Laser induced transitions between internal states of atoms have been playing a fundamental role to manipulate atomic clouds for many decades. In absence of interactions each atom behaves independently and their coherent quantum dynamics is described by the Rabi model. Since the experimental observation of Bose condensation in dilute gases, static and dynamical properties of multicomponent quantum gases have been extensively investigated. Moreover, at very low temperatures quantum fluctuations crucially affect the equation of state of many-body systems. Here we study the effects of quantum fluctuations on a Rabi-coupled two-component Bose gas of interacting alkali atoms. The divergent zero-point energy of gapless and gapped elementary excitations of the uniform system is properly regularized obtaining a meaningful analytical expression for the beyond-mean-field equation of state. In the case of attractive inter-particle interaction we show that the quantum pressure arising from Gaussian fluctuations can prevent the collapse of the mixture with the creation of a self-bound droplet. We characterize the droplet phase and discover an energetic instability above a critical Rabi frequency provoking the evaporation of the droplet. Finally, we suggest an experiment to observe such quantum droplets using Rabi-coupled internal states of K39 atoms.

We systematically investigate the zero temperature phase diagram of bosons interacting via dipolar interactions in three dimensions in free space via path integral Monte Carlo simulations with few hundreds of particles and periodic boundary conditions based on the worm algorithm. Upon increasing the strength of the dipolar interaction and at sufficiently high densities we find a wide region where filaments are stabilized along the direction of the external field. Most interestingly by computing the superfluid fraction we conclude that superfluidity is anisotropic and is greatly suppressed along the orthogonal plane. Finally we perform simulations at finite temperature confirming the stability of filaments against thermal fluctuations and provide an estimate of the superfluid fraction in the weak coupling limit in the framework of the Landau two-fluid model. Superfluidity is an amazing phenomenon of quantum mechanical origin that manifests itself macroscopically as frictionless flow and lack of response to rotation for small enough angular velocity [1,2]. Several experimental platforms have been used to investigate quantum matter in the superfluid regime [3]. Among them, ultracold gases realize a very clean and controllable many-body playground that permit the observation of quantum properties with unprecedented precision [4]. In these systems superfluidity has been observed both with bosonic as well as with fermionic atoms [5][6][7][8]. The superfluid fraction, the ratio of the superfluid density to the total density of the system, has been recently measured in two-component Fermi gas interacting via strong contact potentials [7,9,10].An even richer phenomenology appears when longrange interactions are present. The non-local character of the interparticle potential may induce instabilities of the density that lead to a spontaneous breaking of translational symmetry. A primary example is the long sought supersolid state, where superfluidity is accompanied by a crystalline order [11][12][13][14]. Recent groundbreaking experiments with dipolar condensates demonstrated the existence of dense bosonic droplets in trapped configurations [15][16][17][18] and in free space [19]. Beyond mean-field effects [20][21][22] and three-body interactions [23] have been invoked as the main mechanisms responsible for the stability of these clusters. Large scale simulations based on a non-local non-linear Schrödinger equation have shown very good agreement with the density distribution and the excitation spectra observed in laboratory. Latest experiments paved the way for the search of phase coherence of droplets, demonstrating interference pattern via expansion dynamics of the condensate. The presence of fringes showed that each droplet is individually phase coherent and thus superfluid, leaving yet unresolved the question of global phase coherence of the system [16].In this Letter we report path-integral Monte Carlo (PIMC) results for the low temperature properties of a finite size system of dipolar bosons in three dimensional free...

We study the collective modes of a binary Bose mixture across the soliton to droplet crossover in a quasione-dimensional waveguide with a beyond-mean-field equation of state and a variational Gaussian ansatz for the scalar bosonic field of the corresponding effective action. We observe a sharp difference in the collective modes in the two regimes. Within the soliton regime, modes vary smoothly upon the variation of particle number or interaction strength. On the droplet side, collective modes are inhibited by the emission of particles. This mechanism turns out to be dominant for a wide range of particle numbers and interactions. In a small window of particle number range and for intermediate interactions, we find that monopole frequency is likely to be observed. We focus on the spin-dipole modes for the case of equal intraspecies interactions and equal equilibrium particle numbers in the presence of a weak longitudinal confinement. We find that such modes might be unobservable in the real-time dynamics close to the equilibrium as their frequency is higher than the particle emission spectrum by at least one order of magnitude in the droplet phase. Our results are relevant for experiments with two-component Bose-Einstein condensates for which we provide realistic parameters.

We study the dynamics of dilute and ultracold bosonic gases in a quasi two-dimensional (2D) configuration and in the collisionless regime. We adopt the 2D Landau-Vlasov equation to describe a three-dimensional gas under very strong harmonic confinement along one direction. We use this effective equation to investigate the speed of sound in quasi 2D bosonic gases, i.e. the sound propagation around a Bose-Einstein distribution in collisionless 2D gases. We derive coupled algebraic equations for the real and imaginary parts of the sound velocity, which are then solved taking also into account the equation of state of the 2D bosonic system. Above the Berezinskii-Kosterlitz-Thouless critical temperature we find that there is rapid growth of the imaginary component of the sound velocity which implies a strong Landau damping. Quite remarkably, our theoretical results are in good agreement with very recent experimental data obtained with a uniform 2D Bose gas of 87 Rb atoms.