipolar Fermi gas of erbium atoms. The deformation
is caused by the interplay between strong magnetic dipole-dipole interaction
and the Pauli exclusion principle. We demonstrate the many-body nature of the
effect and its tunability with the Fermi energy. Our observation provides basis
for future studies on anisotropic many-body phenomena in normal and superfluid
phases. | cond-mat_quant-gas |
Mean-field study of itinerant ferromagnetism in trapped ultracold Fermi
gases: Beyond the local density approximation: We theoretically investigate the itinerant ferromagnetic transition of a
spherically trapped ultracold Fermi gas with spin imbalance under strongly
repulsive interatomic interactions. Our study is based on a self-consistent
solution of the Hartree-Fock mean-field equations beyond the widely used local
density approximation. We demonstrate that, while the local density
approximation holds in the paramagnetic phase, after the ferromagnetic
transition it leads to a quantitative discrepancy in various thermodynamic
quantities even with large atom numbers. We determine the position of the phase
transition by monitoring the shape change of the free energy curve with
increasing the polarization at various interaction strengths. | cond-mat_quant-gas |
Analytic models for density of a ground-state spinor condensate: We demonstrate that the ground state of a trapped spin-1 and spin-2 spinor
ferromagnetic Bose-Einstein condensate (BEC) can be well approximated by a
single decoupled Gross-Pitaevskii (GP) equation. Useful analytic models for the
ground-state densities of ferromagnetic BECs are obtained from the Thomas-Fermi
approximation (TFA) to this decoupled equation. Similarly, for the ground
states of spin-1 anti-ferromagnetic and spin-2 anti-ferromagnetic and cyclic
BECs, some of the spin component densities are zero which reduces the coupled
GP equation to a simple reduced form. Analytic models for ground state
densities are also obtained for anti-ferromagnetic and cyclic BECs from the TFA
to the respective reduced GP equations. The analytic densities are illustrated
and compared with the full numerical solution of the GP equation with realistic
experimental parameters. | cond-mat_quant-gas |
Topological phase transitions in finite-size periodically driven
translationally invariant systems: It is known that, in the thermodynamic limit, the Chern number of a
translationally invariant system cannot change under unitary time evolutions
that are smooth in momentum space. Yet a real-space counterpart of the Chern
number, the Bott index, has been shown to change in periodically driven systems
with open boundary conditions. Here we prove that the Bott index and the Chern
number are identical in translationally invariant systems in the thermodynamic
limit. Using the Bott index, we show that, in finite-size translationally
invariant systems, a Fermi sea under a periodic drive that is turned on slowly
can acquire a different topology from that of the initial state. This can
happen provided that the gap-closing points in the thermodynamic limit are
absent in the discrete Brillouin zone of the finite system. Hence, in such
systems, a periodic drive can be used to dynamically prepare topologically
nontrivial states starting from topologically trivial ones. | cond-mat_quant-gas |
Application of the Feshbach-resonance management to a tightly confined
Bose-Einstein condensate: We study suppression of the collapse and stabilization of matter-wave
solitons by means of time-periodic modulation of the effective nonlinearity,
using the nonpolynomial Schroedinger equation (NPSE) for BEC trapped in a tight
cigar-shaped potential. By means of systematic simulations, a stability region
is identified in the plane of the modulation amplitude and frequency. In the
low-frequency regime, solitons feature chaotic evolution, although they remain
robust objects. | cond-mat_quant-gas |
Formation, dynamics and stability of coreless vortex dipoles in
phase-separated binary condensates: We study the motion of the Gaussian obstacle potential created by blue
detuned laser beam through a phase-separated binary condensate in
pancake-shaped traps. For the velocity of the obstacle above a critical
velocity, we observe the generation of vortex dipoles in the outer component
which can penetrate the inner component. This is equivalent to finite, although
small, transport of outer component across the inner component. In the inner
component, the same method can lead to the formation of coreless vortex
dipoles. | cond-mat_quant-gas |
Particle correlations and evidence for dark state condensation in a cold
dipolar exciton fluid: In this paper we show experimental evidence of a few correlation regimes of a
cold dipolar exciton fluid, created optically in a semiconductor bilayer
heterostructure. In the higher temperature regime, the average interaction
energy between the particles shows a surprising temperature dependence which is
an evidence for correlations beyond the mean field model. At a lower
temperature, there is a sharp increase in the interaction energy of optically
active excitons, accompanied by a strong reduction in their apparent
population. This is an evidence for a sharp macroscopic transition to a dark
state as was suggested theoretically. | cond-mat_quant-gas |
Quantum-geometric contribution to the Bogoliubov modes in a two-band
Bose-Einstein condensate: We consider a weakly-interacting Bose-Einstein condensate (BEC) that is
loaded into an optical lattice with a two-point basis, and described by a
two-band Bose-Hubbard model with generic one-body and two-body terms. By first
projecting the system to the lower Bloch band and then applying the Bogoliubov
approximation to the resultant Hamiltonian, we show that the inverse
effective-mass tensor of the superfluid carriers in the Bogoliubov spectrum has
two physically distinct contributions. In addition to the usual inverse
band-mass tensor that is originating from the intraband processes within the
lower Bloch band, there is also a quantum-geometric contribution that is
induced by the two-body interactions through the interband processes. We also
discuss the conditions under which the latter contribution is expressed in
terms of the quantum-metric tensor of the Bloch states, i.e., the natural
Fubini-Study metric on the Bloch sphere. | cond-mat_quant-gas |
All-optical pump-and-probe detection of dynamical correlations in a
two-dimensional Fermi gas: We propose an all-optical scheme to probe the dynamical correlations of a
strongly-interacting gas of ultracold atoms. The proposed technique is based on
a pump-and-probe scheme: a coherent light pulse is initially converted into an
atomic coherence and later retrieved after a variable storage time. The
efficiency of the proposed method to measure the one-particle Green function of
the gas is validated by numerical and analytical calculations of the expected
signal for the two cases of a normal Fermi gas and a BCS superfluid state.
Protocols to extract the superfluid gap and the full quasi-particle dispersions
are discussed. | cond-mat_quant-gas |
Universality of the Three-Body Parameter for Efimov States in Ultracold
Cesium: We report on the observation of triatomic Efimov resonances in an ultracold
gas of cesium atoms. Exploiting the wide tunability of interactions resulting
from three broad Feshbach resonances in the same spin channel, we measure
magnetic-field dependent three-body recombination loss. The positions of the
loss resonances yield corresponding values for the three-body parameter, which
in universal few-body physics is required to describe three-body phenomena and
in particular to fix the spectrum of Efimov states. Our observations show a
robust universal behavior with a three-body parameter that stays essentially
constant. | cond-mat_quant-gas |
Monopole excitations of a harmonically trapped one-dimensional Bose gas
from the ideal gas to the Tonks-Girardeau regime: Using a time-dependent modified nonlinear Schr\"odinger equation (m-NLSE) --
where the conventional chemical potential proportional to the density is
replaced by the one inferred from Lieb-Liniger's exact solution -- we study
frequencies of the collective monopole excitations of a one-dimensional (1D)
Bose gas. We find that our method accurately reproduces the results of a recent
experimental study [E. Haller et al., Science Vol. 325, 1224 (2009)] in the
full spectrum of interaction regimes from the ideal gas, through the mean-field
regime, through the mean-field Thomas-Fermi regime, all the way to the
Tonks-Giradeau gas. While the former two are accessible by the standard
time-dependent NLSE and inaccessible by the time-dependent local density
approximation (LDA), the situation reverses in the latter case. However, the
m-NLSE treats all these regimes within a single numerical method. | cond-mat_quant-gas |
Finite-range effects in the unitary Fermi polaron: Quantum Monte Carlo techniques are employed to study the properties of
polarons in an ultracold Fermi gas, at $T= 0,$ and in the unitary regime using
both a zero-range model and a square-well potential. For a fixed density, the
potential range is varied and results are extrapolated and compared against a
zero-range model. A discussion regarding the choice of an interacting potential
with a finite range is presented. We compute the polaron effective mass, the
polaron binding energy, and the effective coupling between them. The latter is
obtained using the Landau-Pomeranchuk's weakly interacting quasiparticle model.
The contact parameter is estimated by fitting the pair distribution function of
atoms in different spin states. | cond-mat_quant-gas |
Efficient multipole representation for matter-wave optics: Technical optics with matter waves requires a universal description of
three-dimensional traps, lenses, and complex matter-wave fields. In analogy to
the two-dimensional Zernike expansion in beam optics, we present a
three-dimensional multipole expansion for Bose-condensed matter waves and
optical devices. We characterize real magnetic chip traps, optical dipole
traps, and the complex matter-wave field in terms of spherical harmonics and
radial Stringari polynomials. We illustrate this procedure for typical harmonic
model potentials as well as real magnetic and optical dipole traps. Eventually,
we use the multipole expansion to characterize the aberrations of a
ballistically interacting expanding Bose-Einstein condensate in
(3+1)-dimensions. In particular, we find deviations from the quadratic phase
ansatz in the popular scaling approximation. This universal multipole
description of aberrations can be used to optimize matter-wave optics setups,
for example in matter-wave interferometers. | cond-mat_quant-gas |
Distortion of Interference Fringes and the Resulting Vortex Production
of Merging Bose-Einstein Condensates: We investigate the effects of interatomic interactions and expansion on the
distortion of interference fringes of a pair of initially well-separated, but
coherent, condensate clouds trapped in a harmonic trap. The distortion of
interference fringes, which can lead to the spontaneous formation of vortices
in the atom clouds, depends crucially on two relevant parameters: the
center-of-mass velocity and peak density of the initial state. We identify
three qualitatively distinct regimes for the interfering condensates:
collision, expansion, and merging, by the spatial and temporal features of the
fringe spacings. Using a comprehensive set of numerical simulations based on
the Gross-Pitaevskii equation, we specify the cross-overs between these regimes
and propose the optimal the system parameters required for dynamical
instabilities and vortex creation. | cond-mat_quant-gas |
The virial expansion of attractively interacting Fermi gases in 1D, 2D,
and 3D, up to fifth order: The virial expansion characterizes the high-temperature approach to the
quantum-classical crossover in any quantum many-body system. Here, we calculate
the virial coefficients up to the fifth-order of Fermi gases in 1D, 2D, and 3D,
with attractive contact interactions, as relevant for a variety of applications
in atomic and nuclear physics. To that end, we discretize the imaginary-time
direction and calculate the relevant canonical partition functions. In coarse
discretizations, we obtain analytic results featuring relationships between the
interaction-induced changes $\Delta b_3$, $\Delta b_4$, and $\Delta b_5$ as
functions of $\Delta b_2$, the latter being exactly known in many cases by
virtue of the Beth-Uhlenbeck formula. Using automated-algebra methods, we push
our calculations to progressively finer discretizations and extrapolate to the
continuous-time limit. We find excellent agreement for $\Delta b_3$ with
previous calculations in all dimensions and we formulate predictions for
$\Delta b_4$ and $\Delta b_5$ in 1D and 2D. We also provide, for a range of
couplings,the subspace contributions $\Delta b_{31}$, $\Delta b_{22}$, $\Delta
b_{41}$, and $\Delta b_{32}$, which determine the equation of state and static
response of polarized systems at high temperature. As a performance check, we
compare the density equation of state and Tan contact with quantum Monte Carlo
calculations, diagrammatic approaches, and experimental data where available.
Finally, we apply Pad\'e and Pad\'e-Borel resummation methods to extend the
usefulness of the virial coefficients to approach and in some cases go beyond
the unit-fugacity point. | cond-mat_quant-gas |
Mesoscopic spin transport between strongly interacting Fermi gases: We investigate a mesoscopic spin current for strongly interacting Fermi gases
through a quantum point contact. Under the situation where spin polarizations
in left and right reservoirs are same in magnitude but opposite in sign, we
calculate the contribution of quasiparticles to the current by means of the
linear response theory and many-body $T$-matrix approximation. For a small
spin-bias regime, the current in the vicinity of the superfluid transition
temperature is strongly suppressed due to the formation of pseudogaps. For a
large spin-bias regime where the gases become highly polarized, on the other
hand, the current is affected by the enhancement of a minority density of
states due to Fermi polarons. We also discuss the broadening of a quasiparticle
peak associated with an attractive polaron at a large momentum, which is
relevant to the enhancement. | cond-mat_quant-gas |
Mott Insulator-Density Ordered Superfluid Transition and "Shamrock
Transition" in a Frustrated Triangle Lattice: Density order is usually a consequence of the competition between long-range
and short-range interactions. Here we report a density ordered superfluid
emergent from a homogeneous Mott insulator due to the competition between
frustrations and local interactions. This transition is found in a Bose-Hubbard
model on a frustrated triangle lattice with an extra pairing term. Further, we
find a quantum phase transition between two different density ordered
superfluids, which is beyond the Landau-Ginzburg paradigm. Across this
transition, a U(1) symmetry is emergent, while the symmetry in each density
ordered superfluid is Z2*Z3. Because there emerges a shamrock-like degenerate
ground state in parameter space, we call the transition "shamrock transition".
Effective low energy theories are established for the two transitions mentioned
above and we find their resemblance and differences with clock models. | cond-mat_quant-gas |
Local observation of pair-condensation in a Fermi gas at unitarity: We present measurements of the local (homogeneous) density-density response
function of a Fermi gas at unitarity using spatially resolved Bragg
spectroscopy. By analyzing the Bragg response across one axis of the cloud we
extract the response function for a uniform gas which shows a clear signature
of the Bose-Einstein condensation of pairs of fermions when the local
temperature drops below the superfluid transition temperature. The method we
use for local measurement generalizes a scheme for obtaining the local pressure
in a harmonically trapped cloud from the line density and can be adapted to
provide any homogeneous parameter satisfying the local density approximation. | cond-mat_quant-gas |
Robust Vortex Lines, Vortex Rings and Hopfions in 3D Bose-Einstein
Condensates: Performing a systematic Bogoliubov-de Gennes spectral analysis, we illustrate
that stationary vortex lines, vortex rings and more exotic states, such as
hopfions, are robust in three-dimensional atomic Bose-Einstein condensates, for
large parameter intervals. Importantly, we find that the hopfion can be
stabilized in a simple parabolic trap, without the need for trap rotation or
inhomogeneous interactions. We supplement our spectral analysis by studying the
dynamics of such stationary states; we find them to be robust against
significant perturbations of the initial state. In the unstable regimes, we not
only identify the unstable mode, such as a quadrupolar or hexapolar mode, but
we also observe the corresponding instability dynamics. Furthermore, deep in
the Thomas-Fermi regime, we investigate the particle-like behavior of vortex
rings and hopfions. | cond-mat_quant-gas |
Ground states of atomic Fermi gases in a two-dimensional optical lattice
with and without population imbalance: We study the ground state phase diagram of population balanced and imbalanced
ultracold atomic Fermi gases with a short range attractive interaction
throughout the crossover from BCS to Bose-Einstein condensation (BEC), in a
two-dimensional optical lattice (2DOL) comprised of two lattice and one
continuum dimensions. We find that the mixing of lattice and continuum
dimensions, together with population imbalance, has an extraordinary effect on
pairing and the superfluidity of atomic Fermi gases. In the balanced case, the
superfluid ground state prevails the majority of the phase space. However, for
relatively small lattice hopping integral $t$ and large lattice constant $d$, a
pair density wave (PDW) emerges unexpectedly at intermediate coupling strength,
and the nature of the in-plane and overall pairing changes from particle-like
to hole-like in the BCS and unitary regimes, associated with an abnormal
increase in the Fermi volume with the pairing strength. In the imbalanced case,
the stable polarized superfluid phase shrinks to only a small portion of the
entire phase space spanned by $t$, $d$, imbalance $p$ and interaction strength
$U$, mainly in the bosonic regime of low $p$, moderately strong pairing, and
relatively large $t$ and small $d$. Due to the Pauli exclusion between paired
and excessive fermions within the confined momentum space, a PDW phase emerges
and the overall pairing evolves from particle-like into hole-like, as the
pairing strength grows stronger in the BEC regime. In both cases, the ground
state property is largely governed by the Fermi surface topology. These
findings are very different from the cases of pure 3D continuum, 3D lattice or
1DOL. | cond-mat_quant-gas |
Ground-state phases of a mixture of spin-1 and spin-2 Bose-Einstein
condensates: We investigate the ground-state phases of a mixture of spin-1 and spin-2
Bose-Einstein condensates at zero magnetic field. In addition to the intra-spin
interactions, two spin-dependent interaction coefficients are introduced to
describe the inter-spin interaction. We systematically explore the wide
parameter space, and obtain phase diagrams containing a rich variety of phases.
For example, there exists a phase in which the spin-1 and spin-2 vectors are
tilted relative to each other breaking the axial symmetry. | cond-mat_quant-gas |
Heavy polarons in ultracold atomic Fermi superfluids at the BEC-BCS
crossover: formalism and applications: We investigate the system of a heavy impurity embedded in a paired
two-component Fermi gas at the crossover from a Bose-Einstein condensate (BEC)
to a Bardeen--Cooper--Schrieffer (BCS) superfluid via an extension of the
functional determinant approach (FDA). FDA is an exact numerical approach
applied to study manifestations of Anderson\textquoteright s orthogonality
catastrophe (OC) in the system of a static impurity immersed in an ideal Fermi
gas. Here, we extend the FDA to a strongly correlated superfluid background
described by a BCS mean-field wavefunction. In contrast to the ideal Fermi gas
case, the pairing gap in the BCS superfluid prevents the OC and leads to
genuine polaron signals in the spectrum. Thus, our exactly solvable model can
provide a deeper understanding of polaron physics. In addition, we find that
the polaron spectrum can be used to measure the superfluid pairing gap, and in
the case of a magnetic impurity, the energy of the sub-gap Yu-Shiba-Rusinov
(YSR) bound state. Our theoretical predictions can be examined with
state-of-art cold-atom experiments. | cond-mat_quant-gas |
Macroscopic amplification of electroweak effects in molecular
Bose-Einstein condensates: We investigate the possible use of Bose-Einstein condensates of diatomic
molecules to measure nuclear spin-dependent parity violation effects, outlining
a detection method based on the internal Josephson effect between molecular
states of opposite parity. When applied to molecular condensates, the fine
experimental control achieved in atomic bosonic Josephson junctions could
provide data on anapole moments and neutral weak couplings. | cond-mat_quant-gas |
Loschmidt echo in one-dimensional interacting Bose gases: We explore Loschmidt echo in two regimes of one-dimensional (1D) interacting
Bose gases: the strongly interacting Tonks-Girardeau (TG) regime, and the
weakly-interacting mean-field regime. We find that the Loschmidt echo of a TG
gas decays as a Gaussian when small perturbations are added to the Hamiltonian
(the exponent is proportional to the number of particles and the magnitude of a
small perturbation squared). In the mean-field regime the Loschmidt echo decays
faster for larger interparticle interactions (nonlinearity), and it shows
richer behavior than the TG Loschmidt echo dynamics, with oscillations
superimposed on the overall decay. | cond-mat_quant-gas |
Multipolar condensates and multipolar Josephson effects: When single-particle dynamics are suppressed in certain strongly correlated
systems, dipoles arise as elementary carriers of quantum kinetics. These
dipoles can further condense, providing physicists with a rich realm to study
fracton phases of matter. Whereas recent theoretical discoveries have shown
that an unconventional lattice model may host a dipole condensate as the ground
state, fundamental questions arise as to whether dipole condensation is a
generic phenomenon rather than a specific one unique to a particular model and
what new quantum macroscopic phenomena a dipole condensate may bring us with.
Here, we show that dipole condensates prevail in bosonic systems. Because of a
self-proximity effect, where single-particle kinetics inevitably induces a
finite order parameter of dipoles, dipole condensation readily occurs in
conventional normal phases of bosons. Our findings allow experimentalists to
manipulate the phase of a dipole condensate and deliver dipolar Josephson
effects, where supercurrents of dipoles arise in the absence of particle flows.
The self-proximity effects can also be utilized to produce a generic multipolar
condensate. The kinetics of the $n$-th order multipoles unavoidably creates a
condensate of the $(n+1)$-th order multipoles, forming a hierarchy of
multipolar condensates that will offer physicists a whole new class of
macroscopic quantum phenomena. | cond-mat_quant-gas |
Bose-Hubbard realization of fracton defects: Bose-Hubbard models are simple paradigmatic lattice models used to study
dynamics and phases of quantum bosonic matter. We combine the extended
Bose-Hubbard model in the hard-core regime with ring-exchange hoppings. By
investigating the symmetries and low-energy properties of the Hamiltonian we
argue that the model hosts fractonic defect excitations. We back up our claims
with exact numerical simulations of defect dynamics exhibiting mobility
constraints. Moreover, we confirm the robustness of our results against fracton
symmetry breaking perturbations. Finally we argue that this model can be
experimentally realized in recently proposed quantum simulator platforms with
big time crystals, thus paving a way for the controlled study of many-body
dynamics with mobility constraints. | cond-mat_quant-gas |
Strongly Interacting Bose Gases near a $d$-wave Shape Resonance: Many unconventional quantum matters, such as fractional quantum Hall effect
and $d$-wave high-Tc superconductor, are discovered in strongly interacting
systems. Understanding quantum many-body systems with strong interaction and
the unconventional phases therein is one of the most challenging problems in
physics nowadays. Cold atom systems possess a natural way to create strong
interaction by bringing the system to the vicinity of a scattering resonance.
Although this has been a focused topic in cold atom physics for more than a
decade, these studies have so far mostly been limited for $s$-wave resonance.
Here we report the experimental observation of a broad $d$-wave shape resonance
in degenerate ${}^{41}$K gas. We further measure the molecular binding energy
that splits into three branches as a hallmark of $d$-wave molecules, and find
that the lifetime of this many-body system is reasonably long at strongly
interacting regime. From analyzing the breathing mode excited by ramping
through this resonance, it suggests that a quite stable low-temperature atom
and molecule mixture is produced. Putting all the evidence together, our system
offers great promise to reach a $d$-wave molecular superfluid. | cond-mat_quant-gas |
Introduction to quantum turbulence: The term quantum turbulence denotes the turbulent motion of quantum fluids,
systems such as superfluid helium and atomic Bose-Einstein condensates which
are characterized by quantized vorticity, uperfluidity and, at finite
temperatures, two-fluid behavior. This article introduces their basic
properties, describes types and regimes of turbulence which have been observed,
and highlights similarities and differences between quantum turbulence and
classical turbulence in ordinary fluids. Our aim is also to link together the
articles of this special issue, and to provide a perspective of the future
development of a subject which contains aspects of fluid mechanics, atomic
physics, condensed matter and low temperature physics. | cond-mat_quant-gas |
Impurity transport through a strongly interacting bosonic quantum gas: Using near-exact numerical simulations we study the propagation of an
impurity through a one-dimensional Bose lattice gas for varying bosonic
interaction strengths and filling factors at zero temperature. The impurity is
coupled to the Bose gas and confined to a separate tilted lattice. The precise
nature of the transport of the impurity is specific to the excitation spectrum
of the Bose gas which allows one to measure properties of the Bose gas
non-destructively, in principle, by observing the impurity; here we focus on
the spatial and momentum distributions of the impurity as well as its reduced
density matrix. For instance we show it is possible to determine whether the
Bose gas is commensurately filled as well as the bandwidth and gap in its
excitation spectrum. Moreover, we show that the impurity acts as a witness to
the cross-over of its environment from the weakly to the strongly interacting
regime, i.e., from a superfluid to a Mott insulator or Tonks-Girardeau lattice
gas and the effects on the impurity in both of these strongly-interacting
regimes are clearly distinguishable. Finally, we find that the spatial
coherence of the impurity is related to its propagation through the Bose gas,
giving an experimentally controllable example of noise-enhanced quantum
transport. | cond-mat_quant-gas |
Emergent Fermi sea in a system of interacting bosons: An understanding of the possible ways in which interactions can produce
fundamentally new emergent many-body states is a central problem of condensed
matter physics. We ask if a Fermi sea can arise in a system of bosons subject
to contact interaction. Based on exact diagonalization studies and variational
wave functions, we predict that such a state is likely to occur when a system
of two-component bosons in two dimensions, interacting via a species
independent contact interaction, is exposed to a synthetic magnetic field of
strength that corresponds to a filling factor of unity. The fermions forming
the SU(2) singlet Fermi sea are bound states of bosons and quantized vortices,
formed as a result of the repulsive interaction between bosons in the lowest
Landau level. | 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 |
Observation of the Hanbury Brown and Twiss Effect with Ultracold
Molecules: Measuring the statistical correlations of individual quantum objects provides
an excellent way to study complex quantum systems. Ultracold molecules
represent a powerful platform for quantum science due to their rich and
controllable internal degrees of freedom. However, the detection of
correlations between single molecules in an ultracold gas has yet to be
demonstrated. Here we observe the Hanbury Brown and Twiss effect in a gas of
bosonic $^{23}$Na$^{87}$Rb, enabled by the realization of a quantum gas
microscope for molecules. We detect the characteristic bunching correlations in
the density fluctuations of a 2D molecular gas released from and subsequently
recaptured in an optical lattice. The quantum gas microscope allows us to
extract the positions of individual molecules with single-site resolution. As a
result, we obtain a high-contrast two-molecule interference pattern with a
visibility of $54(13)\%$. While these measured correlations arise purely from
the quantum statistics of the molecules, the demonstrated capabilities pave the
way toward site-resolved studies of interacting molecular gases in optical
lattices. | cond-mat_quant-gas |
Quantum Degenerate Majorana Surface Zero Modes in Two-Dimensional Space: We investigate the topological properties of spin polarized fermionic polar
molecules loaded in a multi-layer structure with the electric dipole moment
polarized to the normal direction. When polar molecules are paired by
attractive inter-layer interaction, unpaired Majorana fermions can be
macroscopically generated in the top and bottom layers in dilute density
regime. We show that the resulting topological state is effectively composed by
a bundle of 1D Kitaev ladders labeled by in-plane momenta k and -k, and hence
belongs to BDI class characterized by the winding number Z, protected by the
time reversal symmetry. The Majorana surface modes exhibit a flatband at zero
energy, fully gapped from Bogoliubov excitations in the bulk, and hence becomes
an idea system to investigate the interaction effects on quantum degenerate
Majorana fermions. We further show that additional interference fringes can be
identified as a signature of such 2D Majorana surface modes in the
time-of-flight experiment. | cond-mat_quant-gas |
Virial expansion for a strongly correlated Fermi system and its
application to ultracold atomic Fermi gases: Strongly correlated Fermi system plays a fundamental role in very different
areas of physics, from neutron stars, quark-gluon plasmas, to high temperature
superconductors. Despite the broad applicability, it is notoriously difficult
to be understood theoretically because of the absence of a small interaction
parameter. Recent achievements of ultracold trapped Fermi atoms near a Feshbach
resonance have ushered in enormous changes. The unprecedented control of
interaction, geometry and purity in these novel systems has led to many
exciting experimental results, which are to be urgently understood at both low
and finite temperatures. Here we review the latest developments of virial
expansion for a strongly correlated Fermi gas and their applications on
ultracold trapped Fermi atoms. We show remarkable, quantitative agreements
between virial predictions and various recent experimental measurements at
about the Fermi degenerate temperature. For equation of state, we discuss a
practical way of determining high-order virial coefficients and use it to
calculate accurately the long-sought third-order virial coefficient, which is
now verified firmly in experiments at ENS and MIT. We discuss also virial
expansion of a new many-body paramter - Tan's contact. We then turn to less
widely discussed issues of dynamical properties. For dynamic structure factor,
the virial prediction agrees well with the measurement at the Swinburne
University of Technology. For single-particle spectral function, we show that
the expansion up to the second order accounts for the main feature of
momentum-resolved rf-spectroscopy for a resonantly interacting Fermi gas, as
recently reported by JILA. In the near future, more practical applications with
virial expansion are possible, owing to the ever-growing power in computation. | cond-mat_quant-gas |
Enhanced many-body quantum scars from the non-Hermitian Fock skin effect: In contrast with extended Bloch waves, a single particle can become spatially
localized due to the so-called skin effect originating from non-Hermitian
pumping. Here we show that in a wide class of kinetically constrained many-body
systems, the skin effect can instead manifest as dynamical amplification within
the Fock space, beyond the intuitively expected and previously studied particle
localization and clustering. We exemplify this non-Hermitian Fock skin effect
in an asymmetric version of the PXP model and show that it gives rise to
ergodicity-breaking eigenstates, the non-Hermitian analogs of quantum many-body
scars. A distinguishing feature of these non-Hermitian scars is their enhanced
robustness against external disorders. We propose an experimental realization
of the non-Hermitian scar enhancement in a tilted Bose-Hubbard optical lattice
with laser-induced loss. Our results show that the Fock skin effect provides a
powerful tool for creating robust non-ergodic states in generic open quantum
systems. | cond-mat_quant-gas |
Spatially distributed multipartite entanglement enables
Einstein-Podolsky-Rosen steering of atomic clouds: A key resource for distributed quantum-enhanced protocols is entanglement
between spatially separated modes. Yet, the robust generation and detection of
nonlocal entanglement between spatially separated regions of an ultracold
atomic system remains a challenge. Here, we use spin mixing in a tightly
confined Bose-Einstein condensate to generate an entangled state of
indistinguishable particles in a single spatial mode. We show experimentally
that this local entanglement can be spatially distributed by self-similar
expansion of the atomic cloud. Spatially resolved spin read-out is used to
reveal a particularly strong form of quantum correlations known as
Einstein-Podolsky-Rosen steering between distinct parts of the expanded cloud.
Based on the strength of Einstein-Podolsky-Rosen steering we construct a
witness, which testifies up to genuine five-partite entanglement. | cond-mat_quant-gas |
Opto-mechanical effects in superradiant light scattering by
Bose-Einstein condensate in a cavity: We investigate the effects of a movable mirror (cantilever) of an optical
cavity on the superradiant light scattering from a Bose-Einstein condensate
(BEC) in an optical lattice. We show that the mirror motion has a dynamic
dispersive effect on the cavity-pump detuning. Varying the intensity of the
pump beam, one can switch between the pure superradiant regime and the Bragg
scattering regime. The mechanical frequency of the mirror strongly influences
the time interval between two Bragg peaks. We found that when the system is in
the resolved side band regime for mirror cooling, the superradiant scattering
is enhanced due to coherent energy transfer from the mechanical mirror mode to
the cavity field mode. | cond-mat_quant-gas |
Dynamical many-body phases of the parametrically driven, dissipative
Dicke model: The dissipative Dicke model exhibits a fascinating out-of-equilibrium
many-body phase transition as a function of a coupling between a driven
photonic cavity and numerous two-level atoms. We study the effect of a
time-dependent parametric modulation of this coupling, and discover a rich
phase diagram as a function of the modulation strength. We find that in
addition to the established normal and super-radiant phases, a new phase with
pulsed superradiance which we term dynamical normal phase appears when the
system is parametrically driven. Employing different methods, we characterize
the different phases and the transitions between them. Specific heed is paid to
the role of dissipation in determining the phase boundaries. Our analysis paves
the road for the experimental study of dynamically stabilized phases of
interacting light and matter. | cond-mat_quant-gas |
Continuum of classical-field ensembles from canonical to grand canonical
and the onset of their equivalence: The canonical and grand-canonical ensembles are two usual marginal cases for
ultracold Bose gases, but real collections of experimental runs commonly have
intermediate properties. Here we study the continuum of intermediate cases, and
look into the appearance of ensemble equivalence as interaction rises for
mesoscopic 1d systems. We demonstrate how at sufficient interaction strength
the distributions of condensate and excited atoms become practically identical
regardless of the ensemble used. Importantly, we find that features that are
fragile in the ideal gas and appear only in a strict canonical ensemble can
become robust in all ensembles when interactions become strong. As evidence,
the steep cliff in the distribution of the number of excited atoms is
preserved. To make this study, a straightforward approach for generating
canonical and intermediate classical field ensembles using a modified
stochastic Gross-Pitaevskii equation (SGPE) is developed. | cond-mat_quant-gas |
Quantum Simulators at Negative Absolute Temperatures: We propose that negative absolute temperatures in ultracold atomic clouds in
optical lattices can be used to simulate quantum systems in new regions of
phase diagrams. First we discuss how the attractive SU(3) Hubbard model in
three dimensions can be realized using repulsively interacting 173-Yb atoms,
then we consider how an antiferromagnetic S=1 spin chain could be simulated
using spinor 87-Rb or 23-Na atoms. The general idea to achieve negative
absolute temperatures is to reverse the sign of the external harmonic
potential. Energy conservation in a deep optical lattice imposes a constraint
on the dynamics of the cloud, which can relax toward a T<0 state. As the
process is strongly non-adiabatic, we estimate the change of the entropy. | cond-mat_quant-gas |
Long-range transverse Ising model built with dipolar condensates in
two-well arrays: Dipolar Bose-Einstein condensates in an array of double-well potentials
realize an effective transverse Ising model with peculiar inter-layer
interactions, that may result under proper conditions in an anomalous
first-order ferromagnetic-antiferromagnetic phase transition, and nontrivial
phases due to frustration. The considered setup as well allows the study of
Kibble-Zurek defect formation, whose kink statistics follows that expected from
the universality class of the mean-field transverse Ising model in 1D.
Furthermore, random occupation of each layer of the stack leads to random
effective Ising interactions and generation of local transverse fields, thus
allowing the study of Anderson-like localization of imbalance perturbations in
the two-well stack under controllable conditions. | cond-mat_quant-gas |
Time-Resolved Observation of Spin-Charge Deconfinement in Fermionic
Hubbard Chains: Elementary particles such as the electron carry several quantum numbers, for
example, charge and spin. However, in an ensemble of strongly interacting
particles, the emerging degrees of freedom can fundamentally differ from those
of the individual constituents. Paradigmatic examples of this phenomenon are
one-dimensional systems described by independent quasiparticles carrying either
spin (spinon) or charge (holon). Here we report on the dynamical deconfinement
of spin and charge excitations in real space following the removal of a
particle in Fermi-Hubbard chains of ultracold atoms. Using space- and
time-resolved quantum gas microscopy, we track the evolution of the excitations
through their signatures in spin and charge correlations. By evaluating
multi-point correlators, we quantify the spatial separation of the excitations
in the context of fractionalization into single spinons and holons at finite
temperatures. | cond-mat_quant-gas |
Multistable circular currents of polariton condensates trapped in ring
potentials: We demonstrate the formation and trapping of different stationary solutions,
oscillatory solutions, and rotating solutions of a polariton condensate in a
planar semiconductor microcavity with a built-in ring-shaped potential well.
Multistable ring shaped solutions are trapped in shallow potential wells. These
solutions have the same ring shaped density distribution but different
topological charges, corresponding to different orbital angular momentum (OAM)
of the emitted light. For stronger confinement potentials, besides the
fundamental modes, higher excited (dipole) modes can also be trapped. If two
modes are excited simultaneously, their beating produces a complex oscillation
and rotation dynamics. When the two modes have the same OAM, a double-ring
solution forms for which the density oscillates between the inner and the outer
ring. When the two modes have different OAM, a rotating solution with a
crescent-shaped density and fractional OAM is created. | cond-mat_quant-gas |
Generation and Detection of Atomic Spin Entanglement in Optical Lattices: Ultracold atoms in optical lattices offer a great promise to generate
entangled states for scalable quantum information processing owing to the
inherited long coherence time and controllability over a large number of
particles. We report on the generation, manipulation and detection of atomic
spin entanglement in an optical superlattice. Employing a spin-dependent
superlattice, atomic spins in the left or right sites can be individually
addressed and coherently manipulated by microwave pulses with near unitary
fidelities. Spin entanglement of the two atoms in the double wells of the
superlattice is generated via dynamical evolution governed by spin
superexchange. By observing collisional atom loss with in-situ absorption
imaging we measure spin correlations of atoms inside the double wells and
obtain the lower boundary of entanglement fidelity as $0.79\pm0.06$, and the
violation of a Bell's inequality with $S=2.21\pm 0.08$. The above results
represent an essential step towards scalable quantum computation with ultracold
atoms in optical lattices. | cond-mat_quant-gas |
Phase diagram of one-dimensional earth-alkaline cold fermionic atoms: The phase diagram of one-dimensional earth-alkaline fermionic atoms and
ytterbium 171 atoms is investigated by means of a low-energy approach and
density-matrix renormalization group calculations. For incommensurate filling,
four gapless phases with a spin gap are found and consist of two
superconducting instabilities and two coexisting bond and charge density-waves
instabilities. In the half-filled case, seven Mott-insulating phases arise with
the emergence of four non-degenerate phases with exotic hidden orderings. | cond-mat_quant-gas |
Pairing patterns in one-dimensional spin- and mass-imbalanced Fermi
gases: We study spin- and mass-imbalanced mixtures of spin-$\tfrac{1}{2}$ fermions
interacting via an attractive contact potential in one spatial dimension.
Specifically, we address the influence of unequal particle masses on the pair
formation by means of the complex Langevin method. By computing the
pair-correlation function and the associated pair-momentum distribution we find
that inhomogeneous pairing is present for all studied spin polarizations and
mass imbalances. To further characterize the pairing behavior, we analyze the
density-density correlations in momentum space, the so-called shot noise, which
is experimentally accessible through time-of-flight imaging. At finite spin
polarization, the latter is known to show distinct maxima at momentum
configurations associated with the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)
instability. Besides those maxima, we find that additional features emerge in
the noise correlations when mass imbalance is increased, revealing the
stability of FFLO-type correlations against mass imbalance and furnishing an
experimentally accessible signature to probe this type of pairing. | cond-mat_quant-gas |
Topological Bose-Mott Insulators in a One-Dimensional Optical
Superlattice: We study topological properties of the Bose-Hubbard model with repulsive
interactions in a one-dimensional optical superlattice. We find that the Mott
insulator states of the single-component (two-component) Bose-Hubbard model
under fractional fillings are topological insulators characterized by a nonzero
charge (or spin) Chern number with nontrivial edge states. For ultracold atomic
experiments, we show that the topological Chern number can be detected through
measuring the density profiles of the bosonic atoms in a harmonic trap. | cond-mat_quant-gas |
Dynamical solitons and boson fractionalization in cold-atom topological
insulators: We study the $\mathbb{Z}_2$ Bose-Hubbard model at incommensurate densities,
which describes a one-dimensional system of interacting bosons whose tunneling
is dressed by a dynamical $\mathbb{Z}_2$ field. At commensurate densities, the
model is known to host intertwined topological phases, where long-range order
coexists with non-trivial topological properties. This interplay between
spontaneous symmetry breaking (SSB) and topological symmetry protection gives
rise to interesting fractional topological phenomena when the system is doped
to certain incommensurate fillings. In particular, we hereby show how
topological defects in the $\mathbb{Z}_2$ field can appear in the ground state,
connecting different SSB sectors. These defects are dynamical and can travel
through the lattice carrying both a topological charge and a fractional
particle number. In the hardcore limit, this phenomenon can be understood
through a bulk-defect correspondence. Using a pumping argument, we show that it
survives also for finite interactions, demonstrating how boson
fractionalization can occur in strongly-correlated bosonic systems, the main
ingredients of which have already been realized in cold-atom experiments. | cond-mat_quant-gas |
Synthetic Landau levels for photons: Synthetic photonic materials are an emerging platform for exploring the
interface between microscopic quantum dynamics and macroscopic material
properties[1-5]. Photons experiencing a Lorentz force develop handedness,
providing opportunities to study quantum Hall physics and topological quantum
science[6-8]. Here we present an experimental realization of a magnetic field
for continuum photons. We trap optical photons in a multimode ring resonator to
make a two-dimensional gas of massive bosons, and then employ a non-planar
geometry to induce an image rotation on each round-trip[9]. This results in
photonic Coriolis/Lorentz and centrifugal forces and so realizes the
Fock-Darwin Hamiltonian for photons in a magnetic field and harmonic trap[10].
Using spatial- and energy-resolved spectroscopy, we track the resulting
photonic eigenstates as radial trapping is reduced, finally observing a
photonic Landau level at degeneracy. To circumvent the challenge of trap
instability at the centrifugal limit[10,11], we constrain the photons to move
on a cone. Spectroscopic probes demonstrate flat space (zero curvature) away
from the cone tip. At the cone tip, we observe that spatial curvature increases
the local density of states, and we measure fractional state number excess
consistent with the Wen-Zee theory, providing an experimental test of this
theory of electrons in both a magnetic field and curved space[12-15]. This work
opens the door to exploration of the interplay of geometry and topology, and in
conjunction with Rydberg electromagnetically induced transparency, enables
studies of photonic fractional quantum Hall fluids[16,17] and direct detection
of anyons[18-19]. | cond-mat_quant-gas |
Mean field analysis of quantum phase transitions in a periodic optical
superlattice: In this paper we analyze the various phases exhibited by a system of
ultracold bosons in a periodic optical superlattice using the mean field
decoupling approximation. We investigate for a wide range of commensurate and
incommensurate densities. We find the gapless superfluid phase, the gapped Mott
insulator phase, and gapped insulator phases with distinct density wave orders. | cond-mat_quant-gas |
Two component quantum walk in one-dimensional lattice with hopping
imbalance: We investigate the two-component quantum walk in one-dimensional lattice. We
show that the inter-component interaction strength together with the hopping
imbalance between the components exhibit distinct features in the quantum walk
for different initial states. When the walkers are initially on the same site,
both the slow and fast particles perform independent particle quantum walks
when the interaction between them is weak. However, stronger inter-particle
interactions result in quantum walks by the repulsively bound pair formed
between the two particles. For different initial states when the walkers are on
different sites initially, the quantum walk performed by the slow particle is
almost independent of that of the fast particle, which exhibits reflected and
transmitted components across the particle with large hopping strength for weak
interactions. Beyond a critical value of the interaction strength, the wave
function of the fast particle ceases to penetrate through the slow particle
signalling a spatial phase separation. However, when the two particles are
initially at the two opposite edges of the lattice, then the interaction
facilitates the complete reflection of both of them from each other. We analyze
the above mentioned features by examining various physical quantities such as
the on-site density evolution, two-particle correlation functions and
transmission coefficients. | 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 |
Semiclassical Hartree-Fock theory of a rotating Bose-Einstein
condensation: In this paper, we investigate the thermodynamic behavior of a rotating
Bose-Einstein condensation with non-zero interatomic interactions
theoretically. The analysis relies on a semiclassical Hartree-Fock
approximation where an integral is performed over the phase space and function
of the grand canonical ensemble is derived. Subsequently, we use this result to
derive several thermodynamic quantities including the condensate fraction,
critical temperature, entropy and heat capacity. Thereby, we investigate the
effect of the rotation rate and interactions parameter on the thermodynamic
behavior. The role of finite size is discussed. Our approach can be extended to
consider the rotating condensate in optical potential. | cond-mat_quant-gas |
Thermalisation of Local Observables in Small Hubbard Lattices: We present a study of thermalisation of a small isolated Hubbard lattice
cluster prepared in a pure state with a well-defined energy. We examine how a
two-site subsystem of the lattice thermalises with the rest of the system as
its environment. We explore numerically the existence of thermalisation over a
range of system parameters, such as the interaction strength, system size and
the strength of the coupling between the subsystem and the rest of the lattice.
We find thermalisation over a wide range of parameters and that interactions
are crucial for efficient thermalisation of small systems. We relate this
thermalisation behaviour to the eigenstate thermalisation hypothesis and
quantify numerically the extent to which eigenstate thermalisation holds. We
also verify our numerical results theoretically with the help of previously
established results from random matrix theory for the local density of states,
particularly the finite-size scaling for the onset of thermalisation. | cond-mat_quant-gas |
Synthetic Gauge Fields for Ultra Cold Atoms: A Primer: We start by reviewing the concept of gauge invariance in quantum mechanics,
for Abelian and Non-Ableian cases. Then we idescribe how the various gauge
potential and field can be associated with the geometrical phase acquired by a
quantum mechanical wave function while adiabatically evolving in a parameter
space. Subsequently we show how this concept is exploited to generate light
induced gauge field for neutral ultra cold bosonic atoms. As an example of such
light induced Abelian and Non Abelian gauge field for ultra cold atoms we
disucss ultra cold atoms in a rotating trap and creation of synthetic spin
orbit coupling for ultra cold atomic systems using Raman lasers. | cond-mat_quant-gas |
Experimental realization of a high precision tunable hexagonal optical
lattice: Hexagonal optical lattices offer a tunable platform to study exotic orbital
physics in solid state materials. Here, we present a versatile high-precision
scheme to implement a hexagonal optical lattice potential, which is engineered
by overlapping two independent triangular optical sublattices generated by
laser beams with slightly different wavelengths around 1064 nm. This enables us
to precisely control the detailed structure of the hexagonal lattice by
adjusting the relative position and the relative lattice depth of the two
triangular optical sublattices. Taking advantage of the sensitive dependence of
the second Bloch band on small lattice deformations, we propose a strategy to
optimize the optical lattice geometry with an extremely high precision. This
method can also be extended to other lattice configurations involving more than
two sublattices. Our work provides the experimental requirements in the search
for novel orbital physics of ultracold atoms, for example, in the flat $p$-band
of the hexagonal optical lattice. | cond-mat_quant-gas |
A Non-Equilibrium Kinetic Theory for Trapped Binary Condensates: We derive a non-equilibrium finite-temperature kinetic theory for a binary
mixture of two interacting atomic Bose-Einstein condensates and use it to
explore the degree of hydrodynamicity attainable in realistic experimental
geometries. Based on the standard separation of timescale argument of kinetic
theory, the dynamics of the condensates of the multi-component system are shown
to be described by dissipative Gross-Pitaevskii equations, self-consistently
coupled to corresponding Quantum Boltzmann equations for the non-condensate
atoms: on top of the usual mean field contributions, our scheme identifies a
total of eight distinct collisional processes, whose dynamical interplay is
expected to be responsible for the systems equilibration. In order to provide
their first characterization, we perform a detailed numerical analysis of the
role of trap frequency and geometry on collisional rates for experimentally
accessible mixtures of $^{87}$Rb-$^{41}$K and $^{87}$Rb-$^{85}$Rb, discussing
the extent to which the system may approach the hydrodynamic regime with regard
to some of those processes, as a guide for future experimental investigations
of ultracold Bose gas mixtures. | cond-mat_quant-gas |
Dynamics of matter-wave solutions of Bose-Einstein condensates in a
homogeneous gravitational field: We find a matter-wave solution of Bose-Einstein condensates trapped in a
harmonic-oscillator potential and subjected to a homogeneous gravitational
field, by means of the extended tanhfunction method. This solution has as
special cases the bright and dark solitons. We investigate the dynamics and the
kinematics of these solutions, and the role of gravity is sketched. It is shown
that these solutions can be generated and manipulated by controlling the s-wave
scattering length, without changing the strengths of the magnetic and
gravitational fields. | cond-mat_quant-gas |
Collective dynamical Fermi suppression of optically-induced inelastic
scattering: We observe strong dynamical suppression of optically-induced loss in a weakly
interacting Fermi gas as the $s$-wave scattering length is increased. The
single, cigar-shaped cloud behaves as a large spin lattice in energy space with
a tunable Heisenberg Hamiltonian. The loss suppression occurs as the lattice
transitions into a magnetized state, where the fermionic nature of the atoms
inhibits interactions. The data are quantitatively explained by incorporating
spin-dependent loss into a quasi-classical collective spin vector model, the
success of which enables the application of optical control of effective
long-range interactions to this system. | cond-mat_quant-gas |
Attractive Solution of Binary Bose Mixtures: Liquid-Vapor Coexistence
and Critical Point: We study the thermodynamic behavior of attractive binary Bose mixtures using
exact path-integral Monte-Carlo methods. Our focus is on the regime of
interspecies interactions where the ground state is in a self-bound liquid
phase, stabilized by beyond mean-field effects. We calculate the isothermal
curves in the pressure vs density plane for different values of the attraction
strength and establish the extent of the coexistence region between liquid and
vapor using the Maxwell construction. Notably, within the coexistence region,
Bose-Einstein condensation occurs in a discontinuous way as the density jumps
from the normal gas to the superfluid liquid phase. Furthermore, we determine
the critical point where the line of first-order transition ends and
investigate the behavior of the density discontinuity in its vicinity. We also
point out that the density discontinuity at the transition could be observed in
experiments of mixtures in traps. | cond-mat_quant-gas |
Fermionization of two-component few-fermion systems in a one-dimensional
harmonic trap: The nature of strongly interacting Fermi gases and magnetism is one of the
most important and studied topics in condensed-matter physics. Still, there are
many open questions. A central issue is under what circumstances strong
short-range repulsive interactions are enough to drive magnetic correlations.
Recent progress in the field of cold atomic gases allows to address this
question in very clean systems where both particle numbers, interactions and
dimensionality can be tuned. Here we study fermionic few-body systems in a one
dimensional harmonic trap using a new rapidly converging effective-interaction
technique, plus a novel analytical approach. This allows us to calculate the
properties of a single spin-down atom interacting with a number of spin-up
particles, a case of much recent experimental interest. Our findings indicate
that, in the strongly interacting limit, spin-up and spin-down particles want
to separate in the trap, which we interpret as a microscopic precursor of
one-dimensional ferromagnetism in imbalanced systems. Our predictions are
directly addressable in current experiments on ultracold atomic few-body
systems. | cond-mat_quant-gas |
Quantum Monte Carlo Study of a Resonant Bose-Fermi Mixture: We study a resonant Bose-Fermi mixture at zero temperature by using the
fixed-node diffusion Monte Carlo method. We explore the system from weak to
strong boson-fermion interaction, for different concentrations of the bosons
relative to the fermion component. We focus on the case where the boson density
$n_B$ is smaller than the fermion density $n_F$, for which a first-order
quantum phase transition is found from a state with condensed bosons immersed
in a Fermi sea, to a Fermi-Fermi mixture of composite fermions and unpaired
fermions. We obtain the equation of state and the phase diagram, and we find
that the region of phase separation shrinks to zero for vanishing $n_B$. | cond-mat_quant-gas |
Frustrated quantum antiferromagnetism with ultracold bosons in a
triangular lattice: We propose to realize the anisotropic triangular-lattice Bose-Hubbard model
with positive tunneling matrix elements by using ultracold atoms in an optical
lattice dressed by a fast lattice oscillation. This model exhibits frustrated
antiferromagnetism at experimentally feasible temperatures; it interpolates
between a classical rotor model for weak interaction, and a quantum spin-1/2
$XY$-model in the limit of hard-core bosons. This allows to explore
experimentally gapped spin liquid phases predicted recently [Schmied et al.,
New J. Phys. {\bf 10}, 045017 (2008)]. | cond-mat_quant-gas |
Floquet-heating-induced Bose condensation in a scar-like mode of an open
driven optical-lattice system: Periodically driven quantum systems suffer from heating via resonant
excitation. While such Floquet heating guides a generic isolated system towards
the infinite-temperature state, a driven open system, coupled to a thermal
bath, will approach a non-equilibrium steady state. We show that the interplay
of bath-induced dissipation and controlled Floquet heating can give rise to
non-equilibrium Bose condensation in a mode protected from Floquet heating. In
particular, we consider a one-dimensional (1D) Bose gas in an optical lattice
of finite extent, which is coupled weakly to a three-dimensional thermal bath
given by a second atomic species. The bath temperature $T$ lies well above the
crossover temperature, below which the majority of the system's particles form
a (finite-size) Bose condensate in the ground state. However, when a strong
local potential modulation is switched on, which resonantly excites the system,
a non-equilibrium Bose condensate is formed in a state that decouples from the
drive. Our predictions, which are based on a microscopic model that is solved
using kinetic equations of motion derived from Floquet-Born-Markov theory, can
be probed under realistic experimental conditions. | cond-mat_quant-gas |
Self-bound Bose mixtures: Recent experiments confirmed that fluctuations beyond the mean-field
approximation can lead to self-bound liquid droplets of ultra-dilute binary
Bose mixtures. We proceed beyond the beyond-mean-field approximation, and study
liquid Bose mixtures using the variational hypernetted-chain Euler Lagrange
method, which accounts for correlations non-perturbatively. Focusing on the
case of a mixture of uniform density, as realized inside large saturated
droplets, we study the conditions for stability against evaporation of one of
the components (both chemical potentials need to be negative) and against
liquid-gas phase separation (spinodal instability), the latter being
accompanied by a vanishing speed of sound. Dilute Bose mixtures are stable only
in a narrow range near an optimal ratio $\rho_1/\rho_2$ and near the total
energy minimum. Deviations from a universal dependence on the s-wave scattering
lengths are significant despite the low density. | cond-mat_quant-gas |
Trimer quantum spin liquid in a honeycomb array of Rydberg atoms: Quantum spin liquids are elusive but paradigmatic examples of strongly
correlated quantum states that are characterized by long-range quantum
entanglement. Recently, the direct signatures of a gapped topological
$\mathbb{Z}_2$ spin liquid have been observed in a system of Rydberg atoms
arrayed on the ruby lattice. Here, we illustrate the concrete realization of a
fundamentally different class of spin liquids in a honeycomb array of Rydberg
atoms. Exploring the quantum phase diagram of this system using both
density-matrix renormalization group and exact diagonalization simulations,
several density-wave-ordered phases are characterized and their origins
explained. More interestingly, in the regime where third-nearest-neighbor atoms
lie within the Rydberg blockade radius, we find a novel ground state -- with an
emergent $\mathrm{U}(1)\times \mathrm{U}(1)$ local symmetry -- formed from
superpositions of classical {\it trimer} configurations on the dual triangular
lattice. The fidelity of this trimer spin liquid state can be enhanced via
dynamical preparation, which we explain by a Rydberg-blockade-based projection
mechanism associated with the smooth turnoff of the laser drive. Finally, we
discuss the robustness of the trimer spin liquid phase under realistic
experimental parameters and demonstrate that our proposal can be readily
implemented in current Rydberg atom quantum simulators. | cond-mat_quant-gas |
Reply to the Comment on "Berezinskii-Kosterlitz-Thouless Transition in
Two-Dimensional Dipolar Stripes": This is a Reply to the Comment from F. Cinti and M. Boninsegni on our recent
work on the Berezinskii-Kosterlitz-Thouless (BKT) phase transition in a
two-dimensional dipolar system [R.Bomb\'in, F. Mazzanti and J. Boronat,
Physical Review A 100, 063614 (2019)]. The main criticism about our work,
expressed in that Comment, is that we did not explicitly report the two spatial
contributions to the total superfluid fraction. Here, we analyze our results
for a point of the phase diagram corresponding to the stripe phase, close to
the gas to stripe transition line, and for a temperature below the BKT critical
temperature. The scaling with the system size of the contribution to the
superfluid fraction, coming from the direction in which spatial order appears,
shows that it remains finite in the thermodynamic limit, as we already stated
in our original work. This allow us to state that the stripe phase is
superfluid at low temperatures. Furthermore, we offer some comments that help
to understand where the differences between the results of Cinti and Boninsegni
and ours comes from. | cond-mat_quant-gas |
Macroscopic boundary effects in the one-dimensional extended
Bose-Hubbard model: We study the effect of different open boundary conditions on the insulating
ground states of the one-dimensional extended Bose-Hubbard model at and near
unit filling. To this end, we employ the density matrix renormalization group
method with system sizes up to 250 sites. To characterize the system, various
order parameters and entanglement entropies are calculated. When opposite edge
potentials are added to the two ends of the chain, the inversion symmetry is
explicitly broken, and the regular bulk phases appear. On the other hand,
simple open boundary conditions often exhibit non-degenerate ground states with
a domain wall in the middle of the chain, which induces a sign-flip of an order
parameter. Such a domain wall can lead to an algebraic behavior of the
off-diagonals of the single particle density matrix. We show that this
algebraic behavior adds only a finite contribution to the entanglement entropy,
which does not diverge as the system size increases. Therefore, it is not an
indication of a superfluid phase. We confirm this picture by analytical
calculations based on an effective Hamiltonian for a domain wall. | cond-mat_quant-gas |
Quantum fluctuation induced time of flight correlations of an
interacting trapped Bose gas: We investigate numerically the momentum correlations in a two dimensional,
harmonically trapped interacting Bose system at $T=0$ temperature, by using a
particle number preserving Bogoliubov approximation. Interaction induced
quantum fluctuations of the quasi-condensate lead to a large anti-correlation
dip between particles of wave numbers $\mathbf{k}$ and $-\mathbf{k}$ for
$|\mathbf{k}|\sim 1/R_c$, with $R_c$ typical size of the condensate. The
anti-correlation dip found is a clear fingerprint of coherent quantum
fluctuations of the condensate. In contrast, for larger wave numbers,
$|\mathbf{k}| >> 1/R_c$, a weak positive correlation is found between particles
of wave numbers $\mathbf{k}$ and $-\mathbf{k}$, in accordance with the
Bogoliubov result for homogeneous interacting systems. | cond-mat_quant-gas |
Faraday waves on a bubble Bose-Einstein condensed binary mixture: By studying the dynamic stability of Bose-Einstein condensed binary mixtures
trapped on the surface of an ideal two-dimensional spherical bubble, we show
how the Rabi coupling between the species can modulate the interactions leading
to parametric resonances. In this spherical geometry, the discrete unstable
angular modes drive both phase separations and spatial patterns, with Faraday
waves emerging and coexisting with an immiscible phase. Noticeable is the fact
that, in the context of discrete kinetic energy spectrum, the only parameters
to drive the emergence of Faraday waves are the $s-wave$ contact interactions
and the Rabi coupling. Once analytical solutions for population dynamics are
obtained, the stability of homogeneous miscible species is investigated through
Bogoliubov-de Gennes and Floquet methods, with predictions being analysed by
full numerical solutions applied to the corresponding time-dependent coupled
formalism. | cond-mat_quant-gas |
Superfluidity of Bose-Einstein condensates in toroidal traps with
nonlinear lattices: Superfluid properties of Bose-Einstein condensates (BEC) in toroidal
quasi-one-dimensional traps are investigated in the presence of periodic
scattering length modulations along the ring. The existence of several types of
stable periodic waves, ranging from almost uniform to very fragmented chains of
weakly interacting and equally spaced solitons, is demonstrated. We show that
these waves may support persistent atomic currents and sound waves with spectra
of Bogoliubov type. Fragmented condensates can be viewed as arrays of Josephson
junctions and the current as a BEC manifestation of the dc-Josephson effect.
The influence of linear defects on BEC superfluidity has been also
investigated. We found that for subcritical velocities, linear defects that are
static with respect to the lattice (while the condensate moves in respect to
both the optical lattice and the defect) preserve the BEC superfluidity. | cond-mat_quant-gas |
Thermodynamic engine with a quantum degenerate working fluid: Can quantum mechanical thermodynamic engines outperform their classical
counterparts? To address one aspect of this question, we experimentally realize
and characterize an isentropic thermodynamic engine that uses a Bose-condensed
working fluid. In this engine, an interacting quantum degenerate gas of bosonic
lithium is subjected to trap compression and relaxation strokes interleaved
with strokes strengthening and weakening interparticle interactions. We observe
a significant enhancement in efficiency and power when using a Bose-condensed
working fluid, compared to the case of a non-degenerate thermal gas. We
demonstrate reversibility, and measure power and efficiency as a function of
engine parameters including compression ratio and cycle time. Results agree
quantitatively with interacting finite temperature field-theoretic simulations
that closely replicate the length and energy scales of the working fluid. | cond-mat_quant-gas |
One Dimensional 1H, 2H and 3H: The ground-state properties of one-dimensional electron-spin-polarized
hydrogen $^1$H, deuterium $^2$H, and tritium $^3$H are obtained by means of
quantum Monte Carlo methods. The equations of state of the three isotopes are
calculated for a wide range of linear densities. The pair correlation function
and the static structure factor are obtained and interpreted within the
framework of the Luttinger liquid theory. We report the density dependence of
the Luttinger parameter and use it to identify different physical regimes:
Bogoliubov Bose gas, super-Tonks-Girardeau gas, and quasi-crystal regimes for
bosons; repulsive, attractive Fermi gas, and quasi-crystal regimes for
fermions. We find that the tritium isotope is the one with the richest
behaviour. Our results show unambiguously the relevant role of the isotope mass
in the properties of this quantum system. | cond-mat_quant-gas |
Measuring topology in a laser-coupled honeycomb lattice: From Chern
insulators to topological semi-metals: Ultracold fermions trapped in a honeycomb optical lattice constitute a
versatile setup to experimentally realize the Haldane model [Phys. Rev. Lett.
61, 2015 (1988)]. In this system, a non-uniform synthetic magnetic flux can be
engineered through laser-induced methods, explicitly breaking time-reversal
symmetry. This potentially opens a bulk gap in the energy spectrum, which is
associated with a non-trivial topological order, i.e., a non-zero Chern number.
In this work, we consider the possibility of producing and identifying such a
robust Chern insulator in the laser-coupled honeycomb lattice. We explore a
large parameter space spanned by experimentally controllable parameters and
obtain a variety of phase diagrams, clearly identifying the accessible
topologically non-trivial regimes. We discuss the signatures of Chern
insulators in cold-atom systems, considering available detection methods. We
also highlight the existence of topological semi-metals in this system, which
are gapless phases characterized by non-zero winding numbers, not present in
Haldane's original model. | cond-mat_quant-gas |
Measure synchronization in quantum many-body systems: The concept of measure synchronization between two coupled quantum many-body
systems is presented. In general terms we consider two quantum many-body
systems whose dynamics gets coupled through the contact particle-particle
interaction. This coupling is shown to produce measure synchronization, a
generalization of synchrony to a large class of systems which takes place in
absence of dissipation. We find that in quantum measure synchronization, the
many-body quantum properties for the two subsystems, e.g. condensed fractions
and particle fluctuations, behave in a coordinated way. To illustrate the
concept we consider a simple case of two species of bosons occupying two
distinct quantum states. Measure synchronization can be readily explored with
state-of-the-art techniques in ultracold atomic gases and, if propertly
controlled, be employed to share quantum correlations between different degrees
of freedom. | cond-mat_quant-gas |
Impact of photo-assisted collisions on superradiant light scattering
with Bose condensates: We present experimental evidence supporting the postulation that the
secondary effects of light-assisted collisions are the main reason that the
superradiant light scattering efficiency in condensates is asymmetric with
respect to the sign of the pump-laser detuning. Contrary to the recent
experimental study, however, we observe severe and comparable heating with all
three pump-laser polarizations. We also perform two-color, double-pulse
measurements to directly study the degradation of condensate coherence and the
resulting impact on the superradiant scattering efficiency. | cond-mat_quant-gas |
Quantum Monte Carlo study of ultracold gases (PhD thesis): This Dissertation presents results of a thorough study of ultracold bosonic
and fermionic gases in three-dimensional and quasi-one-dimensional systems.
Although the analyses are carried out within various theoretical frameworks
(Gross-Pitaevskii, Bethe ansatz, local density approximation, etc.) the main
tool of the study is the Quantum Monte Carlo method in different modifications
(variational Monte Carlo, diffusion Monte Carlo, fixed-node Monte Carlo
methods). We benchmark our Monte Carlo calculations by recovering known
analytical results (perturbative theories in dilute limits, exactly solvable
models, etc.) and extend calculations to regimes, where the results are so far
unknown. In particular we calculate the equation of state and correlation
functions for gases in various geometries and with various interatomic
interactions. | cond-mat_quant-gas |
Many-body Aharonov-Bohm caging in a lattice of rings: We study a system of a few ultracold bosons loaded into the states with
orbital angular momentum $l=1$ of a one-dimensional staggered lattice of rings.
Local eigenstates with winding numbers $+l$ and $-l$ form a Creutz ladder with
a real dimension and a synthetic one. States with opposite winding numbers in
adjacent rings are coupled through complex tunnelings, which can be tuned by
modifying the central angle $\phi$ of the lattice. We analyze both the
single-particle case and the few boson bound-state subspaces for the regime of
strong interactions using perturbation theory, showing how the geometry of the
system can be engineered to produce an effective $\pi$-flux through the
plaquettes. We find non-trivial topological band structures and many-body
Aharonov-Bohm caging in the $N$-particle subspaces even in the presence of a
dispersive single-particle spectrum. Additionally, we study the family of
models where the angle $\phi$ is introduced at an arbitrary lattice periodicity
$\Gamma$. For $\Gamma>2$, the $\pi$-flux becomes non-uniform, which enlarges
the spatial extent of the Aharonov-Bohm caging as the number of flat bands in
the spectrum increases. All the analytical results are benchmarked through
exact diagonalization. | cond-mat_quant-gas |
Prethermalization in the cooling dynamics of an impurity in a BEC: We discuss the cooling dynamics of heavy impurity atoms in a Bose-Einstein
condensate (BEC) by emission of Cherenkov phonons from scattering with the
condensate. In a weakly interacting, low-temperature condensate the
superfluidity of the condensate results in a separation of time-scales of the
thermalization dynamics. Pre-thermalized states are formed with distinct
regions of impurity momenta determined by the mass ratio of impurity and BEC
atoms. This can be employed to detect the mass renormalization of the impurity
upon the formation of a polaron and paves the way to preparing non-equilibrium
impurity-momentum distributions. | cond-mat_quant-gas |
Pair condensation of polarized fermions in the BCS-BEC crossover: We investigate a two-component Fermi gas with unequal spin populations along
the BCS-BEC crossover. By using the extended BCS equations and the concept of
off-diagonal-long-range-order we derive a formula for the condensate number of
Cooper pairs as a function of energy gap, average chemical potential, imbalance
chemical potential and temperature. Then we study the zero-temperature
condensate fraction of Cooper pairs by varying interaction strength and
polarization, finding a depletion of the condensate fraction by increasing the
population imbalance. We also consider explicitly the presence of an external
harmonic confinement and we study, within the local-density approximation, the
phase separation between superfluid and normal phase regions of the polarized
fermionic cloud. In particular, we calculate both condensate density profiles
and total density profiles from the inner superfluid core to the normal region
passing for the interface, where a finite jump in the density is a clear
manifestation of this phase-separated regime. Finally, we compare our
theoretical results with the available experimental data on the condensate
fraction of polarized 6Li atoms [Science 311, 492 (2006)]. These experimental
data are in reasonable agreement with our predictions in a suitable range of
polarizations, but only in the BCS side of the crossover up to unitarity. | cond-mat_quant-gas |
One-dimensional Bose-Einstein condensation of photons in a microtube: This paper introduces a quasiequilibrium one-dimensional Bose-Einstein
condensation of photons trapped in a microtube. Light modes with a cut-off
frequency (a photon's "mass") interact through different processes of
absorption, emission, and scattering on molecules and atoms. In this paper, we
study the conditions for the one-dimensional condensation of light and the role
of photon-photon interactions in the system. The technique in use is the
Matsubara's Green's functions formalism modified for the quasiequilibrium
system under study. | cond-mat_quant-gas |
Detection of coherent superpositions of phase states by full counting
statistics in a Bose Josephson junction: For a Bose Josephson junction realized with a double-well potential we
propose a strategy to observe the coherent superpositions of phase states
occurring during the time evolution after a sudden rise of the barrier
separating the two wells. We show that their phase content can be obtained by
the full-counting statistics of the spin-boson operators characterizing the
junction, which could be mapped out by repeated measurements of the population
imbalance after rotation of the state. This measurement can distinguish between
coherent superpositions and incoherent mixtures, and can be used for a
two-dimensional, tomographic reconstruction of the phase content of the state. | cond-mat_quant-gas |
Probing the Hall Voltage in Synthetic Quantum Systems: In the context of experimental advances in the realization of artificial
magnetic fields in quantum gases, we discuss feasible schemes to extend
measurements of the Hall polarization to a study of the Hall voltage, allowing
for direct comparison with solid state systems. Specifically, for the
paradigmatic example of interacting flux ladders, we report on characteristic
zero crossings and a remarkable robustness of the Hall voltage with respect to
interaction strengths, particle fillings, and ladder geometries, which is
unobservable in the Hall polarization. Moreover, we investigate the
site-resolved Hall response in spatially inhomogeneous quantum phases. | cond-mat_quant-gas |
FACt: FORTRAN toolbox for calculating fluctuations in atomic condensates: We develop a FORTRAN code to compute fluctuations in atomic condensates
(FACt) by solving the Bogoliubov-de Gennes (BdG) equations for two component
Bose-Einstein condensate (TBEC) in quasi two dimensions. The BdG equations are
recast as matrix equations and solved self consistently. The code is suitable
for handling quantum fluctuations as well as thermal fluctuations at
temperatures below the critical point of Bose-Einstein condensation. The code
is versatile, and the ground state density profile and low energy excitation
modes obtained from the code can be easily adapted to compute different
properties of TBECs -- ground state energy, overlap integral, quasi particle
amplitudes of BdG spectrum, dispersion relation and structure factor and other
related experimental observables. | cond-mat_quant-gas |
Dissipative Distillation of Supercritical Quantum Gases: We experimentally realize a method to produce non-equilibrium Bose Einstein
condensates with condensed fraction exceeding those of equilibrium samples with
the same parameters. To do this, we immerse an ultracold Bose gas of 87Rb in a
cloud of 39K with substantially higher temperatures, providing a controlled
source of dissipation. By combining the action of the dissipative environment
with evaporative cooling, we are able to progressively distil the
non-equilibrium Bose-Einstein condensate from the thermal cloud. We show that
by increasing the strength of the dissipation it is even possible to produce
condensates above the critical temperature. We finally demonstrate that our
out-of-equilibrium samples are long-lived and do not reach equilibrium in a
time that is accessible for our experiment. Due to its high degree of control,
our distillation process is a promising tool for the engineering of open
quantum systems. | cond-mat_quant-gas |
Emergent Quasicrystalline Symmetry in Light-Induced Quantum Phase
Transitions: The discovery of quasicrystals with crystallographically forbidden rotational
symmetries has changed the notion of the ordering in materials, yet little is
known about the dynamical emergence of such exotic forms of order. Here we
theoretically study a nonequilibrium cavity-QED setup realizing a
zero-temperature quantum phase transition from a homogeneous Bose-Einstein
condensate to a quasicrystalline phase via collective superradiant light
scattering. Across the superradiant phase transition, collective light
scattering creates a dynamical, quasicrystalline optical potential for the
atoms. Remarkably, the quasicrystalline potential is "emergent" as its
eight-fold rotational symmetry is not present in the Hamiltonian of the system,
rather appears solely in the low-energy states. For sufficiently strong
two-body contact interactions between atoms, a quasicrystalline order is
stabilized in the system, while for weakly interacting atoms the condensate is
localized in one or few of the deepest minima of the quasicrystalline
potential. | cond-mat_quant-gas |
Floquet Dynamical Decoupling at Zero Bias: Dynamical decoupling (DD) is an efficient method to decouple systems from
environmental noises and to prolong the coherence time of systems. In contrast
to discrete and continuous DD protocols in the presence of bias field, we
propose a Floquet DD at zero bias to perfectly suppress both the zeroth and
first orders of noises according to the Floquet theory. Specifically, we
demonstrate the effectiveness of this Floquet DD protocol in two typical
systems including a spinor atomic Bose-Einstein condensate decohered by
classical stray magnetic fields and a semiconductor quantum dot electron spin
coupled to nuclear spins. Furthermore, our protocol can be used to sense
high-frequency noises. The Floquet DD protocol we propose shines new light on
low-cost and high-portable DD technics without bias field and with low
controlling power, which may have wide applications in quantum computing,
quantum sensing, nuclear magnetic resonance and magnetic resonance imaging. | cond-mat_quant-gas |
Field-induced topological pair-density wave states in a multilayer
optical lattice: We study the superfluid phases of a Fermi gas in a multilayer optical lattice
system in the presence of out-of-plane Zeeman field, as well as spin-orbit (SO)
coupling. We show that the Zeeman field combined with the SO coupling leads to
exotic topological pair-density wave (PDW) phases in which different layers
possess different superfluid order parameters, even though each layer
experiences the same Zeeman field and the SO coupling. We elucidate the
mechanism of the emerging PDW phases, and characterize their topological
properties by calculating the associated Chern numbers. | cond-mat_quant-gas |
Phase Transitions in Three-Dimensional Bosonic Systems in Optical
Lattices: We formulate the Collective Quantum Field Theory for three-dimensional
bosonic optical lattices and evaluate its consequences in a mean-field
approximation to two collective fields, proposed by Fred Cooper et al. and in a
lowest-order Variational Perturbation Theory (VPT). It is shown that present
mean-field approximation predicts some essential features of the experimentally
observed dependence of the critical temperature on the coupling strength and a
second - order quantum phase transition.In contrast to a recent prediction for
atomic gases by Fred Cooper et. al., we find no superfluid state with zero
condensate fraction. | cond-mat_quant-gas |
Quench between a Mott insulator and a Lieb-Liniger liquid: In this work we study a quench between a Mott insulator and a repulsive
Lieb-Liniger liquid. We find explicitly the stationary state when a long time
has passed after the quench. It is given by a GGE density matrix which we
completely characterize, calculating the quasiparticle density describing the
system after the quench. In the long time limit we find an explicit form for
the local three body density density density correlation function and the
asymptotic long distance limit of the density density correlation function. The
later is shown to have a Gaussian decay at large distances. | cond-mat_quant-gas |
The optimal frequency window for Floquet engineering in optical lattices: The concept of Floquet engineering is to subject a quantum system to
time-periodic driving in such a way that it acquires interesting novel
properties. It has been employed, for instance, for the realization of
artificial magnetic fluxes in optical lattices and, typically, it is based on
two approximations. First, the driving frequency is assumed to be low enough to
suppress resonant excitations to high-lying states above some energy gap
separating a low energy subspace from excited states. Second, the driving
frequency is still assumed to be large compared to the energy scales of the
low-energy subspace, so that also resonant excitations within this space are
negligible. Eventually, however, deviations from both approximations will lead
to unwanted heating on a time scale $\tau$. Using the example of a
one-dimensional system of repulsively interacting bosons in a shaken optical
lattice, we investigate the optimal frequency (window) that maximizes $\tau$.
As a main result, we find that, when increasing the lattice depth, $\tau$
increases faster than the experimentally relevant time scale given by the
tunneling time $\hbar/J$, so that Floquet heating becomes suppressed. | cond-mat_quant-gas |
Miscibility in coupled dipolar and non-dipolar Bose-Einstein condensates: We perform a full three-dimensional study on miscible-immiscible conditions
for coupled dipolar and non-dipolar Bose-Einstein condensates (BEC), confined
within anisotropic traps. Without loosing general miscibility aspects that can
occur for two-component mixtures, our main focus was on the atomic
erbium-dysprosium ($^{168}$Er-$^{164}$Dy) and dysprosium-dysprosium
($^{164}$Dy-$^{162}$Dy) mixtures. Our analysis for pure-dipolar BEC was limited
to coupled systems confined in pancake-type traps, after considering a study on
the stability regime of such systems. In case of non-dipolar systems with
repulsive contact intneeractions we are able to extend the miscibility analysis
to coupled systems with cigar-type symmetries. For a coupled condensate with
repulsive inter- and intra-species two-body interactions, confined by an
external harmonic trap, the transition from a miscible to an immiscible phase
is verified to be much softer than in the case the system is confined by a
symmetric hard-wall potential. Our results, presented by density plots, are
pointing out the main role of the trap symmetry and inter-species interaction
for the miscibility. A relevant parameter to measure the overlap between the
two densities was defined and found appropriate to quantify the miscibility of
a coupled system. | cond-mat_quant-gas |
Quantum Melting of a Wigner crystal of Rotating Dipolar Fermions in the
Lowest Landau Level: We have investigated the behavior and stability of a Wigner crystal of
rotating dipolar fermions in two dimensions. Using an ansatz wave function for
the ground state of rotating two-dimensional dipolar fermions, which occupy
only partially the lowest Landau level, we study the correlation energy,
elastic moduli and collective modes of Wigner crystals in the lowest Landau
level. We then calculate the mean square of the displacement vector of Wigner
crystals. The critical filling factor, below which the crystalline state is
expected, is evaluated at absolute zero by use of the Lindeman's criterion. We
find that the particle (hole) crystal is locally stable for filling factor is
less than 1/15 (between filling factors 14/15 and 1), where the stable regime
of the crystal is much narrower than the result from Baranov, Fehrmann and
Lewenstein, [Phys. Rev. Lett. 100, 200402 (2008)]. | cond-mat_quant-gas |
Dissipative nonlinear waves in a gravitating quantum fluid: Nonlinear wave propagation is studied analytically in a dissipative,
self-gravitating Bose Einstein condensate, in the framework of Gross-Pitaevskii
model. The linear dispersion relation shows that the effect of dissipation is
to suppress dynamical instabilities that destabilize the system. The small
amplitude analysis using reductive perturbation technique is found to yield a
modified form of KdV equation. The soliton energy, amplitude and velocity are
found to decay with time, whereas the soliton width increases, such that the
soliton exists for a finite time only | cond-mat_quant-gas |
Viscosity of strongly interacting quantum fluids: spectral functions and
sum rules: The viscosity of strongly interacting systems is a topic of great interest in
diverse fields.
We focus here on the bulk and shear viscosities of \emph{non-relativistic}
quantum fluids, with particular emphasis on strongly interacting ultracold
Fermi gases. We use Kubo formulas for the bulk and shear viscosity spectral
functions, $\zeta(\omega)$ and $\eta(\omega)$ respectively, to derive exact,
non-perturbative results. Our results include: a microscopic connection between
the shear viscosity $\eta$ and the normal fluid density $\rho_n$; sum rules for
$\zeta(\omega)$ and $\eta(\omega)$ and their evolution through the BCS-BEC
crossover; universal high-frequency tails for $\eta(\omega)$ and the dynamic
structure factor $S({\bf q}, \omega)$. We use our sum rules to show that, at
unitarity, $\zeta(\omega)$ is identically zero and thus relate $\eta(\omega)$
to density-density correlations. We predict that frequency-dependent shear
viscosity $\eta(\omega)$ of the unitary Fermi gas can be experimentally
measured using Bragg spectroscopy. | cond-mat_quant-gas |
Ramsey interferometry of non-Hermitian quantum impurities: We introduce a Ramsey pulse scheme which extracts the non-Hermitian
Hamiltonian associated to an arbitrary Lindblad dynamics. We propose a realted
protocol to measure via interferometry a generalised Loschmidt echo of a
generic state evolving in time with the non-Hermitian Hamiltonian itself, and
we apply the scheme to a one-dimensional weakly interacting Bose gas coupled to
a stochastic atomic impurity. The Loschmidt echo is mapped into a functional
integral from which we calculate the long-time decohering dynamics at arbitrary
impurity strengths. For strong dissipation we uncover the phenomenology of a
quantum many-body Zeno effect: corrections to the decoherence exponent
resulting from the impurity self-energy becomes purely imaginary, in contrast
to the regime of small dissipation where they instead enhance the decay of
quantum coherences. Our results illustrate the prospects for experiments
employing Ramsey interferometry to study dissipative quantum impurities in
condensed matter and cold atoms systems. | cond-mat_quant-gas |
Chirp Control of Sinusoidal Lattice Modes in Bose-Einstein Condensate: A parametrically forced Bose-Einstein condensate (BEC) is studied in the mean
field approach for the case of a general moving optical lattice. The
interaction between the atoms in the condensate and the time dependent lattice
potential leads to a novel propagating superfluid matter wave, which can be
controlled through chirp management. This system, when placed in a trap,
accelerates and undergoes rapid nonlinear compression, controlled by the chirp.
The density achieves its maximum, precisely when the matter wave changes
direction. A dynamical phase transition is identified, which takes the
superfluid phase to an insulating state. The exact expression for energy is
obtained and analyzed in detail to gain physical understanding of the chirp
management of the sinusoidal excitations and also the dynamical phase
transition. | cond-mat_quant-gas |
Quantum Fluctuation Driven First-order Phase Transitions in Optical
Lattices: We study quantum fluctuation driven first-order phase transitions of a
two-species bosonic system in a three-dimensional optical lattice. Using
effective potential method we find that the superfluid-Mott insulator phase
transition of one type of bosons can be changed from second-order to
first-order by the quantum fluctuations of the other type of bosons. The study
of the scaling behaviors near the quantum critical point shows that the
first-order phase transition has a different universality from the second-order
one. We also discuss the observation of this exotic phenomenon in the realistic
cold-atom experiments. | cond-mat_quant-gas |
From few to many bosons inside the unitary window: a transition between
universal to non-universal behavior: Universal behaviour in few-bosons systems close to the unitary limit, where
two bosons become unbound, has been intensively investigated in recent years
both experimentally and theoretically. In this particular region, called the
unitary window, details of the inter-particle interactions are not important
and observables, such as binding energies, can be characterized by a few
parameters. With an increasing number of particles the short-range repulsion,
present in all atomic, molecular or nuclear interactions, gradually induces
deviations from the universal behaviour. In the present letter we discuss for
the first time a simple way of incorporating non-universal behaviour through
one specific parameter which controls the smooth transition of the system from
universal to non-universal regime. Using a system of $N$ helium atoms as an
example we calculate their ground state energies as trajectories within the
unitary window and also show that the control parameters can be used to
determine the energy per particle in homogeneous systems when $N \rightarrow
\infty$. | cond-mat_quant-gas |
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