We report on the observation of a highly-degenerate, strongly-interacting Fermi gas of atoms. Fermionic 6 Li atoms in an optical trap are evaporatively cooled to degeneracy using a magnetic field to induce strong, resonant interactions. Upon abruptly releasing the cloud from the trap, the gas is observed to expand rapidly in the transverse direction while remaining nearly stationary in the axial. We interpret the expansion dynamics in terms of collisionless superfluid and collisional hydrodynamics. For the data taken at the longest evaporation times, we find that collisional hydrodynamics does not provide a satisfactory explanation, while superfluidity is plausible.As the fundamental constituents of matter are interacting fermions, the experimental study of stronglyinteracting, degenerate Fermi gases will impact theories in fields from particle physics to materials science. Although the interactions between fermions are understood when they are weak (e.g., quantum electrodynamics), the treatment of very strong interactions requires the development of new theoretical approaches. To test these new approaches, there is a need for experimental systems with widely tunable interaction strengths, densities, and temperatures. Ultracold atomic Fermi gases have exactly these properties, and thus enable tests of calculational techniques for fundamental systems ranging from quarks in nuclear matter to electrons in high temperature superconductors [1,2, 3,4,5,6,7,8,9,10,11]. For this reason, a number of groups are developing methods for creating and exploring ultracold atomic Fermi gases [12,13,14,15,16,17]. We report on the study of a strongly-interacting, degenerate Fermi gas. In contrast to the isotropic expansion previously observed for a noninteracting degenerate Fermi gas [12], we observe anisotropic expansion when the gas is released from an optical trap.An exciting feature of strongly-interacting atomic Fermi gases is the possibility of high-temperature superfluids that are analogs of very high temperature superconductors [8,9,10,11]. Our experiments produce the conditions predicted for this type of superfluid transition. Further, the anisotropic expansion we observe has been suggested as a signature of the onset of superfluidity in a Fermi gas [18]. We interpret the observed anisotropic expansion in terms of both collisionless superfluid hydrodynamics [18] and a new form of collisional hydrodynamics.Strong, magnetically-tunable interactions are achieved in our experiments by employing a Fermi gas comprising a 50-50 mixture of the two lowest hyperfine states of 6 Li, i.e., the |F = 1/2, M = ±1/2 states in the low-magnetic-field basis. This mixture has a predicted broad Feshbach resonance near an applied magnetic field of 860 G [19,20], where the energy of a bound 6 Li-6 Li molecular state is tuned into coincidence with the total energy of the colliding atoms. This enables the interaction strength to be widely varied [19,20,21,22]. It has also been suggested that interactions between fermions can be modified by immers...

We observe collective oscillations of a trapped, degenerate Fermi gas of 6Li atoms at a magnetic field just above a Feshbach resonance, where the two-body physics does not support a bound state. The gas exhibits a radial breathing mode at a frequency of 2837(05) Hz, in excellent agreement with the frequency of nu(H) identical with sqrt[10nu(x)nu(y)/3]=2830(20) Hz predicted for a hydrodynamic Fermi gas with unitarity-limited interactions. The measured damping times and frequencies are inconsistent with predictions for both the collisionless mean field regime and for collisional hydrodynamics. These observations provide the first evidence for superfluid hydrodynamics in a resonantly interacting Fermi gas.

This paper describes a single-shot spectral imaging approach based on the concept of compressive sensing. The primary features of the system design are two dispersive elements, arranged in opposition and surrounding a binary-valued aperture code. In contrast to thin-film approaches to spectral filtering, this structure results in easily-controllable, spatially-varying, spectral filter functions with narrow features. Measurement of the input scene through these filters is equivalent to projective measurement in the spectral domain, and hence can be treated with the compressive sensing frameworks recently developed by a number of groups. We present a reconstruction framework and demonstrate its application to experimental data.

We achieve degeneracy in a mixture of the two lowest hyperfine states of 6Li by direct evaporation in a CO2 laser trap, yielding the first all optically produced degenerate Fermi gas. More than 10(5) atoms are confined at temperatures below 4 microK at full trap depth, where the Fermi temperature for each state is 8 microK. This degenerate two-component mixture is ideal for exploring mechanisms of superconductivity ranging from Cooper pairing to Bose-Einstein condensation of strongly bound pairs.

A Fokker-Planck equation is derived for the energy distribution of atoms in a three-dimensional harmonic trap with fluctuations in the spring constant and the equilibrium position. Using this model, we predict trap lifetimes based on the measurable noise spectra of the fluctuations. The energy distributions evolve into a single eigenmode where the apparent temperature of the distribution remains constant while the population decays as a consequence of the energy input. The method of analysis and the corresponding results are applicable to any optical, magnetic, or ion trap that is approximately harmonic, and offer useful insights into both noise-induced and optical heating processes. ͓S1050-2947͑98͒05211-1͔