We have observed Bose-Einstein condensation of pairs of fermionic atoms in an ultracold 6Li gas at magnetic fields above a Feshbach resonance, where no stable 6Li2 molecules would exist in vacuum. We accurately determined the position of the resonance to be 822+/-3 G. Molecular Bose-Einstein condensates were detected after a fast magnetic field ramp, which transferred pairs of atoms at close distances into bound molecules. Condensate fractions as high as 80% were obtained. The large condensate fractions are interpreted in terms of preexisting molecules which are quasistable even above the two-body Feshbach resonance due to the presence of the degenerate Fermi gas.

We have observed Bose-Einstein condensation of molecules. When a spin mixture of fermionic 6Li atoms was evaporatively cooled in an optical dipole trap near a Feshbach resonance, the atomic gas was converted into 6Li2 molecules. Below 600 nK, a Bose-Einstein condensate of up to 900 000 molecules was identified by the sudden onset of a bimodal density distribution. This condensate realizes the limit of tightly bound fermion pairs in the crossover between BCS superfluidity and Bose-Einstein condensation.

We have produced a macroscopic quantum system in which a 6Li Fermi sea coexists with a large and stable 23Na Bose-Einstein condensate. This was accomplished using interspecies sympathetic cooling of fermionic 6Li in a thermal bath of bosonic 23Na. The system features rapid thermalization and long lifetimes.

The study of superfluid fermion pairs in a periodic potential has important ramifications for understanding superconductivity in crystalline materials. By using cold atomic gases, various models of condensed matter can be studied in a highly controllable environment. Weakly repulsive fermions in an optical lattice could undergo d-wave pairing at low temperatures, a possible mechanism for high temperature superconductivity in the copper oxides. The lattice potential could also strongly increase the critical temperature for s-wave superfluidity. Recent experimental advances in bulk atomic gases include the observation of fermion-pair condensates and high-temperature superfluidity. Experiments with fermions and bosonic bound pairs in optical lattices have been reported but have not yet addressed superfluid behaviour. Here we report the observation of distinct interference peaks when a condensate of fermionic atom pairs is released from an optical lattice, implying long-range order (a property of a superfluid). Conceptually, this means that s-wave pairing and coherence of fermion pairs have now been established in a lattice potential, in which the transport of atoms occurs by quantum mechanical tunnelling and not by simple propagation. These observations were made for interactions on both sides of a Feshbach resonance. For larger lattice depths, the coherence was lost in a reversible manner, possibly as a result of a transition from superfluid to insulator. Such strongly interacting fermions in an optical lattice can be used to study a new class of hamiltonians with interband and atom-molecule couplings.

Radio-frequency techniques were used to study ultracold fermions. We observed the absence of mean-field "clock" shifts, the dominant source of systematic error in current atomic clocks based on bosonic atoms. This absence is a direct consequence of fermionic antisymmetry. Resonance shifts proportional to interaction strengths were observed in a three-level system. However, in the strongly interacting regime, these shifts became very small, reflecting the quantum unitarity limit and many-body effects. This insight into an interacting Fermi gas is relevant for the quest to observe superfluidity in this system.

We have observed three Feshbach resonances in collisions between6 Li and 23 Na atoms. The resonances were identified as narrow loss features when the magnetic field was varied. The molecular states causing these resonances have been identified, and additional 6 Li-23 Na resonances are predicted. These resonances will allow the study of degenerate Bose-Fermi mixtures with adjustable interactions, and could be used to generate ultracold heteronuclear molecules.PACS numbers: 32.80. Pj, Feshbach resonances [1,2,3,4] have made it possible to control interactions in ultracold atomic gases. By tuning the magnetic field near a value where the energy of two free atoms coincides with a molecular bound state, the sign and strength of the atomic interactions can be varied. Such tunable interactions were used to Bose-Einstein condense atomic species with unfavorable collisional properties [5,6,7], for cooling fermionic mixtures to degeneracy [8], and to produce bright solitons [6,9]. Measurements of Feshbach resonances led to precise determinations of interatomic potentials [10]. An important recent application of Feshbach resonances was the production of ultracold molecules from ultracold atoms [11], and Bose-Einstein condensation of molecules [12,13,14,15].So far, all experiments on Feshbach resonances studied collisions between two atoms of the same species. A few theoretical papers predicted Feshbach resonances between different atomic species [16,17], but these have not been observed. Interspecies Feshbach resonances should lead to a host of new scientific phenomena, including the study of ultracold Fermi-Bose mixtures with tunable interactions, for which boson-mediated Cooper pairing [18,19], phase separation [20] and supersolid order [21] have been predicted. These resonances may also be used to produce polar molecules, at phase-space densities higher than those obtained by heteronuclear photoassociation [22,23,24,25]. Ultracold polar molecules could be used for quantum computation [26] for studies of correlated many-body systems [27,28], and for searches for an electronic dipole moment [29].In this work, we studied collisions between fermionic 6 Li and bosonic 23 Na. In the absence of any theoretical prediction, it was not clear if there were any resonances in the accessible range of magnetic fields. Three s-wave Feshbach resonances were observed and assigned.An ultracold mixture of 6 Li in the |F, m F = |3/2, 3/2 and 23 Na in the |F, m F = |2, 2 hyperfine states was produced by forced microwave evaporation of 23 Na in a magnetic trap as previously described in [30]. The evaporation was stopped before reaching quantum degeneracy, and the mixture was transferred into a single focus 1064 nm optical dipole trap with a waist of 25 µm and a maximum power of 9 W. The 6 Li and 23 Na atoms were then transferred to the |1/2, 1/2 and |1, 1 hyperfine states, respectively, by simultaneous RF Landau-Zener sweeps. The sweeps were done by ramping the magnetic field from 9 to 10 G in 10 ms while keeping the RF frequencies constant a...

Critical velocities have been observed in an ultracold superfluid Fermi gas throughout the BEC-BCS crossover. A pronounced peak of the critical velocity at unitarity demonstrates that superfluidity is most robust for resonant atomic interactions. Critical velocities were determined from the abrupt onset of dissipation when the velocity of a moving one dimensional optical lattice was varied. The dependence of the critical velocity on lattice depth and on the inhomogeneous density profile was studied.PACS numbers: 03.75. Kk, 03.75.Lm, 03.75.Ss The recent realization of the BEC-BCS crossover in ultracold atomic gases [1] allows one to study how bosonic superfluidity transforms into fermionic superfluidity. The critical velocity for superfluid flow is determined by the low-lying excitations of the superfluid. For weakly bound fermions, the (Landau) critical velocity is proportional to the binding energy of the pairs, which increases monotonically along the crossover into the BEC regime. However, the speed of sound, which sets the critical velocity for phonon excitations, is almost constant in the BCS regime, but then decreases monotonically on the BEC side, since the strongly bound molecules are weakly interacting. At the BEC-BCS crossover, one expects a rather narrow transition from a region where excitation of sound limits superfluid flow, to a region where pair breaking dominates. In this transition region, the critical velocity is predicted to reach a maximum [2,3,4]. This makes the critical velocity one of the few quantities which show a pronounced peak across the BEC-BCS crossover in contrast to the chemical potential, the transition temperature [5], the speed of sound [6,7] and the frequencies of shape oscillations [8], which all vary monotonically.In this paper, we report the first study of critical velocities across the BEC-BCS crossover, where a Feshbach resonance allows the magnetic tuning of the atomic interactions, and find that superfluid flow is most robust near the resonance. Our observation of a pronounced maximum of the critical velocity is in agreement with the predicted crossover between the two different mechanisms for dissipation.Critical velocities have been determined before in atomic BECs perturbed by a stirring beam [9,10,11] as well as by a 1D moving optical lattice [12]. In both cases, the inhomogeneous density of the harmonically trapped sample had to be carefully accounted for in order to make quantitative comparisons to theory. Here * Present address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138 † Website: cua.mit.edu/ketterle group we mitigate this problem by probing only the central region of our sample with a tightly focused moving lattice formed from two intersecting laser beams. For decreasing lattice depths, the critical velocity increases and, at very small depths, approaches a value which is in agreement with theoretical predictions.In our experiments, we first create a superfluid of 6 Li pairs according to the procedure de...

The formation time of a condensate of fermionic atom pairs close to a Feshbach resonance was studied. This was done using a phase-shift method in which the delayed response of the many-body system to a modulation of the interaction strength was recorded. The observable was the fraction of condensed molecules in the cloud after a rapid magnetic field ramp across the Feshbach resonance. The measured response time was slow compared to the rapid ramp, which provides final proof that the molecular condensates reflect the presence of fermion pair condensates before the ramp.PACS numbers: 03.75. Ss, 05.30.Fk Atomic Fermi gases close to a Feshbach resonance [1,2,3,4] offer the unique possibility of studying many-body phenomena in a strongly interacting system with tunable interactions. Recently a major focus has been on condensates of pairs of fermionic atoms [5,6,7,8,9,10,11]. By changing the magnetic field the interaction strength between atoms in two spin states can be varied. That way, condensates of either tightly bound molecules or of extended pairs of fermions can be created, whose size can become comparable or even larger than the interparticle spacing. The description of this so-called BEC-BCS crossover [12,13,14] is an active frontier in many-body physics with still controversial interpretations [15,16,17,18].The control of interactions via magnetic fields does not only give access to very different physical regimes, it also allows to apply a time-varying interaction strength [19,20] and to study the dynamics of a many-body system in novel ways. This was used in recent experiments in which molecular condensates were observed after a rapid field ramp from the BCS to the BEC side of the Feshbach resonance [9,10]. It was argued that if the ramp time was faster than the formation time of a molecular condensate, its presence after the sweep necessarily reflected a preexisting condensate of fermion pairs. However, without access to that formation time, secondary evidence was gathered, namely the invariance of the condensate fraction under variations of the sweep rate [9] or of the density immediately before the ramp [10]. This excluded simple models of the molecular condensate formation during the ramp, but left room for more sophisticated many-body effects. In particular, the time to cross the Feshbach resonance in these experiments was not faster than the unitarity limited collision time ∝ E −1 F , and therefore dynamics during the sweep could not be ruled out.Here we present an experimental study of the formation time of a fermionic condensate on the BCS side of the Feshbach resonance [21]. We employ a novel phase-shift method, which records the delayed response of the manybody system to a modulation of the magnetic field that changes periodically its interaction strength. The observable is again the molecular condensate fraction after a rapid sweep to the BEC side of the Feshbach resonance. Its sensitivity to changes in the scattering length on the BCS side [9,10] arises through the dependence of the critical...