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Optimizing Quantum Gas Production by an Evolutionary Algorithm: We report on the application of an evolutionary algorithm (EA) to enhance
performance of an ultra-cold quantum gas experiment. The production of a
$^{87}$Rubidium Bose-Einstein condensate (BEC) can be divided into fundamental
cooling steps, specifically magneto optical trapping of cold atoms, loading of
atoms to a far detuned crossed dipole trap and finally the process of
evaporative cooling. The EA is applied separately for each of these steps with
a particular definition for the feedback the so-called fitness. We discuss the
principles of an EA and implement an enhancement called differential evolution.
Analyzing the reasons for the EA to improve \eg, the atomic loading rates and
increase the BEC phase-space density, yields an optimal parameter set for the
BEC production and enables us to reduce the BEC production time significantly.
Furthermore, we focus on how additional information about the experiment and
optimization possibilities can be extracted and how the correlations revealed
allow for further improvement. Our results illustrate that EAs are powerful
optimization tools for complex experiments and exemplify that the application
yields useful information on the dependence of these experiments on the
optimized parameters. | cond-mat_quant-gas |
Signatures of non-trivial pairing in the quantum walk of two-component
bosons: Nearest neighbour bosons possessing only onsite interactions do not form
onsite bound pairs in their quantum walk due to fermionization. We obtain
signatures of non-trivial onsite pairing in the quantum walk of strongly
interacting two component bosons in a one dimensional lattice. By considering
an initial state with particles from different components located at the
nearest-neighbour sites in the central region of the lattice, we show that in
the dynamical evolution of the system, competing intra- and inter-component
onsite repulsion leads to the formation of onsite inter-component bound states.
We find that when the total number of particles is three, an inter-component
pair is favoured in the limit of equal intra- and inter-component interaction
strengths. However, when two bosons from each species are considered,
inter-component pairs and trimer are favoured depending on the ratios of the
intra- and inter-component interactions. In both the cases, we find that the
quantum walks exhibit a re-entrant behaviour as a function of inter-component
interaction. | cond-mat_quant-gas |
Gas-to-soliton transition of attractive bosons on a spherical surface: We investigate the ground state properties of $N$ bosons with attractive
zero-range interactions characterized by the scattering length $a>0$ and
confined to the surface of a sphere of radius $R$. We present the analytic
solution of the problem for $N=2$, mean-field analysis for $N\rightarrow
\infty$, and exact diffusion Monte-Carlo results for intermediate $N$. For
finite $N$ we observe a smooth crossover from the uniform state in the limit
$a/R\gg 1$ (weak attraction) to a localized state at small $a/R$ (strong
attraction). With increasing $N$ this crossover narrows down to a discontinuous
transition from the uniform state to a soliton of size $\sim R/\sqrt{N}$. The
two states are separated by an energy barrier, tunneling under which is
exponentially suppressed at large $N$. The system behavior is marked by a
peculiar competition between space-curvature effects and beyond-mean-field
terms, both breaking the scaling invariance of a two-dimensional mean-field
theory. | cond-mat_quant-gas |
Two-dimensional imbalanced Fermi gas in antiparallel magnetic fields: We study a two-dimensional Fermi gas with an attractive interaction subjected
to synthetic magnetic fields, which are assumed to be mutually antiparallel for
two different spin components with population imbalance. By employing the
mean-field approximation, we show that the Fulde-Ferrell state is energetically
favored over the Larkin-Ovchinnikov state in the weak-coupling limit. We then
elucidate the zero-temperature phase diagram in the space of attraction and two
chemical potentials analytically at weak coupling as well as numerically beyond
it. Rich structures consisting of quantum Hall insulator, unpolarized
superfluid, and Fulde-Ferrell phases separated by various second-order and
first-order quantum phase transitions are found. | cond-mat_quant-gas |
Quantum many-body effects on Rydberg excitons in cuprous oxide: We investigate quantum many-body effects on Rydberg excitons in cuprous oxide
induced by the surrounding electron-hole plasma. Line shifts and widths are
calculated by full diagonalisation of the plasma Hamiltonian and compared to
results in first order perturbation theory, and the oscillator strength of the
exciton lines is analysed. | cond-mat_quant-gas |
Condensate fraction and critical temperature of interacting Bose gas in
anharmonic trap: By using a correlated many body method and using the realistic van der Waals
potential we study several statistical measures like the specific heat,
transition temperature and the condensate fraction of the interacting Bose gas
trapped in an anharmonic potential. As the quadratic plus a quartic confinement
makes the trap more tight, the transition temperature increases which makes
more favourable condition to achieve Bose-Einstein condensation (BEC)
experimentally. BEC in 3D isotropic harmonic potential is also critically
studied, the correction to the critical temperature due to finite number of
atoms and also the correction due to inter-atomic interaction are calculated by
the correlated many-body method. Comparison and discussion with the mean-field
results are presented. | cond-mat_quant-gas |
Vortex stream generation and enhanced propagation in a polariton
superfluid: In this work, we implement a new experimental configuration which exploits
the specific properties of the optical bistability exhibited by the polariton
system and we demonstrate the generation of a superfluid turbulent flow in the
wake of a potential barrier. The propagation and direction of the turbulent
flow are sustained by a support beam on distances an order of magnitude longer
than previously reported. This novel technique is a powerful tool for the
controlled generation and propagation of quantum turbulences and paves the way
to the study of the hydrodynamic of quantum turbulence in driven-dissipative 2D
polariton systems. | cond-mat_quant-gas |
Interaction effects on dynamic correlations in non-condensed Bose gases: We consider dynamic, i.e., frequency-dependent, correlations in non-condensed
ultracold atomic Bose gases. In particular, we consider the single-particle
correlation function and its power spectrum. We compute this power spectrum for
a one-component Bose gas, and show how it depends on the interatomic
interactions that lead to a finite single-particle relaxation time. As another
example, we consider the power spectrum of spin-current fluctuations for a
two-component Bose gas and show how it is determined by the spin-transport
relaxation time. | cond-mat_quant-gas |
Ground state phase diagram of the repulsive SU(3) Hubbard model in
Gutzwiller approximation: We perform a variational Gutzwiller calculation to study the ground state of
the repulsive SU(3) Hubbard model on the Bethe lattice with infinite
coordination number. We construct a ground-state phase diagram focusing on
phases with a two-sublattice structure and find five relevant phases: (1) a
paramagnet, (2) a completely polarized ferromagnet, (3) a two-component
antiferromagnet where the third component is depleted, (4) a two-component
antiferromagnet with a metallic third component (an "orbital selective" Mott
insulator), and (5) a density-wave state where two components occupy dominantly
one sublattice and the last component the other one. First-order transitions
between these phases lead to phase separation. A comparison of the SU(3)
Hubbard model to the better-known SU(2) model shows that the effects of doping
are completely different in the two cases. | cond-mat_quant-gas |
Observation of Floquet band topology change in driven ultracold Fermi
gases: Periodic driving of a quantum system can significantly alter its energy bands
and even change the band topology, opening a completely new avenue for
engineering novel quantum matter. Although important progress has been made
recently in measuring topological properties of Floquet bands in different
systems, direct experimental measurement of Floquet band dispersions and their
topology change is still demanding. Here we directly measure Floquet band
dispersions in a periodically driven spin-orbit coupled ultracold Fermi gas.
Using spin injection radio-frequency spectroscopy, we observe that the Dirac
point originating from two dimensional spin-orbit coupling can be manipulated
to emerge at the lowest or highest two dressed bands by fast modulating Raman
laser frequencies, demonstrating topological change of Floquet bands. Our work
will provide a powerful tool for understanding fundamental Floquet physics as
well as engineering exotic topological quantum matter. | cond-mat_quant-gas |
Apparent low-energy scale invariance in two-dimensional Fermi gases: Recent experiments on a $\2d$ Fermi gas find an undamped breathing mode at
twice the trap frequency over a wide range of parameters. To understand this
seemingly scale-invariant behavior in a system with a scale, we derive two
exact results valid across the entire BCS-BEC crossover at all temperatures.
First, we relate the shift of the mode frequency from its scale-invariant value
to $\gamma_d \equiv (1+2/d)P-\rho(\partial P/\partial\rho)_s$ in $d$
dimensions. Next, we relate $\gamma_d$ to dissipation via a new low-energy bulk
viscosity sum rule. We argue that $\2d$ is special, with its logarithmic
dependence of the interaction on density, and thus $\gamma_2$ is small in both
the BCS and BEC regimes, even though $P - 2\varepsilon/d$, sensitive to the
dimer binding energy that breaks scale invariance, is not. | cond-mat_quant-gas |
Quantum quenches in the anisotropic spin-1/2 Heisenberg chain: different
approaches to many-body dynamics far from equilibrium: Recent experimental achievements in controlling ultracold gases in optical
lattices open a new perspective on quantum many-body physics. In these
experimental setups it is possible to study coherent time evolution of isolated
quantum systems. These dynamics reveal new physics beyond the low-energy
properties usually relevant in solid-state many-body systems. In this paper we
study the time evolution of antiferromagnetic order in the Heisenberg chain
after a sudden change of the anisotropy parameter, using various numerical and
analytical methods. As a generic result we find that the order parameter, which
can show oscillatory or non-oscillatory dynamics, decays exponentially except
for the effectively non-interacting case of the XX limit. For weakly ordered
initial states we also find evidence for an algebraic correction to the
exponential law. The study is based on numerical simulations using a numerical
matrix product method for infinite system sizes (iMPS), for which we provide a
detailed description and an error analysis. Additionally, we investigate in
detail the exactly solvable XX limit. These results are compared to
approximative analytical approaches including an effective description by the
XZ-model as well as by mean-field, Luttinger-liquid and sine-Gordon theories.
This reveals which aspects of non-equilibrium dynamics can as in equilibrium be
described by low-energy theories and which are the novel phenomena specific to
quantum quench dynamics. The relevance of the energetically high part of the
spectrum is illustrated by means of a full numerical diagonalization of the
Hamiltonian. | cond-mat_quant-gas |
Bosonic Josephson effect in the Fano-Anderson model: We investigate the coherent dynamics of a non-interacting Bose-Einstein
condensate in a system consisting of two bosonic reservoirs coupled via a
spatially localized mode. We describe this system by a two-terminal
Fano-Anderson model and investigate analytically the time evolution of
observables such as the bosonic Josephson current. In doing so, we find that
the Josephson current sensitively depends on the on-site energy of the
localized mode. This facilitates to use this setup as a transistor for a
Bose-Einstein condensate. We identify two regimes. In one regime, the system
exhibits well-behaved long-time dynamics with a slowly oscillating and undamped
Josephson current. In a second regime, the Josephson current is a superposition
of an extremely weakly damped slow oscillation and an undamped fast
oscillation. Our results are confirmed by finite-size simulations. | cond-mat_quant-gas |
Number squeezed and fragmented states of strongly interacting bosons in
a double well: We present a systematic study of the phenomena of number squeezing and
fragmentation for a repulsive Bose-Einstein condensate (BEC) in a three
dimensional double well potential over a range of interaction strengths and
barrier heights, including geometries that exhibit appreciable overlap in the
one-body wavefunctions localized in the left and right wells. We compute the
properties of the condensate with numerically exact, full dimensional path
integral ground state (PIGS) Quantum Monte Carlo simulations and compare with
results obtained from using two- and eight-mode truncated basis models. The
truncated basis models are found to agree with the numerically exact PIGS
simulations for weak interactions, but fail to correctly predict the amount of
number squeezing and fragmentation exhibited by the PIGS simulations for strong
interactions. We find that both number squeezing and fragmentation of the BEC
show non-monotonic behavior at large values of interaction strength a. The
number squeezing shows a universal scaling with the product of number of
particles and interaction strength (Na) but no such universal behavior is found
for fragmentation. Detailed analysis shows that the introduction of repulsive
interactions not only suppresses number fluctuations to enhance number
squeezing, but can also enhance delocalization across wells and tunneling
between wells, each of which may suppress number squeezing. This results in a
dynamical competition whose resolution shows a complex dependence on all three
physical parameters defining the system: interaction strength, number of
particles, and barrier height. | cond-mat_quant-gas |
Losses in interacting quantum gases: ultra-violet divergence and its
regularization: We investigate the effect of losses on an interacting quantum gas. We show
that, for gases in dimension higher than one, assuming together a vanishing
correlation time of the reservoir where dissipation occurs, and contact
interactions leads to a divergence of the energy increase rate. This divergence
is a combined effect of the contact interactions, which impart arbitrary large
momenta to the atoms, and the infinite energy width of the reservoir associated
to its vanishing correlation time. We show how the divergence is regularized
when taking into account the finite energy width of the reservoir, and, for
large energy width, we give an expression for the energy increase rate that
involves the contact parameter. We then consider the specific case of a weakly
interacting Bose Einstein condensate, that we describe using the Bogoliubov
theory. Assuming slow losses so that the gas is at any time described by a
thermal equilibrium, we compute the time evolution of the temperature of the
gas. Using a Bogoliubov analysis, we also consider the case where the
regularization of the divergence is due to the finite range of the interaction
between atoms. | cond-mat_quant-gas |
Low-lying energy levels of a one-dimensional weakly interacting Bose gas
under zero boundary conditions: We diagonalize the second-quantized Hamiltonian of a one-dimensional Bose gas
with a nonpoint repulsive interatomic potential and zero boundary conditions.
At weak coupling the solutions for the ground-state energy $E_{0}$ and the
dispersion law $E(k)$ coincide with the Bogoliubov solutions for a periodic
system. In this case, the single-particle density matrix $F_{1}(x,x^{\prime})$
at $T=0$ is close to the solution for a periodic system and, at $T>0$, is
significantly different from it. We also obtain that the wave function $\langle
\hat{\psi}(x,t) \rangle$ of the effective condensate is close to a constant
$\sqrt{N_{0}/L}$ inside the system and vanishes on the boundaries (here,
$N_{0}$ is the number of atoms in the effective condensate, and $L$ is the size
of the system). We find the criterion of applicability of the method, according
to which the method works for a finite system at very low temperature and with
a weak coupling (a weak interaction or a large concentration). | cond-mat_quant-gas |
Spatiotemporal scaling of two-dimensional nonequilibrium
exciton-polariton systems with weak interactions: We perform a numerical study on the two-dimensional nonequilibrium
exciton-polariton systems driven by incoherent pumping based on the stochastic
generalized Gross-Pitaevskii equation. We calculate the density fluctuation,
coherence function, and scaling function. It is found that the correlations at
short range agree with the Bogoliubov linear theory. While at large distance,
both static and dynamic correlations are characterized by the nonlinear scaling
behaviors of Kardar-Parisi-Zhang (KPZ) universality class, especially when the
interaction is weak. In this regime, scaling analyses are crucial to capture
the universal KPZ scaling features. In addition, the interaction between
vortices is modified in the strong KPZ regime and leads to complex
nonequilibrium vortex patterns. | cond-mat_quant-gas |
Formation of nonlinear X-waves in condensed matter systems: X-waves are an example of a localized wave packet solution of the homogeneous
wave equation, and can potentially arise in any area of physics relating to
wave phenomena, such as acoustics, electromagnetism, or quantum mechanics. They
have been predicted in condensed matter systems such as atomic Bose-Einstein
condensates in optical lattices, and were recently observed in
exciton-polariton condensates. Here we show that polariton X-waves result from
an interference between two separating wave packets that arise from the
combination of a locally hyperbolic dispersion relation and nonlinear
interactions. We show that similar X-wave structures could also be observed in
expanding spin-orbit coupled Bose-Einstein condensates. | cond-mat_quant-gas |
Early Stage of Superradiance from Bose-Einstein Condensates: We investigate the dynamics of matter and optical waves at the early stage of
superradiant Rayleigh scattering from Bose-Einstein Condensates. Our analysis
is within a spatially dependent quantum model which is capable of providing
analytic solutions for the operators of interest. The predictions of the
present model are compared to the predictions of a closely related mean field
model, and we provide a procedure that allows one to calculate quantum
expectation values by averaging over semiclassical solutions. The coherence
properties of the outgoing scattered light are also analyzed, and it is shown
that the corresponding correlation functions may provide detailed information
about the internal dynamics of the system. | cond-mat_quant-gas |
Numerical analysis of spin-orbit coupled one dimensional Fermi gas in
the magnetic field: We use the density matrix renormalization group method(DMRG) and the infinite
time evolved block decimation method(iTEBD) to investigate the ground states of
the spin-orbit coupled Fermi gas in a one dimensional optical lattice with a
transverse magnetic field. We discover that the system with attractive
interaction can have a polarized insulator(PI), a superconducting phase(SC), a
Luther-Emery(LE) phase and a band insulator(BI) phase as we vary the chemical
potential and the strength of magnetic field. We find that spin-orbit coupling
induces a triplet pairing order at zero momentum with the same critical
exponent as that of the singlet pairing one in both the SC and the LE phase. In
contrast to the FFLO phase found in the spin imbalanced system without
spin-orbit coupling, pairings at finite momentum in these two phases have a
larger exponent hence do not dictate the long range behavior. We also find good
agreements of the dominant correlations between numerical results and the
prediction from the bosonization method. The presence of Majorana fermions is
tested. However, unlike results from the mean field study, we do not find
positive evidence of Majorana fermions in our system. | cond-mat_quant-gas |
Spin dynamics and domain formation of a spinor Bose-Einstein condensate
in an optical cavity: We consider a ferromagnetic spin-1 Bose-Einstein condensate (BEC)
dispersively coupled to a unidirectional ring cavity. We show that the ability
of a cavity to modify, in a highly nonlinear fashion, matter-wave phase shifts
adds a new dimension to the study of spinor condensates both within and beyond
the single-mode approximation. In addition to demonstrating strong matter-wave
bistability as in our earlier publication [L. Zhou et al., Phys. Rev. Lett.
103, 160403 (2009)], we show that the interplay between atomic and cavity
fields can greatly enrich both the physics of critical slowing down in spin
mixing dynamics and the physics of spin-domain formation in spinor condensates. | cond-mat_quant-gas |
Dynamical response of ultracold interacting fermion-boson mixtures: We analyze the dynamical response of a ultracold binary gas mixture in
presence of strong boson-fermion couplings. Mapping the problem onto that of
the optical response of a metal/semiconductor electronic degrees of freedom to
electromagnetic perturbation we calculate the corresponding dynamic linear
response susceptibility in the non-perturbative regimes of strong boson-fermion
coupling using diagrammatic resummation technique as well as quantum Monte
Carlo simulations. We evaluate the Bragg spectral function as well as the
optical conductivity and find a pseudogap, which forms in certain parameter
regimes. | cond-mat_quant-gas |
Spontaneous symmetry breaking in rotating condensates of ultracold atoms: We describe an equilibrium state of a rotating trapped atomic condensate,
which is characterized by a non-zero internal circulation and spontaneous
breaking of the rotational O(2) symmetry with all three major semiaxes of the
condensate having different values. The macroscopic rotation of the condensate
is supported by a mesh of quantized vortices, whose number density is a
function of internal circulation. The oscillation modes of this state are
computed and the Goldstone mode associated with the loss of the symmetry is
identified. The possible avenues for experimental identification this state are
discussed. | cond-mat_quant-gas |
Expansion of the strongly interacting superfluid Fermi gas: symmetries
and self-similar regimes: We consider an expansion of the strongly interacting superfluid Fermi gas in
a vacuum, assuming absence of the trapping potential, in the so-called unitary
regime (see, for instance, \cite{pitaevskii2008superfluid}) when the chemical
potential $\mu \propto \hbar^2n^{2/3}/m$ where $n$ is the density of the
Bose-Einstein condensate of Cooper pairs of fermionic atoms. In low
temperatures, $T\to 0$, such expansion can be described in the framework of the
Gross-Pitaevskii equation (GPE). Because of the chemical potential dependence
on the density, $\sim n^{2/3}$, the GPE has additional symmetries, resulting in
the existence of the virial theorem \cite% {vlasov1971averaged}, connecting the
mean size of the gas cloud and its Hamiltonian. It leads asymptotically at
$t\to\infty$ to the gas cloud expansion, linearly growing in time. We study
such asymptotics, and reveal the perfect match between the quasi-classical
self-similar solution and the asymptotic expansion of the non-interacting gas.
This match is governed by the virial theorem, derived through utilizing the
Talanov transformation \cite{talanov1970focusing}, which was first obtained for
the stationary self-focusing of light in media with a cubic nonlinearity due to
the Kerr effect. In the quasi-classical limit, the equations of motion coincide
with 3D hydrodynamics for the perfect monoatomic gas with $\gamma=5/3$. Their
self-similar solution describes, on the background of the gas expansion, the
angular deformities of the gas shape in the framework of the Ermakov--Ray--Reid
type system. | cond-mat_quant-gas |
Superglass formation in an atomic BEC with competing long-range
interactions: The complex dynamical phases of quantum systems are dictated by atomic
interactions that usually evoke an emergent periodic order. Here, we study a
quantum many-body system with two competing and substantially different
long-range interaction potentials where the dynamical instability towards
density order can give way to a superglass phase, i. e., a superfluid
disordered amorphous solid, which exhibits local density modulations but no
long-range periodic order. We consider a two-dimensional BEC in the
Rydberg-dressing regime coupled to an optical standing wave resonator. The
dynamic pattern formation in this system is governed by the competition between
the two involved interaction potentials: repulsive soft-core interactions
arising due to Rydberg dressing and infinite-range sign changing interactions
induced by the cavity photons. The superglass phase is found when the two
interaction potentials introduce incommensurate length scales. The dynamic
formation of this peculiar phase without any externally added disorder is
driven by quantum fluctuations and can be attributed to frustration induced by
the two competing interaction energies and length scales. | cond-mat_quant-gas |
Universality in rotating strongly interacting gases: We analytically determine the properties of two interacting particles in a
harmonic trap subject to a rotation or a uniform synthetic magnetic field,
where the spherical symmetry of the relative Hamiltonian is preserved.
Thermodynamic quantities such as the entropy and energy are calculated via the
second order quantum cluster expansion. We find that in the strongly
interacting regime the energy is universal, however the entropy changes as a
function of the rotation or synthetic magnetic field strength. | cond-mat_quant-gas |
Transport, atom blockade and output coupling in a Tonks-Girardeau gas: Recent experiments have demonstrated how quantum-mechanical impurities can be
created within strongly correlated quantum gases and used to probe the
coherence properties of these systems [S. Palzer, C. Zipkes, C. Sias, and M.
K\"ohl, Phys. Rev. Lett. 103, 150601 (2009).]. Here we present a
phenomenological model to simulate such an output coupler for a Tonks-Girardeau
gas that shows qualitative agreement with the experimental results for atom
transport and output coupling. Our model allows us to explore nonequilibrium
transport phenomena in ultracold quantum gases and leads us to predict a regime
of atom blockade, where the impurity component becomes localized in the parent
cloud despite the presence of gravity. We show that this provides a stable
mixed-species quantum gas in the strongly correlated limit. | cond-mat_quant-gas |
Magnetic lattices for ultracold atoms and degenerate quantum gases: We review recent developments in the use of magnetic lattices as a
complementary tool to optical lattices for trapping periodic arrays of
ultracold atoms and degenerate quantum gases. Recent advances include the
realisation of Bose-Einstein condensation in multiple sites of a magnetic
lattice of one-dimensional microtraps, the trapping of ultracold atoms in
square and triangular magnetic lattices, and the fabrication of magnetic
lattice structures with sub-micron period suitable for quantum tunnelling
experiments. Finally, we describe a proposal to utilise long-range interacting
Rydberg atoms in a large spacing magnetic lattice to create interactions
between atoms on neighbouring sites. | cond-mat_quant-gas |
Deformation of a quantum many-particle system by a rotating impurity: During the last 70 years, the quantum theory of angular momentum has been
successfully applied to describing the properties of nuclei, atoms, and
molecules, their interactions with each other as well as with external fields.
Due to the properties of quantum rotations, the angular momentum algebra can be
of tremendous complexity even for a few interacting particles, such as valence
electrons of an atom, not to mention larger many-particle systems. In this
work, we study an example of the latter: a rotating quantum impurity coupled to
a many-body bosonic bath. In the regime of strong impurity-bath couplings the
problem involves addition of an infinite number of angular momenta which
renders it intractable using currently available techniques. Here, we introduce
a novel canonical transformation which allows to eliminate the complex angular
momentum algebra from such a class of many-body problems. In addition, the
transformation exposes the problem's constants of motion, and renders it
solvable exactly in the limit of a slowly-rotating impurity. We exemplify the
technique by showing that there exists a critical rotational speed at which the
impurity suddenly acquires one quantum of angular momentum from the
many-particle bath. Such an instability is accompanied by the deformation of
the phonon density in the frame rotating along with the impurity. | cond-mat_quant-gas |
Multidimensional hybrid Bose-Einstein condensates stabilized by
lower-dimensional spin-orbit coupling: We show that attractive spinor Bose-Einstein condensates under the action of
spin-orbit coupling (SOC) and Zeeman splitting form self-sustained stable two-
and three-dimensional (2D and 3D) states in free space, even when SOC acts in a
lower-dimensional form. We find that two-dimensional states are stabilized by
one-dimensional (1D) SOC in a broad range of chemical potentials, for atom
numbers (or norm of the spinor wavefunction) exceeding a threshold value, which
strongly depends on the SOC strength and vanishes at a critical point. The
zero-threshold point is a boundary between single-peaked and striped states,
realizing hybrids combining 2D and 1D structural features. In a vicinity of
such point, an asymptotic equation describing the bifurcation of the solitons
from the linear spectrum is derived and investigated analytically. We show that
striped 3D solitary states are as well stabilized by 2D SOC, albeit in a
limited range of chemical potentials and norms. | cond-mat_quant-gas |
Few-boson tunneling in a double well with spatially modulated
interaction: We study few-boson tunneling in a one-dimensional double well with a
spatially modulated interaction. The dynamics changes from Rabi oscillations in
the non-interacting case to a highly suppressed tunneling for intermediate
coupling strengths followed by a revival near the fermionization limit. With
extreme interaction inhomogeneity in the regime of strong correlations we
observe tunneling between the higher bands. The dynamics is explained on the
basis of the few-body spectrum and stationary eigenstates. For higher number of
particles, N > 2, it is shown that the inhomogeneity of the interaction can be
tuned to generate tunneling resonances. Finally, a tilted double-well and its
interplay with the interaction asymmetry is discussed. | cond-mat_quant-gas |
Patterned Supersolids in Dipolar Bose Systems: We study by means of first principle Quantum Monte Carlo simulations the
ground state phase diagram of a system of dipolar bosons with aligned dipole
moments, and with the inclusion of a two-body repulsive potential of varying
range. The system is shown to display a supersolid phase in a relatively broad
region of the phase diagram, featuring different crystalline patterns depending
on the density and on the range of the repulsive part of the interaction
(scattering length). The supersolid phase is sandwiched between a classical
crystal of parallel filaments and a homogeneous superfluid phase. We show that
a "roton" minimum appears in the elementary excitation spectrum of the
superfluid as the system approaches crystallization. | cond-mat_quant-gas |
A pathway to ultracold bosonic $^{23}\textrm{Na}^{39}\textrm{K}$ ground
state molecules: We spectroscopically investigate a pathway for the conversion of
$^{23}\textrm{Na}^{39}\textrm{K}$ Feshbach molecules into rovibronic ground
state molecules via STImulated Raman Adiabatic Passage (STIRAP). Using
photoassociation spectroscopy from the diatomic scattering threshold in the
$a^3\Sigma^+$ potential, we locate the resonantly mixed electronically excited
intermediate states $|B^1\Pi, v=8\rangle$ and $|c^3\Sigma^+, v=30\rangle$
which, due to their singlet-triplet admixture, serve as an ideal bridge between
predominantly $a^3\Sigma^+$ Feshbach molecules and pure $X^1\Sigma^+$ ground
state molecules. We investigate their hyperfine structure and present a simple
model to determine the singlet-triplet coupling of these states. Using
Autler-Townes spectroscopy, we locate the rovibronic ground state of the
$^{23}\textrm{Na}^{39}\textrm{K}$ molecule ($|X^1\Sigma^+, v=0, N=0\rangle$)
and the second rotationally excited state $N=2$ to unambiguously identify the
ground state. We also extract the effective transition dipole moment from the
excited to the ground state. Our investigations result in a fully characterized
scheme for the creation of ultracold bosonic $^{23}\textrm{Na}^{39}\textrm{K}$
ground state molecules. | cond-mat_quant-gas |
Superfluidity breakdown of periodic matter waves in quasi
one-dimensional annular traps via resonant scattering with moving defects: We investigate, both analytically and numerically, the quasi-superfluidity
properties of periodic Bose-Einstein condensates (BECs) in a
quasi-one-dimensional (1D) ring with optical lattices (OL) of different kinds
(linear and nonlinear) and with a moving defect of an infinite mass inside. To
study the dynamics of the condensate we used a mean-field approximation
describing the condensate by use of the Gross-Pitaevskii equation for the order
parameter. We show that the resonant scattering of sound Bloch waves with the
defect profoundly affect BEC superfluidity. In particular, a moving defect
always leads to the breakdown of superfluidity independently of the value of
its velocity. For weak periodic potentials the superfluidity breakdown may
occur on a very long time scale (quasisuperfluidity) but the breakdown process
can be accelerated by increasing the strength of the OL. Quite remarkably, we
find that when the length of the ring is small enough to imply the discreteness
of the reciprocal space, it becomes possible to avoid the resonant scattering
and to restore quasi-superfluidity. | cond-mat_quant-gas |
Sub-micron period lattice structures of magnetic microtraps for
ultracold atoms on an atom chip: We report on the design, fabrication and characterization of magnetic
nanostructures to create a lattice of magnetic traps with sub--micron period
for trapping ultracold atoms. These magnetic nanostructures were fabricated by
patterning a Co/Pd multilayered magnetic film grown on a silicon substrate
using high precision e-beam lithography and reactive ion etching. The Co/Pd
film was chosen for its small grain size and high remanent magnetization and
coercivity. The fabricated structures are designed to magnetically trap
$^{87}$Rb atoms above the surface of the magnetic film with 1D and 2D
(triangular and square) lattice geometries and sub-micron period. Such magnetic
lattices can be used for quantum tunneling and quantum simulation experiments,
including using geometries and periods that may be inaccessible with optical
lattice. | cond-mat_quant-gas |
Critical velocity in resonantly driven polariton superfluids: We study the necessary condition under which a resonantly driven exciton
polariton superfluid flowing against an obstacle can generate turbulence. The
value of the critical velocity is well estimated by the transition from
elliptic to hyperbolic of an operator following ideas developed by Frisch,
Pomeau, Rica for a superfluid flow around an obstacle, though the nature of
equations governing the polariton superfluid is quite different. We find
analytical estimates depending on the pump amplitude and on the pump energy
detuning, quite consistent with our numerical computations. | cond-mat_quant-gas |
Efficiently Extracting Multi-Point Correlations of a Floquet Thermalized
System: Nonequilibrium dynamics of many-body systems is challenging for classical
computing, providing opportunities for demonstrating practical quantum
computational advantage with analogue quantum simulators. It is proposed to be
classically intractable to sample driven thermalized many-body states of
Bose-Hubbard systems, and further extract multi-point correlations for
characterizing quantum phases. Here, leveraging dedicated precise manipulations
and number-resolved detection through a quantum gas microscope, we implement
and sample a 32-site driven Hubbard chain in the thermalized phase. Multi-point
correlations of up to 14th-order extracted from experimental samples offer
clear distinctions between the thermalized and many-body-localized phases. In
terms of estimated computational powers, the quantum simulator is comparable to
the fastest supercomputer with currently known best algorithms. Our work paves
the way towards practical quantum advantage in simulating Floquet dynamics of
many-body systems. | cond-mat_quant-gas |
Snake instability of dark solitons across the BEC-BCS crossover: an
effective field theory perspective: In the present article the snake instability mechanism for dark solitons in
superfluid Fermi gases is studied in the context of a recently developed
effective field theory [Eur. Phys. J. B 88, 122 (2015)]. This theoretical
treatment has proven to be suitable to study stable dark solitons in quasi-1D
setups across the BEC-BCS crossover. In this manuscript the nodal plane of the
stable soliton solution is perturbed by adding a transverse modulation. The
numerical solution of the system of coupled nonlinear differential equations
describing the amplitude of the perturbation leads to the instability spectra
which are calculated for a wide range of interaction regimes and compared to
other theoretical predictions. The maximum transverse size that the atomic
cloud can have in order to preserve the stability is estimated, and the effects
of spin-imbalance on this critical length are examined, revealing a
stabilization of the soliton with increasing imbalance. | cond-mat_quant-gas |
Inhomogeneities and impurities in a dense one-dimensional Rydberg
lattice gas: We consider a dense one-dimensional laser-driven Rydberg lattice gas with
perfect nearest-neighbor blockade. The ground state of this system can be found
analytically in certain parameter regimes even when the applied fields are
inhomogeneous in space. We will use this unique feature to investigate the
effect of an impurity - introduced by the local variation of the laser
parameters - on the correlations of the many-body ground state. Moreover, we
explore the role of a staggered laser field which alternates from site to site
thereby breaking the sublattice symmetry. We demonstrate that this technique,
which can be applied experimentally, reveals insights into the role of
long-range interactions on the critical properties of a Rydberg gas. Our work
highlight novel possibilities for the exploration of many-body physics in
Rydberg lattice gases based on locally tuneable laser fields. | cond-mat_quant-gas |
Topologically protected edge gap solitons of interacting Bosons in
one-dimensional superlattices: We comprehensively investigate the nontrivial states of interacting Bose
system in one-dimensional optical superlattices under the open boundary
condition. Our results show that there exists a kind of stable localized
states: edge gap solitons. We argue that the states originate from the
eigenstates of independent edge parabolas. In particular, the edge gap solitons
exhibit a nonzero topological invariant. The topological nature is due to the
connection of the present model to the quantized adiabatic particle transport
problem. In addition, the composition relations between the gap solitons and
the extend states under the open boundary condition are discussed. | cond-mat_quant-gas |
Fulde-Ferrell Superfluids without Spin Imbalance in Driven Optical
Lattices: Spin-imbalanced ultracold Fermi gases have been widely studied recently as a
platform for exploring the long-sought Fulde-Ferrell-Larkin-Ovchinnikov
superfluid phases, but so far conclusive evidence has not been found. Here we
propose to realize an Fulde-Ferrell (FF) superfluid without spin imbalance in a
three-dimensional fermionic cold atom optical lattice, where $s$- and
$p$-orbital bands of the lattice are coupled by another weak moving optical
lattice. Such coupling leads to a spin-independent asymmetric Fermi surface,
which, together with the $s$-wave scattering interaction between two spins,
yields an FF type of superfluid pairing. Unlike traditional schemes, our
proposal does not rely on the spin imbalance (or an equivalent Zeeman field) to
induce the Fermi surface mismatch and provides a completely new route for
realizing FF superfluids. | cond-mat_quant-gas |
Composite structure of vortices in two-component Bose-Einstein
condensate: In contrast to one-component Bose-Einstein condensate case, the vortices in
two-component condensate can have various complicated structures. The vortices
in a space-homogeneous Bose-Einstein condensate have been studied in this
paper. It is shown that the vortex structure is described by three
dimensionless parameters. This is totally different from the usual
one-component condensate case, where an isolated vortex is described by a
parameter-less dimensionless equation. The two-component vortex structure
strongly depends on the sign of "interaction" constant of the components. A few
types of vortices with different qualitative structure are explored. We show
that the super-density vortices can exist, when the "interaction" constant is
positive. The super-density vortices have the near-axis density greater than
the equilibrium density of a homogeneous space Bose-Einstein condensate. We
also show that the vortices with opposite direction of the condensate component
rotation near the axis and far off the axis can exist. | cond-mat_quant-gas |
Spatial pattern formation and polarization dynamics of a nonequilibrium
spinor polariton condensate: Quasiparticles in semiconductors -- such as microcavity polaritons -- can
form condensates in which the steady-state density profile is set by the
balance of pumping and decay. By taking account of the polarization degree of
freedom for a polariton condensate, and considering the effects of an applied
magnetic field, we theoretically discuss the interplay between polarization
dynamics, and the spatial structure of the pumped decaying condensate. If
spatial structure is neglected, this dynamics has attractors that are linearly
polarized condensates (fixed points), and desynchronized solutions (limit
cycles), with a range of bistability. Considering spatial fluctuations about
the fixed point, the collective spin modes can either be diffusive, linearly
dispersing, or gapped. Including spatial structure, interactions between the
spin components can influence the dynamics of vortices; produce stable
complexes of vortices and rarefaction pulses with both co- and counter-rotating
polarizations; and increase the range of possible limit cycles for the
polarization dynamics, with different attractors displaying different spatial
structures. | cond-mat_quant-gas |
Controlling spontaneous-emission noise in measurement-based feedback
cooling of a Bose-Einstein Condensate: Off-resonant optical imaging is the most popular method for continuous
monitoring of a Bose-Einstein condensate (BEC). However, the disturbance caused
by scattered photons places a serious limitation on the lifetime of such
continuously-monitored condensates. In this paper, we demonstrate that a new
choice of feedback control can overcome the heating effects of the measurement
backaction. In particular, we show that the measurement backaction caused by
off-resonant optical imaging is a multimode quantum-field effect, as the entire
heating process is not seen in single-particle or mean-field models of the
system. Correctly simulating such continuously-monitored systems is only
possible using the number-phase Wigner (NPW) particle filter, which is a hybrid
between the leading techniques for simulating non-equilibrium dynamics in
condensates and particle filters for simulating high-dimensional non-Gaussian
filters in the field of engineering. The new control scheme will enable
long-term continuous measurement and feedback on one of the leading platforms
for precision measurement and the simulation of quantum fields, allowing for
the possibility of single-shot experiments, adaptive measurements and robust
state-preparation and manipulation. | cond-mat_quant-gas |
Association of Efimov trimers from a three-atom continuum: We develop an experimental technique for rf-association of Efimov trimers
from three-atoms continuum. We apply it to probe the lowest accessible Efimov
energy level in bosonic lithium in the region where strong deviations from the
universal behavior are expected, and provide quantitative study of this effect.
Position of the Efimov resonance at the atom-dimer threshold, measured with a
different experimental technique, concurs with the rf-association results. | cond-mat_quant-gas |
Universal many-body response of heavy impurities coupled to a Fermi sea: In this work we discuss the dynamical response of heavy quantum impurities
immersed in a Fermi gas at zero and at finite temperature. Studying both the
frequency and the time domain allows one to identify interaction regimes that
are characterized by distinct many-body dynamics. From this theoretical study a
picture emerges in which impurity dynamics is universal on essentially all time
scales, and where the high-frequency few-body response is related to the
long-time dynamics of the Anderson orthogonality catastrophe by Tan relations.
Our theoretical description relies on different and complementary approaches:
functional determinants give an exact numerical solution for time- and
frequency-resolved responses, bosonization provides analytical expressions at
low temperatures, and the theory of Toeplitz determinants allows one to
analytically predict response up to high temperatures. Using these approaches
we predict the thermal decoherence rate and prove that within the considered
model the fastest rate of long-time decoherence is given by $\gamma=\pi
k_BT/4$. We show that Feshbach resonances in cold atomic systems give access to
new interaction regimes where quantum effects prevail even in the thermal
regime of many-body dynamics. The key signature of this phenomenon is a
crossover between exponential decay rates of the real-time Ramsey signal. It is
shown that the physics of the orthogonality catastrophe is experimentally
observable up to temperatures $T/T_F\lesssim 0.2$ where it leaves its
fingerprint in a power-law temperature dependence of thermal spectral weight
and we review how this phenomenon is related to the physics of heavy ions in
liquid $^3$He and the formation of Fermi polarons. The presented results are in
excellent agreement with recent experiments on LiK mixtures, and we predict
several phenomena that can be tested using currently available experimental
technology. | cond-mat_quant-gas |
Self-induced entanglement resonance in a disordered Bose-Fermi mixture: Different regimes of entanglement growth under measurement have been
demonstrated for quantum many-body systems, with an entangling phase for low
measurement rates and a disentangling phase for high rates (quantum Zeno
effect). Here we study entanglement growth on a disordered Bose-Fermi mixture
with the bosons playing the role of the effective self-induced measurement for
the fermions. Due to the interplay between the disorder and a non-Abelian
symmetry, the model features an entanglement growth resonance when the
boson-fermion interaction strength is varied. With the addition of a magnetic
field, the model acquires a dynamical symmetry leading to experimentally
measurable long-time local oscillations. At the entanglement growth resonance,
we demonstrate the emergence of the cleanest oscillations. Furthermore, we show
that this resonance is distinct from both noise enhanced transport and a
standard stochastic resonance. Our work paves the way for experimental
realizations of self-induced correlated phases in multi-species systems. | cond-mat_quant-gas |
Dissipative topological superconductors in number-conserving systems: We discuss the dissipative preparation of p-wave superconductors in
number-conserving one-dimensional fermionic systems. We focus on two setups:
the first one entails a single wire coupled to a bath, whereas in the second
one the environment is connected to a two-leg ladder. Both settings lead to
stationary states which feature the bulk properties of a p-wave superconductor,
identified in this number-conserving setting through the long-distance behavior
of the proper p-wave correlations. The two schemes differ in the fact that the
steady state of the single wire is not characterized by topological order,
whereas the two-leg ladder hosts Majorana zero modes, which are decoupled from
damping and exponentially localized at the edges. Our analytical results are
complemented by an extensive numerical study of the steady-state properties, of
the asymptotic decay rate and of the robustness of the protocols. | cond-mat_quant-gas |
Optical-plug-assisted spin vortex in a $^{87}$Rb dipolar spinor
Bose-Einstein condensate: Generating a spin vortex in a $^{87}$Rb dipolar spinor Bose-Einstein
condensate in a controllable way is still experimentally challenging. We
propose an experimentally easy and tunable way to produce spin vortex by
varying the potential barrier height and the width of an additionally applied
optical plug. A topological phase transition occurs from the trivial single
mode approximation phase to the optical-plug-assisted-vortex one, as the
barrier height increases and the width lies in an appropriate range. The
optical plug causes radial density variation thus the spin vortex is favored by
significantly lowering the intrinsic magnetic dipolar energy. A type of
coreless spin vortex, different from the conventional polar core vortex, is
predicted by our numerical results. Our proposal removes a major obstacle to
investigate the topological phase transition in a $^{87}$Rb dipolar spinor BEC. | cond-mat_quant-gas |
Connecting Topological Anderson and Mott Insulators in Disordered
Interacting Fermionic Systems: The topological Anderson and Mott insulators are two phases that have so far
been separately and widely explored beyond topological band insulators. Here we
combine the two seemingly different topological phases into a system of
spin-1/2 interacting fermionic atoms in a disordered optical lattice. We find
that the topological Anderson and Mott insulators in the noninteracting and
clean limits can be adiabatically connected without gap closing in the phase
diagram of our model. Lying between the two phases, we uncover a disordered
correlated topological insulator, which is induced from a trivial band
insulator by the combination of disorder and interaction, as the generalization
of topological Anderson insulators to the many-body interacting regime. The
phase diagram is determined by computing various topological properties and
confirmed by unsupervised and automated machine learning. We develop an
approach to provide a unified and clear description of topological phase
transitions driven by interaction and disorder. The topological phases can be
detected from disorder/interaction induced edge excitations and charge pumping
in optical lattices. | cond-mat_quant-gas |
Position- and momentum-space two-body correlations in a weakly
interacting trapped condensate: We investigate the position- and momentum-space two--body correlations in a
weakly interacting, harmonically trapped atomic Bose-Einstein condensed gas at
low temperatures. The two-body correlations are computed within the Bogoliubov
approximation and the peculiarities of the trapped gas are highlighted in
contrast to the spatially homogeneous case. In the position space, we recover
the anti-bunching induced by the repulsive inter-atomic interaction in the
condensed fraction localized around the trap center and the bunching in the
outer thermal cloud. In the momentum space, bunching signatures appear for
either equal or opposite values of the momentum and display peculiar features
as a function of the momentum and the temperature. In analogy to the optical
Hanbury Brown and Twiss effect, the amplitude of the bunching signal at
close-by momenta is fixed by the chaotic nature of the matter field state and
its linewidth is shown to be set by the (inverse of the) finite spatial size of
the associated in-trap momentum components. In contrast, the linewidth of the
bunching signal at opposite-momenta is only determined by the condensate size. | cond-mat_quant-gas |
Phase Winding a Two-Component BEC in an Elongated Trap: Experimental
Observation of Moving Magnetic Orders and Dark-bright Solitons: We experimentally investigate the phase winding dynamics of a harmonically
trapped two-component BEC subject to microwave induced Rabi oscillations
between two pseudospin components. While the single particle dynamics can be
explained by mapping the system to a two-component Bose-Hubbard model,
nonlinearities due to the interatomic repulsion lead to new effects observed in
the experiments: In the presence of a linear magnetic field gradient, a
qualitatively stable moving magnetic order that is similar to antiferromagnetic
order is observed after critical winding is achieved. We also demonstrate how
the phase winding can be used as a new tool to generate copious dark-bright
solitons in a two-component BEC, opening the door for new experimental studies
of these nonlinear features. | cond-mat_quant-gas |
Experimental Methods for Generating Two-Dimensional Quantum Turbulence
in Bose-Einstein Condensates: Bose-Einstein condensates of dilute gases are well-suited for investigations
of vortex dynamics and turbulence in quantum fluids, yet there has been little
experimental research into the approaches that may be most promising for
generating states of two-dimensional turbulence in these systems. Here we give
an overview of techniques for generating the large and disordered vortex
distributions associated with two-dimensional quantum turbulence. We focus on
describing methods explored in our Bose-Einstein condensation laboratory, and
discuss the suitability of these methods for studying various aspects of
two-dimensional quantum turbulence. We also summarize some of the open
questions regarding our own understanding of these mechanisms of
two-dimensional quantum turbulence generation in condensates. We find that
while these disordered distributions of vortices can be generated by a variety
of techniques, further investigation is needed to identify methods for
obtaining quasi-steady-state quantum turbulence in condensates. | cond-mat_quant-gas |
Mesoscopic quantum superpositions in bimodal Bose-Einstein condensates:
decoherence and strategies to counteract it: We study theoretically the interaction-induced generation of mesoscopic
coherent spin state superpositions (small cat states) from an initial coherent
spin state in bimodal Bose-Einstein condensates and the subsequent phase
revival, including decoherence due to particle losses and fluctuations of the
total particle number. In a full multimode description, we propose a
preparation procedure of the initial coherent spin state and we study the
effect of preexisting thermal fluctuations on the phase revival, and on the
spin and orbito-spinorial cat fidelities. | cond-mat_quant-gas |
Universal contact and collective excitations of a strongly interacting
Fermi gas: We study the relationship between Tan's contact parameter and the macroscopic
dynamic properties of an ultracold trapped gas, such as the frequencies of the
collective oscillations and the propagation of sound in one-dimensional (1D)
configurations. We find that the value of the contact, extracted from the most
recent low-temperature measurements of the equation of state near unitarity,
reproduces with accuracy the experimental values of the collective frequencies
of the radial breathing mode at the lowest temperatures. The available
experiment results for the 1D sound velocities near unitarity are also
investigated. | cond-mat_quant-gas |
Detecting quadrupole interactions in ultracold Fermi gases: We propose to detect quadrupole interactions of neutral ultra-cold atoms via
their induced mean-field shift. We consider a Mott insulator state of
spin-polarized atoms in a two-dimensional optical square lattice. The
quadrupole moments of the atoms are aligned by an external magnetic field. As
the alignment angle is varied, the mean-field shift shows a characteristic
angular dependence, which constitutes the defining signature of the quadrupole
interaction. For the $^{3}P_{2}$ states of Yb and Sr atoms, we find a frequency
shift of the order of tens of Hertz, which can be realistically detected in
experiment with current technology. We compare our results to the mean-field
shift of a spin-polarized quasi-2D Fermi gas in continuum. | cond-mat_quant-gas |
Superfluidity of a Raman spin-orbit-coupled Bose gas at finite
temperature: We investigate the superfluidity of a three-dimensional weakly interacting
Bose gas with a one-dimensional Raman-type spin-orbit coupling at both zero and
finite temperatures. Using the imaginary-time Green's function within the
Bogoliubov approximation, we explicitly derive analytic expressions of the
current-current response functions in the plane-wave and zero-momentum phases,
from which we extract the superfluid density in the limits of long wavelength
and zero frequency. At zero temperature, we check that the resultant superfluid
density agrees exactly with our previous analytic prediction obtained from a
phase-twist approach. Both results also satisfy a generalized Josephson
relation in the presence of spin-orbit coupling. At finite temperature, we find
a significant non-monotonic temperature dependence of superfluid density near
the transition from the plane-wave phase to the zero-momentum phase. We show
that this non-trivial behavior might be understood from the sound velocity,
which has a similar temperature dependence. The non-monotonic temperature
dependence is also shared by Landau critical velocity, above which the
spin-orbit-coupled Bose gas loses its superfluidity. Our results would be
useful for further theoretical and experimental studies of superfluidity in
exotic spin-orbit coupled quantum gases. | cond-mat_quant-gas |
Sudden and slow quenches into the antiferromagnetic phase of ultracold
fermions: We propose a method to reach the antiferromagnetic state of two-dimensional
Fermi gases trapped in optical lattices: Independent subsystems are prepared in
suitable initial states and then connected by a sudden or slow quench of the
tunneling between the subsystems. Examples of suitable low-entropy subsystems
are double wells or plaquettes, which can be experimentally realised in Mott
insulating shells using optical super-lattices. We estimate the effective
temperature T* of the system after the quench by calculating the distribution
of excitations created using the spin wave approximation in a Heisenberg model.
We investigate the effect of an initial staggered magnetic field and find that
for an optimal polarisation of the initial state the effective temperature can
be significantly reduced from T*$\approx$1.7 Tc at zero polarisation to
T*<0.65Tc, where Tc is the crossover temperature to the antiferromagnetic
state. The temperature can be further reduced using a finite quench time. We
also show that T* decreases logarithmically with the linear size of the
subsystem. | cond-mat_quant-gas |
Itinerant ferromagnetism of two-dimensional repulsive fermions with Rabi
coupling: We study a two-dimensional fermionic cloud of repulsive alkali-metal atoms
characterized by two hyperfine states which are Rabi coupled. Within a
variational Hartree-Fock scheme, we calculate analytically the ground-state
energy of the system. Then we determine the conditions under which there is a
quantum phase transition with spontaneous symmetry breaking from a
spin-balanced configuration to a spin-polarized one, an effect known as
itinerant ferromagnetism. Interestingly, we find that the transition appears
when the interaction energy per particle exceedes both the kinetic energy per
particle and the Rabi coupling energy. The itinerant ferromagnetism of the
polarized phase is analyzed, obtaining the population imbalance as a function
of interaction strength, Rabi coupling, and number density. Finally, the
inclusion of a external harmonic confinement is investigated by adopting the
local density approximation. We predict that a single atomic cloud can display
population imbalance near the center of the trap and a fully balanced
configuration at the periphery. | cond-mat_quant-gas |
Three-body repulsive forces among identical bosons in one dimension: I consider non-relativistic bosons interacting via pairwise potentials with
infinite scattering length and supporting no two-body bound states. To lowest
order in effective field theory, these conditions lead to non-interacting
bosons, since the coupling constant of the Lieb-Liniger model vanishes
identically in this limit. Since any realistic pairwise interaction is not a
mere delta function, the non-interacting picture is an idealisation indicating
that the effect of interactions is weaker than in the case of off-resonant
potentials. I show that the leading order correction to the ground state energy
for more than two bosons is accurately described by the lowest order three-body
force in effective field theory that arises due to the off-shell structure of
the two-body interaction. For natural two-body interactions with a
short-distance repulsive core and an attractive tail, the emergent three-body
interaction is repulsive and, therefore, three bosons do not form any bound
states. This situation is analogous to the two-dimensional repulsive Bose gas,
when treated using the lowest-order contact interaction, where the scattering
amplitude exhibits an unphysical Landau pole. The avoidance of this state in
the three-boson problem proceeds in a way that parallels the two-dimensional
case. These results pave the way for the experimental realisation of
one-dimensional Bose gases with pure three-body interactions using ultracold
atomic gases. | cond-mat_quant-gas |
Measuring entropy and mutual information in the two-dimensional Hubbard
model: We measure pressure and entropy of ultracold fermionic atoms in an optical
lattice for a range of interaction strengths, temperatures and fillings. Our
measurements demonstrate that, for low enough temperatures, entropy-rich
regions form locally in the metallic phase which are in contact with a
Mott-insulating phase featuring lower entropy. In addition, we also measure the
reduced density matrix of a single lattice site, and from the comparison
between the local and thermodynamic entropies we determine the mutual
information between a single lattice site and the rest of the system. For low
lattice fillings, we find the mutual information to be independent of
interaction strength, however, for half filling we find that strong
interactions suppress the correlations between a single site and the rest of
the system. | cond-mat_quant-gas |
Synthetic Dimensions for Cold Atoms from Shaking a Harmonic Trap: We introduce a simple scheme to implement synthetic dimensions in ultracold
atomic gases, which only requires two basic and ubiquitous ingredients: the
harmonic trap, which confines the atoms, combined with a periodic shaking. In
our approach, standard harmonic oscillator eigenstates are reinterpreted as
lattice sites along a synthetic dimension, while the coupling between these
lattice sites is controlled by the applied time-modulation. The phase of this
modulation enters as a complex hopping phase, leading straightforwardly to an
artificial magnetic field upon adding a second dimension. We show that this
artificial gauge field has important consequences, such as the counterintuitive
reduction of average energy under resonant driving, or the realisation of
quantum Hall physics. Our approach offers significant advantages over previous
implementations of synthetic dimensions, providing an intriguing route towards
higher-dimensional topological physics and strongly-correlated states. | cond-mat_quant-gas |
Compressibility and entropy of cold fermions in one dimensional optical
lattices: We calculate several thermodynamic quantities for repulsively interacting
one-dimensional fermions.We solve the Hubbard model at both zero and finite
temperatures using the Bethe-ansatz method. For arbitrary values of the
chemical potential, we calculate the particle number density, the double
occupancy, various compressibilities, and the entropy as a function of
temperature and interaction. We find that these thermodynamic quantities show a
characteristic behavior so that measurements of these quantities can be used as
a detection of temperature, the metal-insulator transition, and metallic and
insulating phases in the trap environment. Further, we discuss an experimental
scheme to extract these thermodynamic quantities from the column density
profiles. The entropy and the compressibility of the entire trapped atomic
cloud also reveal characteristic features indicating whether insulating and/or
metallic phases coexist in the trap. | cond-mat_quant-gas |
Interacting bosonic flux ladders with a synthetic dimension:
Ground-state phases and quantum quench dynamics: Flux ladders constitute the minimal setup enabling a systematic understanding
of the rich physics of interacting particles subjected simultaneously to strong
magnetic fields and a lattice potential. In this paper, the ground-state phase
diagram of a flux-ladder model is mapped out using extensive density-matrix
renormalization-group simulations. The emphasis is put on parameters which can
be experimentally realized exploiting the internal states of potassium atoms as
a synthetic dimension. The focus is on accessible observables such as the
chiral current and the leg-population imbalance. Considering a particle filling
of one boson per rung, we report the existence of a Mott-insulating Meissner
phase as well as biased-ladder phases on top of superfluids and Mott
insulators. Furthermore, we demonstrate that quantum quenches from suitably
chosen initial states can be used to probe the equilibrium properties in the
transient dynamics. Concretely, we consider the instantaneous turning on of
hopping matrix elements along the rungs or legs in the synthetic flux-ladder
model, with different initial particle distributions. We show that clear
signatures of the biased-ladder phase can be observed in the transient
dynamics. Moreover, the behavior of the chiral current in the transient
dynamics is discussed. The results presented in this paper provide guidelines
for future implementations of flux ladders in experimental setups exploiting a
synthetic dimension. | cond-mat_quant-gas |
Reliable equation of state for composite bosons in the 2D BCS-BEC
crossover: We briefly discuss recent experiments on the BCS-BEC crossover with ultracold
alkali-metal atoms both in three-dimensional configurations and two-dimensional
ones. Then we analyze the quantum-field-theory formalism used to describe an
attractive $D$-dimensional Fermi gas taking into account Gaussian fluctuations.
Finally, we apply this formalism to obtain a reliable equation of state of the
2D system at low temperaratures in the BEC regime of the crossover by
performing a meaningful dimensional regularization of the divergent zero-point
energy of collective bosonic excitations. | cond-mat_quant-gas |
Loss-induced phase separation and pairing for 3-species atomic lattice
fermions: We study the physics of a three-component Fermi gas in an optical lattice, in
the presence of a strong three-body constraint arising due to three-body loss.
Using analytical and numerical techniques, we show that an atomic color
superfluid phase is formed in this system and undergoes phase separation
between unpaired fermions and superfluid pairs. This phase separation survives
well above the critical temperature, giving a clear experimental signature of
the three-body constraint. | cond-mat_quant-gas |
A mobile ion in a Fermi sea: The remarkable single particle control of individual ions combined with the
versatility of ultracold atomic gases makes hybrid ion-atom system an exciting
new platform for quantum simulation of few- and many-body quantum physics.
Here, we study theoretically the properties of a mobile ion immersed in a
quantum degenerate gas of fermionic atoms. Using an effective low-energy
atom-ion interaction together with a well established approach that includes
exactly two-body correlations, we calculate the full spectral response of the
ion and demonstrate the existence of several quasiparticle branches, which are
charged analogues of the Fermi polaron observed in neutral atomic gases. Due to
the long-range nature of the atom-ion interaction, these ionic Fermi polarons
have several properties distinct from their neutral counterparts such as the
simultaneous presence of several stable states and smooth transitions from
repulsive to attractive polarons with increasing interaction strength.
Surprisingly, the residue of the ionic polaron is shown to increase with the
Fermi density for fixed interaction strength, which is in marked contrast to
the neutral polaron. The properties of the ionic polaron approach that of the
neutral polaron only in the low density limit where the average interparticle
spacing is larger than the characteristic length of the atom-ion interaction.
We finally analyse the effects of the Fermi gas on the molecular ions, which
are bound atom-dimer states. | cond-mat_quant-gas |
Quantum Chaos in Ultracold Collisions of Erbium: Atomic and molecular samples reduced to temperatures below 1 microkelvin, yet
still in the gas phase, afford unprecedented energy resolution in probing and
manipulating how their constituent particles interact with one another. For
simple atoms, such as alkalis, scattering resonances are extremely
well-characterized. However, ultracold physics is now poised to enter a new
regime, where far more complex species can be cooled and studied, including
magnetic lanthanide atoms and even molecules. For molecules, it has been
speculated that a dense forest of resonances in ultracold collision cross
sections will likely express essentially random fluctuations, much as the
observed energy spectra of nuclear scattering do. According to the
Bohigas-Giannoni-Schmit conjecture, these fluctuations would imply chaotic
dynamics of the underlying classical motion driving the collision. This would
provide a paradigm shift in ultracold atomic and molecular physics,
necessitating new ways of looking at the fundamental interactions of atoms in
this regime, as well as perhaps new chaos-driven states of ultracold matter. In
this report we provide the first experimental demonstration that random spectra
are indeed found at ultralow temperatures. In the experiment, an ultracold gas
of erbium atoms is shown to exhibit many Fano-Feshbach resonances, for bosons
on the order of 3 per gauss. Analysis of their statistics verifies that their
distribution of nearest-neighbor spacings is what one would expect from random
matrix theory. The density and statistics of these resonances are explained by
fully-quantum mechanical scattering calculations that locate their origin in
the anisotropy of the atoms' potential energy surface. Our results therefore
reveal for the first time chaotic behavior in the native interaction between
ultracold atoms. | cond-mat_quant-gas |
Different models of gravitating Dirac fermions in optical lattices: In this paper I construct the naive lattice Dirac Hamiltonian describing the
propagation of fermions in a generic 2D optical metric for different lattice
and flux-lattice geometries. First, I apply a top-down constructive approach
that we first proposed in [Boada {\it et al.,New J. Phys.} {\bf 13} 035002
(2011)] to the honeycomb and to the brickwall lattices. I carefully discuss how
gauge transformations that generalize momentum (and Dirac cone) shifts in the
Brillouin zone in the Minkowski homogeneous case can be used in order to change
the phases of the hopping. In particular, I show that lattice Dirac Hamiltonian
for Rindler spacetime in the honeycomb and brickwall lattices can be realized
by considering real and isotropic (but properly position dependent) tunneling
terms. For completeness, I also discuss a suitable formulation of Rindler Dirac
Hamiltonian in semi-synthetic brickwall and $\pi$-flux square lattices (where
one of the dimension is implemented by using internal spin states of atoms as
we originally proposed in [Boada {\it et al.,Phys. Rev. Lett. } {\bf 108}
133001 (2012)] and [Celi {\it et al.,Phys. Rev. Lett. } {\bf 112} 043001
(2012)]). | cond-mat_quant-gas |
Realizing Hopf Insulators in Dipolar Spin Systems: The Hopf insulator is a weak topological insulator characterized by an
insulating bulk with conducting edge states protected by an integer-valued
linking number invariant. The state exists in three-dimensional two-band
models. We demonstrate that the Hopf insulator can be naturally realized in
lattices of dipolar-interacting spins, where spin exchange plays the role of
particle hopping. The long-ranged, anisotropic nature of the dipole-dipole
interactions allows for the precise detail required in the momentum-space
structure, while different spin orientations ensure the necessary structure of
the complex phases of the hoppings. Our model features robust gapless edge
states at both smooth edges, as well as sharp edges obeying a certain
crystalline symmetry, despite the breakdown of the two-band picture at the
latter. In a companion manuscript [2105.10504], we provide a specific
experimental blueprint for implementing our proposal using ultracold polar
molecules of $^{40}$K$^{87}$Rb. | cond-mat_quant-gas |
State selective cooling of $\mathrm{SU}(N)$ Fermi-gases: We investigate a species selective cooling process of a trapped
$\mathrm{SU}(N)$ Fermi gas using entropy redistribution during adiabatic
loading of an optical lattice. Using high-temperature expansion of the Hubbard
model, we show that when a subset $N_A < N$ of the single-atom levels
experiences a stronger trapping potential in a certain region of space, the
dimple, it leads to improvement in cooling as compared to a $\mathrm{SU}(N_A)$
Fermi gas only. We show that optimal performance is achieved when all atomic
levels experience the same potential outside the dimple and we quantify the
cooling for various $N_A$ by evaluating the dependence of the final entropy
densities and temperatures as functions of the initial entropy. Furthermore,
considering ${}^{87}{\rm Sr}$ and ${}^{173}{\rm Yb}$ for specificity, we
provide a quantitative discussion of how the state selective trapping can be
achieved with readily available experimental techniques. | cond-mat_quant-gas |
Anomalous supersolidity in a weakly interacting dipolar Bose mixture on
a square lattice: We calculate the mean-field phase diagram of a zero-temperature, binary Bose
mixture on a square optical lattice, where one species possesses a
non-negligible dipole moment. Remarkably, this system exhibits supersolidity
for anomalously weak dipolar interaction strengths, which are readily
accessible with current experimental capabilities. The supersolid phases are
robust, in that they occupy large regions in the parameter space. Further, we
identify a first-order quantum phase transition between supersolid and
superfluid phases. Our results demonstrate the rich features of the dipolar
Bose mixture, and suggest that this system is well-suited for exploring
supersolidity in the experimental setting. | cond-mat_quant-gas |
Supersolid phases of lattice dipoles tilted in three-dimensions: By means of quantum Monte Carlo simulations we study phase diagrams of
dipolar bosons in a square optical lattice. The dipoles in the system are
parallel to each other and their orientation can be fixed in any direction of
the three-dimensional space. Starting from experimentally tunable parameters
like scattering length and dipolar interaction strength, we derive the
parameters entering the effective Hamiltonian. Depending on the direction of
the dipoles, various types of supersolids (e.g. checkerboard, stripe) and
solids (checkerboard, stripe, diagonal stripe, and an incompressible phase) can
be stabilized. Remarkably, we find a cluster supersolid characterized by the
formation of horizontal clusters of particles. These clusters order along a
direction at an angle with the horizontal. Moreover, we find what we call a
grain-boundary superfluid. In this phase, regions with solid order are
separated by extended defects -- grain boundaries -- which support
superfluidity. We also investigate the robustness of the stripe supersolid
against thermal fluctuations. Finally, we comment on the experimental
realization of the phases found. | cond-mat_quant-gas |
Realizing quantum Ising models in tunable two-dimensional arrays of
single Rydberg atoms: Spin models are the prime example of simplified manybody Hamiltonians used to
model complex, real-world strongly correlated materials. However, despite their
simplified character, their dynamics often cannot be simulated exactly on
classical computers as soon as the number of particles exceeds a few tens. For
this reason, the quantum simulation of spin Hamiltonians using the tools of
atomic and molecular physics has become very active over the last years, using
ultracold atoms or molecules in optical lattices, or trapped ions. All of these
approaches have their own assets, but also limitations. Here, we report on a
novel platform for the study of spin systems, using individual atoms trapped in
two-dimensional arrays of optical microtraps with arbitrary geometries, where
filling fractions range from 60 to 100% with exact knowledge of the initial
configuration. When excited to Rydberg D-states, the atoms undergo strong
interactions whose anisotropic character opens exciting prospects for
simulating exotic matter. We illustrate the versatility of our system by
studying the dynamics of an Ising-like spin-1/2 system in a transverse field
with up to thirty spins, for a variety of geometries in one and two dimensions,
and for a wide range of interaction strengths. For geometries where the
anisotropy is expected to have small effects we find an excellent agreement
with ab-initio simulations of the spin-1/2 system, while for strongly
anisotropic situations the multilevel structure of the D-states has a
measurable influence. Our findings establish arrays of single Rydberg atoms as
a versatile platform for the study of quantum magnetism. | cond-mat_quant-gas |
Fate of the Mollow triplet in strongly-coupled atomic arrays: Subwavelength arrays of quantum emitters have emerged as an interesting
platform displaying prominent collective effects. Here we study such arrays
under coherent driving, realizing an open quantum many-body problem in a
strongly non-linear regime. We show that the combination of dipolar
interactions and regular geometry have a dramatic effect on the spectrum of
emitted light: the famous Mollow triplet characterizing the emission of a
single atom develops a structured broadening with flat sidebands, with a
bandwidth determined by the dipolar interactions. This emission spectrum
characterizes atomic arrays and distinguishes them from disordered ensembles as
well as non-interacting emitters. Our predictions are based on a novel
dynamical mean-field theory (DMFT) approach to the problem, paving the way for
further studies of these systems. | cond-mat_quant-gas |
Spatially Modulated Interaction Induced Bound States and Scattering
Resonances: We study the two-body problem with a spatially modulated interaction
potential using a two-channel model, in which the inter-channel coupling is
provided by an optical standing wave and its strength modulates periodically in
space. As the modulation amplitudes increases, there will appear a sequence of
bound states. Part of them will cause divergence of the effective scattering
length, defined through the phase shift in the asymptotic behavior of
scattering states. We also discuss how the local scattering length, defined
through short-range behavior of scattering states, modulates spatially in
different regimes. These results provide a theoretical guideline for new
control technique in cold atom toolbox, in particular, for alkali-earth-(like)
atoms where the inelastic loss is small. | cond-mat_quant-gas |
Quantum-geometric perspective on spin-orbit-coupled Bose superfluids: We employ the Bogoliubov approximation to study how the quantum geometry of
the helicity states affects the superfluid properties of a spin-orbit-coupled
Bose gas in continuum. In particular we derive the low-energy Bogoliubov
spectrum for a plane-wave condensate in the lower helicity band and show that
the geometric contributions to the sound velocity are distinguished by their
linear dependences on the interaction strength, i.e., they are in sharp
contrast to the conventional contribution which has a square-root dependence.
We also discuss the roton instability of the plane-wave condensate against the
stripe phase and determine their phase transition boundary. In addition we
derive the superfluid density tensor by imposing a phase-twist on the
condensate order parameter and study the relative importance of its
contribution from the interband processes that is related to the quantum
geometry. | cond-mat_quant-gas |
Properties of the Superfluid in the Disordered Bose-Hubbard Model: We investigate the properties of the superfluid phase in the
three-dimensional disordered Bose-Hubbard model using Quantum Monte-Carlo
simulations. The phase diagram is generated using Gaussian disorder on the
on-site potential. Comparisons with box and speckle disorder show qualitative
similarities leading to the re-entrant behavior of the superfluid. Quantitative
differences that arise are controlled by the specific shape of the disorder.
Statistics pertaining to disorder distributions are studied for a range of
interaction strengths and system sizes, where strong finite-size effects are
observed. Despite this, both the superfluid fraction and compressibility remain
self-averaging throughout the superfluid phase. Close to the
superfluid-Bose-glass phase boundary, finite-size effects dominate but still
suggest that self-averaging holds. Our results are pertinent to experiments
with ultracold atomic gases where a systematic disorder averaging procedure is
typically not possible. | cond-mat_quant-gas |
Diffusion Monte Carlo methods for Spin-Orbit-Coupled ultracold Bose
gases: We present two Diffusion Monte Carlo (DMC) algorithms for systems of
ultracold quantum gases featuring synthetic spin-orbit interactions. The first
one is a discrete spin generalization of the T- moves spin-orbit DMC, which
provides an upper bound to the fixed-phase energy. The second is a
spin-integrated DMC method which recovers the fixed-phase property by avoiding
the definition of the effective Hamiltonian involved in the T-moves approach.
The latter is a more accurate method but it is restricted to spin-independent
two-body interactions. We report a comparison between both algorithms for
different systems. As a check of the efficiency of both methods, we compare the
DMC energies with results obtained with other numerical methods, finding
agreement between both estimation | cond-mat_quant-gas |
Many-body approach to low-lying collective excitations in a BEC
approaching collapse: An approximate many-body theory incorporating two-body correlations has been
employed to calculate low-lying collective multipole frequencies in a
Bose-Einstein condensate containing $A$ bosons, for different values of the
interaction parameter $\lambda=\frac{Aa_{s}}{a_{ho}}$. Significant difference
from the variational estimate of the Gross-Pitaevskii equation has been found
near the collapse region. This is attributed to two-body correlations and
finite range attraction of the realistic interatomic interaction. A large
deviation from the hydrodynamic model is also seen for the second monopole
breathing mode and the quadrupole mode for large positive $\lambda$. | cond-mat_quant-gas |
Multicriticality, Metastability, and Roton Feature in Bose-Einstein
Condensates with Three-Dimensional Spin-Orbit Coupling: We theoretically study homogeneously trapped atomic Bose-Einstein condensates
where all three momentum components couple to a pseudo-spin-$1/2$ degree of
freedom. Tuning the anisotropies of spin-orbit coupling and the spin-dependent
interactions is shown to provide access to a rich phase diagram with a
tetracritical point, first-order phase transitions, and multiple metastable
phases of stripe and plane-wave character. The elementary excitation spectrum
of the axial plane-wave phase features an anisotropic roton feature and can be
used to probe the phase diagram. In addition to providing a versatile
laboratory for studying fundamental concepts in statistical physics, the
emergence of metastable phases creates new opportunities for observing
false-vacuum decay and bubble nucleation in ultra-cold-atom experiments. | cond-mat_quant-gas |
Instability of superfluid Fermi gases induced by a roton-like density
mode in optical lattices: We study the stability of superfluid Fermi gases in deep optical lattices in
the BCS--Bose-Einstein condensation (BEC) crossover at zero temperature. Within
the tight-binding attractive Hubbard model, we calculate the spectrum of the
low-energy Anderson-Bogoliubov (AB) mode as well as the single-particle
excitations in the presence of superfluid flow in order to determine the
critical velocities. To obtain the spectrum of the AB mode, we calculate the
density response function in the generalized random-phase approximation
applying the Green's function formalism developed by C\^ot\'e and Griffin to
the Hubbard model. We find that the spectrum of the AB mode is separated from
the particle-hole continuum having the characteristic rotonlike minimum at
short wavelength due to the strong charge-density-wave fluctuations. The energy
of the rotonlike minimum decreases with increasing the lattice velocity and it
reaches zero at the critical velocity which is smaller than the pair breaking
velocity. This indicates that the superfluid state is energetically unstable
due to the spontaneous emission of the short-wavelength rotonlike excitations
of the AB mode instead due to pair-breaking. We determine the critical
velocities as functions of the interaction strength across the BCS-BEC
crossover regime. | cond-mat_quant-gas |
Local correlations in the attractive 1D Bose gas: from Bethe ansatz to
the Gross-Pitaevskii equation: We consider the ground-state properties of an extended one-dimensional Bose
gas with pointwise attractive interactions. We take the limit where the
interaction strength goes to zero as the system size increases at fixed
particle density. In this limit the gas exhibits a quantum phase transition. We
compute local correlation functions at zero temperature, both at finite and
infinite size. We provide analytic formulas for the experimentally relevant
one-point functions $g_2$, $g_3$ and analyze their finite-size corrections. Our
results are compared to the mean-field approach based on the Gross-Pitaevskii
equation which yields the exact results in the infinite system size limit, but
not for finite systems. | cond-mat_quant-gas |
Degenerate approach to the mean field Bose- Hubbard Hamiltonian: A degenerate variant of mean field perturbation theory for the on-site
Bose-Hubbard Hamiltonian is presented. We split the perturbation into two terms
and perform exact diagonalization in the two-dimensional subspace corresponding
to the degenerate states. The final relations for the second order ground state
energy and first order wave function do not contain singularities at integer
values of the chemical potentials. The resulting equation for the phase
boundary between superfluid and Mott states coincides with the prediction based
on the conventional mean field perturbation approach. | cond-mat_quant-gas |
A long-lived Higgs mode in a two-dimensional confined Fermi gas: The Higgs mode corresponds to the collective motion of particles due to the
vibrations of an invisible field. It plays a fundamental role for our
understanding of both low and high energy physics, giving elementary particles
their mass and leading to collective modes in condensed matter and nuclear
systems. The Higgs mode has been observed in a limited number of table-top
systems, where it however is characterised by a short lifetime due to decay
into a continuum of modes. A major goal which has remained elusive so far, is
therefore to realise a long-lived Higgs mode in a controllable system. Here, we
show how an undamped Higgs mode can be observed unambiguously in a Fermi gas in
a two-dimensional trap, close to a quantum phase transition between a normal
and a superfluid phase. We develop a first-principles theory of the pairing and
the associated collective modes, which is quantitatively reliable when the
pairing energy is much smaller than the trap level spacing, yet simple enough
to allow the derivation of analytical results. The theory includes the trapping
potential exactly, which is demonstrated to stabilize the Higgs mode by making
its decay channels discrete. Our results show how atoms in micro-traps can
unravel properties of a long-lived Higgs mode, including the role of
confinement and finite size effects. | cond-mat_quant-gas |
Modulational instability and soliton generation in chiral Bose-Einstein
condensates with zero-energy nonlinearity: By means of analytical and numerical methods, we address the modulational
instability (MI) in chiral condensates governed by the Gross-Pitaevskiiequation
including the current nonlinearity. The analysis shows that this nonlinearity
partly suppresses off the MI driven by the cubic self-focusing, although the
current nonlinearity is not represented in the system's energy (although it
modifies the momentum), hence it may be considered as zero-energy nonlinearity.
Direct simulations demonstrate generation of trains of stochastically
interacting chiral solitons by MI. In the ring-shaped setup, the MI creates a
single traveling solitary wave. The sign of the current nonlinearity determines
the direction of propagation of the emerging solitons. | cond-mat_quant-gas |
Thermodynamics of a Bose gas near the superfluid--Mott-insulator
transition: We study the thermodynamics near the generic (density-driven)
superfluid--Mott-insulator transition in the three-dimensional Bose-Hubbard
model using the nonperturbative renormalization-group approach. At low energy
the physics is controlled by the Gaussian fixed point and becomes universal.
Thermodynamic quantities can then be expressed in terms of the universal
scaling functions of the dilute Bose gas universality class while the
microscopic physics enters only {\it via} two nonuniversal parameters, namely
the effective mass $m^*$ and the "scattering length" $a^*$ of the elementary
excitations at the quantum critical point between the superfluid and
Mott-insulating phase. A notable exception is the condensate density in the
superfluid phase which is proportional to the quasi-particle weight $\Zqp$ of
the elementary excitations. The universal regime is defined by $m^*a^*{}^2 T\ll
1$ and $m^*a^*{}^2|\delta\mu|\ll 1$, or equivalently $|\bar n-\bar
n_c|a^*{}^3\ll 1$, where $\delta\mu=\mu-\mu_c$ is the chemical potential shift
from the quantum critical point $(\mu=\mu_c,T=0)$ and $\bar n-\bar n_c$ the
doping with respect to the commensurate density $\bar n_c$ of the T=0 Mott
insulator. We compute $\Zqp$, $m^*$ and $a^*$ and find that they vary strongly
with both the ratio $t/U$ between hopping amplitude and on-site repulsion and
the value of the (commensurate) density $\bar n_c$. Finally, we discuss the
experimental observation of universality and the measurement of $\Zqp$, $m^*$
and $a^*$ in a cold atomic gas in an optical lattice. | cond-mat_quant-gas |
Criticality-enhanced quantum sensing in ferromagnetic Bose-Einstein
condensates: role of readout measurement and detection noise: We theoretically investigate estimation of the control parameter in a
ferromagnetic Bose-Einstein condensate near second order quantum phase
transitions. We quantify sensitivity by quantum and classical Fisher
information and using the error-propagation formula. For these different
metrics, we find the same, beyond-standard-quantum-limit (SQL) scaling with
atom number near critical points, and SQL scaling away from critical points. We
find that both depletion of the $m_f=0$ Zeeman sub-level and transverse
magnetization provide signals of sufficient quality to saturate the sensitivity
scaling. To explore the effect of experimental imperfections, we study the
scaling around criticality at nonzero temperature and with nonzero detection
noise. Our results suggest the feasibility of sub-SQL sensing in ferromagnetic
condensates with current experimental capabilities. | cond-mat_quant-gas |
Intermediate super-exponential localization with Aubry-André chains: We demonstrate the existence of an intermediate super-exponential
localization regime for eigenstates of the Aubry-Andr\'e chain. In this regime,
the eigenstates localize factorially similarly to the eigenstates of the
Wannier-Stark ladder. The super-exponential decay emerges on intermediate
length scales for large values of the $\textit{winding length}$ -- the
quasi-period of the Aubry-Andr\'e potential. This intermediate localization is
present both in the metallic and insulating phases of the system. In the
insulating phase, the super-exponential localization is periodically
interrupted by weaker decaying tails to form the conventional asymptotic
exponential decay predicted for the Aubry-Andr\'e model. In the metallic phase,
the super-exponential localization happens for states with energies away from
the center of the spectrum and is followed by a super-exponential growth into
the next peak of the extended eigenstate. By adjusting the parameters it is
possible to arbitrarily extend the validity of the super-exponential
localization. A similar intermediate super-exponential localization regime is
demonstrated in quasiperiodic discrete-time unitary maps. | cond-mat_quant-gas |
Theoretical Prediction of Non-Hermitian Skin Effect in Ultracold Atom
Systems: Non-Hermitian skin effect, which refers to the phenomenon that an extensive
number of eigenstates are localized at the boundary, has been widely studied in
lattice models and experimentally observed in several classical systems. In
this work, we predict that the existence of the non-Hermitian skin effect in
the dissipative ultracold fermions with spin-orbit coupling, a continuous model
that has been implemented by the Hong-Kong group in a recent experiment. This
skin effect is robust against the variation of external parameters and trapping
potentials. We further reveal a dynamic sticky effect in our system, which has
a common physical origin with the non-Hermitian skin effect. Our work paves the
way for studying novel physical responses of non-Hermitian skin effect in
quantum systems. | cond-mat_quant-gas |
Signatures of non-trivial pairing in the quantum walk of two-component
bosons: Nearest neighbour bosons possessing only onsite interactions do not form
onsite bound pairs in their quantum walk due to fermionization. We obtain
signatures of non-trivial onsite pairing in the quantum walk of strongly
interacting two component bosons in a one dimensional lattice. By considering
an initial state with particles from different components located at the
nearest-neighbour sites in the central region of the lattice, we show that in
the dynamical evolution of the system, competing intra- and inter-component
onsite repulsion leads to the formation of onsite inter-component bound states.
We find that when the total number of particles is three, an inter-component
pair is favoured in the limit of equal intra- and inter-component interaction
strengths. However, when two bosons from each species are considered,
inter-component pairs and trimer are favoured depending on the ratios of the
intra- and inter-component interactions. In both the cases, we find that the
quantum walks exhibit a re-entrant behaviour as a function of inter-component
interaction. | cond-mat_quant-gas |
Virial coefficients for Bose and Fermi trapped gases beyond the unitary
limit: an S-Matrix approach: We study the virial expansion for three-dimensional Bose and Fermi gases at
finite temperature using an approximation that only considers two-body
processes and is valid for high temperatures and low densities. The first
virial coefficients are computed and the second is exact. The results are
obtained for the full range of values of the scattering length and the unitary
limit is recovered as a particular case. A weak coupling expansion is performed
and the free case is also obtained as a proper limit. The influence of an
anisotropic harmonic trap is considered using the Local Density Approximation -
LDA, analytical results are obtained and the special case of the isotropic trap
is discussed in detail. | cond-mat_quant-gas |
The Higgs mode in a superfluid of Dirac fermions: We study the Higgs amplitude mode in the s-wave superfluid state on the
honeycomb lattice inspired by recent cold atom experiments. We consider the
attractive Hubbard model and focus on the vicinity of a quantum phase
transition between semi-metal and superfluid phases. On either side of the
transition, we find collective mode excitations that are stable against decay
into quasiparticle-pairs. In the semi-metal phase, the collective modes have
"Cooperon" and exciton character. These modes smoothly evolve across the
quantum phase transition, and become the Anderson-Bogoliubov mode and the Higgs
mode of the superfluid phase. The collective modes are accommodated within a
window in the quasiparticle-pair continuum, which arises as a consequence of
the linear Dirac dispersion on the honeycomb lattice, and allows for sharp
collective excitations. Bragg scattering can be used to measure these
excitations in cold atom experiments, providing a rare example wherein
collective modes can be tracked across a quantum phase transition. | cond-mat_quant-gas |
Revealing Hidden Antiferromagnetic Correlations in Doped Hubbard Chains
via String Correlators: Topological phases, like the celebrated Haldane phase in spin-1 chains, defy
characterization through local order parameters. Instead, non-local string
order parameters can be employed to reveal their hidden order. Similar diluted
magnetic correlations appear in doped one-dimensional lattice systems due to
the phenomenon of spin-charge separation. Here we report on the direct
observation of such hidden magnetic correlations via quantum gas microscopy of
hole-doped ultracold Fermi-Hubbard chains. The measurement of non-local
spin-density correlation functions reveals a hidden finite-range
antiferromagnetic order, a direct consequence of spin-charge separation. Our
technique demonstrates how topological order can directly be measured in
experiments and it can be extended to higher dimensions to study the complex
interplay between magnetic order and density fluctuations. | cond-mat_quant-gas |
Thermodynamic signatures for topological phase transitions to Majorana
and Weyl superfluids in ultracold Fermi gases: We discuss the thermodynamic signatures for the topological phase transitions
into Majorana and Weyl superfluid phases in ultracold Fermi gases in two and
three dimensions in the presence of Rashba spin-orbit coupling and a Zeeman
field. We analyze the thermodynamic properties exhibiting the distinct nature
of the topological phase transitions linked with the Majorana fermions (2D
Fermi gas) and Weyl fermions (3D Fermi gas) which can be observed
experimentally, including pressure, chemical potential, isothermal
compressibility, entropy, and specific heat, as a function of the interaction
and the Zeeman field at both zero and finite temperatures. We conclude that
among the various thermodynamic quantities, the isothermal compressibility and
the chemical potential as a function of the artificial Zeeman field have the
strongest signatures of the topological transitions in both two and three
dimensions. | cond-mat_quant-gas |
Majorana edge-modes in a spinful particle conserving model: We show the presence of Majorana edge modes in an interacting fermionic
ladder with spin in a number conserved setting. The interchain single particle
hopping is suppressed and only a pair hopping is present between the different
chains of the ladder. Additionally, the hopping along the chains is spin
imbalanced and a transverse magnetic field is applied breaking time-reversal
invariance. We study the robustness of the topological phase with respect to an
on-site interaction between the spin-up and spin-down fermions and the spin
dependent imbalance of the hopping. The main result of the present work is that
the topological phase survives for a finite region in the parameter space in
the presence of interactions. The localized Majorana edge modes seems to be
more stable in the case when the on-site interaction is an attraction. | cond-mat_quant-gas |
Long-range transverse Ising model built with dipolar condensates in
two-well arrays: Dipolar Bose-Einstein condensates in an array of double-well potentials
realize an effective transverse Ising model with peculiar inter-layer
interactions, that may result under proper conditions in an anomalous
first-order ferromagnetic-antiferromagnetic phase transition, and nontrivial
phases due to frustration. The considered setup as well allows the study of
Kibble-Zurek defect formation, whose kink statistics follows that expected from
the universality class of the mean-field transverse Ising model in 1D.
Furthermore, random occupation of each layer of the stack leads to random
effective Ising interactions and generation of local transverse fields, thus
allowing the study of Anderson-like localization of imbalance perturbations in
the two-well stack under controllable conditions. | cond-mat_quant-gas |
Interactions and dynamics of one-dimensional droplets, bubbles and kinks: We explore the dynamics and interactions of multiple bright droplets and
bubbles, as well as the interactions of kinks with droplets and with antikinks,
in the extended one-dimensional Gross-Pitaevskii model including the
Lee-Huang-Yang correction. Existence regions are identified for the
one-dimensional droplets and bubbles in terms of their chemical potential,
verifying the stability of the droplets and exposing the instability of the
bubbles. The limiting case of the droplet family is a stable kink. The
interactions between droplets demonstrate in-phase (out-of-phase) attraction
(repulsion), with the so-called Manton's method explicating the observed
dynamical response, and mixed behavior for intermediate values of the phase
shift. Droplets bearing different chemical potentials experience mass-exchange
phenomena. Individual bubbles exhibit core expansion and mutual attraction
prior to their destabilization. Droplets interacting with kinks are absorbed by
them, a process accompanied by the emission of dispersive shock waves and gray
solitons. Kink-antikink interactions are repulsive, generating
counter-propagating shock waves. Our findings reveal dynamical features of
droplets and kinks that can be detected in current experiments. | cond-mat_quant-gas |
Above-Barrier Reflection of Cold Atoms by Resonant Laser Light within
the Gross-Pitaevskii Approximation: Above-barrier reflection of cold alkali atoms by resonant laser light was
considered analytically within the Gross-Pitaevskii approximation. Correction
for the reflection coefficient because of a weak nonlinearity of the stationary
Schroedinger equation has been derived using multiscale analysis as a form of
perturbation theory. The nonlinearity adds spatial harmonics to linear incident
and reflecting waves. It was shown that the role of nonlinearity increases when
the kinetic energy of an atom is nearly to the height of the potential barrier.
Results are compared to the known numerical derivations for wave functions of
the Gross-Pitaevskii equation with the step potential. | cond-mat_quant-gas |
A proof to Biswas-Mitra-Bhattacharyya conjecture for ideal quantum gas
trapped under generic power law potential $U=\sum_{i=1} ^d c_i |\frac
{x_i}{a_i}|^{n_i}$ in $d$ dimension: The well known relation for ideal classical gas $\Delta \epsilon^2=kT^2 C_V$
which does not remain valid for quantum system is revisited. A new connection
is established between energy fluctuation and specific heat for quantum gases,
valid in the classical limit and the degenerate quantum regime as well. Most
importantly the proposed Biswas-Mitra-Bhattacharyya (BMB) conjecture (Biswas
$et.$ $al.$, J. Stat. Mech. P03013, 2015.) relating hump in energy fluctuation
and discontinuity of specific heat is proved and precised in this manuscript. | cond-mat_quant-gas |
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