Ketterle group presents two papers on properties of ultra-cold gases

Wolfgang Ketterle of MIT reports that his group has published two recent papers in Physical Review Letters. The first of these ("Collisions in zero temperature Fermi gases" by S. Gupta, Z. Hadzibabic, J.R. Anglin, and W. Ketterle, Phys. Rev. Lett. 92, 100401 (2004)) examines the collisional behavior of two-component Fermi gases released at zero temperature from a harmonic trap. Using a phase-space formalism to calculate the collision rate during expansion, the authors find that Pauli blocking plays only a minor role for momentum-changing collisions. As a result, for a large scattering cross section, Pauli blocking will not prevent the gas from entering the collisionally hydrodynamic regime. In contrast to the bosonic case, hydrodynamic expansion at very low temperatures is therefore not evidence for fermionic superfluidity.

The 2nd paper ("Atom interferometry with Bose-Einstein condensates in a double-well potential" by Y. Shin, M. Saba, T. Pasquini, W. Ketterle, D.E. Pritchard, and A.E. Leanhardt, Phys. Rev. Lett. 92, 050405 (2004)) deals with the behavior of an ultra-cold gas of bosons placed in a double-well potential. This construction demonstrates a trapped-atom interferometer using gaseous Bose-Einstein condensates (BEC). The relative phase between the two BEC samples was determined from the spatial phase of the matter-wave interference pattern formed upon releasing the condensates from the separated potential wells. Coherent phase evolution was observed for condensates held separated by 13 µm for up to 5 milliseconds. The development of the interference pattern was controlled by applying ac Stark shift potentials to either of the two separated condensate samples.