A cesium gas strongly confined in one dimension : sideband cooling and collisional properties

We study one-dimensional sideband cooling of Cesium atoms strongly confined in a far-detuned optical lattice. The Lamb-Dicke regime is achieved in the lattice direction whereas the transverse confinement is much weaker. The employed sideband cooling method, first studied by Vuletic et al.\cite{Vule98}, uses Raman transitions between Zeeman levels and produces a spin-polarized sample. We present a detailed study of this cooling method and investigate the role of elastic collisions in the system. We accumulate $83(5)$ of the atoms in the vibrational ground state of the strongly confined motion, and elastic collisions cool the transverse motion to a temperature of $2.8 \mu$K=$0.7 \hbar\omega_{\rm osc}/k_{\rm B}$, where $\omega_{\rm osc}$ is the oscillation frequency in the strongly confined direction. The sample then approaches the regime of a quasi-2D cold gas. We analyze the limits of this cooling method and propose a dynamical change of the trapping potential as a mean of cooling the atomic sample to still lower temperatures. Measurements of the rate of thermalization between the weakly and strongly confined degrees of freedom are compatible with the zero energy scattering resonance observed previously in weak 3D traps. For the explored temperature range the measurements agree with recent calculations of quasi-2D collisions\cite{Petr01}. Transparent analytical models reproduce the expected behavior for $k_{\rm B}T \gg \hbar \omega_{\rm osc}$ and also for $k_{\rm B}T \ll \hbar \omega_{\rm osc}$ where the 2D features are prominent.Comment: 18 pages, 12 figure

Collective many-body bounce in the breathing-mode oscillations of a Tonks-Girardeau gas

We analyse the breathing-mode oscillations of a harmonically quenched Tonks-Giradeau (TG) gas using an exact finite-temperature dynamical theory. We predict a striking collective manifestation of impenetrability---a collective many-body bounce effect. The effect, while being invisible in the evolution of the in-situ density profile of the gas, can be revealed through a nontrivial periodic narrowing of its momentum distribution, taking place at twice the rate of the fundamental breathing-mode frequency. We identify physical regimes for observing the many-body bounce and construct the respective nonequilibrium phase diagram as a function of the quench strength and the initial temperature of the gas. We also develop a finite-temperature hydrodynamic theory of the TG gas, wherein the many-body bounce is explained by an increased thermodynamic pressure of the gas during the isentropic compression, which acts as a potential barrier at the inner turning points of the breathing cycle.Comment: 5 pages, 4 figures, and Supplemental Material. arXiv admin note: substantial text overlap with arXiv:1608.0872

Exact nonequilibrium dynamics of finite-temperature Tonks-Girardeau gases

Describing finite-temperature nonequilibrium dynamics of interacting many-particle systems is a notoriously challenging problem in quantum many-body physics. Here we provide an exact solution to this problem for a system of strongly interacting bosons in one dimension in the Tonks-Girardeau regime of infinitely strong repulsive interactions. Using the Fredholm determinant approach and the Bose-Fermi mapping we show how the problem can be reduced to a single-particle basis, wherein the finite-temperature effects enter the solution via an effective "dressing" of the single-particle wavefunctions by the Fermi-Dirac occupation factors. We demonstrate the utility of our approach and its computational efficiency in two nontrivial out-of-equilibrium scenarios: collective breathing mode oscillations in a harmonic trap and collisional dynamics in the Newton's cradle setting involving real-time evolution in a periodic Bragg potential.Comment: Final published version in PRA style; moved Supplemental Material into main text; 6 pages, 3 figure

Direct observation of quantum phonon fluctuations in a one dimensional Bose gas

We report the first direct observation of collective quantum fluctuations in a continuous field. Shot-to-shot atom number fluctuations in small sub-volumes of a weakly interacting ultracold atomic 1D cloud are studied using \textit{in situ} absorption imaging and statistical analysis of the density profiles. In the cloud centers, well in the \textit{quantum quasicondensate} regime, the ratio of chemical potential to thermal energy is $\mu/ k_B T\simeq4$, and, owing to high resolution, up to 20% of the microscopically observed fluctuations are quantum phonons. Within a non-local analysis at variable observation length, we observe a clear deviation from a classical field prediction, which reveals the emergence of dominant quantum fluctuations at short length scales, as the thermodynamic limit breaks down.Comment: 4 pages, 3 figures (Supplementary material 3 pages, 3 figures

Experimental evidence for the breakdown of a Hartree-Fock approach in a weakly interacting Bose gas

We study the formation of a quasi-condensate in a nearly one dimensional, weakly interacting trapped atomic Bose gas. We show that a Hartree Fock (mean-field) approach fails to explain the presence of the quasi-condensate in the center of the cloud: the quasi-condensate appears through an interaction-driven cross-over and not a saturation of the excited states. Numerical calculations based on Bogoliubov theory give an estimate of the cross-over density in agreement with experimental results.Comment: submitted to Phys. Rev. Letter

Preparation of spin squeezed atomic states by optical phase shift measurement

In this paper we present a state vector analysis of the generation of atomic spin squeezing by measurement of an optical phase shift. The frequency resolution is improved when a spin squeezed sample is used for spectroscopy in place of an uncorrelated sample. When light is transmitted through an atomic sample some photons will be scattered out of the incident beam, and this has a destructive effect on the squeezing. We present quantitative studies for three limiting cases: the case of a sample of atoms of size smaller than the optical wavelength, the case of a large dilute sample and the case of a large dense sample.Comment: 18 page

Limitation of the modulation method to smooth wire guide roughness

It was recently demonstrated that wire guide roughness can be suppressed by modulating the wire currents so that the atoms experience a time-averaged potential without roughness. We theoretically study the limitations of this technique. At low modulation frequency, we show that the longitudinal potential modulation produces a heating of the cloud and we compute the heating rate. We also give a quantum derivation of the rough conservative potential associated with the micro-motion of the atoms. At large modulation frequency, we compute the loss rate due to non adiabatic spin flip and show it presents resonnances at multiple modulation frequencies. These studies show that the modulation technique works for a wide range of experimental parameters. We also give conditions to realise radio-frequency evaporative cooling in such a modulated trap.Comment: 11 page

Mapping out the quasicondensate transition through the dimensional crossover from one to three dimensions

By measuring the density fluctuations in a highly elongated weakly interacting Bose gas, we observe and quantify the transition from the ideal gas to a quasicondensate regime throughout the dimensional crossover from a purely one-dimensional (1D) to an almost three-dimensional (3D) gas. We show that that the entire transition region and the dimensional crossover are described surprisingly well by the modified Yang-Yang model. Furthermore, we find that at low temperatures the linear density at the quasicondensate transition scales according to an interaction-driven scenario of a longitudinally uniform 1D Bose gas, whereas at high temperatures it scales according to the degeneracy-driven critical scenario of transverse condensation of a 3D ideal gas