Using a species-selective dipole potential, we create initially localized impurities and investigate their interactions with a majority species of bosonic atoms in a one-dimensional configuration during expansion. We find an interaction-dependent amplitude reduction of the oscillation of the impurities' size with no measurable frequency shift, and study it as a function of the interaction strength. We discuss possible theoretical interpretations of the data. We compare, in particular, with a polaronic mass shift model derived following Feynman variational approach.

We investigate experimentally the entropy transfer between two distinguishable atomic quantum gases at ultralow temperatures. Exploiting a species-selective trapping potential, we are able to control the entropy of one target gas in presence of a second auxiliary gas. With this method, we drive the target gas into the degenerate regime in conditions of controlled temperature by transferring entropy to the auxiliary gas. We envision that our method could be useful both to achieve the low entropies required to realize new quantum phases and to measure the temperature of atoms in deep optical lattices. We verified the thermalization of the two species in a 1D lattice.PACS numbers: 03.75. Hh, 67.85.Pq, 05.30.Jp In recent years an intense research of quantum phases common to condensed matter systems and atomic quantum gases has made remarkable progresses [1]. Some of these phases can only be reached provided that the temperature is suitably low. However, in strongly correlated quantum systems, even the temperature measurement can be a challenging task. If so, to ascertain whether a given quantum phase is accessible, it is convenient to focus on the critical value of entropy, rather than temperature. The advantage is especially clear when the strongly correlated regime is reached by sufficiently slow, entropy-preserving, transformations of the trapping potential, as it is often the case for atoms in deep optical lattices [2]. For these reasons, it is important to determine and grasp control of the entropy of degenerate quantum gases [3,4,5]. In this work, we demonstrate the reversible and controlled transfer of entropy between the two ultracold, harmonically trapped Bose gases, which is based on the use of a species-selective dipole potential (SSDP), i.e., an optical potential experienced exclusively by one species (Fig. 1) [6,7]. In particular, we drive the target gas across the threshold for Bose-Einstein condensation, by a reversible transfer of entropy to the auxiliary gas.The main idea can be understood from textbook thermodynamics. Let us consider two distinguishable gases filling an isolated box, exchanging neither particles nor energy with the outside, and imagine that only one gas (target) is compressed, e.g. through a piston permeable to the second gas (auxiliary). The temperature will increase and, in thermal equilibrium, heat, hence entropy, will transfer from the target to the auxiliary uncompressed gas. In the limit of the auxiliary gas containing a large number of particles, it stands as a thermal bath. In a more formal way, for an ideal gas of N particles, the entropy S is proportional to N log(Σ/N ), where the number of accessible single-particle states Σ increases with the energy density of states and with the average energy, i.e., the temperature. In an adiabatic compression of one single gas, the reduction of the energy density of states is compensated by a temperature raising such that Σ, hence S, remains constant. If we add the uncompressed auxiliary gas in thermal contact, the temperature rai...

We present a new measurement of the Newtonian gravitational constant G based on cold-atom interferometry. Freely falling samples of laser-cooled rubidium atoms are used in a gravity gradiometer to probe the field generated by nearby source masses. In addition to its potential sensitivity, this method is intriguing as gravity is explored by a quantum system. We report a value of G = 6.667 x 10(-11) m(3) kg(-1) s(-2), estimating a statistical uncertainty of +/-0.011 x 10(-11) m(3) kg(-1) s(-2) and a systematic uncertainty of +/-0.003 x 10(-11) m(3) kg(-1) s(-2). The long-term stability of the instrument and the signal-to-noise ratio demonstrated here open interesting perspectives for pushing the measurement accuracy below the 100 ppm level.

When a system crosses a second-order phase transition on a finite timescale, spontaneous symmetry breaking can cause the development of domains with independent order parameters, which then grow and approach each other creating boundary defects. This is known as Kibble-Zurek mechanism. Originally introduced in cosmology, it applies both to classical and quantum phase transitions, in a wide variety of physical systems. Here we report on the spontaneous creation of solitons in Bose-Einstein condensates via the Kibble-Zurek mechanism. We measure the power-law dependence of defects number with the quench time, and provide a check of the Kibble-Zurek scaling with the sonic horizon. These results provide a promising test bed for the determination of critical exponents in Bose-Einstein condensates.The Kibble-Zurek mechanism (KZM) describes the spontaneous formation of defects in systems that cross a second-order phase transition at finite rate [1][2][3][4]. The mechanism was first proposed in the context of cosmology to explain how during the expansion of the early Universe the rapid cooling below a critical temperature induced a cosmological phase transition resulting in the formation domain structures. In fact, the KZM is ubiquitous in nature and regards both classical and quantum phase transitions [5,6]. Experimental evidences have been observed in superfluid 4 He [7,8] and 3 He [9,10], in superconducting films [11] and rings [12][13][14][15][16] and in ion chains [17,18]. Bose-Einstein condensation in trapped cold gases has been considered as an ideal platform for the KZM [19][20][21][22][23]; the system is extremely clean and controllable and particularly suitable for the investigation of interesting effects arising from the spatial inhomogeneities induced by the confinement. Quantized vortices produced in a pancake-shaped condensate by a fast quench across the transition temperature have been already observed [24], but their limited statistics prevented the test of the KZM scaling. The KZM has been studied across the quantum superfluid to Mott insulator transition with atomic gases trapped in optical lattices [25]. Here we report on the observation of solitons resulting from phase defects of the order parameter, spontaneously created in an elongated Bose-Einstein condensate (BEC) of sodium atoms. We show that the number of solitons in the final condensate grows according to a power-law as a function of the rate at which the BEC transition is crossed, consistent with the expectations of the KZM, and provide the first check of the KZM scaling with the sonic horizon. We support our observations by comparing the estimated speed of the transition front in the gas to the speed of the sonic causal horizon, showing that solitons are produced in a regime of inhomogeneous Kibble-Zurek mechanism (IKZM) [21]. Our measurements can open the way to the determination of the critical exponents of the BEC transition in trapped gases, for which so far little information is available [26].The KZM predicts the formation of independen...

We experimentally investigate the mix-dimensional scattering occurring when the collisional partners live in different dimensions. We employ a binary mixture of ultracold atoms and exploit a species-selective 1D optical lattice to confine only one atomic species in 2D. By applying an external magnetic field in proximity of a Feshbach resonance, we adjust the free-space scattering length to observe a series of resonances in mixed dimensions. By monitoring 3-body inelastic losses, we measure the magnetic field values corresponding to the mix-dimensional scattering resonances and find a good agreement with the theoretical predictions based on simple energy considerations. Degenerate atomic gases have provided quantum systems with unprecedented possibilities of manipulation and control, achieved by combining magnetic and optical potentials as well as scattering resonances. The capability to model and control tightly confining potentials sparked the experimental investigation on quantum systems of reduced dimensionality, since particles can be forced to occupy a single quantum level along specific directions. Spectacular achievements, such as the observation of the BKT crossover [1] in 2D and the realization of Tonks-Girardeau gases [2] in 1D, confirmed the importance of quantum gases as testbench for fundamental low-energy physical phenomena. Moreover, low dimensional ultracold atomic gases show further peculiar scattering properties leading to the appearance of confinement-induced resonances depending on the dimensionality of the system [3-6]. Interestingly, while much of the work done so far deals with well-defined dimensionality, systems composed of interacting parts living in different dimensions have received little attention and, besides recent theoretical analysis [7,8], experimental investigation is still lacking. Such mix-dimensional systems are relevant in several physical domains, ranging from cosmology to condensed matter physics. In brane theory, for example, particles and fields are confined to the ordinary 3D space and interact with gravitons that can propagate in extra dimensions [9].