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

We show that the degeneracy parameter of a trapped Bose gas can be cha nged adiabatically in a reversible way, both in the Boltzmann regime a nd in the degenerate Bose regime. We have performed measurements on sp in-polarized atomic hydrogen in the Boltzmann regime, demonstrating re versible 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 regim e we predict that, starting close enough to the Bose-Einstein phase tr ansition, one can cross the transition by an adiabatic change of the t rap shape.