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First-order dissipative phase transition in an exciton-polariton
condensate: We investigate the phase diagram of a two-dimensional driven-dissipative
system of polaritons coupled to the excitonic reservoir. We find that two
critical points exists. The first corresponds to the quasi-condensation and the
second to a first-order phase transition from the non-uniform state with
spatially modulated density to a uniform state. The latter is related to the
modulational instability of a homogeneous state due to the repulsive
interactions with the non-condensed reservoir. The first-order character of the
transition is evidenced by a discontinuity in the density and the correlation
length as well as the phase coexistence and metastability. Moreover, we show
that a signature of a Berezinskii-Kosterlitz-Thouless-like transition can be
observed in the non-uniform phase. | cond-mat_quant-gas |
Behavior of heat capacity of an attractive Bose-Einstein Condensate
approaching collapse: We report calculation of heat capacity of an attractive Bose-Einstein
condensate, with the number N of bosons increasing and eventually approaching
the critical number Ncr for collapse, using the correlated potential harmonics
(CPH) method. Boson pairs interact via the realistic van der Waals potential.
It is found that the transition temperature Tc increases initially slowly, then
rapidly as N becomes closer to Ncr . The peak value of heat capacity for a
fixed N increases slowly with N, for N far away from Ncr . But after reaching a
maximum, it starts decreasing when N approaches Ncr . The effective potential
calculated by CPH method provides an insight into this strange behavior. | cond-mat_quant-gas |
All-optical cooling of Fermi gases via Pauli inhibition of spontaneous
emission: A technique is proposed to cool Fermi gases to the regime of quantum
degeneracy based on the expected inhibition of spontaneous emission due to the
Pauli principle. The reduction of the linewidth for spontaneous emission
originates a corresponding reduction of the Doppler temperature, which under
specific conditions may give rise to a runaway process through which fermions
are progressively cooled. The approach requires a combination of a
magneto-optical trap as a cooling system and an optical dipole trap to enhance
quantum degeneracy. This results in expected Fermi degeneracy factors $T/T_F$
comparable to the lowest values recently achieved, with potential for a direct
implementation in optical lattices. The experimental demonstration of this
technique should also indirectly provide a macroscopic manifestation of the
Pauli exclusion principle at the atomic physics level. | cond-mat_quant-gas |
Gapped spectrum in pair-superfluid bosons: We study the ground state of a bilayer system of dipolar bosons with dipoles
oriented by an external field perpendicularly to the two parallel planes. By
decreasing the interlayer distance, for a fixed value of the strength of the
dipolar interaction, the system undergoes a quantum phase transition from an
atomic to a pair superfluid. We investigate the excitation spectrum across this
transition by using microscopic approaches. Quantum Monte Carlo methods are
employed to obtain the static structure factors and intermediate scattering
functions in imaginary time. The dynamic response is calculated using both the
correlated basis functions (CBF) method and the approximate inversion of the
Laplace transform of the quantum Monte Carlo imaginary time data. In the atomic
phase, both density and spin excitations are gapless. However, in the
pair-superfluid phase a gap opens in the excitation energy of the spin mode.
For small separation between layers, the minimal spin excitation energy equals
the binding energy of a dimer and is twice the gap value. | cond-mat_quant-gas |
Single-Atom Resolved Fluorescence Imaging of an Atomic Mott Insulator: The reliable detection of single quantum particles has revolutionized the
field of quantum optics and quantum information processing. For several years,
researchers have aspired to extend such detection possibilities to larger scale
strongly correlated quantum systems, in order to record in-situ images of a
quantum fluid in which each underlying quantum particle is detected. Here we
report on fluorescence imaging of strongly interacting bosonic Mott insulators
in an optical lattice with single-atom and single-site resolution. From our
images, we fully reconstruct the atom distribution on the lattice and identify
individual excitations with high fidelity. A comparison of the radial density
and variance distributions with theory provides a precise in-situ temperature
and entropy measurement from single images. We observe Mott-insulating plateaus
with near zero entropy and clearly resolve the high entropy rings separating
them although their width is of the order of only a single lattice site.
Furthermore, we show how a Mott insulator melts for increasing temperatures due
to a proliferation of local defects. Our experiments open a new avenue for the
manipulation and analysis of strongly interacting quantum gases on a lattice,
as well as for quantum information processing with ultracold atoms. Using the
high spatial resolution, it is now possible to directly address individual
lattice sites. One could, e.g., introduce local perturbations or access regions
of high entropy, a crucial requirement for the implementation of novel cooling
schemes for atoms on a lattice. | cond-mat_quant-gas |
On quantum time crystals and interacting gauge theories in atomic
Bose-Einstein condensates: We study the dynamics of a Bose-Einstein condensate trapped circumferentially
on a ring, and which is governed by an interacting gauge theory. We show that
the associated density-dependent gauge potential and concomitant current
nonlinearity permits a ground state in the form of a rotating chiral bright
soliton. This chiral soliton is constrained to move in one direction by virtue
of the current nonlinearity, and represents a time crystal in the same vein as
Wilczek's original proposal. | cond-mat_quant-gas |
Life Cycle of Superfluid Vortices and Quantum Turbulence in the Unitary
Fermi Gas: The unitary Fermi gas (UFG) offers an unique opportunity to study quantum
turbulence both experimentally and theoretically in a strongly interacting
fermionic superfluid. It yields to accurate and controlled experiments, and
admits the only dynamical microscopic description via time-dependent density
functional theory (DFT) - apart from dilute bosonic gases - of the crossing and
reconnection of superfluid vortex lines conjectured by Feynman in 1955 to be at
the origin of quantum turbulence in superfluids at zero temperature. We
demonstrate how various vortex configurations can be generated by using well
established experimental techniques: laser stirring and phase imprinting. New
imagining techniques demonstrated by the MIT group [Ku et al. arXiv:1402.7052]
should be able to directly visualize these crossings and reconnections in
greater detail than performed so far in liquid helium. We demonstrate the
critical role played by the geometry of the trap in the formation and dynamics
of a vortex in the UFG and how laser stirring and phase imprint can be used to
create vortex tangles with clear signatures of the onset of quantum turbulence. | cond-mat_quant-gas |
Realizing bright matter-wave soliton collisions with controlled relative
phase: We propose a method to split the ground state of an attractively interacting
atomic Bose-Einstein condensate into two bright solitary waves with controlled
relative phase and velocity. We analyze the stability of these waves against
their subsequent re-collisions at the center of a cylindrically symmetric,
prolate harmonic trap as a function of relative phase, velocity, and trap
anisotropy. We show that the collisional stability is strongly dependent on
relative phase at low velocity, and we identify previously unobserved
oscillations in the collisional stability as a function of the trap anisotropy.
An experimental implementation of our method would determine the validity of
the mean field description of bright solitary waves, and could prove an
important step towards atom interferometry experiments involving bright
solitary waves. | cond-mat_quant-gas |
Quench Dynamics of Three-Dimensional Disordered Bose Gases:
Condensation, Superfluidity and Fingerprint of Dynamical Bose Glass: In an equilibrium three-dimensional (3D) disordered condensate, it's well
established that disorder can generate an amount of normal fluid equaling to
$\frac{4}{3}$ of the condensate depletion. The concept that the superfluid is
more volatile to the existence of disorder than the condensate is crucial to
the understanding of Bose glass phase. In this Letter, we show that, by
bringing a weakly disordered 3D condensate to nonequilibrium regime via a
quantum quench in the interaction, disorder can destroy superfluid
significantly more, leading to a steady state in which the normal fluid density
far exceeds $\frac{4}{3}$ of the condensate depletion. This suggests a
possibility of engineering Bose Glass in the dynamic regime. As both the
condensate density and superfluid density are measurable quantities, our
results allow an experimental demonstration of the dramatized interplay between
the disorder and interaction in the nonequilibrium scenario. | cond-mat_quant-gas |
Exact Solution for 1D Spin-Polarized Fermions with Resonant Interactions: Using the asymptotic Bethe Ansatz, we obtain an exact solution of the
many-body problem for 1D spin-polarized fermions with resonant p-wave
interactions, taking into account the effects of both scattering volume and
effective range. Under typical experimental conditions, accounting for the
effective range, the properties of the system are significantly modified due to
the existence of "shape" resonances. The excitation spectrum of the considered
model has unexpected features, such as the inverted position of the particle-
and hole-like branches at small momenta, and roton-like minima. We find that
the frequency of the "breathing" mode in the harmonic trap provides an
unambiguous signature of the effective range. | cond-mat_quant-gas |
Chaotic Einstein-Podolsky-Rosen pairs, measurements and time reversal: We consider a situation when evolution of an entangled
Einstein-Podolsky-Rosen (EPR) pair takes place in a regime of quantum chaos
being chaotic in the classical limit. This situation is studied on an example
of chaotic pair dynamics described by the quantum Chirikov standard map. The
time evolution is reversible even if a presence of small errors breaks time
reversal of classical dynamics due to exponential growth of errors induced by
exponential chaos instability. However, the quantum evolution remains
reversible since a quantum dynamics instability exists only on a
logarithmically short Ehrenfest time scale. We show that due to EPR pair
entanglement a measurement of one particle at the moment of time reversal
breaks exact time reversal of another particle which demonstrates only an
approximate time reversibility. This result is interpreted in the framework of
the Schmidt decomposition and Feynman path integral formulation of quantum
mechanics. The time reversal in this system has already been realized with cold
atoms in kicked optical lattices in absence of entanglement and measurements.
On the basis of the obtained results we argue that the experimental
investigations of time reversal of chaotic EPR pairs is within reach of present
cold atom capabilities. | cond-mat_quant-gas |
Experimental Observation of the 2D-1D Dimensional Crossover in Strongly
Interacting Ultracold Bosons: Dimensionality plays an essential role in determining the nature and
properties of a physical system. For quantum systems the impact of interactions
and fluctuations is enhanced in lower dimensions, leading to a great diversity
of genuine quantum effects for reduced dimensionality. In most cases, the
dimension is fixed to some integer value. Here, we experimentally probe the
dimensional crossover from two to one dimension using strongly interacting
ultracold bosons in variable lattice potentials and compare the data to
ab-initio theory that takes into account non-homogeneous trapping and non-zero
temperature. From a precise measurement of the momentum distribution we analyze
the characteristic decay of the one-body correlation function in the two
dimensionalities and then track how the decay is modified in the crossover. A
varying two-slope structure is revealed, reflecting the fact that the particles
see their dimensionality as being one or two depending on whether they are
probed on short or long distances, respectively. Our observations demonstrate
how quantum properties in the strongly-correlated regime evolve in the
dimensional crossover as a result of the interplay between dimensionality,
interactions, and temperature. | cond-mat_quant-gas |
Observability of Quantum Criticality and a Continuous Supersolid in
Atomic Gases: We analyze the Bose-Hubbard model with a three-body hardcore constraint by
mapping the system to a theory of two coupled bosonic degrees of freedom. We
find striking features that could be observable in experiments, including a
quantum Ising critical point on the transition from atomic to dimer
superfluidity at unit filling, and a continuous supersolid phase for strongly
bound dimers. | cond-mat_quant-gas |
Ultracold molecules for quantum simulation: rotational coherences in CaF
and RbCs: We explore the uses of ultracold molecules as a platform for future
experiments in the field of quantum simulation, focusing on two molecular
species, $^{40}$Ca$^{19}$F and $^{87}$Rb$^{133}$Cs. We report the development
of coherent quantum state control using microwave fields in both molecular
species; this is a crucial ingredient for many quantum simulation applications.
We demonstrate proof-of-principle Ramsey interferometry measurements with
fringe spacings of $\sim 1~\rm kHz$ and investigate the dephasing time of a
superposition of $N=0$ and $N=1$ rotational states when the molecules are
confined. For both molecules, we show that a judicious choice of molecular
hyperfine states minimises the impact of spatially varying transition-frequency
shifts across the trap. For magnetically trapped $^{40}$Ca$^{19}$F we use a
magnetically insensitive transition and observe a coherence time of 0.61(3) ms.
For optically trapped $^{87}$Rb$^{133}$Cs we exploit an avoided crossing in the
AC Stark shift and observe a maximum coherence time of 0.75(6) ms. | cond-mat_quant-gas |
From non equilibrium quantum Brownian motion to impurity dynamics in 1D
quantum liquids: Impurity motion in one dimensional ultra cold quantum liquids confined in an
optical trap has attracted much interest recently. As a step towards its full
understanding, we construct a generating functional from which we derive the
position non equilibrium correlation function of a quantum Brownian particle
with general Gaussian non-factorizing initial conditions. We investigate the
slow dynamics of a particle confined in a harmonic potential after a position
measurement; the rapid relaxation of a particle trapped in a harmonic potential
after a quantum quench realized as a sudden change in the potential parameters;
and the evolution of an impurity in contact with a one dimensional bosonic
quantum gas. We argue that such an impurity-Luttinger liquid system, that has
been recently realized experimentally, admits a simple modeling as quantum
Brownian motion in a super Ohmic bath. | cond-mat_quant-gas |
Response of fermions in Chern bands to spatially local quenches: We study the dynamical evolution of Chern-band systems after subjecting them
to local quenches. For open-boundary systems, we show for half-filling that the
chiral nature of edge states is manifested in the time-dependent chiral
response to local density quenches on the edge. In the presence of power-law
traps, we show how to mimic the half-filling situation by choosing the
appropriate number of fermions depending on the trap size, and explore chiral
responses of edges to local quenches in such a configuration. We find that
perturbations resulting from the quenches propagate at different group
velocities depending on the bulk band gap. Our results provide different routes
to check dynamically the non-trivial nature of Chern bands. | cond-mat_quant-gas |
Generating scalable entanglement of ultracold bosons in superlattices
through resonant shaking: Based on a one-dimensional double-well superlattice with a unit filling of
ultracold atoms per site, we propose a scheme to generate scalable entangled
states in the superlattice through resonant lattice shakings. Our scheme
utilizes periodic lattice modulations to entangle two atoms in each unit cell
with respect to their orbital degree of freedom, and the complete atomic system
in the superlattice becomes a cluster of bipartite entangled atom pairs. To
demonstrate this we perform $ab \ initio$ quantum dynamical simulations using
the Multi-Layer Multi-Configuration Time-Dependent Hartree Method for Bosons,
which accounts for all correlations among the atoms. The proposed clusters of
bipartite entanglements manifest as an essential resource for various quantum
applications, such as measurement based quantum computation. The lattice
shaking scheme to generate this cluster possesses advantages such as a high
scalability, fast processing speed, rich controllability on the target
entangled states, and accessibility with current experimental techniques. | cond-mat_quant-gas |
Topological inheritance in half-SSH Hubbard models: The interplay between interparticle interactions and topological features may
result in unusual phenomena. Interestingly, interactions may induce topological
features in an originally trivial system, as we illustrate for the case of a
one-dimensional two-component Hubbard model in which one component is subjected
to Su-Schrieffer-Heeger(SSH) dimerization, whereas the other one is not. We
show that due to inter-component interactions the topological properties of one
component are induced in the originally trivial one. Although for large
interactions topological inheritance may be readily explained by on-site
pairing, we show that the threshold for full inheritance occurs at weak
interactions, for which the components are not yet paired. We illustrate this
inheritance by discussing both bulk and edge properties, as well as dynamical
observables as mean chiral displacement and charge pumping. | cond-mat_quant-gas |
Quasi-1D atomic gases across wide and narrow
confinement-induced-resonances: We study quasi-one-dimensional atomic gases across wide and narrow
confinement-induced-resonances (CIR). We show from Virial expansion that by
tuning the magnetic field, the repulsive scattering branch initially prepared
at low fields can continuously go across CIR without decay; instead, the decay
occurs when approaching the non-interacting limit. The interaction properties
essentially rely on the resonance width of CIR. Universal thermodynamics holds
for scattering branch right at wide CIR, but is smeared out in narrow CIR due
to strong energy-dependence of coupling strength. In wide and narrow CIR, the
interaction energy of scattering branch shows different types of strong
asymmetry when approaching the decay from opposite sides of magnetic field.
Finally we discuss the stability of repulsive branch for a repulsively
interacting Fermi gas in different trapped geometries at low temperatures. | cond-mat_quant-gas |
The interaction-sensitive states of a trapped two-component ideal Fermi
gas and application to the virial expansion of the unitary Fermi gas: We consider a two-component ideal Fermi gas in an isotropic harmonic
potential. Some eigenstates have a wavefunction that vanishes when two
distinguishable fermions are at the same location, and would be unaffected by
s-wave contact interactions between the two components. We determine the other,
interaction-sensitive eigenstates, using a Faddeev ansatz. This problem is
nontrivial, due to degeneracies and to the existence of unphysical Faddeev
solutions. As an application we present a new conjecture for the fourth-order
cluster or virial coefficient of the unitary Fermi gas, in good agreement with
the numerical results of Blume and coworkers. | cond-mat_quant-gas |
Thermal and quantum noncondensate particles near the superfluid to Mott
insulator transition: We investigate the finite temperature momentum distribution of bosonic
noncondensate particles inside a 3D optical lattice near the superfluid to Mott
insulator transition point, treating the quantum fluctuation and thermal
fluctuation effects on equal footing. We explicitly address the different
momentum ($q$) dependence of quasi-particles excitations resulted from thermal
and quantum origin: the former scales as $|\bfq|^{-2}$ and hence is dominant in
the small momentum region, while the later scales as $|\bfq|^{-1}$ and hence
dominant in the large momentum limit. Analytic and semi-analytic results are
derived, providing a unique method to determine the temperature, condensate
density, coherent length and/or single particle gap etc. inside the optical
lattice. Our results also agree with the scaling theory of a quantum $XY$ model
near the transition point. Experimental implication of the TOF measurement is
also discussed. | cond-mat_quant-gas |
Bosonic orbital Su-Schrieffer-Heeger model in a lattice of rings: We study the topological properties of interacting and non-interacting bosons
loaded in the orbital angular momentum states $l=1$ in a lattice of rings with
alternating distances. At the single-particle level, the two circulation states
within each site lead to two decoupled Su-Schrieffer-Heeger lattices with
correlated topological phases. We characterize the topological configuration of
these lattices in terms of the alternating distances, as well as their
single-particle spectrum and topologically protected edge states. Secondly, we
add on-site interactions for the two-boson case, which lead to the appearance
of multiple bound states and edge bound states. We investigate the doublon
bands in terms of a strong-link model and we analyze the resulting subspaces
using perturbation theory in the limit of strong interactions. All analytical
results are benchmarked against exact diagonalization simulations. | cond-mat_quant-gas |
Spin-orbit coupled fermions in ladder-like optical lattices at
half-filling: We study the ground-state phase diagram of two-component fermions loaded in a
ladder-like lattice at half filling in the presence of spin-orbit coupling. For
repulsive fermions with unidirectional spin-orbit coupling along the legs we
identify a N\'{e}el state which is separated from rung-singlet and
ferromagnetic states by Ising phase transition lines. These lines cross for
maximal spin-orbit coupling and a direct Gaussian phase transition between
rung-singlet and ferro phases is realized. For the case of Rashba-like
spin-orbit coupling, besides the rung singlet phases two distinct striped
ferromagnetic phases are formed. In case of attractive fermions with spin-orbit
coupling at half-filling for decoupled chains we identify a dimerized state
that separates a singlet superconductor and a ferromagnetic states. | cond-mat_quant-gas |
Boosting the Rotational Sensitivity of Matter-wave Interferometry with
Nonlinearity: We propose a mechanism to use nonlinearity arising from inter-particle
interactions to significantly enhance rotation sensitivity of matter-wave
interferometers. The method relies on modifying Sagnac interferometers by
introducing a weak circular lattice potential that couples modes with opposite
orbital angular momenta (OAM). The primary observable comprises of the modal
population distributions measured at particular times. This provides an
alternate mechanism for rotation sensing that requires substantially smaller
ring size, even in the linear non-interacting regime. Nonlinearity can improve
the sensitivity, as well as operation timescales, by several orders of
magnitude. | cond-mat_quant-gas |
Efficient production of a narrow-line erbium magneto-optical trap with
two-stage slowing: We describe an experimental setup for producing a large cold erbium (Er)
sample in a narrow-line magneto-optical trap (MOT) in a simple and efficient
way. We implement a pair of angled slowing beams with respect to the Zeeman
slower axis, and further slow down atoms exiting from the Zeeman slower. The
second-stage slowing beams enable the narrow-line MOT to trap atoms exiting
from the Zeeman slower with higher velocity. This scheme is particularly useful
when the Zeeman slower is at low optical power without the conventional
transverse cooling between an oven and a Zeeman slower, in which case we
significantly improve the loading efficiency into the MOT and are able to trap
more than $10^8$ atoms in the narrow-line MOT of $^{166}$Er. This work
highlights our implementation, which greatly simplifies laser cooling and
trapping of Er atoms and also should benefit other similar elements. | cond-mat_quant-gas |
The pinning quantum phase transition in a Tonks Girardeau gas:
diagnostics by ground state fidelity and the Loschmidt echo: We study the pinning quantum phase transition in a Tonks-Girardeau gas, both
in equilibrium and out-of-equilibrium, using the ground state fidelity and the
Loschmidt echo as diagnostic tools. The ground state fidelity (GSF) will have a
dramatic decrease when the atomic density approaches the commensurate density
of one particle per lattice well. This decrease is a signature of the pinning
transition from the Tonks to the Mott insulating phase. We study the
applicability of the fidelity for diagnosing the pinning transition in
experimentally realistic scenarios. Our results are in excellent agreement with
recent experimental work. In addition, we explore the out of equilibrium
dynamics of the gas following a sudden quench with a lattice potential. We find
all properties of the ground state fidelity are reflected in the Loschmidt echo
dynamics i.e., in the non equilibrium dynamics of the Tonks-Girardeau gas
initiated by a sudden quench of the lattice potential. | cond-mat_quant-gas |
Fermionic transport in a homogeneous Hubbard model: Out-of-equilibrium
dynamics with ultracold atoms: Transport properties are among the defining characteristics of many important
phases in condensed matter physics. In the presence of strong correlations they
are difficult to predict even for model systems like the Hubbard model. In real
materials they are in general obscured by additional complications including
impurities, lattice defects or multi-band effects. Ultracold atoms in contrast
offer the possibility to study transport and out-of-equilibrium phenomena in a
clean and well-controlled environment and can therefore act as a quantum
simulator for condensed matter systems. Here we studied the expansion of an
initially confined fermionic quantum gas in the lowest band of a homogeneous
optical lattice. While we observe ballistic transport for non-interacting
atoms, even small interactions render the expansion almost bimodal with a
dramatically reduced expansion velocity. The dynamics is independent of the
sign of the interaction, revealing a novel, dynamic symmetry of the Hubbard
model. | cond-mat_quant-gas |
Universal Superfluid Transition and Transport Properties of
Two-Dimensional Dirty Bosons: We study the phase diagram of two-dimensional, interacting bosons in the
presence of a correlated disorder in continuous space, using large-scale finite
temperature quantum Monte Carlo simulations. We show that the superfluid
transition is strongly protected against disorder. It remains of the
Berezinskii-Kosterlitz-Thouless type up to disorder strengths comparable to the
chemical potential. Moreover, we study the transport properties in the strong
disorder regime where a zero-temperature Bose-glass phase is expected. We show
that the conductance exhibits a thermally activated behavior vanishing only at
zero temperature. Our results point towards the existence of Bose bad-metal
phase as a precursor of the Bose-glass phase. | cond-mat_quant-gas |
Quasi-long-range order in trapped two-dimensional Bose gases: We study the fate of algebraic decay of correlations in a harmonically
trapped two-dimensional degenerate Bose gas. The analysis is inspired by recent
experiments on ultracold atoms where power-law correlations have been observed
despite the presence of the external potential. We generalize the spin wave
description of phase fluctuations to the trapped case and obtain an analytical
expression for the one-body density matrix within this approximation. We show
that algebraic decay of the central correlation function persists to lengths of
about 20% of the Thomas--Fermi radius. We establish that the trap-averaged
correlation function decays algebraically with a strictly larger exponent
weakly changing with trap size and find indications that the recently observed
enhanced scaling exponents receive significant contributions from the normal
component of the gas. We discuss radial and angular correlations and propose a
local correlation approximation which captures the correlations very well. Our
analysis goes beyond the usual local density approximation and the developed
summation techniques constitute a powerful tool to investigate correlations in
inhomogeneous systems. | cond-mat_quant-gas |
Glassy properties of the Bose-glass phase of a one-dimensional
disordered Bose fluid: We study a one-dimensional disordered Bose fluid using bosonization, the
replica method and a nonperturbative functional renormalization-group approach.
The Bose-glass phase is described by a fully attractive strong-disorder fixed
point characterized by a singular disorder correlator whose functional
dependence assumes a cuspy form that is related to the existence of metastable
states. At nonzero momentum scale, quantum tunneling between these metastable
states leads to a rounding of the nonanalyticity in a quantum boundary layer
that encodes the existence of rare superfluid regions responsible for the
$\omega^2$ behavior of the (dissipative) conductivity in the low-frequency
limit. These results can be understood within the "droplet" picture put forward
for the description of glassy (classical) systems. | cond-mat_quant-gas |
Dynamical properties of the unitary Fermi gas: collective modes and
shock waves: We discuss the unitary Fermi gas made of dilute and ultracold atoms with an
infinite s-wave inter-atomic scattering length. First we introduce an efficient
Thomas-Fermi-von Weizsacker density functional which describes accurately
various static properties of the unitary Fermi gas trapped by an external
potential. Then, the sound velocity and the collective frequencies of
oscillations in a harmonic trap are derived from extended superfluid
hydrodynamic equations which are the Euler-Lagrange equations of a
Thomas-Fermi-von Weizsacker action functional. Finally, we show that this
amazing Fermi gas supports supersonic and subsonic shock waves. | cond-mat_quant-gas |
Magnetism and pairing of two-dimensional trapped fermions: The emergence of local phases in a trapped two-component Fermi gas in an
optical lattice is studied using quantum Monte Carlo simulations. We treat
temperatures that are comparable or lower than those presently achievable in
experiments and large enough systems that both magnetic and paired phases can
be detected by inspection of the behavior of suitable short-range correlations.
We use the latter to suggest the interaction strength and temperature range at
which experimental observation of incipient magnetism and d-wave pairing are
more likely and evaluate the relation between entropy and temperature in
two-dimensional confined fermionic systems. | cond-mat_quant-gas |
Spin-Flipping Half Vortex in a Macroscopic Polariton Spinor Ring
Condensate: We report the observation of vorticity in a macroscopic Bose-Einstein
condensate of polaritons in a ring geometry. Because it is a spinor condensate,
the elementary excitations are "half vortices" in which there is a phase
rotation of $\pi$ in connection with a polarization vector rotation of $\pi$
around a closed path. This is clearly seen in the experimental observations of
the polarization rotation around the ring. In the ring geometry, a new type of
half vortex is allowed in which the handedness of the spin flips from one side
of the ring to the other, in addition to the rotation of the linear
polarization component; such a state is not allowed in a simply-connected
geometry. Theoretical calculation of the energy of this state shows that when
many-body interactions are taken into account, it is lower in energy than a
simple half vortex. The direction of circulation of the flow around the ring
fluctuates randomly between clockwise and counterclockwise from one shot to the
next; this corresponds to spontaneous breaking of time-reversal symmetry in the
system. These new, macroscopic polariton ring condensates allow for the
possibility of direct control of the vorticity of the condensate. | cond-mat_quant-gas |
Quantum phases of incommensurate optical lattices due to cavity
backaction: Ultracold bosonic atoms are confined by an optical lattice inside an optical
resonator and interact with a cavity mode, whose wave length is incommensurate
with the spatial periodicity of the confining potential. We predict that the
intracavity photon number can be significantly different from zero when the
atoms are driven by a transverse laser whose intensity exceeds a threshold
value and whose frequency is suitably detuned from the cavity and the atomic
transition frequency. In this parameter regime the atoms form clusters in which
they emit in phase into the cavity. The clusters are phase locked, thereby
maximizing the intracavity photon number. These predictions are based on a
Bose-Hubbard model, whose derivation is here reported in detail. The
Bose-Hubbard Hamiltonian has coefficients which are due to the cavity field and
depend on the atomic density at all lattice sites. The corresponding phase
diagram is evaluated using Quantum Monte Carlo simulations in one-dimension and
mean-field calculations in two dimensions. Where the intracavity photon number
is large, the ground state of the atomic gas lacks superfluidity and possesses
finite compressibility, typical of a Bose-glass. | cond-mat_quant-gas |
From weak to strong: constrained extrapolation of perturbation series
with applications to dilute Fermi systems: We develop a method that uses truncation-order-dependent re-expansions
constrained by generic strong-coupling information to extrapolate perturbation
series to the nonperturbative regime. The method is first benchmarked against a
zero-dimensional model field theory and then applied to the dilute Fermi gas in
one and three dimensions. Overall, our method significantly outperforms Pad\'e
and Borel extrapolations in these examples. The results for the ground-state
energy of the three-dimensional Fermi gas are robust with respect to changes of
the form of the re-expansion and compare well with quantum Monte Carlo
simulations throughout the BCS regime and beyond. | cond-mat_quant-gas |
Effects of spatially inhomogeneous atomic interactions on Bose-Einstein
condensates in optical lattices: An interplay of optical lattices and nonlinear impurities in controlling the
dynamics of Bose-Einstein condensate bright solitons is investigated using
effective potential approach. The ability of pushing the solitons into or away
from the impurity region by changing both lattice and impurity parameters is
suggested. A possibility for the existence of stable fundamental gap solitons,
which appear to satisfy an inverted Vakhitov-Kolokolov criterion, is examined. | cond-mat_quant-gas |
Ferromagnetism in tilted fermionic Mott insulators: We investigate the magnetism in tilted fermionic Mott insulators. With a
small tilt, the fermions are still localized and form a Mott-insulating state,
where the localized spins interact via antiferromagnetic exchange coupling.
While the localized state is naively expected to be broken with a large tilt,
in fact, the fermions are still localized under a large tilt due to the
Wannier-Stark localization and it can be regarded as a localized spin system.
We find that the sign of the exchange coupling is changed and the ferromagnetic
interaction is realized under the large tilt. To show this, we employ the
perturbation theory and the real-time numerical simulation with the fermionic
Hubbard chain. Our simulation exhibits that it is possible to effectively
control the speed and time direction of the dynamics by changing the size of
tilt, which may be useful for experimentally measuring the out-of-time ordered
correlators. Finally, we address the experimental platforms, such as ultracold
atoms in an optical lattice, to observe these phenomena. | cond-mat_quant-gas |
Collective fermion excitation in a warm massless Dirac system: Basing on a self-consistent method, we predict theoretically that there
occurs not only a normal (quasi) fermion mode, but also a collective fermion
mode, plasmino, in a warm 2D massless Dirac system, especially in a warm
intrinsic graphene system. Results of Landau damping show that both fermion and
plasmino are well defined modes. We find that there are sharp differences
between the discussed system and the QCD/QED system. Firstly, the thermal mass
is proportional to $\alpha_g^{3/4}T$ but not $\alpha_g T$. Secondly, at
$0<q<q_c$, the fermion channel and plasmino channel are nearly degenerate and
furthermore, the energy difference between fermion and plasmino becomes more
and more larger with increasing $q$ at the region $q>q_c$. Thirdly, the fermion
behaves as a "relativity particles" with none zero mass and the plasmino
exhibits an anormal dispersion at moderate momentum. | cond-mat_quant-gas |
Realizing a scalable building block of a U(1) gauge theory with cold
atomic mixtures: In the fundamental laws of physics, gauge fields mediate the interaction
between charged particles. An example is quantum electrodynamics -- the theory
of electrons interacting with the electromagnetic field -- based on U(1) gauge
symmetry. Solving such gauge theories is in general a hard problem for
classical computational techniques. While quantum computers suggest a way
forward, it is difficult to build large-scale digital quantum devices required
for complex simulations. Here, we propose a fully scalable analog quantum
simulator of a U(1) gauge theory in one spatial dimension. To engineer the
local gauge symmetry, we employ inter-species spin-changing collisions in an
atomic mixture. We demonstrate the experimental realization of the elementary
building block as a key step towards a platform for large-scale quantum
simulations of continuous gauge theories. | cond-mat_quant-gas |
Phase diagram and optimal control for n-tupling discrete time crystal: A remarkable consequence of spontaneously breaking the time translational
symmetry in a system, is the emergence of time crystals. In periodically driven
systems, discrete time crystals (DTC) can be realized which have a periodicity
that is n times the driving period. However, all of the experimental
observations have been performed for period-doubling and period-tripling
discrete time crystals. Novel physics can arise by simulating many-body physics
in the time domain, which would require a genuine realisation of the n-tupling
DTC. A system of ultra-cold bosonic atoms bouncing resonantly on an oscillating
mirror is one of the models that can realise large period DTC. The preparation
of DTC demands control in creating the initial distribution of the ultra-cold
bosonic atoms along with the mirror frequency. In this work, we demonstrate
that such DTC is robust against perturbations to the initial distribution of
atoms. We show how Bayesian methods can be used to enhance control in the
preparation of the initial state as well as to efficiently calculate the phase
diagram for such a model. Moreover, we examine the stability of DTCs by
analyzing quantum many-body fluctuations and show that they do not reveal
signatures of heating. | cond-mat_quant-gas |
Finite-rate quenches of site bias in the Bose-Hubbard dimer: For a Bose-Hubbard dimer, we study quenches of the site energy imbalance,
taking a highly asymmetric Hamiltonian to a fully symmetric one. The ramp is
carried out over a finite time that interpolates between the instantaneous and
adiabatic limits. We provide results for the excess energy of the final state
compared to the ground state energy of the final Hamiltonian, as a function of
the quench rate. This excess energy serves as the analog of the defect density
that is considered in the Kibble-Zurek picture of ramps across phase
transitions. We also examine the fate of quantum `self-trapping' when the ramp
is not instantaneous. | cond-mat_quant-gas |
Competing Supersolid and Haldane Insulator phases in the extended
one-dimensional bosonic Hubbard model: The Haldane Insulator is a gapped phase characterized by an exotic non-local
order parameter. The parameter regimes at which it might exist, and how it
competes with alternate types of order, such as supersolid order, are still
incompletely understood. Using the Stochastic Green Function (SGF) quantum
Monte Carlo (QMC) and the Density Matrix Renormalization Group (DMRG), we study
numerically the ground state phase diagram of the one-dimensional bosonic
Hubbard model (BHM) with contact and near neighbor repulsive interactions. We
show that, depending on the ratio of the near neighbor to contact interactions,
this model exhibits charge density waves (CDW), superfluid (SF), supersolid
(SS) and the recently identified Haldane insulating (HI) phases. We show that
the HI exists only at the tip of the unit filling CDW lobe and that there is a
stable SS phase over a very wide range of parameters. | cond-mat_quant-gas |
Experimental realization of a long-range antiferromagnet in the Hubbard
model with ultracold atoms: Many exotic phenomena in strongly correlated electron systems emerge from the
interplay between spin and motional degrees of freedom. For example, doping an
antiferromagnet gives rise to interesting phases including pseudogap states and
high-temperature superconductors. A promising route towards achieving a
complete understanding of these materials begins with analytic and
computational analysis of simplified models. Quantum simulation has recently
emerged as a complementary approach towards understanding these models.
Ultracold fermions in optical lattices offer the potential to answer open
questions on the low-temperature regime of the doped Hubbard model, which is
thought to capture essential aspects of the cuprate superconductor phase
diagram but is numerically intractable in that parameter regime. A new
perspective is afforded by quantum gas microscopy of fermions, which allows
readout of magnetic correlations at the site-resolved level. Here we report the
realization of an antiferromagnet in a repulsively interacting Fermi gas on a
2D square lattice of approximately 80 sites. Using site-resolved imaging, we
detect (finite-size) antiferromagnetic long-range order (LRO) through the
development of a peak in the spin structure factor and the divergence of the
correlation length that reaches the size of the system. At our lowest
temperature of T/t = 0.25(2) we find strong order across the entire sample. Our
experimental platform enables doping away from half filling, where pseudogap
states and stripe ordering are expected, but theoretical methods become
numerically intractable. In this regime we find that the antiferromagnetic LRO
persists to hole dopings of about 15%, providing a guideline for computational
methods. Our results demonstrate that quantum gas microscopy of ultracold
fermions in optical lattices can now address open questions on the
low-temperature Hubbard model. | cond-mat_quant-gas |
Microscopic Approach to Shear Viscosities in Superfluid Gases: From BCS
to BEC: We compute the shear viscosity, $\eta$, at general temperatures $T$, in a
BCS-BEC crossover scheme which is demonstrably consistent with conservation
laws. The study of $\eta$ is important because it constrains microscopic
theories by revealing the excitation spectra. The onset of a normal state
pairing gap and the contribution from pair degrees of freedom imply that $\eta$
at low $T$ becomes small, rather than exhibiting the upturn predicted by most
others. Using the local density approximation, we find quite reasonable
agreement with just-published experiments. | cond-mat_quant-gas |
Quasi-particle Lifetime in a Mixture of Bose and Fermi Superfluids: In this letter, to reveal the effect of quasi-particle interactions in a
Bose-Fermi superfluid mixture, we consider the lifetime of quasi-particle of
Bose superfluid due to its interaction with quasi-particles in Fermi
superfluid. We find that this damping rate, i.e. inverse of the lifetime, has
quite different threshold behavior at the BCS and the BEC side of the Fermi
superfluid. The damping rate is a constant nearby the threshold momentum in the
BCS side, while it increases rapidly in the BEC side. This is because in the
BCS side the decay processe is restricted by constant density-of-state of
fermion quasi-particle nearby Fermi surface, while such a restriction does not
exist in the BEC side where the damping process is dominated by bosonic
quasi-particles of Fermi superfluid. Our results are related to collective mode
experiment in recently realized Bose-Fermi superfluid mixture. | cond-mat_quant-gas |
Direct observation of chiral currents and magnetic reflection in atomic
flux lattices: The prospect of studying topological matter with the precision and control of
atomic physics has driven the development of many techniques for engineering
artificial magnetic fields and spin-orbit interactions. Recently, the idea of
introducing nontrivial topology through the use of internal (or external)
atomic states as effective "synthetic dimensions" has garnered attraction for
its versatility and possible immunity from heating. Here, we directly engineer
tunable artificial gauge fields through the local control of tunneling phases
in an effectively two-dimensional manifold of discrete atomic momentum states.
We demonstrate the ability to create homogeneous gauge fields of arbitrary
value, directly imaging the site-resolved dynamics of induced chiral currents.
We furthermore engineer the first inhomogeneous artificial gauge fields for
cold atoms, observing the magnetic reflection of atoms incident upon a
step-like variation of an artificial vector potential. These results open up
new possibilities for the study of topological phases and localization
phenomena in atomic gases. | cond-mat_quant-gas |
Properties of dipolar bosonic quantum gases at finite temperatures: The properties of ultracold quantum gases of bosons with dipole-dipole
interaction is investigated at finite temperature in the frame of the
representative ensembles theory. Self-consistent coupled equations of motion
are derived for the condensate and the non-condensate components. Corrections
due to the dipolar interaction to the condensate depletion, the anomalous
density and thermodynamic quantities such as the ground state energy, the
equation of state, the compressibility and the presure are calculated in the
homogeneous case at both zero and finite temperatures. Effects of interaction
and temperature on the structure factor are also discussed. Within the realm of
the local density approximation, we generalize our results to the case of a
trapped dipolar gas. | cond-mat_quant-gas |
Diversified vortex phase diagram for a rotating trapped two-band Fermi
gas in the BCS-BEC crossover: We report the equilibrium vortex phase diagram of a rotating two-band Fermi
gas confined to a cylindrically symmetric parabolic trapping potential, using
the recently developed finite-temperature effective field theory [Phys. Rev. A
$\bf{94}$, 023620 (2016)]. A non-monotonic resonant dependence of the free
energy as a function of the temperature and the rotation frequency is revealed
for a two-band superfluid. We particularly focus on novel features that appear
as a result of interband interactions and can be experimentally resolved. The
resonant dependence of the free energy is directly manifested in vortex phase
diagrams, where areas of stability for both integer and fractional vortex
states are found. The study embraces the BCS-BEC crossover regime and the
entire temperature range below the critical temperature $T_{c}$. Significantly
different behavior of vortex matter as a function of the interband coupling is
revealed in the BCS and BEC regimes. | cond-mat_quant-gas |
Rigorous mean-field dynamics of lattice bosons: Quenches from the Mott
insulator: We provide a rigorous derivation of Gutzwiller mean-field dynamics for
lattice bosons, showing that it is exact on fully connected lattices. We apply
this formalism to quenches in the interaction parameter from the Mott insulator
to the superfluid state. Although within mean-field the Mott insulator is a
steady state, we show that a dynamical critical interaction $U_d$ exists, such
that for final interaction parameter $U_f>U_d$ the Mott insulator is
exponentially unstable towards emerging long-range superfluid order, whereas
for $U_f<U_d$ the Mott insulating state is stable. We discuss the implications
of this prediction for finite-dimensional systems. | cond-mat_quant-gas |
Interorbital Interactions in an SU(2)xSU(6)-Symmetric Fermi-Fermi
Mixture: We characterize inter- and intraisotope interorbital interactions between
atoms in the 1S0 ground state and the 3P0 metastable state in interacting
Fermi-Fermi mixtures of 171Yb and 173Yb. We perform high-precision clock
spectroscopy to measure interaction-induced energy shifts in a deep 3D optical
lattice and determine the corresponding scattering lengths. We find the elastic
interaction of the interisotope mixtures 173Yb_e-171Yb_g and 173Yb_g-171Yb_e to
be weakly attractive and very similar, while the corresponding two-body loss
coefficients differ by more than two orders of magnitude. By comparing
different spin mixtures we experimentally demonstrate the SU(2)xSU(6) symmetry
of all elastic and inelastic interactions. Furthermore, we measure the
spin-exchange interaction in 171Yb and confirm its previously observed
antiferromagnetic nature. | cond-mat_quant-gas |
Controllable high-speed polariton waves in a PT-symmetric lattice: Parity-time (PT) symmetry gives rise to unusual phenomena in many physical
systems, presently attracting a lot of attention. One essential and non-trivial
task is the fabrication and design of the PT-symmetric lattices in different
systems. Here we introduce a method to realize such a lattice in an
exciton-polariton condensate in a planar semiconductor microcavity. We
theoretically demonstrate that in the regime, where lattice profile is nearly
PT-symmetric, a polariton wave can propagate at very high velocity resulting
from the beating of a ground state condensate created in the lowest energy band
at very small momentum and a condensate simultaneously created in higher energy
states with large momentum. The spontaneous excitation of these two states in
the nonlinear regime due to competition between multiple eigenmodes becomes
possible since the spectrum of nearly PT-symmetric structure reveals
practically identical amplification for Bloch waves from the entire Brillouin
zone. There exists a wide velocity range for the resulting polariton wave. This
velocity can be controlled by an additional coherent pulse carrying a specific
momentum. We also discuss the breakup of the PT-symmetry when the polariton
lifetime exceeds a certain threshold value. | cond-mat_quant-gas |
Mean field study of 2D quasiparticle condensate formation in presence of
strong decay: Bose-condensation in a system of 2D quasiparticles is considered in the scope
of a microscopic model. Mean-field dynamical equations are derived with the
help of the Schwinger-Keldysh formalism and a simple model is proposed which
allows to describe key features of condensate formation in systems with various
quasiparticle decay rates. By analysing stationary solutions of this equation,
we obtain the phase diagram of quasiparticle gas, finding a bistability region
in the parameter space of the system. Finally, as an application of our theory,
we study the phase diagram of a 2D exciton-polariton system in CdTe
microcavity. | cond-mat_quant-gas |
Finite-Temperature Auxiliary-Field Quantum Monte Carlo for Bose-Fermi
Mixtures: We present a quantum Monte Carlo (QMC) technique for calculating the exact
finite-temperature properties of Bose-Fermi mixtures. The Bose-Fermi
Auxiliary-Field Quantum Monte Carlo (BF-AFQMC) algorithm combines two methods,
a finite-temperature AFQMC algorithm for bosons and a variant of the standard
AFQMC algorithm for fermions, into one algorithm for mixtures. We demonstrate
the accuracy of our method by comparing its results for the Bose-Hubbard and
Bose-Fermi-Hubbard models against those produced using exact diagonalization
for small systems. Comparisons are also made with mean-field theory and the
worm algorithm for larger systems. As is the case with most fermion
Hamiltonians, a sign or phase problem is present in BF-AFQMC. We discuss the
nature of these problems in this framework and describe how they can be
controlled with well-studied approximations to expand BF-AFQMC's reach. The new
algorithm can serve as an essential tool for answering many unresolved
questions about many-body physics in mixed Bose-Fermi systems. | cond-mat_quant-gas |
Quantum simulation of the Hubbard model with ultracold fermions in
optical lattices: Ultracold atomic gases provide a fantastic platform to implement quantum
simulators and investigate a variety of models initially introduced in
condensed matter physics or other areas. One of the most promising applications
of quantum simulation is the study of strongly-correlated Fermi gases, for
which exact theoretical results are not always possible with state-of-the-art
approaches. Here, we review recent progress of the quantum simulation of the
emblematic Fermi-Hubbard model with ultracold atoms. After introducing the
Fermi-Hubbard model in the context of condensed matter, its implementation in
ultracold atom systems, and its phase diagram, we review landmark experimental
achievements, from the early observation of the onset of quantum degeneracy and
superfluidity to demonstration of the Mott insulator regime and the emergence
of long-range anti-ferromagnetic order. We conclude by discussing future
challenges, including the possible observation of high-Tc superconductivity,
transport properties, and the interplay of strong correlations and disorder or
topology. | cond-mat_quant-gas |
Collectively pair-driven-dissipative bosonic arrays: exotic and
self-oscillatory condensates: Modern quantum platforms such as superconducting circuits provide exciting
opportunities for the experimental exploration of driven-dissipative many-body
systems in unconventional regimes. One of such regimes occurs in bosonic
systems, where nowadays one can induce driving and dissipation through pairs of
excitations, rather than the conventional single-excitation processes.
Moreover, modern platforms can be driven in a way in which the modes of the
bosonic array decay collectively rather than locally, such that the pairs of
excitations recorded by the environment come from a coherent superposition of
all sites. In this work we analyze the superfluid phases accessible to bosonic
arrays subject to these novel mechanisms more characteristic of quantum optics,
which we prove to lead to remarkable spatiotemporal properties beyond the
traditional scope of pattern formation in condensed-matter systems or nonlinear
optics alone. We show that, even in the presence of residual local loss, the
system is stabilized into an exotic state with bosons condensed along the modes
of a closed manifold in Fourier space, with a distribution of the population
among these Fourier modes that can be controlled via a weak bias (linear)
drive. This gives access to a plethora of different patterns, ranging from
periodic and quasi-periodic ones with tunable spatial wavelength, to
homogeneously-populated closed-Fourier-manifold condensates that are thought to
play an important role in some open problems of condensed-matter physics.
Moreover, we show that when any residual local linear dissipation is balanced
with pumping, new constants of motion emerge that can force the superfluid to
oscillate in time, similarly to the mechanism behind the recently discovered
superfluid time crystals. We propose specific experimental implementations with
which this rich and unusual spatiotemporal superfluid behavior can be explored. | cond-mat_quant-gas |
Cavity-Controlled Collective Scattering at the Recoil Limit: We study collective scattering with Bose-Einstein condensates interacting
with a high-finesse ring cavity. The condensate scatters the light of a
transverse pump beam superradiantly into modes which, in contrast to previous
experiments, are not determined by the geometrical shape of the condensate, but
specified by a resonant cavity mode. Moreover, since the recoil-shifted
frequency of the scattered light depends on the initial momentum of the
scattered fraction of the condensate, we show that it is possible to employ the
good resolution of the cavity as a filter selecting particular quantized
momentum states. | cond-mat_quant-gas |
Goos-Hänchen shifts in spin-orbit-coupled cold atoms: We consider a matter wave packet of cold atom gas impinging upon a step
potential created by the optical light field. In the presence of spin-orbit
(SO) coupling, the atomic eigenstates contain two types of evanescent states,
one of which is the ordinary evanescent state with pure imaginary wave vector
while the other possesses complex wave vector and is recognized as oscillating
evanescent state. We show that the presence and interplay of these two types of
evanescent states can give rise to two different mechanisms for total internal
reflection (TIR), and thus lead to unusual Goos-H\"anchen (GH) effect. As a
result, not only large positive but also large negative GH shift can be
observed in the reflected atomic beam. The dependence of the GH shift on the
incident angle, energy and height of the step potential is studied numerically. | cond-mat_quant-gas |
Roton excitations in a trapped dipolar Bose-Einstein condensate: We consider the quasi-particle excitations of a trapped dipolar Bose-Einstein
condensate. By mapping these excitations onto radial and angular momentum we
show that the roton modes are clearly revealed as discrete fingers in parameter
space, whereas the other modes form a smooth surface. We examine the properties
of the roton modes and characterize how they change with the dipole interaction
strength. We demonstrate how the application of a perturbing potential can be
used to engineer angular rotons, i.e. allowing us to controllably select modes
of non-zero angular momentum to become the lowest energy rotons. | cond-mat_quant-gas |
Antiferromagnetic behavior in self-bound one-dimensional composite
bosons: The structure of self-bound one-dimensional droplets containing a mixture of
Ytterbium fermionic isotopes ($^{173}$Yb, $^{171}$Yb) is calculated by means of
a diffusion Monte Carlo technique. We considered only balanced setups in which
all the atoms of one isotope are spin-polarized, while the atoms of the other
can have up to three different spin values, that difference being a necessary
requirement to achieve stable systems. Our results indicate that these droplets
consist of consecutive "molecules" made up of one $^{173}$Yb and one $^{171}$Yb
atom. In other words, we have up to three different kinds of composite bosons,
corresponding to the number of spin components in the non-polarized isotope.
The fermionic nature of those Yb atoms makes pairs with identical spin
composition avoid each other, creating a Pauli-like-hole filed by another
molecule in which at least one of the Yb atoms has a different spin from that
of their closest neighbors. This effective repulsion is akin to an
antiferromagnetic short-range interaction between different kinds of composite
bosons. | cond-mat_quant-gas |
Fast phase-modulated optical lattice for wave packet engineering: We investigate experimentally a Bose Einstein condensate placed in a 1D
optical lattice whose phase is modulated at a frequency large compared to all
characteristic frequencies. As a result, the depth of the periodic potential is
renormalized by a Bessel function which only depends on the amplitude of
modulation, a prediction that we have checked quantitatively using a careful
calibration scheme. This renormalization provides an interesting tool to
engineer in time optical lattices. For instance, we have used it to perform
simultaneously a sudden $\pi$-phase shift (without phase residual errors)
combined with a change of lattice depth, and to study the subsequent
out-of-equilibrium dynamics. | cond-mat_quant-gas |
Stoner-Wohlfarth switching of the condensate magnetization in a dipolar
spinor gas and the metrology of excitation damping: We consider quasi-one-dimensional dipolar spinor Bose-Einstein condensates in
the homogeneous-local-spin-orientation approximation, that is with
unidirectional local magnetization. By analytically calculating the exact
effective dipole-dipole interaction, we derive a Landau-Lifshitz-Gilbert
equation for the dissipative condensate magnetization dynamics, and show how it
leads to the Stoner-Wohlfarth model of a uni-axial ferro-magnetic particle,
where the latter model determines the stable magnetization patterns and
hysteresis curves for switching between them. For an external magnetic field
pointing along the axial, long direction, we analytically solve the
Landau-Lifshitz-Gilbert equation. The solution explicitly demonstrates that the
magnetic dipole-dipole interaction {\it accelerates} the dissipative dynamics
of the magnetic moment distribution and the associated dephasing of the
magnetic moment direction. Under suitable conditions, dephasing of the
magnetization direction due to dipole-dipole interactions occurs within time
scales up to two orders of magnitude smaller than the lifetime of currently
experimentally realized dipolar spinor condensates, e.g., produced with the
large magnetic-dipole-moment atoms ${}^{166} \textrm{Er}$. This enables
experimental access to the dissipation parameter $\Gamma$ in the
Gross-Pitaevski\v\i~mean-field equation, for a system currently lacking a
complete quantum kinetic treatment of dissipative processes and, in particular,
an experimental check of the commonly used assumption that $\Gamma$ is a single
scalar independent of spin indices. | cond-mat_quant-gas |
A cavity-induced artificial gauge field in a Bose-Hubbard ladder: We consider theoretically ultracold interacting bosonic atoms confined to
quasi-one-dimensional ladder structures formed by optical lattices and coupled
to the field of an optical cavity. The atoms can collect a spatial phase
imprint during a cavity-assisted tunneling along a rung via Raman transitions
employing a cavity mode and a transverse running wave pump beam. By adiabatic
elimination of the cavity field we obtain an effective Hamiltonian for the
bosonic atoms, with a self-consistency condition. Using the numerical density
matrix renormalization group method, we obtain a rich steady state diagram of
self-organized steady states. Transitions between superfluid to Mott-insulating
states occur, on top of which we can have Meissner, vortex liquid, and vortex
lattice phases. Also a state that explicitly breaks the symmetry between the
two legs of the ladder, namely the biased-ladder phase is dynamically
stabilized. | cond-mat_quant-gas |
Quantum magnetism and topological ordering via enhanced Rydberg-dressing
near Förster-resonances: We devise a cold-atom approach to realizing a broad range of bi-linear
quantum magnets. Our scheme is based on off-resonant single-photon excitation
of Rydberg $P$-states (Rydberg-dressing), whose strong interactions are shown
to yield controllable XYZ-interactions between effective spins, represented by
different atomic ground states. The distinctive features of F\"orster-resonant
Rydberg atom interactions are exploited to enhance the effectiveness of
Rydberg-dressing and, thereby, yield large spin-interactions that greatly
exceed corresponding decoherence rates. We illustrate the concept on a spin-1
chain implemented with cold Rubidium atoms, and demonstrate that this permits
the dynamical preparation of topological magnetic phases. Generally, the
described approach provides a viable route to exploring quantum magnetism with
dynamically tuneable (an)isotropic interactions as well as variable space- and
spin-dimensions in cold-atom experiments. | cond-mat_quant-gas |
Universal nonanalytic behavior of the Hall conductance in a Chern
insulator at the topologically driven nonequilibrium phase transition: We study the Hall conductance of a Chern insulator after a global quench of
the Hamiltonian. The Hall conductance in the long time limit is obtained by
applying the linear response theory to the diagonal ensemble. It is expressed
as the integral of the Berry curvature weighted by the occupation number over
the Brillouin zone. We identify a topologically driven nonequilibrium phase
transition, which is indicated by the nonanalyticity of the Hall conductance as
a function of the energy gap m_f in the post-quench Hamiltonian H_f. The
topological invariant for the quenched state is the winding number of the
Green's function W, which equals the Chern number for the ground state of H_f.
In the limit that m_f goes to zero, the derivative of the Hall conductance with
respect to m_f is proportional to ln(|m_f|), with the constant of
proportionality being the ratio of the change of W at m_f = 0 to the energy gap
in the initial state. This nonanalytic behavior is universal in two-band Chern
insulators such as the Dirac model, the Haldane model, or the Kitaev honeycomb
model in the fermionic basis. | cond-mat_quant-gas |
Quantum fluctuations in atomic Josephson junctions: the role of
dimensionality: We investigate the role of quantum fluctuations in the dynamics of a bosonic
Josephson junction in $D$ spatial dimensions, by using beyond mean-field
Gaussian corrections. We derive some key dynamical properties in a systematic
way for $D=3, 2, 1$. In particular, we compute the Josephson frequency in the
regime of low population imbalance. We also obtain the critical strength of the
macroscopic quantum self-trapping. Our results show that quantum corrections
increase the Josephson frequency in spatial dimensions $D=2$ and $D=3$, but
they decrease it in the $D=1$ case. The critical strength of macroscopic
quantum self-trapping is instead reduced by quantum fluctuations in $D=2$ and
$D=3$ cases, while it is enhanced in the $D=1$ configuration. We show that the
difference between the cases of D = 2 and D = 3 on one side, and D = 1 on the
other, can be related to the qualitatively different dependence of the
interaction strength on the scattering length in the different dimensions. | cond-mat_quant-gas |
Shear viscosity of a universal Fermi gas near the superfluid phase
transition: We precisely measure the shear viscosity for a resonantly interacting Fermi
gas as a function of temperature, from nearly the ground state through the
superfluid phase transition at a critical temperature $T_c$. Using an iterative
method to invert the data, we extract the {\it local} shear viscosity
coefficient $\alpha_S(\theta)$ versus reduced temperature $\theta$, revealing
previously hidden features. We find that $\alpha_S$ begins to decrease rapidly
with decreasing $\theta$ well above $T_c$, suggesting that preformed pairs play
an important role. Further, we observe that the derivative $\alpha_S'(\theta)$
has a maximum at $T_c$. We compare the local data to several microscopic
theories. Finally, we determine the local ratio of the shear viscosity to the
entropy density. | cond-mat_quant-gas |
Numerical and variational solutions of the dipolar Gross-Pitaevskii
equation in reduced dimensions: We suggest a simple Gaussian Lagrangian variational scheme for the reduced
time-dependent quasi-one- and quasi-two-dimensional Gross-Pitaevskii (GP)
equations of a dipolar Bose-Einstein condensate (BEC) in cigar and disk
configurations, respectively. The variational approximation for stationary
states and breathing oscillation dynamics in reduced dimensions agrees well
with the numerical solution of the GP equation even for moderately large
short-range and dipolar nonlinearities. The Lagrangian variational scheme also
provides much physical insight about soliton formation in dipolar BEC. | cond-mat_quant-gas |
Matching universal behavior with potential models: Two-, three-, and four-boson systems are studied close to the unitary limit
using potential models constructed to reproduce the minimal information given
by the two-body scattering length $a$ and the two-body binding energy or
virtual state energy $E_2$. The particular path used to reach the unitary limit
is given by varying the potential strength. In this way the energy spectrum in
the three- and four-boson systems is computed. The lowest energy states show
finite-range effects absorbed in the construction of level functions that can
be used to study real systems. Higher energy levels are free from finite-range
effects, therefore the corresponding level functions tend to the zero-range
universal function. Using this property a zero-range equation for the
four-boson system is proposed and the four-boson universal function is
computed. | cond-mat_quant-gas |
Equatorial Waves in Rotating Bubble-Trapped Superfluids: As the Earth rotates, the Coriolis force causes several oceanic and
atmospheric waves to be trapped along the equator, including Kelvin, Yanai,
Rossby, and Poincar\'e modes. It has been demonstrated that the mathematical
origin of these waves is related to the nontrivial topology of the underlying
hydrodynamic equations. Inspired by recent observations of Bose-Einstein
condensation (BEC) in bubble-shaped traps in microgravity ultracold quantum gas
experiments, we show that equatorial modes are supported by a rapidly rotating
condensate in a spherical geometry. Based on a zero-temperature coarse-grained
hydrodynamic framework, we reformulate the coupled oscillations of the
superfluid and the Abrikosov vortex lattice resulting from rotation by a
Schr\"odinger-like eigenvalue problem. The obtained non-Hermitian Hamiltonian
is topologically nontrivial. Furthermore, we solve the hydrodynamic equations
for a spherical geometry and find that the rotating superfluid hosts Kelvin,
Yanai, and Poincar\'e equatorial modes, but not the Rossby mode. Our
predictions can be tested with state-of-the-art bubble-shaped trapped BEC
experiments. | cond-mat_quant-gas |
Three-body correlations in a two-dimensional SU(3) Fermi gas: We consider a three-component Fermi gas that has SU(3) symmetry and is
confined to two dimensions (2D). For realistic cold atomic gas experiments, we
show that the phase diagram of the quasi-2D system can be characterized using
two 2D scattering parameters: the scattering length and the effective range.
Unlike the case in 3D, we argue that three-body bound states (trimers) in the
quasi-2D system can be stable against three-body losses. Using a low-density
expansion coupled with a variational approach, we investigate the fate of such
trimers in the many-body system as the attractive interactions are decreased
(or, conversely, as the density of particles is increased). We find that
remnants of trimers can persist in the form of strong three-body correlations
in the weak-coupling (high-density) limit. | cond-mat_quant-gas |
Engineering infinite-range SU($n$) interactions with spin-orbit-coupled
fermions in an optical lattice: We study multilevel fermions in an optical lattice described by the Hubbard
model with on site SU($n$)-symmetric interactions. We show that in an
appropriate parameter regime this system can be mapped onto a spin model with
all-to-all SU($n$)-symmetric couplings. Raman pulses that address internal spin
states modify the atomic dispersion relation and induce spin-orbit coupling,
which can act as a synthetic inhomogeneous magnetic field that competes with
the SU($n$) exchange interactions. We investigate the mean-field dynamical
phase diagram of the resulting model as a function of $n$ and different initial
configurations that are accessible with Raman pulses. Consistent with previous
studies for $n=2$, we find that for some initial states the spin model exhibits
two distinct dynamical phases that obey simple scaling relations with $n$.
Moreover, for $n>2$ we find that dynamical behavior can be highly sensitive to
initial intra-spin coherences. Our predictions are readily testable in current
experiments with ultracold alkaline-earth(-like) atoms. | cond-mat_quant-gas |
Phase fluctuations in anisotropic Bose condensates: from cigars to rings: We study the phase-fluctuating condensate regime of ultra-cold atoms trapped
in a ring-shaped trap geometry, which has been realized in recent experiments.
We first consider a simplified box geometry, in which we identify the
conditions to create a state that is dominated by thermal phase-fluctuations,
and then explore the experimental ring geometry. In both cases we demonstrate
that the requirement for strong phase fluctuations can be expressed in terms of
the total number of atoms and the geometric length scales of the trap only. For
the ring-shaped trap we discuss the zero temperature limit in which a
condensate is realized where the phase is fluctuating due to interactions and
quantum fluctuations. We also address possible ways of detecting the phase
fluctuating regime in ring condensates. | cond-mat_quant-gas |
Geometrical quench and dynamical quantum phase transition in the
$α-T_3$ lattice: We investigate quantum quenches and the Loschmidt echo in the two
dimensional, three band $\alpha-T_3$ model, a close descendant of the dice
lattice. By adding a chemical potential to the central site, the integral of
the Berry curvature of the bands in different valleys is continously tunable by
the ratio of the hopping integrals between the sublattices. By investigating
one and two filled bands, we find that dynamical quantum phase transition
(DQPT), i.e. nonanalytical temporal behaviour in the rate function of the
return amplitude, occurs for a certain range of parameters, independent of the
band filling. By focusing on the effective low energy description of the model,
we find that DQPTs happen not only in the time derivative of the rate function,
which is a common feature in two dimensional models, but in the rate function
itself. This feature is not related to the change of topological properties of
the system during the quench, but rather follows from population inversion for
all momenta. This is accompanied by the appearance of dynamical vortices in the
time-momentum space of the Pancharatnam geometric phase. The positions of the
vortices form an infinite vortex ladder, i.e. a macroscopic phase structure,
which allows us to identify the dynamical phases that are separated by the
DQPT. | cond-mat_quant-gas |
Fermi-Fermi crossover in the ground state of 1D few-body systems with
anomalous three-body interactions: In one spatial dimension, quantum systems with an attractive three-body
contact interaction exhibit a scale anomaly. In this work, we examine the
few-body sector for up to six particles. We study those systems with a
self-consistent, non-perturbative, iterative method, in the subspace of zero
total momentum. Exploiting the structure of the contact interaction, the method
reduces the complexity of obtaining the wavefunction by three powers of the
dimension of the Hilbert space. We present results on the energy, and momentum
and spatial structure, as well as Tan's contact. We find a Fermi-Fermi
crossover interpolating between large, weakly bound trimers and compact, deeply
bound trimers: at weak coupling, the behavior is captured by degenerate
perturbation theory; at strong coupling, the system is governed by an effective
theory of heavy trimers (plus free particles in the case of asymmetric
systems). Additionally, we find that there is no trimer-trimer attraction and
therefore no six-body bound state. | cond-mat_quant-gas |
Vortex reconnections between coreless vortices in binary condensates: Vortex reconnections plays an important role in the turbulent flows
associated with the superfluids. To understand the dynamics, we examine the
reconnections of vortex rings in the superfluids of dilute atomic gases
confined in trapping potentials using Gross-Petaevskii equation. Furthermore we
study the reconnection dynamics of coreless vortex rings, where one of the
species can act as a tracer. | cond-mat_quant-gas |
Superfluid drag in the two-component Bose-Hubbard model: In multicomponent superfluids and superconductors, co- and counter-flows of
components have in general different properties. It was discussed in 1975 by
Andreev and Bashkin, in the context of He$^3$/He$^4$ superfluid mixtures, that
inter-particle interactions produce a dissipationless drag. The drag can be
understood as a superflow of one component induced by phase gradients of the
other component. Importantly the drag can be both positive (entrainment) and
negative (counter-flow). The effect is known to be of crucial importance for
many properties of diverse physical systems ranging from the dynamics of
neutron stars, rotational responses of Bose mixtures of ultra-cold atoms to
magnetic responses of multicomponent superconductors. Although there exists a
substantial literature that includes the drag interaction phenomenologically,
much fewer regimes are covered by quantitative studies of the microscopic
origin of the drag and its dependence on microscopic parameters. Here we study
the microscopic origin and strength of the drag interaction in a quantum system
of two-component bosons on a lattice with short-range interaction. By
performing quantum Monte-Carlo simulations of a two-component Bose-Hubbard
model we obtain dependencies of the drag strength on the boson-boson
interactions and properties of the optical lattice. Of particular interest are
the strongly-correlated regimes where the ratio of co-flow and counter-flow
superfluid stiffnesses can diverge, corresponding to the case of saturated
drag. | cond-mat_quant-gas |
Exact results on the two-particle Green's function of a Bose-Einstein
condensate: Starting from the Dyson-Beliaev and generalized Gross-Pitaevskii equations
with an extra nonlocal potential, we derive an exact expression of the
two-particle Green's function K for an interacting Bose-Einstein condensate in
terms of unambiguously defined self-energies and vertices. The formula can be a
convenient basis for approximate calculations of K. It also tells us that poles
of K are not shared with (i.e. shifted from) those of the single-particle
Green's function, contrary to the conclusion of previous studies. | cond-mat_quant-gas |
Parametric Instabilities in Resonantly-Driven Bose-Einstein Condensates: Shaking optical lattices in a resonant manner offers an efficient and
versatile method to devise artificial gauge fields and topological band
structures for ultracold atomic gases. This was recently demonstrated through
the experimental realization of the Harper-Hofstadter model, which combined
optical superlattices and resonant time-modulations. Adding inter-particle
interactions to these engineered band systems is expected to lead to
strongly-correlated states with topological features, such as fractional Chern
insulators. However, the interplay between interactions and external
time-periodic drives typically triggers violent instabilities and
uncontrollable heating, hence potentially ruling out the possibility of
accessing such intriguing states of matter in experiments. In this work, we
study the early-stage parametric instabilities that occur in systems of
resonantly-driven Bose-Einstein condensates in optical lattices. We apply and
extend an approach based on Bogoliubov theory [PRX 7, 021015 (2017)] to a
variety of resonantly-driven band models, from a simple shaken Wannier-Stark
ladder to the more intriguing driven-induced Harper-Hofstadter model. In
particular, we provide ab initio numerical and analytical predictions for the
stability properties of these topical models. This work sheds light on general
features that could guide current experiments to stable regimes of operation. | cond-mat_quant-gas |
Dynamical formation of the unitary Bose gas: We study the structure of a Bose-condensed gas after quenching interactions
to unitarity. Using the method of cumulants, we decompose the evolving gas in
terms of clusters. Within the quantum depletion we observe the emergence of
two-body clusters bound purely by many-body effects, scaling continuously with
the atomic density. As the unitary Bose gas forms, three-body Efimov clusters
are first localized and then sequentially absorbed into the embedded
atom-molecule scattering continuum of the surrounding depletion. These results
motivate future experimental probes of a quenched Bose-condensate during
evolution at unitarity. | cond-mat_quant-gas |
Interaction-driven dynamical quantum phase transitions in a strongly
correlated bosonic system: We study dynamical quantum phase transitions (DQPTs) in the extended
Bose-Hubbard model after a sudden quench of the nearest-neighbor interaction
strength. Using the time-dependent density matrix renormalization group, we
demonstrate that interaction-driven DQPTs can appear after quenches between two
topologically trivial insulating phases -- a phenomenon that has so far only
been studied between gapped and gapless phases. These DQPTs occur when the
interaction strength crosses a certain threshold value that does not coincide
with the equilibrium phase boundaries, which is in contrast to quenches that
involve a change of topology. In order to elucidate the nonequilibrium
excitations during the time evolution, we define a new set of string and parity
order parameters. We find a close connection between DQPTs and these newly
defined order parameters for both types of quenches. In the interaction-driven
case, the order parameter exhibits a singularity at the time of the DQPT only
when the quench parameter is close to the threshold value. Finally, the
timescales of DQPTs are scrutinized and different kinds of power laws are
revealed for the topological and interaction-driven cases. | cond-mat_quant-gas |
Effective interaction in an unbalanced Fermion mixture: A one dimensional Fermi mixture with delta--interaction is investigated in
the limit of extreme imbalance. In particular we consider the cases of only one
or two minority Fermions which interact with the Fermi-sea of the majority
Fermions. We calculate dispersion relation and polaron mass for the minority
Fermions as well as equal time density-density correlators. Within a cluster
expansion we derive an expression for the effective interaction potential
between minority Fermions. For our calculations we use a reformulation of the
exact wave functions, originally obtained by Yang and Gaudin by a nested Bethe
ansatz, in terms of determinants. | cond-mat_quant-gas |
Observation of Nonlinear Response and Onsager Regression in a Photon
Bose-Einstein Condensate: The quantum regression theorem states that the correlations of a system at
two different times are governed by the same equations of motion as the
temporal response of the average values. Such a relation provides a powerful
framework for the investigation of physical systems by establishing a formal
connection between intrinsic microscopic behaviour and a macroscopic 'effect'
due to an external 'cause'. Measuring the response to a controlled perturbation
in this way allows to determine, for example, structure factors in condensed
matter systems as well as other correlation functions of material systems. Here
we experimentally demonstrate that the two-time particle number correlations in
a photon Bose-Einstein condensate inside a dye-filled microcavity exhibit the
same dynamics as the response of the condensate to a sudden perturbation of the
dye molecule bath. This confirms the regression theorem for a quantum gas and,
moreover, establishes a test of this relation in an unconventional form where
the perturbation acts on the bath and only the condensate response is
monitored. For strong perturbations, we observe nonlinear relaxation dynamics
which our microscopic theory relates to the equilibrium fluctuations, thereby
extending the regression theorem beyond the regime of linear response. The
demonstrated nonlinearity of the condensate-bath system paves the way for
studies of novel elementary excitations in lattices of driven-dissipative
photon condensates. | cond-mat_quant-gas |
Mixtures of dipolar gases in two dimensions: a quantum Monte Carlo study: We studied the miscibility of two dipolar quantum gases in the limit of zero
temperature. The system under study is composed by a mixture of two Bose gases
with dominant dipolar interaction in a two-dimensional harmonic confinement.
The dipolar moments are considered all to be perpendicular to the plane,
turning the dipolar potential in a purely repulsive and isotropic model. Our
analysis is carried out by using the diffusion Monte Carlo method which allows
for an exact solution to the many-body problem within some statistical noise.
Our results show that the miscibility between the two species is rather
constrained as a function of the relative dipolar moments and masses of the two
components. A narrow regime is predicted where both species mix and we
introduce an adimensional parameter whose value predicts quite accurately the
miscibility of the two dipolar gases. | cond-mat_quant-gas |
Flavour-selective localization in interacting lattice fermions via SU(N)
symmetry breaking: A large repulsion between particles in a quantum system can lead to their
localization, as it happens for the electrons in Mott insulating materials.
This paradigm has recently branched out into a new quantum state, the
orbital-selective Mott insulator, where electrons in some orbitals are
predicted to localize, while others remain itinerant. We provide a direct
experimental realization of this phenomenon, that we extend to a more general
flavour-selective localization. By using an atom-based quantum simulator, we
engineer SU(3) Fermi-Hubbard models breaking their symmetry via a tunable
coupling between flavours, observing an enhancement of localization and the
emergence of flavour-dependent correlations. Our realization of
flavour-selective Mott physics opens the path to the quantum simulation of
multicomponent materials, from superconductors to topological insulators. | cond-mat_quant-gas |
Exotic superfluidity in cold atoms: We derived the low energy effective action for the collective modes in
asymmetric fermionic systems with attractive interaction. We obtained the phase
diagram in terms of the chemical potentials. It features a stable gapless
superfluidity with one Fermi surface on the BEC side of the resonance. Also we
predict a sharp increase in outer core of a vortex, i.e. vortex size, upon
entering into the gapless phase. This may serve as a signature of a gapless
phase. | cond-mat_quant-gas |
Revealing the Condensate and Non-Condensate Distributions in the
Inhomogeneous Bose-Hubbard Model: We calculate the condensate fraction and the condensate and non-condensate
spatial and momentum distribution of the Bose-Hubbard model in a trap. From our
results, it is evident that using approximate distributions can lead to
erroneous experimental estimates of the condensate. Strong interactions cause
the condensate to develop pedestal-like structures around the central peak that
can be mistaken as non-condensate atoms. Near the transition temperature, the
peak itself can include a significant non-condensate component. Using
distributions generated from QMC simulations, experiments can map their
measurements for higher accuracy in identifying phase transitions and
temperature. | cond-mat_quant-gas |
Fractional quantization of charge and spin in topological quantum pumps: Topological quantum pumps are topologically equivalent to the quantum Hall
state: In these systems, the charge pumped during each pumping cycle is
quantized and coincides with the Chern invariant. However, differently from
quantum Hall insulators, quantum pumps can exhibit novel phenomena such as the
fractional quantization of the charge transport, as a consequence of their
distinctive symmetries in parameter space. Here, we report the analogous
fractional quantization of the spin transport in a topological spin pump
realized in a one-dimensional lattice via a periodically modulated Zeeman
field. In the proposed model, which is a spinfull generalization of the
Harper-Hofstadter model, the amount of spin current pumped during well-defined
fractions of the pumping cycle is quantized as fractions of the spin Chern
number. This fractional quantization of spin is topological, and is a direct
consequence of the additional symmetries ensuing from the commensuration of the
periodic field with the underlying lattice. | cond-mat_quant-gas |
Zero-temperature equation of state of mass-imbalanced resonant Fermi
gases: We calculate the zero-temperature equation of state of mass-imbalanced
resonant Fermi gases in an ab initio fashion, by implementing the recent
proposal of imaginary-valued mass difference to bypass the sign problem in
lattice Monte Carlo calculations. The fully non-perturbative results thus
obtained are analytically continued to real mass imbalance to yield the
physical equation of state, providing predictions for upcoming experiments with
mass-imbalanced atomic Fermi gases. In addition, we present an exact relation
for the rate of change of the equation of state at small mass imbalances,
showing that it is fully determined by the energy of the mass-balanced system. | cond-mat_quant-gas |
Truncated many-body dynamics of interacting bosons: A variational
principle with error monitoring: We develop a method to describe the temporal evolution of an interacting
system of bosons, for which the field operator expansion is truncated after a
finite number $M$ of modes, in a rigorously controlled manner. Using
McLachlan's principle of least error, we find a self-consistent set of
equations for the many-body state. As a particular benefit, and in distinction
to previously proposed approaches, the presently introduced method facilitates
the dynamical increase of the number of orbitals during the temporal evolution,
due to the fact that we can rigorously monitor the error made by increasing the
truncation dimension $M$. The additional orbitals, determined by the condition
of least error of the truncated evolution relative to the exact one, are
obtained from an initial trial state by steepest $constrained$ descent. | cond-mat_quant-gas |
Multichannel Molecular State and Rectified Short-range Boundary
Condition for Spin-orbit Coupled Ultracold Fermions Near p-wave Resonances: We study the interplay of spin-orbit coupling (SOC) and strong p-wave
interaction to the scattering property of spin-1/2 ultracold Fermi gases. Based
on a two-channel square-well potential generating p-wave resonance, we show
that the presence of an isotropic SOC, even for its length much longer than the
potential range, can greatly modify the p-wave short-range boundary
condition(BC). As a result, the conventional p-wave BC cannot predict the
induced molecules near p-wave resonance, which can be fully destroyed to vanish
due to strong interference between s- and p-wave channels. By analyzing the
intrinsic reasons for the breakdown of conventional BC, we propose a new p-wave
BC that can excellently reproduce the exact molecule solutions and also equally
apply for a wide class of single-particle potentials besides SOC. This work
reveals the significant effect of SOC to both the short- and long-range
properties of fermions near p-wave resonance, paving the way for future
exploring interesting few- and many-body physics in such system. | cond-mat_quant-gas |
Condensation of Cooper Triples: The condensation of Cooper pairs, originating from the Fermi-surface
instability due to a weakly attractive interaction between two fermions, opened
a new frontier for exploring many-body physics in interdisciplinary contexts.
In this work, we discuss the possible condensation of Cooper triples, which are
three-body counterparts of Cooper pairs for three-component fermions with a
three-body attraction. Although each composite trimer-like state obeys the
Fermi-Dirac statistics, its aggregate can form a condensate at zero
center-of-mass momentum in the presence of the internal degrees of freedom
associated with the relative momenta of constituent particles of momenta close
to the Fermi surface. Such condensation can be regarded as bosonization in
infinite-component fermions. We propose a variational wave function for the
condensate of Cooper triples in analogy with the Bardeen-Cooper-Schrieffer
ground state and obtain the ground-state energy. | cond-mat_quant-gas |
Casimir force of a dilute Bose gas confined by a parallel plate geometry
in improved Hatree-Fock approximation: Within framework of quantum field theory, in improved Hatree-Fock (IHF)
approximation, we have considered a dilute single Bose-Einstein condensate
(BEC) confined between two parallel plates. We found that the effective mass
and order parameter of BEC strongly depend on distance separating two plates.
Our results shows that the effective mass, order parameter and the Casimir
force in IHF approximation equal to their values in one-loop approximation
added a corrected term due to contribution of two-loop diagrams. We also show
that the one-loop approximation is enough for calculating Casimir effect in an
ideal Bose gas. | cond-mat_quant-gas |
Large-N ground state of the Lieb-Liniger model and Yang-Mills theory on
a two-sphere: We derive the large particle number limit of the Bethe equations for the
ground state of the attractive one-dimensional Bose gas (Lieb-Liniger model) on
a ring and solve it for arbitrary coupling. We show that the ground state of
this system can be mapped to the large-N saddle point of Euclidean Yang-Mills
theory on a two-sphere with a U(N) gauge group, and the phase transition that
interpolates between the homogeneous and solitonic regime is dual to the
Douglas-Kazakov confimenent-deconfinement phase transition. | cond-mat_quant-gas |
Vortex Lattices in Strongly Confined Quantum Droplets: Bose mixture quantum droplets display a fascinating stability that relies on
quantum fluctuations to prevent collapse driven by mean-field effects. Most
droplet research focuses on untrapped or weakly trapped scenarios, where the
droplets exhibit a liquid-like flat density profile. When weakly trapped
droplets rotate, they usually respond through center-of-mass motion or
splitting instability. Here, we study rapidly rotating droplets in the strong
external confinement limit where the external potential prevents splitting and
the center-of-mass excitation. We find that quantum droplets form a triangular
vortex lattice as in single-component repulsive Bose-Einstein condensates
(BEC), but the overall density follows the analytical Thomas-Fermi profile
obtained from a cubic equation. We observe three significant differences
between rapidly rotating droplets and repulsive BECs. First, the vortex core
size changes markedly at finite density, visible in numerically obtained
density profiles. We analytically estimate the vortex core sizes from the
droplets' coherence length and find good agreement with the numerical results.
Second, the change in the density profile gives a slight but observable
distortion to the lattice, which agrees with the distortion expected due to
nonuniform superfluid density. Lastly, unlike a repulsive BEC, which expands
substantially as the rotation frequency approaches the trapping frequency,
rapidly rotating droplets show only a fractional change in their size. We argue
that this last point can be used to create clouds with lower filling factors,
which may facilitate reaching the elusive strongly correlated regime. | cond-mat_quant-gas |
Improved Silbey-Harris polaron ansatz for the spin-boson model: In this paper, the well-known Silbey-Harris (SH) polaron ansatz for the
spin-boson model is improved by adding orthogonal displaced Fock states. The
obtained results for the ground state in all baths converge very quickly within
finite displaced Fock states and corresponding SH results are corrected
considerably. Especially for the sub-Ohmic spin-boson model, the converging
results are obtained for 0 < s < 1/2 in the fourth-order correction and very
accurate critical coupling strengths of the quantum phase transition are
achieved. Converging magnetization in the biased spin-boson model is also
arrived at. Since the present improved SH ansatz can yield very accurate, even
almost exact results, it should have wide applications and extensions in
various spin-boson model and related fields. | cond-mat_quant-gas |
Generalized Effective Potential Landau Theory for Bosonic Quadratic
Superlattices: We study the properties of the Bose-Hubbard model for a quadratic optical
superlattice. To this end we generalize a recently established effective
potential Landau theory for a single component to the case of multi components
and find not only the characteristic incompressible solid phases with
fractional filling, but also obtain the underlying quantum phase diagram in the
whole parameter region at zero temperature. Comparing our analytic results with
corresponding ones from quantum Monte Carlo simulations demonstrates the high
accuracy of the generalized effective potential Landau theory (GEPLT). Finally,
we comment on the advantages and disadvantages of the GEPLT in view of a direct
comparison with a corresponding decoupled mean-field theory. | cond-mat_quant-gas |
Fractional domain walls from on-site softening in dipolar bosons: We study dipolar bosons in a 1D optical lattice and identify a region in
parameter space---strong coupling but relatively weak on-site
repulsion---hosting a series of stable charge-density-wave (CDW) states whose
low-energy excitations, built from "fractional domain walls," have remarkable
similarities to those of non-Abelian fractional quantum Hall states. Here, a
conventional domain wall between translated CDW's may be split by inserting
strings of degenerate, but inequivalent, CDW states. Outside these insulating
regions, we find numerous supersolids as well as a superfluid regime. The
mentioned phases should be accessible experimentally and, in particular, the
fractional domain walls can be created in the ground state using single-site
addressing, i.e., by locally changing the chemical potential. | cond-mat_quant-gas |
Localization driven superradiant instability: The prominent Dicke superradiant phase arises from coupling an ensemble of
atoms to cavity optical field when external optical pumping exceeds a threshold
strength. Here we report a prediction of the superrandiant instability driven
by Anderson localization, realized with a hybrid system of Dicke and
Aubry-Andre (DAA) model for bosons trapped in a one-dimensional (1D)
quasiperiodic optical lattice and coupled to a cavity. Our central finding is
that for bosons condensed in localized phase given by the DAA model, the
resonant superradiant scattering is induced, for which the critical optical
pumping of superradiant phase transition approaches zero, giving an instability
driven by Anderson localization. The superradiant phase for the DAA model with
or without a mobility edge is investigated, showing that the localization
driven superradiant instability is in sharp contrast to the superradiance as
widely observed for Bose condensate in extended states, and should be
insensitive to temperature of the system. This study unveils an insightful
effect of localization on the Dicke superradiance, and is well accessible based
on the current experiments. | cond-mat_quant-gas |
Universal nonanalytic behavior of the Hall conductance in a Chern
insulator at the topologically driven nonequilibrium phase transition: We study the Hall conductance of a Chern insulator after a global quench of
the Hamiltonian. The Hall conductance in the long time limit is obtained by
applying the linear response theory to the diagonal ensemble. It is expressed
as the integral of the Berry curvature weighted by the occupation number over
the Brillouin zone. We identify a topologically driven nonequilibrium phase
transition, which is indicated by the nonanalyticity of the Hall conductance as
a function of the energy gap m_f in the post-quench Hamiltonian H_f. The
topological invariant for the quenched state is the winding number of the
Green's function W, which equals the Chern number for the ground state of H_f.
In the limit that m_f goes to zero, the derivative of the Hall conductance with
respect to m_f is proportional to ln(|m_f|), with the constant of
proportionality being the ratio of the change of W at m_f = 0 to the energy gap
in the initial state. This nonanalytic behavior is universal in two-band Chern
insulators such as the Dirac model, the Haldane model, or the Kitaev honeycomb
model in the fermionic basis. | cond-mat_quant-gas |
A nonlinear dynamics approach to Bogoliubov excitations of Bose-Einstein
condensates: We assume the macroscopic wave function of a Bose-Einstein condensate as a
superposition of Gaussian wave packets, with time-dependent complex width
parameters, insert it into the mean-field energy functional corresponding to
the Gross-Pitaevskii equation (GPE) and apply the time-dependent variational
principle. In this way the GPE is mapped onto a system of coupled equations of
motion for the complex width parameters, which can be analyzed using the
methods of nonlinear dynamics. We perform a stability analysis of the fixed
points of the nonlinear system, and demonstrate that the eigenvalues of the
Jacobian reproduce the low-lying quantum mechanical Bogoliubov excitation
spectrum of a condensate in an axisymmetric trap. | cond-mat_quant-gas |
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