ADIABATICALLY CHANGING THE PHASE-SPACE DENSITY OF A TRAPPED BOSE GAS

We show that the degeneracy parameter of a trapped Bose gas can be changed adiabatically in a reversible way, both in the Boltzmann regime and in the degenerate Bose regime. We have performed measurements on spin-polarized atomic hydrogen in the Boltzmann regime, demonstrating reversible changes of the degeneracy parameter (phase-space density) by more than a factor of 2. This result is in good agreement with theory. By extending our theoretical analysis to the quantum degenerate regime we predict that, starting close enough to the Bose-Einstein phase transition, one can cross the transition by an adiabatic change of the trap shape. [S0031-9007(97)02357-0] PACS numbers: 03.75.Fi, 67.65.+z, 32.80.Pj