  A method is proposed to cool down atoms in a harmonic trap without
phase-space compression as in a perfectly slow adiabatic expansion, i.e.,
keeping the populations of the instantaneous initial and final levels
invariant, but in a much shorter time. This may require that the harmonic trap
becomes an expulsive parabolic potential in some time interval. The cooling
times achieved are also shorter than previous minimal times using
optimal-control bang-bang methods and real frequencies.
