Behind the Scenes with Ultracold Atom Gases and BEC – Brian DeMarco

Date: Fri. May 2nd, 2003, 12:30 pm-1:30 pm
Location: Rockefeller 221

The field of ultra-cold quantum atom gases began in 1995 with the realization of Bose-Einstein condensation in a dilute alkali atom gas. Four years later, in Deborah Jin’s group at JILA, we created the first atomic Fermi gas. Not only interesting systems in their own right, ultra-cold atom gases have many parallels with condensed matter systems. Ultra-cold atom gases are unique in that the atomic internal and external states can be manipulated with great precision and the interactions between constituent particles are theoretically accessible and can be controlled experimentally.

The traditional route to alkali atom Bose-Einstein condensation (BEC) involves first collecting roughly billions of atoms from a room temperature vapor and cooling them to temperatures between 10-100 mK. The atoms are then optically pumped and transferred into a magnetic trap. Evaporative cooling then proceeds by forcibly removing the highest energy atoms from the trap; the remaining atoms rethermalize to lower temperature through collisions. At temperatures below typically 1 mK and sufficient density, the gas of (typically) millions of bosonic atoms undergoes the BEC quantum phase transition, which is characterized by macroscopic occupation of the ground state of the confining potential. This traditional “BEC recipe” will be discussed in detail.

Two experiments from JILA will be used to illustrate some of the physics of alkali atom BEC. In the first experiment, the strength and nature (effectively repulsive or attractive) of the collisions between the atoms in a 85Rb BEC is manipulated using a Feshbach resonance. By changing the interactions to be strongly attractive, mechanical collapse of the BEC was observed and an atomic-molecular BEC superposition was created. In the second experiment, vortices are created in a condensate of 87Rb atoms. Originally, single vortices were created using a wavefunction engineering technique involving multiple internal states. Currently, vortex arrays are generated by rotationally “spinning-up” the gas.