Session J4: Atom Circuits

Beyond the thermodynamic limit of number fluctuations

The number fluctuations of quantum gases at finite temperature are discussed, pointing out the deviations with respect to the thermodynamic limit in both weakly and highly compressible fluids. A dramatic enhancement of the fluctuations is predicted in 2D dipolar Bose gases when the size of the sample cell is of the order of the wavelength of the rotonic excitation induced by the long range of the interaction.   

Stirring Up Excitations in Toroidal BECs

The advent of experimentally realizable toroidal BECs in optical traps (\textit{G. K. Campbell et. al., this session}) will provide not only investigations which parallel those of long studied superconducting loops but, with the unique tunability of both trap and super-fluid properties of the BEC itself, also open up new classes of experimental questions and opportunities. Here we present an outgrowth of work [1] using condensate phase rigidity to create super-currents in ring geometries by stirring, allowing simple and robust control of winding numbers, and on release of the trap center, study of the decay of high vorticity condensates. Within the ring trap itself, these studies quickly revealed new classes of elementary collective excitations created by controlled ``mismatching'' of the phase winding number, including dark solitons, whose life times would be a natural object of study in a such uniform environment with coherence times of up to 40 seconds, as well as novel excitations which may be though of as combinations of soliton and vortex behavior, which we have previously called s'vortices. \\[4pt] [1] J. Brand and W. P. Reinhardt, \textit{J. Phys. B}, \underline {\textbf{34}}, L113 (2001).   

Superflow in Toroidal Condensates

Persistent currents are a hallmark of both superfluidity and superconductivity. Just as a current in a superconducting circuit will flow forever, if a current is created in a superfluid condensate, the flow will not decay. Recently, we have created long-lived persistent currents ($>$30s) in toroid-shaped Bose-Einstein Condensates. The all- optical toroidal trap is created using a Laguerre-Gaussian beam, and circulation is created by transferring quantized angular momentum from optical fields using a two- photon Raman process. A repulsive optical barrier intersects a portion of the torus, creating a tunable weak link in the condensate circuit, which can be used to control the current around the loop. As the barrier strength is increased, we find that the superflow stops abruptly when the local flow velocity at the barrier exceeds a critical velocity. These results demonstrate an essential step toward realizing an atomic SQUID analog.   

Bose-Einstein Condensates in Painted Potentials

We have developed a robust and straightforward method to create arbitrary and dynamic two-dimensional potentials for manipulating Bose-Einstein condensates (BECs) [1]. The technique uses a rapidly-moving laser beam that ``paints'' a time-averaged optical dipole potential in which we create BECs in a variety of geometries, including toroids, ring lattices, and square lattices. Matter wave interference patterns confirm that the trapped gas is a condensate. We have used the painted potential technique to study the rotation of toroidal BECs. In a toroidal trap, Bose-condensed atoms should flow with a well defined winding number, making it an ideal system to demonstrate the quantized nature of circulation We created BECs in traps rotating at different frequencies and then studied the resulting superfluid flow in time-of-flight images. Our results show that the rotation of a toroidal BEC is indeed quantized, and that our painted torus will support superfluid flows with winding number up to five.\\[0pt] [1] K Henderson, C Ryu, C MacCormick, and M G Boshier, New J. Phys. 11, 043030 (2009)