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We theoretically calculate and experimentally measure the beyond-mean-field (BMF) equation of state in a coherently coupled two-component Bose-Einstein condensate (BEC) in the regime where averaging of the interspecies and intraspecies coupling constants over the hyperfine composition of the single-particle dressed state predicts the exact cancellation of the two-body interaction. We show that with increasing the Rabi-coupling frequency $\mathrm{\ensuremath{\Omega}}$, the BMF energy density crosses over from the nonanalytic Lee-Huang-Yang scaling $\ensuremath{\propto}{n}^{5/2}$ to an expansion in integer powers of density, where, in addition to a two-body BMF term $\ensuremath{\propto}{n}^{2}\sqrt{\mathrm{\ensuremath{\Omega}}}$, there emerges a repulsive three-body contribution $\ensuremath{\propto}{n}^{3}/\sqrt{\mathrm{\ensuremath{\Omega}}}$. We experimentally evidence these two contributions, thanks to their different scaling with $\mathrm{\ensuremath{\Omega}}$, in the expansion of a Rabi-coupled two-component $^{39}\mathrm{K}$ condensate in a waveguide. By studying the expansion with and without Rabi coupling, we reveal an important feature relevant for observing BMF effects and associated phenomena in mixtures with spin-asymmetric losses: Rabi coupling helps preserve the spin composition and thus prevents the system from drifting away from the point of the vanishing mean field.

We theoretically investigate the elusive Andreev-Bashkin collisionless drag for a two-component onedimensional Bose-Hubbard model on a ring. By means of tensor network algorithms, we calculate the superfluid stiffness matrix as a function of intra- and interspecies interactions and of the lattice filling. We then focus on the most promising region close to the so-called pair-superfluid phase, where we observe that the drag can become comparable with the total superfluid density. We elucidate the importance of the drag in determining the long-range behavior of the correlation functions and the spin speed of sound. In this way, we are able to provide an expression for the spin Luttinger parameter $K_S$ in terms of drag and the spin susceptibility. Our results are promising in view of implementing the system by using ultracold Bose mixtures trapped in deep optical lattices, where the size of the sample is of the same order of the number of particles we simulate. Importantly, the mesoscopicity of the system, far from being detrimental, appears to favor a large drag, avoiding the Berezinskii-Kosterlitz-Thouless jump at the transition to the pair-superfluid phase which would reduce the region where a large drag can be observed., Comment: 7 pages, 6 figures. Final version published as Letter in PRR

We report on the experimental characterization of a spatially extended Josephson junction realized with a coherently-coupled two-spin-component Bose-Einstein condensate. The cloud is trapped in an elongated potential such that that transverse spin excitations are frozen. We extract the non-linear parameter with three different manipulation protocols. The outcomes are all consistent with a simple local density approximation of the spin hydrodynamics, i.e., of the so-called Bose-Josephson junction equations. We also identify a method to produce states with a well defined uniform magnetization., 8 pages, 4 figures

A rich variety of physical effects in spin dynamics arises at the interface between different magnetic materials. Engineered systems with interlaced magnetic structures have been used to implement spin transistors, memories and other spintronic devices. However, experiments in solid state systems can be difficult to interpret because of disorder and losses. Here, we realize analogues of magnetic junctions using a coherently-coupled mixture of ultracold bosonic gases. The spatial inhomogeneity of the atomic gas makes the system change its behavior from regions with oscillating magnetization -- resembling a magnetic material in the presence of an external transverse field -- to regions with a defined magnetization, as in magnetic materials with a ferromagnetic anisotropy stronger than external fields. Starting from a far-from-equilibrium fully polarized state, magnetic interfaces rapidly form. At the interfaces, we observe the formation of short-wavelength magnetic waves. They are generated by a quantum torque contribution to the spin current and produce strong spatial anticorrelations in the magnetization. Our results establish ultracold gases as a platform for the study of far-from-equilibrium spin dynamics in regimes that are not easily accessible in solid-state systems., 9 pages, 5 figures

We investigate the properties of quantized vortices in a dipolar Bose-Einstein condensed gas by means of a generalized Gross-Pitaevskii equation. The size of the vortex core hugely increases by increasing the weight of the dipolar interaction, approaching the transition to the supersolid phase. The critical angular velocity for the existence of an energetically stable vortex decreases in the supersolid, due to the reduced value of the density in the interdroplet region. The angular momentum per particle associated with the vortex line is shown to be smaller than $\ensuremath{\hbar}$, reflecting the reduction of the global superfluidity. The real-time vortex nucleation in a rotating trap is shown to be triggered, as for a standard condensate, by the softening of the quadrupole mode. For large angular velocities, when the distance between vortices becomes comparable to the interdroplet distance, the vortices are arranged into a honeycomb structure, which coexists with the triangular geometry of the supersolid lattice and persists during the free expansion of the atomic cloud.

Distintictive features of supersolids show up in their rotational properties. We calculate the moment of inertia of a harmonically trapped dipolar Bose-Einstein condensed gas as a function of the tunable scattering length parameter, providing the transition from the (fully) superfluid to the supersolid phase and eventually to an incoherent crystal of self-bound droplets. The transition from the superfluid to the supersolid phase is characterized by a jump in the moment on inertia, revealing its first order nature. In the case of elongated trapping in the plane of rotation we show that the the moment of inertia determines the value of the frequency of the scissors mode, which is significantly affected by the reduction of superfluidity in the supersolid phase. The case of isotropic trapping is instead well suited to study the formation of quantized vortices, which are shown to be characterized, in the supersolid phase, by a sizeable deformed core, caused by the presence of the sorrounding density peaks., Comment: Improved text and pictures. Version accepted to be published in Phys. Rev. Lett

We theoretically investigate the spin-dipole oscillation of a strongly interacting Fermi gas in a harmonic trap. By using a combined diagrammatic strong-coupling theory with a local density approximation and a sum rule approach, we clarify the temperature dependence of the spin-dipole frequency near the unitarity, which is deeply related to the spin susceptibility, as well as pairing correlations. While the spin-dipole frequency exactly coincides with the trap frequency in a non-interacting Fermi gas, it is shown to remarkably be enhanced in the superfluid state, because of the suppression of the spin degree of freedom due to the spin-singlet Cooper-pair formation. In strongly interacting Fermi gases, this enhancement occurs even above the superfluid phase transition temperature, due to the strong pairing correlations., 15 pages, 7 figures

A domain wall of relative phase in a flattened harmonically-trapped Bose-Einstein condensed mixture of two atomic hyperfine states, subject to a stationary Rabi coupling of intensity $\Omega$, is predicted to decay through two different mechanisms. For small values of $\Omega$ the instability has an energetic nature and is associated with the formation of a vortex-antivortex pair of the same atomic hyperfine states, whose motion inside the trap causes the emergence of magnetization, the bending of the domain wall and its consequent fragmentation. For large values of $\Omega$ the domain wall instead undergoes a dynamic snake instability, caused by the negative value of its effective mass and results in the fast fragmentation of the wall into smaller domain walls confining vortex pairs of different atomic species. Numerical predictions are given by solving the time-dependent Gross-Pitaevskii equation in experimentally available configurations of mixtures of sodium atomic gases., Comment: 9 pages (5 of main text and 4 of supplementary material), 4 figures

The existence of a paradoxical supersolid phase of matter, possessing the apparently incompatible properties of crystalline order and superfluidity, was predicted 50 years ago. Solid helium was the natural candidate, but there supersolidity has not been observed yet, despite numerous attempts. Ultracold quantum gases have recently shown the appearance of the periodic order typical of a crystal, due to various types of controllable interactions. A crucial feature of a D-dimensional supersolid is the occurrence of up to D+1 gapless excitations reflecting the Goldstone modes associated with the spontaneous breaking of two continuous symmetries: the breaking of phase invariance, corresponding to the locking of the phase of the atomic wave functions at the origin of superfluid phenomena, and the breaking of translational invariance due to the lattice structure of the system. The occurrence of such modes has been the object of intense theoretical investigations, but their experimental observation is still missing. Here we demonstrate the supersolid symmetry breaking through the appearance of two distinct compressional oscillation modes in a harmonically trapped dipolar Bose-Einstein condensate, reflecting the gapless Goldstone excitations of the homogeneous system. We observe that the two modes have different natures, with the higher frequency mode associated with an oscillation of the periodicity of the emergent lattice and the lower one characterizing the superfluid oscillations. Our work paves the way to explore the two quantum phase transitions between the superfluid, supersolid and solid-like configurations that can be accessed by tuning a single interaction parameter.

We analyze the dynamics of a set of two-level atoms coupled to the electromagnetic environment within a waveguide. This problem is often tackled by assuming a weak coupling between the atoms and the environment as well as the associated Markov approximation. We show that the accuracy of such an approximation may be more limited than in the single-atom case and may also be strongly determined by the presence of collective effects produced by atom-atom interactions. To this aim, we solve the full problem with exact diagonalization and also the time-dependent density matrix renormalization group method, and we compare the result to that obtained within a weak-coupling master equation and with the Dicke approximation. Finally, we study the dynamics of the entanglement within the system when considering several interatomic distances and atomic frequencies.

We report on the study of binary collisions between quantum droplets formed by an attractive mixture of ultracold atoms. We distinguish two main outcomes of the collision, i.e. merging and separation, depending on the velocity of the colliding pair. The critical velocity $v_c$ that discriminates between the two cases displays a different dependence on the atom number $N$ for small and large droplets. By comparing our experimental results with numerical simulations, we show that the non-monotonic behavior of $v_c(N)$ is due to the crossover from a compressible to an incompressible regime, where the collisional dynamics is governed by different energy scales, i.e. the droplet binding energy and the surface tension. These results also provide the first evidence of the liquid-like nature of quantum droplets in the large $N$ limit, where their behavior closely resembles that of classical liquid droplets.

We study the dynamics of a two-level impurity embedded in a two-dimensional Bose-Hubbard model at zero temperature from an open quantum system perspective. Results for the decoherence across the whole phase diagram are presented, with a focus on the critical region close to the transition between superfluid and Mott insulator. In particular, we show how the decoherence and the deviation from a Markovian behaviour are sensitive to whether the transition is crossed at commensurate or incommensurate densities. The role of the spectrum of the Bose-Hubbard environment and its non-Gaussian statistics, beyond the standard independent boson model, is highlighted. Our analysis resorts on a recently developed method [Phys. Rev. Research 2, 033276 (2020)] - closely related to slave boson approaches - that enables us to capture the correlations across the whole phase diagram. This semi-analytical method provides us with a deep insight into the physics of the spin decoherence in the superfluid and Mott phases as well as close to the phase transitions., 27 pages (main text: 14 pages), 6 figures

A recent experiment by P. Bordia et al. (Periodically Driving a Many Body Localized Quantum System, Nat Phys, Jan 2017) has demonstrated that periodically modulating the potential of a localised many-body quantum system described by the Aubry-Andr\'e Hamiltonian with on-site interactions can lead to a many-body localisation-delocalisation transition, provided the modulation amplitude is big enough. Here, we consider the noninteracting counterpart of that model in order to explore its phase diagram as a function of the strength of the disordered potential, the driving frequency and its amplitude. We will first of all mimic the experimental procedure of the experiment and use the even-odd sites imbalance as a parameter in order to discern between different phases. Then we compute the Floquet eigenstates and relate the localisation-delocalisation transition to their Inverse Participation Ratio. Both these approaches show that the delocalisation transition occurs for frequencies that are low compared to the bandwidth of the time independent model. Moreover, in agreement with the results of the experiment there is an amplitude threshold below which no delocalisation transition occurs. We estimate both the critical values for the frequency and the amplitude.

We present a strategy based on two-dimensional arrays of coupled linear optical resonators to investigate the two-body physics of interacting bosons in one-dimensional lattices. In particular, we want to address the bound pairs in topologically nontrivial Su-Schrieffer-Heeger arrays. Taking advantage of the driven-dissipative nature of the resonators, we propose spectroscopic protocols to detect and tomographically characterize bulk doublon bands and doublon edge states from the spatially resolved transmission spectra, and to highlight Feshbach resonance effects in two-body collision processes. We discuss the experimental feasibility using state-of-the-art devices, with a specific eye on arrays of semiconductor micropillar cavities., SCOPUS: ar.j, info:eu-repo/semantics/published

When the spectrum of magnetic excitations of a quantum mixture is much softer than the density spectrum, the system becomes effectively incompressible and can host magnetic defects. These are characterized by the presence of a topological defect in one of the two species and by a local modification of the density in the second one, the total density being practically unaffected. For miscible mixtures interacting with equal intraspecies coupling constants the width of these magnetic defects is fixed by the difference between the intraspecies and interspecies coupling constants and becomes larger and larger as one approaches the demixing transition. When the density of the filling component decreases, the incompressibility condition breaks down and we predict the existence of a critical filling, below which all the atoms of the minority component remain bound in the core of the topological defect. Applications to the sodium case both in uniform and harmonically trapped configurations are considered and a protocol to produce experimentally these defects is discussed. The case of binary mixtures interacting with unequal intraspecies forces and experiencing buoyancy is also addressed., Comment: 10 pages, 6 figures

We study a mixture of two superfluids with density-density and current-current (Andreev-Bashkin) interspecies interactions. The Andreev-Bashkin coupling gives rise to a dissipationless drag (or entrainment) between the two superfluids. Within the quantum hydrodynamics approximation, we study the relations between speeds of sound, susceptibilities and static structure factors, in a generic model in which the density and spin dynamics decouple. Due to translational invariance, the density channel does not feel the drag. The spin channel, instead, does not satisfy the usual Bijl-Feynman relation, since the f-sum rule is not exhausted by the spin phonons. The very same effect on one dimensional Bose mixtures and their Luttinger liquid description is analysed within perturbation theory. Using diffusion quantum Monte Carlo simulations of a system of dipolar gases in a double layer configuration, we confirm the general results. Given the recent advances in measuring the counterflow instability, we also study the effect of the entrainment on the dynamical stability of a superfluid mixture with non-zero relative velocity., Comment: 12 pages, 4 figures

Ferromagnetism is a manifestation of strong repulsive interactions between itinerant fermions in condensed matter. Whether short-ranged repulsion alone is sufficient to stabilize ferromagnetic correlations in the absence of other effects, like peculiar band dispersions or orbital couplings, is however unclear. Here, we investigate ferromagnetism in the minimal framework of an ultracold Fermi gas with short-range repulsive interactions tuned via a Feshbach resonance. While fermion pairing characterises the ground state, our experiments provide signatures suggestive of a metastable Stoner-like ferromagnetic phase supported by strong repulsion in excited scattering states. We probe the collective spin response of a two-spin mixture engineered in a magnetic domain-wall-like configuration, and reveal a substantial increase of spin susceptibility while approaching a critical repulsion strength. Beyond this value, we observe the emergence of a time-window of domain immiscibility, indicating the metastability of the initial ferromagnetic state. Our findings establish an important connection between dynamical and equilibrium properties of strongly-correlated Fermi gases, pointing to the existence of a ferromagnetic instability., Comment: 8 + 17 pages, 4 + 8 figures, 44 + 19 references

We study the ground state and excitations of a one-dimensional trapped polarized Fermi gas interacting with a single impurity. First, we study the tunnelling dynamics of the impurity through a potential barrier, such as one effectively created by a double-well trap. To this end, we perform an exact diagonalization of the full few-body Hamiltonian and analyze the results in a Local Density Approximation. Off-diagonal and one-particle correlation matrices are studied and are shown to be useful for discerning between different symmetries of the states. Second, we consider a radio-frequency (RF) spectroscopy of our system and the resulting spectral function. These calculations can motivate future experiments, which can provide a further insight into the physics of a Fermi polaron., 9 pages, 12 figures

We study the low-energy excitations of the Bose-Hubbard model in the strongly-interacting superfluid phase using a Gutzwiller approach and extract the single-particle and single-hole excitation amplitudes for each mode. We report emergent mode-dependent particle-hole symmetry on specific arc-shaped lines in the phase diagram connecting the well-known Lorentz-invariant limits of the Bose-Hubbard model. By tracking the in-phase particle-hole symmetric oscillations of the order parameter, we provide an answer to the long-standing question about the fate of the pure amplitude Higgs mode away from the integer-density critical point. Furthermore, we point out that out-of-phase oscillations are responsible for a full suppression of the condensate density oscillations of the gapless Goldstone mode. Possible detection protocols are also discussed., Comment: 6 pages, 3 figures

We consider two interacting bosons in a dimerized Su-Schrieffer-Heeger (SSH) lattice. We identify a rich variety of two-body states. In particular, for open boundary conditions and moderate interactions, edge bound states (EBS) are present even for the dimerization that does not sustain single-particle edge states. Moreover, for large values of the interactions, we find a breaking of the standard bulk-boundary correspondence. Based on the mapping of two interacting particles in one dimension onto a single particle in two dimensions, we propose an experimentally realistic coupled optical fibers setup as quantum simulator of the two-body SSH model. This setup is able to highlight the localization properties of the states as well as the presence of a resonant scattering mechanism provided by a bound state that crosses the scattering continuum, revealing the closed-channel population in real time and real space., Comment: 14 pages; 14 figures

We theoretically propose an experimentally viable scheme to use an impurity atom in an atomic Bose-Einstein condensate, in order to realize condensed-matter analogs of quantum vacuum effects. In a suitable atomic level configuration, the collisional interaction between the impurity atom and the density fluctuations in the condensate can be tailored to closely reproduce the electric-dipole coupling of quantum electrodynamics. By virtue of this analogy, we recover and extend the paradigm of electromagnetic vacuum forces to the domain of cold atoms, showing in particular the emergence, at supersonic atomic speeds, of a novel power-law scaling of the Casimir force felt by the atomic impurity, as well as the occurrence of a quantum frictional force, accompanied by the Ginzburg emission of Bogoliubov quanta. Observable consequences of these quantum vacuum effects in realistic spectroscopic experiments are discussed.

We study the two-body bound and scattering states of two particles in a one dimensional optical lattice in the presence of a coherent coupling between two internal atomic levels. Due to the interplay between periodic potential, interactions and coherent coupling, the internal structure of the bound states depends on their center of mass momentum. This phenomenon corresponds to an effective momentum-dependent magnetic field for the dimer pseudo-spin, which could be observed in a chirping of the precession frequency during Bloch oscillations. The essence of this effect can be easily interpreted in terms of an effective bound state Hamiltonian. Moreover for indistinguishable bosons, the two-body eigenstates can present simultaneously attractive and repulsive bound-state nature or even bound and scattering properties., 11 pages, 9 figures

We study the decay rate $\mathrm{\ensuremath{\Gamma}}(k)$ of density excitations of two-component Bose-Einstein condensates at zero temperature. Those excitations, where the two components oscillate in phase, include the Goldstone mode resulting from condensation. While within Bogoliubov approximation the density sector and the spin (out-of-phase) sector are independent, they couple at the three-phonon level. For a Bose-Bose mixture we find that the Belyaev decay is slightly modified due to the coupling with the gapless spin mode. At the phase separation point the decay rate changes instead from the standard ${k}^{5}$ to a ${k}^{5/2}$ behavior due to the parabolic nature of the spin mode. If instead a coherent coupling between the two components is present, the spin sector is gapped and, away from the ferromagnetic-like phase transition point, the decay of the density mode is not affected. On the other hand, at the transition point, when the spin fluctuations become critical, the Goldstone mode is not well defined anymore since $\mathrm{\ensuremath{\Gamma}}(k)\ensuremath{\propto}k$. As a consequence, we show that the friction induced by a moving impurity is enhanced---a feature which could be experimentally tested. Our results apply to every nonlinear 2-component quantum hydrodynamic Hamiltonian which is time-reversal invariant and possesses an $U(1)\ifmmode\times\else\texttimes\fi{}{\mathbb{Z}}_{2}$ symmetry.

We study the bosonic two-body problem in a Su-Schrieffer-Heeger dimerized chain with on-site and nearest-neighbor interactions. We find two classes of bound states. The first, similar to the one induced by on-site interactions, has its center of mass on the strong link, whereas the second, existing only thanks to nearest-neighbors interactions, is centered on the weak link. We identify energy crossings between these states and analyse them using exact diagonalization and perturbation theory. In the presence of open boundary conditions, novel strongly-localized edge-bound states appear in the spectrum as a consequence of the interplay between lattice geometry, on-site and nearest-neighbor interactions. Contrary to the case of purely on-site interactions, such EBS persist even in the strongly interacting regime., Comment: 12 pages, 8 figures; Submitted to EPJ Special Topics, Quantum Gases and Quantum Coherence

The dynamic behavior of vortex pairs in two-component coherently (Rabi) coupled Bose-Einstein condensates is investigated in the presence of harmonic trapping. We discuss the role of the surface tension associated with the domain wall connecting two vortices in condensates of atoms occupying different spin states and its effect on the precession of the vortex pair. The results, based on the numerical solution of the Gross-Pitaevskii equations, are compared with the predictions of an analytical macroscopic model and are discussed as a function of the size of the pair, the Rabi coupling and the inter-component interaction. We show that the increase of the Rabi coupling results in the disintegration of the domain wall into smaller pieces, connecting vortices of new-created vortex pairs. The resulting scenario is the analogue of quark confinement and string breaking in quantum chromodynamics., Comment: 5 pages, 5 figures

We study the electromagnetic force exerted on a pair of parallel slab waveguides by the light propagating through them. We have calculated the dependence of the force on the slab separation by means of the Maxwell--Stress tensor formalism and we have discussed its main features for the different propagation modes: spatially symmetric (antisymmetric) modes give rise to an attractive (repulsive) interaction. We have derived the asymptotic behaviors of the force at small and large separation and we have quantitatively estimated the mechanical deflection induced on a realistic air-bridge structure., Comment: 10 pages, 6 figures

We describe the ground state of a gas of bosonic atoms with two coherently coupled internal levels in a deep optical lattice in a one dimensional geometry. In the single-band approximation this system is described by a Bose-Hubbard Hamiltonian. The system has a superfluid and a Mott insulating phase which can be either paramagnetic or ferromagnetic. We characterize the quantum phase transitions at unit filling by means of a density matrix renormalization group technique and compare it with a mean-field approach. The presence of the ferromagnetic Ising-like transition modifies the Mott lobes. In the Mott insulating region the system maps to the ferromagnetic spin-1/2 XXZ model in a transverse field and the numerical results compare very well with the analytical results obtained from the spin model. In the superfluid regime quantum fluctuations strongly modify the phase transition with respect to the well established mean-field three dimensional classical bifurcation., Comment: 6 pages, 3 figures

We study the role of the dipolar-induced resonance (DIR) in a quasi-one-dimensional system of ultracold bosons. We first describe the effect of the DIR on two particles in a harmonic trap. Then, we consider a deep optical lattice loaded with ultracold dipolar bosons. In order to describe this system, we introduce a novel atom-dimer extended Bose-Hubbard model, which is the minimal model correctly accounting for the DIR. We analyze the impact of the DIR on the phase diagram at T = 0 by exact diagonalization of a small-sized system. We show that the DIR strongly affects this phase diagram. In particular, we predict the mass density wave to occur in a narrow domain corresponding to weak nearest-neighbor interactions, and we predict the occurrence of a collapse phase for stronger dipolar interactions.

We investigate a dilute Fermi gas of polar molecules confined into a bilayer setup with dipole moments polarized perpendicular to the layers. In particular, we consider the extreme case of population imbalance, where we have only one particle in one layer and many particles in the other one. The single molecule is attracted by the dilute Fermi-gas through the inter-layer dipole-dipole force presenting an interesting impurity problem with longrange anistropic interaction. We calulate the chemical potential of the impurity, in second order perturbation theory and in ladder approximation with a Brueckner-Hartree-Fock approach. Moreover, we determine the momentum relaxation rate of the impurity, which is related to the "dipolar" drag effect. For a confined system we relate the results for the chemical potential with the measurement of the collective modes of the impurity. The momentum relaxation rate provide instead an estimate on how quickly the oscillations are damped., Comment: 7 pages, 3 figures

We present a self-consistent study of coherently coupled two-component Bose-Einstein condensates. Finite spin-flipping coupling changes the first order demixing phase transition for Bose-Bose mixtures to a second order phase transition between an unpolarized and a polarized state. We analise the excitation spectrum and the structure factor along the transition for a homogeneous system. We discuss the main differences at the transition between a coherent coupled gas and a two-component mixture. We finally study the ground state when spin-(in)dependent trapping potentials are added to the system, focusing on optical lattices, which give rise to interesting new configurations., Comment: 11 pages, 11 figures, minor changes in figures and references, accepted for publication in EPJD

International audience; We study several realistic configurations allowing to realize an acoustic horizon in the flow of a one dimensional Bose-Einstein condensate. In each case we give an analytical description of the flow pattern, of the spectrum of Hawking radiation and of the associated quantum fluctuations. Our calculations confirm that the non local correlations of the density fluctuations previously studied in a simplified model provide a clear signature of Hawking radiation also in realistic configurations. In addition we explain by direct computation how this non local signal relates to short range modifications of the density correlations.

The observation of significant dipolar effects in gases of ultra-cold polar molecules typically demands a strong external electric field to polarize the molecules. We show that even in the absence of a significant polarization, dipolar effects may play a crucial role in the physics of polar molecules in bilayers, provided that the molecules in each layer are initially prepared in a different rotational state. Then, inter-layer dipolar interactions result in a non-local swap of the rotational state between molecules in different layers, even for weak applied electric fields. The inter-layer scattering due to the dipole-dipole interaction leads to a non-trivial dependence of the swapping rate on density, temperature, inter-layer spacing, and population imbalance. For reactive molecules like KRb, chemical recombination immediately follows a non-local swap and dominates the losses even for temperatures well above quantum degeneracy, and could be hence observed under current experimental conditions. PACS numbers: 67.85.-d, 34.50.Cx A new generation of experiments has started to explore the remarkable novel physics of dipolar gases, in which dipole-dipole interactions play a key role [1]. These interactions, being long-range and anisotropic, differ from the short-range isotropic interactions which have dominated up to now ultra-cold atomic physics. Polar molecules are expected to provide fascinating new scenarios for quantum gases due to their large electric dipole moments. Recent experiments on preparation and control of ro-vibrational and hyperfine states of KRb at JILA [2] open new perspectives towards a degenerate quantum gas of polar molecules. Unfortunately chemical recombination due to the reactive character of KRb has up to now prevented to reach quantum degeneracy [3]. However, dipolar interactions between partially polarized

We investigate the metastability associated with the first order transition from normal to superfluid phases in the phase diagram of two-component polarised Fermi gases.We begin by detailing the dominant decay processes of single quasiparticles.Having determined the momentum thresholds of each process and calculated their rates, we apply this understanding to a Fermi sea of polarons by linking its metastability to the stability of individual polarons, and predicting a region of metastability for the normal partially polarised phase. In the limit of a single impurity, this region extends from the interaction strength at which a polarised phase of molecules becomes the groundstate, to the one at which the single quasiparticle groundstate changes character from polaronic to molecular. Our argument in terms of a Fermi sea of polarons naturally suggests their use as an experimental probe. We propose experiments to observe the threshold of the predicted region of metastability, the interaction strength at which the quasiparticle groundstate changes character, and the decay rate of polarons., 12 pages, 5 figures

We investigate the number fluctuations in small cells of quantum gases pointing out important deviations from the thermodynamic limit fixed by the isothermal compressibility. Both quantum and thermal fluctuations in weakly as well as highly compressible fluids are considered. For the two-dimensional (2D) superfluid Bose gas we find a significant quenching of fluctuations with respect to the thermodynamic limit, in agreement with recent experimental findings. An enhancement of the thermal fluctuations is instead predicted for the 2D dipolar superfluid Bose gas, which becomes dramatic when the size of the sample cell is of the order of the wavelength of the rotonic excitation induced by the interaction., 6 pages, 5 figures

International audience; We study acoustic white holes in a steadily flowing atomic Bose-Einstein condensate. A white hole configuration is obtained when the flow velocity goes from a super-sonic value in the upstream region to a sub-sonic one in the downstream region. The scattering of phonon wavepackets on a white hole horizon is numerically studied in terms of the Gross-Pitaevskii equation of mean-field theory: dynamical stability of the acoustic white hole is found, as well as a signature of a nonlinear back-action of the incident phonon wavepacket onto the horizon. The correlation pattern of density fluctuations is numerically studied by means of the truncated-Wigner method which includes quantum fluctuations. Signatures of the white hole radiation of correlated phonon pairs by the horizon are characterized; analogies and differences with Hawking radiation from acoustic black holes are discussed. In particular, a short wavelength feature is identified in the density correlation function, whose amplitude steadily grows in time since the formation of the horizon. The numerical observations are quantitatively interpreted by means of an analytical Bogoliubov theory of quantum fluctuations for a white hole configuration within the step-like horizon approximation.

We apply the microscopic Bogoliubov theory of dilute Bose-Einstein condensates to analyze quantum and thermal fluctuations in a flowing atomic condensate in the presence of a sonic horizon. For the simplest case of a step-like horizon, closed-form analytical expressions are found for the spectral distribution of the analog Hawking radiation and for the density correlation function. The peculiar long-distance density correlations that appear as a consequence of the Hawking emission features turns out to be reinforced by a finite initial temperature of the condensate. The analytical results are in good quantitative agreement with first principle numerical calculations., 11 pages, 7 figures

We discuss the effective interactions between two localized perturbations in one-dimensional (1D) quantum liquids. For non-interacting fermions, the interactions exhibit Friedel oscillations, giving rise to a RKKY-type interaction familiar from impurity spins in metals. In the interacting case, at low energies, a Luttinger liquid description applies. In the case of repulsive fermions, the Friedel oscillations of the interacting system are replaced, at long distances, by a universal Casimir-type interaction which depends only on the sound velocity and decays inversely with the separation. The Casimir-type interaction between localized perturbations embedded in a fermionic environment gives rise to a long range coupling between quantum dots in ultracold Fermi gases, opening a novel alternative to couple qubits with neutral atoms. We also briefly discuss the case of bosonic quantum liquids in which the interaction between weak impurities turns out to be short ranged, decaying exponentially on the scale of the healing length., Comment: 14 pages, 1 figure, to appear in Phys. Rev. A

We present an experimental and theoretical study of the response of a dipolar supersolid to a Bragg excitation at high-energy defined by the impulse approximation regime. We experimentally observe a continuous reduction of the response when tuning the contact interaction from an ordinary Bose-Einstein condensate to a supersolid state and ultimately to an incoherent array of droplets. Already in the supersolid regime, the observed reduction is faster than the one theoretically predicted by the Bogoliubov--de Gennes theory. By comparing experiments and theories, we are able to attribute this discrepancy to the presence of coherent phase dynamics induced by the crossing of the phase transition. The phase variations are found to change character along the phase diagram and become predominantly incoherent only when reaching the incoherent-droplet regime.