We map out the critical velocity in the crossover from Bose-Einstein condensation to Bardeen-Cooper-Schrieffer superfluidity with ultracold ^{6}Li gases. A small attractive potential is dragged along lines of constant column density. The rate of the induced heating increases steeply above a critical velocity v_{c}. In the same samples, we measure the speed of sound v_{s} by exciting density waves and compare the results to the measured values of v_{c}. We perform numerical simulations in the Bose-Einstein condensation regime and find very good agreement, validating the approach. In the strongly correlated regime our measurements of v_{c} provide a testing ground for theoretical approaches.

We report on the experimental realization of homogeneous two-dimensional (2D) Fermi gases trapped in a box potential. In contrast to harmonically trapped gases, these homogeneous 2D systems are ideally suited to probe local as well as nonlocal properties of strongly interacting many-body systems. As a first benchmark experiment, we use a local probe to measure the density of a noninteracting 2D Fermi gas as a function of the chemical potential and find excellent agreement with the corresponding equation of state. We then perform matter wave focusing to extract the momentum distribution of the system and directly observe Pauli blocking in a near unity occupation of momentum states. Finally, we measure the momentum distribution of an interacting homogeneous 2D gas in the crossover between attractively interacting fermions and bosonic dimers.

We demonstrate rapid loading of a small array of optical tweezers with a single 87 Rb atom per site. We find that loading efficiencies of up to 90% per tweezer are achievable in less than 170 ms for traps separated by more than 1.7 µm. Interestingly, we find the load efficiency is affected by nearby traps and present the efficiency as a function of the spacing between two optical tweezers. This enhanced loading, combined with subsequent rearranging of filled sites, will enable the study of quantum many-body systems via quantum gas assembly.A frontier in atomic physics is the study of quantum many-body physics on a microscopic scale. Recent experiments have shown the power of microscopy of degenerate quantum gases in optical lattices [1,2]. An exciting prospect is not only imaging quantum gases, but assembling them into a well-known initial configuration from single-atom building blocks and then observing the resulting dynamics with single-atom resolution. Wavelength-scale optical dipole traps, or optical tweezers, are an attractive platform for control of neutral atoms because they allow repositioning of the atoms after state preparation and site-resolved imaging. Using optical tweezers, long-range interactions between neutral atoms have been harnessed via Rydberg blockade [3-5], and it is now possible to observe controlled interactions and interference between bosonic and fermionic atoms placed individually in their motional ground state [6][7][8]. While optical tweezer traps can be scaled to arrays [9][10][11][12], realizing an ordered array with a single atom per trap is difficult and is a problem of long-standing interest [13][14][15][16].Early experiments with optical tweezers demonstrated sub-Poissonian atom-number statistics using light-assisted collisions that rapidly expel pairs of atoms, a process known as collisional blockade [17][18][19][20]. This has become a reliable method to isolate single atoms, as well as the basis for parity imaging in quantum-gas microscopes [1,2,17]. However, the collisional blockade also limits loading efficiencies to approximately 50%, making the probability to uniformly-fill large arrays prohibitively small [18][19][20].Careful studies of light-assisted collisions in optical dipole traps hold promise for realizing deterministic loading of arrays of atoms [16,21]. Light-assisted collisions are successfully described by transitions between molecular potentials that become resonant with the light at specific interatomic separations, R C [ Fig. 1(a)] [22]. In the case of light that is red-detuned from the bare atomic transition, the atoms associate to an attractive potential and can gain a large kinetic energy, leading to loss of both atoms from the trap. Conversely, when the light is blue-detuned, the atoms associate to a repulsive potential where the maximum kinetic energy gained is set by the detuning [23]. This control has been used to preferentially expel single 85 Rb atoms from a trap, enabling the isolation of single atoms with high probability [16,21]. However, open q...