Supersonic rotation of a superfluid: a long-lived dynamical ring

We present the experimental realization of a long-lived superfluid flow of a quantum gas rotating in an anharmonic potential, sustained by its own angular momentum. The gas is set into motion by rotating an elliptical deformation of the trap. An evaporation selective in angular momentum yields an acceleration of rotation until the density vanishes at the trap center, resulting in a dynamical ring with approx. 350 hbar angular momentum per particle. The density profile of the ring corresponds to the one of a quasi two-dimensional superfluid, with a linear velocity reaching Mach 18 and a rotation lasting more than a minute.

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Oscillations and decay of superfluid currents in a one-dimensional Bose gas on a ring

We study the time evolution of a supercurrent imprinted on a one-dimensional ring of interacting bosons in the presence of a defect created by a localized barrier. Depending on interaction strength and temperature, we identify various dynamical regimes where the current oscillates, is self-trapped or decays with time. We show that the dynamics is captured by a dual Josephson model, and involves phase slips of thermal or quantum nature.

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Probing superfluidity in a quasi two-dimensional Bose gas through its local dynamics

We propose a method to prepare states of given quantized circulation in annular Bose-Einstein condensates (BEC) confined in a ring trap using the method of phase imprinting without relying on a two-photon angular momentum transfer. The desired phase profile is imprinted on the atomic wave function using a short light pulse with a tailored intensity pattern generated with a spatial light modulator. We demonstrate the realization of "helicoidal" intensity profiles suitable for this purpose. Due to the diffraction limit, the theoretical steplike intensity profile is not achievable in practice. We investigate the effect of imprinting an intensity profile smoothed by a finite optical resolution onto the annular BEC with a numerical simulation of the time-dependent Gross-Pitaevskii equation. This allows us to optimize the intensity pattern for a given target circulation to compensate for the limited resolution.

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Probing superfluidity in a quasi two-dimensional Bose gas through its local dynamics

We have evidenced directly the superfluid character of a quasi two-dimensional Bose gas by observing its dynamical response to a collective excitation, the scissors mode. Relying on a novel local average analysis, we are able to probe inhomogeneous clouds and reveal their local dynamics. The analysis is restricted to a thin annulus of average radius ra. We identify in this way the superfluid (left, with a single scissors frequency (a)) and thermal (right, two frequencies, an upper branch in (a) and a lower branch in (b)) phases inside the gas and locate the boundary at which the Berezinskii-Kosterlitz-Thouless crossover occurs. This new analysis also allows to evidence the coupling of the two fluids which induces at finite temperatures damping rates larger than the usual Landau damping.

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Imaging the collective excitations of an ultracold gas using statistical correlations

Advanced data analysis techniques have proved to be crucial for extracting information from noisy images. Here we show that principal component analysis can be successfully applied to ultracold gases to unveil their collective excitations. By analyzing the correlations in a series of images we are able to identify the collective modes which are excited, determine their population, image their eigenfunction, and measure their frequency. Our method allows to discriminate the relevant modes from other noise components and is robust with respect to the data sampling procedure. It can be extended to other dynamical systems including cavity polariton quantum gases or trapped ions.

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Inductive dressed ring traps for ultracold atoms

In collaboration with Aidan Arnold and Barry Garraway.

We present two novel dressed inductive ring trap geometries, ideal for atom interferometry or studies of superfluidity and well-suited to utilization in atom chip architectures. The design permits ring radii currently only accessible via near-diffraction-limited optical traps, whilst retaining the ultra-smooth magnetic potential afforded by inductive traps. One geometry offers simple parallel implementation of multiple rings, whereas the other geometry permits axial beam-splitting of the torus suitable for whole-ring atom interferometry.

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Breakdown of scale invariance in the vicinity of the Tonks-Girardeau limit

In this work, we consider the monopole excitations of a harmonically trapped Bose gas in the vicinity of the Tonks-Girardeau limit. Using Girardeau's Fermi-Bose duality and subsequently an effective fermion-fermion odd-wave interaction, we obtain the dominant correction to the scale-invariance-protected value of the excitation frequency, for microscopically small excitation amplitudes. We produce a series of diffusion Monte Carlo results that confirm our analytic prediction for three particles. And less expectedly, our result stands in excellent agreement with the result of a hydrodynamic simulation (with the Lieb-Liniger equation of state as an input) of the microscopically large but macroscopically small excitations.

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Breakdown of scale invariance in a quasi-two-dimensional Bose gas due to the presence of the third dimension

We describe how the presence of the third, "hidden", dimension may break the scale invariance in a two-dimensional Bose gas in a pan-cake trap. From the two-dimensional perspective, a possibility of a weak spilling of the atomic density beyond the ground state of the confinement alters the two-dimensional chemical potential; in turn, this correction no longer supports scale invariance. We compare experimental data with numerical and analytic perturbative results and find a fair agreement.

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A two-dimensional quantum gas in a magnetic trap

We present the first experimental realization of a two-dimensional quantum gas in a purely magnetic trap dressed by a radio frequency field in the presence of gravity. The resulting potential is extremely smooth and very close to harmonic in the two-dimensional plane of confinement. We fully characterize the trap and demonstrate the confinement of a quantum gas to two dimensions. The trap geometry can be modified to a large extent, in particular in a dynamical way. Taking advantage of this possibility, we study the monopole and the quadrupole modes of a two-dimensional Bose gas. Left: anisotropic time-of-flight expansion of a two dimensional quantum gas.

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Critical rotation of an annular superfluid Bose gas

We analyze the excitation spectrum of a superfluid Bose-Einstein condensate rotating in a ring trap. We identify two important branches of the spectrum related to external and internal surface modes that lead to the instability of the superfluid. Depending on the initial circulation of the annular condensate, either the external or the internal modes become first unstable. This instability is crucially related to the superfluid nature of the rotating gas. In particular we point out the existence of a maximal circulation above which the superflow decays spontaneously, which cannot be explained by invoking the average speed of sound.

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Rubidium 87 Bose-Einstein condensate in an optically plugged quadrupole trap

We describe an experiment to produce 87Rb Bose-Einstein condensates in an optically plugged magnetic quadrupole trap, using a standard 532 nm laser. Due to the large detuning of the plug laser with respect to the atomic transition, the evaporation has to be carefully optimized in order to efficiently overcome the Majorana losses. We provide a complete theoretical and experimental study of the trapping potential at low temperatures and show that this simple model describes well our data. In particular we demonstrate methods to reliably measure the trap oscillation frequencies and the bottom frequency, based on periodic excitation of the trapping potential and on radio-frequency spectroscopy, respectively. We show that this hybrid trap can be operated in a well controlled regime that allows a reliable production of degenerate gases.

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An Example of Quantum Anomaly in the Physics of Ultra-Cold Gases

We propose an experimental scheme for observation of a quantum anomaly — quantum-mechanical symmetry breaking — in a two-dimensional harmonically trapped Bose gas. The anomaly manifests itself in a shift of the monopole excitation frequency away from the value dictated by the Pitaevskii-Rosch dynamical symmetry. While the corresponding classical Gross-Pitaevskii equation and the derived from it hydrodynamic equations do exhibit this symmetry, it is violated under quantization. The resulting frequency shift is of the order of 1% of the carrier, well in reach for modern experimental techniques. We propose using the dipole oscillations as a frequency gauge.

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RF spectroscopy in a resonant RF-dressed trap

We study the spectroscopy of atoms dressed by a resonant radiofrequency (RF) field inside an inhomogeneous magnetic field and confined in the resulting adiabatic potential. The spectroscopic probe is a second, weak, RF field. The observed line shape is related to the temperature of the trapped cloud. We demonstrate evaporative cooling of the RF-dressed atoms by sweeping the frequency of the second RF field around the Rabi frequency of the dressing field.

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Influence of the Radio-Frequency source properties on RF-based atom traps

We discuss the quality required for the RF source used to trap neutral atoms in RF-dressed potentials. We illustrate this discussion with experimental results obtained on a Bose-Einstein condensation
experiment with different RF sources.

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Trapping and cooling of rf-dressed atoms in a quadrupole magnetic field

We observe the spontaneous evaporation of atoms confined in a bubble-like rf-dressed trap. The atoms are confined in a quadrupole magnetic trap and are dressed by a linearly polarized rf field. The evaporation is related to the presence of holes in the trap, at the positions where the rf coupling vanishes, due to its vectorial character. The final temperature results from a competition between residual heating and evaporation efficiency, which is controlled via the height of the holes with respect to the bottom of the trap. The experimental data are modeled by a Monte-Carlo simulation.

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Continuous transfer and laser guiding between two cold atom traps

In collaboration with a group at Laboratoire Aimé Cotton, we have demonstrated and modeled a simple and efficient method to transfer atoms from a first Magneto-Optical Trap (MOT) to a second one. A high power and slightly diverging laser beam optimizes the transfer between the two traps when its frequency is red-detuned from the atomic transition. This pushing laser extracts a continuous beam of slow and cold atoms out of the first MOT and also provides a guiding to the second one through the dipolar force. In order to optimize the transfer efficiency, the dependence of the atomic flux on the pushing laser parameters (power, detuning, divergence and waist) is investigated. We present a simple analysis of the atomic motion inside the pushing-guiding laser, in good agreement with the experimental data.

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Diffuse reflection of a Bose-Einstein condensate from a rough evanescent wave mirror

These experimental results show the diffuse reflection of a Bose-Einstein condensate from a rough mirror, consisting of a dielectric substrate supporting a blue-detuned evanescent wave. The scattering is anisotropic, more pronounced in the direction of the surface propagation of the evanescent wave. These results agree very well with theoretical predictions.

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Evaporative cooling in a radio-frequency trap

A theoretical investigation for implementing a scheme of forced evaporative cooling in radio-frequency (rf) adiabatic potentials is presented. Supposing the atoms to be trapped by a rf field RF1, the cooling procedure is facilitated using a second rf source RF2. This second rf field produces a controlled coupling between the spin states dressed by RF1. The evaporation is then possible in a pulsed or continuous mode. Our results also show that when the frequencies of the fields RF1 and RF2 are separated by at least the Rabi frequency associated with RF1, additional evaporation zones appear which can make this process more efficient.

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Ring trap for ultracold atoms

We propose a toroidal trap designed for ultracold atoms. It relies on a combination of a magnetic trap for rf-dressed atoms, which creates a bubble-like trap, and a standing wave of light. This trap is well-suited for investigating questions of low dimensionality in a ring potential. We study the trap characteristics for a set of experimentally accessible parameters. A loading procedure from a conventional magnetic trap is also proposed. The flexible nature of this ring trap, including an adjustable radius and adjustable transverse oscillation frequencies, will allow the study of superfluidity in variable geometries and dimensionalities.

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Diffraction of a Bose-Einstein Condensate in the Time Domain

We have observed the diffraction of a Bose-Einstein condensate of rubidium atoms on a vibrating mirror potential. The matter wave packet bounces back at normal incidence on a blue-detuned evanescent light field after a 3.6 mm free fall. The mirror vibrates at a frequency of 500 kHz with an amplitude of 3.0 nm. The atomic carrier and sidebands are directly imaged during their ballistic expansion. The locations and the relative weights of the diffracted atomic wave packets are in very good agreement with the theoretical prediction of Carsten Henkel et al.

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Ultracold atoms confined in rf-induced two-dimensional trapping potentials

We present the experimental implementation of a new trap for cold atoms proposed by O. Zobay and B. M. Garraway. It relies on adiabatic potentials for atoms dressed by a rf field in an inhomogeneous magnetic field. This trap is well suited to confine atoms tightly along one direction to produce a two-dimensional atomic gas. We transferred ultracold atoms into this trap, starting either from thermal samples or Bose--Einstein condensates. In the latter case, technical noise during the loading stage caused heating and prevented us from observing 2D BECs.

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