Frédéric Chevy

Vortex formation in a stirred Bose-Einstein condensate

Using a focused laser beam we stir a Bose-Einstein condensate of 87Rb confined in a magnetic trap and observe the formation of a vortex for a stirring frequency exceeding a critical value. At larger rotation frequencies we produce states of the condensate for which up to four vortices are simultaneously present. We have also measured the lifetime of the single vortex state after turning off the stirring laser beam.

Exploring the thermodynamics of a universal Fermi gas

One of the greatest challenges in modern physics is to understand the behaviour of an ensemble of strongly interacting particles. A class of quantum many-body systems (such as neutron star matter and cold Fermi gases) share the same universal thermodynamic properties when interactions reach the maximum effective value allowed by quantum mechanics, the so-called unitary limit. This makes it possible in principle to simulate some astrophysical phenomena inside the highly controlled environment of an atomic physics laboratory. Previous work on the thermodynamics of a two-component Fermi gas led to thermodynamic quantities averaged over the trap, making comparisons with many-body theories developed for uniform gases difficult. Here we develop a general experimental method that yields the equation of state of a uniform gas, as well as enabling a detailed comparison with existing theories. The precision of our equation of state leads to new physical insights into the unitary gas. For the unpolarized gas, we show that the low-temperature thermodynamics of the strongly interacting normal phase is well described by Fermi liquid theory, and we localize the superfluid transition. For a spin-polarized system, our equation of state at zero temperature has a 2 per cent accuracy and extends work on the phase diagram to a new regime of precision. We show in particular that, despite strong interactions, the normal phase behaves as a mixture of two ideal gases: a Fermi gas of bare majority atoms and a non-interacting gas of dressed quasi-particles, the fermionic polarons.

Stationary States of a Rotating Bose-Einstein Condensate: Routes to Vortex Nucleation

Using a focused laser beam we stir a 87Rb Bose-Einstein condensate confined in a magnetic trap. We observe that the steady states of the condensate correspond to an elliptic cloud, stationary in the rotating frame. These steady states depend nonlinearly on the stirring parameters (amplitude and frequency), and various solutions can be reached experimentally depending on the path followed in this parameter space. These states can be dynamically unstable and we observe that such instabilities lead to vortex nucleation in the condensate.

The Equation of State of a Low-Temperature Fermi Gas with Tunable Interactions

Determining the Crossover Point The equation of state of an equilibrated system at zero temperature relates its pressure to other macroscopic parameters (such as the chemical potential) and can be used to deduce all relevant thermodynamic properties. For a quantum interacting gas, both the measurement and the theoretical derivation of the equation of state have been challenging. Now, Navon et al. (p. 729, published online 15 April) have used a two-component ultracold Fermi gas of lithium atoms with tunable interactions to quantify the corrections to the mean-field predictions for the equation of state in the crossover between Bose-Einstein condensation and Bardeen-Cooper-Schrieffer limits at near-zero temperature. The polaron mass in the spin-imbalanced gas was also measured. The results agree with known beyond-the-mean-field corrections and present a challenge to future theoretical efforts. A Fermi gas is characterized along the crossover regime between its weak and strongly interacting limits. Interacting fermions are ubiquitous in nature, and understanding their thermodynamics is an important problem. We measured the equation of state of a two-component ultracold Fermi gas for a wide range of interaction strengths at low temperature. A detailed comparison with theories including Monte-Carlo calculations and the Lee-Huang-Yang corrections for low-density bosonic and fermionic superfluids is presented. The low-temperature phase diagram of the spin-imbalanced gas reveals Fermi liquid behavior of the partially polarized normal phase for all but the weakest interactions. Our results provide a benchmark for many-body theories and are relevant to other fermionic systems such as the crust of neutron stars.

Collective Oscillations of an Imbalanced Fermi Gas: Axial Compression Modes and Polaron Effective Mass

We investigate the low-lying compression modes of a unitary Fermi gas with imbalanced spin populations. For low polarization, the strong coupling between the two spin components leads to a hydrodynamic behavior of the cloud. For large population imbalance we observe a decoupling of the oscillations of the two spin components, giving access to the effective mass of the Fermi polaron, a quasiparticle composed of an impurity dressed by particle-hole pair excitations in a surrounding Fermi sea. We find m*/m = 1.17(10), in agreement with the most recent theoretical predictions.

Measurement of the angular momentum of a rotating bose-einstein condensate

We study the quadrupole oscillation of a Bose-Einstein condensate of 87Rb atoms confined in an axisymmetric magnetic trap, after it has been stirred by an auxiliary laser beam. The stirring may lead to the nucleation of one or more vortices, whose presence is revealed unambiguously by the precession of the axes of the quadrupolar mode. For a stirring frequency Omega below the single vortex nucleation threshold Omega(c), no measurable precession occurs. Just above Omega(c), the angular momentum deduced from the precession is approximately Plancks over 2pi. For stirring frequencies above Omega(c) the angular momentum is a smooth and increasing function of Omega, until an angular frequency is reached at which the vortex lattice disappears.

Universal phase diagram of a strongly interacting Fermi gas with unbalanced spin populations

We present a theoretical interpretation of a recent experiment presented by Zwierlein et al. [Nature (London) 442, 54 (2006)] on the density profile of Fermi gases with unbalanced spin populations. We show that in the regime of strong interaction, the boundaries of the three phases observed by Zwierlein et al. can be characterized by two dimensionless numbers {eta}{sub {alpha}} and {eta}{sub {beta}}. Using a combination of a variational treatment and a study of the experimental results, we infer rather precise bounds for these two parameters.

Vortices in a stirred Bose-Einstein condensate

Abstract We stir with a focused laser beam a Bose-Einstein condensate of 87Rb atoms confined in a magnetic trap. We observe the formation of a single vortex for a stirring frequency exceeding a critical value. At larger rotation frequencies we produce states of the condensate for which up to eleven vortices are simultaneously present. We present measurements of the decay of a vortex array once the stirring laser beam is removed.

Vortex lattices in a stirred Bose-Einstein condensate

Abstract We stir with a focused laser beam a Bose-Einstein condensate of 87Rb atoms confined in a magnetic trap. We observe the formation of a single vortex for a stirring frequency exceeding a critical value. At larger rotation frequencies we produce states of the condensate for which up to eleven vortices are simultaneously present. We present measurements of the decay of a vortex array once the stirring laser beam is removed.

Quadrupole Oscillation of a Single-Vortex Bose-Einstein Condensate: Evidence for Kelvin Modes

We study the two transverse quadrupole modes of a cigar-shaped Bose-Einstein condensate with a single centered vortex. We show that the counterrotating mode is more strongly damped than in the absence of a vortex, whereas the corotating mode is not affected appreciably by the vortex. We interpret this result as a decay of the counterrotating quadrupole mode into two excitations of the vortex line, the so-called Kelvin modes. This is supported by direct observation of the vortex line.