We theoretically consider the formation of bright solitons in a mixture of Bose and Fermi degenerate gases. While we assume the forces between atoms in a pure Bose component to be effectively repulsive, their character can be changed from repulsive to attractive in the presence of fermions provided the Bose and Fermi gases attract each other strongly enough. In such a regime the Bose component becomes a gas of effectively attractive atoms. Hence, generating bright solitons in the bosonic gas is possible. Indeed, after a sudden increase of the strength of attraction between bosons and fermions (realized by using a Feshbach resonance technique or by firm radial squeezing of both samples) soliton trains appear in the Bose-Fermi mixture.Solitonic solutions are a very general feature of nonlinear wave equations. Solitons have been studied in many different physical systems ranging from particle physics to optics. They differ from ordinary wave packets as they retain their shape while propagating instead of spreading due to dispersion. This intriguing feature is based on the existence of a nonlinear interaction which compensates for dispersion and produces a self-focusing effect on the propagating wave packet.Dilute atomic quantum gases offer a unique environment to study fundamental solitonic excitations in a pure quantum system with intrinsic nonlinearity. Since the interparticle interaction causing this nonlinearity can be both attractive and repulsive, the Gross-Pitaevskii equation describing the evolution of the condensate wave function exhibits both dark and bright solitonic solutions [1]. Dark solitons as a fundamental excitation in stable BoseEinstein condensates with repulsive interparticle interaction have been studied in different geometries [2,3,4].Bright solitons have been observed in Bose-Einstein condensates of 7 Li in quasi-one-dimensional geometry [5,6]. However, in three-dimensional geometry usually used to prepare the sample the necessary large and negative scattering length leads to density-limited particle numbers (dynamical instability -collapse). The observation of bright solitons was therefore only possible due to magnetic tuning of the interactions from repulsive (used to form a stable Bose-Einstein condensate) to attractive during the experiments.Another experimental approach to bright matter wave solitons was realized in the recently reported observation of gap solitons [7] in a condensate with repulsive interactions by engineering of the matter wave dispersion relation via sophisticated manipulation in a periodic potential (concept of negative effective mass [8]).In this Letter we propose a novel scheme to realize bright solitons in one-dimensional atomic quantum gases. In particular we study the formation of bright solitons in a Bose-Einstein condensate embedded in a quantum degenerate Fermi gas. One important feature is that this mixture allows tuning of the one-dimensional interactions not only by Feshbach resonances but also by simply changing the trap geometry.We consider the bare inte...

Numerical simulations show that, at the onset of Anderson localization, the momentum distribution of a coherent wave packet launched inside a random potential exhibits, in the forward direction, a novel interference peak that complements the coherent backscattering peak. An explanation of this phenomenon in terms of maximally crossed diagrams predicts that the signal emerges around the localization time and grows on the scale of the Heisenberg time associated with the localization volume. Together, coherent back and forward scattering provide evidence for the occurrence of Anderson localization.

Using analytical and numerical methods, it is shown that the momentum distribution of a matter wave packet launched in a random potential exhibits a pronounced coherent backscattering (CBS) peak. By analyzing the momentum distribution, key transport times can be directly measured. The CBS peak can be used to prove that transport occurs in the phase-coherent regime, and measuring its time dependence permits monitoring the transition from classical diffusion to Anderson localization.

We show that solitons occur generically in the thermal equilibrium state of a weakly-interacting elongated Bose gas, without the need for external forcing or perturbations. This reveals a major new quality to the experimentally widespread quasicondensate state, usually thought of as primarily phase-fluctuating. Thermal solitons are seen in uniform 1D, trapped 1D, and elongated 3D gases, appearing as shallow solitons at low quasicondensate temperatures, becoming widespread and deep as temperature rises. This behaviour can be understood via thermal occupation of the Type II excitations in the Lieb-Liniger model of a uniform 1D gas. Furthermore, we find that the quasicondensate phase includes very appreciable density fluctuations, while leaving phase fluctuations largely unaltered from the standard picture derived from a density-fluctuation-free treatment.Solitons, or non-destructible local disturbances, are important features of many one-dimensional (1D) nonlinear wave phenomena. In ultra-cold gases, they have long been sought, and were first observed to be generated by phaseimprinting [1, 2]. More recently, their spontaneous formation in 1D gases was predicted as a result of the Kibble-Zurek mechanism [3, 4], rapid evaporative cooling [5], and dynamical processes after a quantum quench [6]. Here we show that they actually occur generically in the thermal equilibrium state of a weakly-interacting elongated Bose gas, without the need for external forcing or perturbations. This reveals a major new quality to the experimentally widespread quasicondensate state. It can be understood via thermal occupation of the famous and somewhat elusive Type II excitations in the Lieb-Liniger model of a uniform 1D gas [7].A mathematically distinct class of soliton equations are the completely integrable systems. Among them, the GrossPitaevskii[8, 9] equation describes weakly interacting bosons in a 1D geometry in the mean field approximation. The corresponding multi-atom Lieb-Liniger model of N bosons on the circumference of a circle interacting by contact forces [10] has elementary excitations of two kinds: those of a Bogoliubov type and an additional "Type II" branch [7]. These additional excitations have been associated with solitons of the mean field [11][12][13][14]. Although the trapping potential removes integrability, from the early days experimenters have searched for gray solitons. See [15] for a review.Typically, by irradiating one part of the condensate, one engineers a phase difference with the remainder, and a dark soliton forms at the interface between the phase domains[1, 2]. Other proposed schemes involve taking the system away from equilibrium [3][4][5][6]. Our results show that solitons are in fact present spontaneously even in equilibrium. However, engineered solitons have been easier to identify with the standard destructive imaging measurements because their position does not vary from shot to shot.Here we generate a classical field ensemble that describes the weakly interacting Bose gas at thermal equilibri...

We show that the momentum distribution of a nonlinear matter wave suddenly released with a finite velocity in a speckle potential converges, after an out-of-equilibrium evolution, to a universal Rayleigh-Jeans thermal distribution. By exploring the complete phase diagram of the equilibrated wave, we discover that for low but nonzero values of the disorder strength, a large-scale structure -a condensate- appears in the equilibrium distribution.Comment: 5 pages, 3 figures. Supplementary material adde

The quantum mechanical states of electrons in atoms and molecules are distinct orbitals, which are fundamental for our understanding of atoms, molecules and solids. Electronic orbitals determine a wide range of basic atomic properties, allowing also for the explanation of many chemical processes. Here, we propose a novel technique to optically image the shape of electron orbitals of neutral atoms using electron-phonon coupling in a Bose-Einstein condensate. To validate our model we carefully analyze the impact of a single Rydberg electron onto a condensate and compare the results to experimental data. Our scheme requires only well-established experimental techniques that are readily available and allows for the direct capture of textbook-like spatial images of single electronic orbitals in a single shot experiment. OPEN ACCESS RECEIVED

We study experimentally and numerically the quasi-bidimensional transport of a 87 Rb Bose-Einstein condensate launched with a velocity v0 inside a disordered optical potential created by a speckle pattern. A time-of-flight analysis reveals a pronounced enhanced density peak in the backscattering direction −v0, a feature reminiscent of coherent backscattering. Detailed numerical simulations indicate however that other effects also contribute to this enhancement, including a "backscattering echo" due to the position-momentum correlations of the initial wave packet.

We study the stability of a zero temperature mixture of attractively interacting degenerate bosons and spin-polarized fermions in the absence of confinement. We demonstrate that higher order corrections to the standard mean-field energy can lead to a formation of Bose-Fermi liquid droplets -self-bound systems in threedimensional space. The stability analysis of the homogeneous case is supported by numerical simulations of finite systems by explicit inclusion of surface effects. We discuss the experimental feasibility of formation of quantum droplets and indicate the main obstacle -inelastic three-body collisions.