Experiments with Supersonic Beams as a Source of Cold Atoms

wall starts at the edge of the trap, and the atoms do not encounter the wall during
their oscillations. To cool the atoms, the wall is slowly swept towards the center
of the trap. By moving the wall slowly, each atom in the trap will encounter the
one-way wall close to the classical limit of its trajectory in the trap. Since the atoms
are near their classical turning points, they have very little kinetic energy when they
interact with the barrier. Encountering the one-way barrier, the atoms pass through
the wall and are trapped near the minimum of the potential formed by the one-way
wall and the original trap. In this manner the atoms have been transferred from a
high potential energy state in one trap, to a low potential energy state in a different
trap. By sweeping the wall adiabatically, atoms trapped by the one-way barrier are
not heated by the sweep, and the entire 1D trap can be cooled. The one-way wall
only cools the atoms in one dimension, even when placed in a three dimensional
trap, unless trap ergodicity mixes the degrees of freedom sufficiently for each atom
to encounter the wall at nearly zero kinetic energy.
The experimental technique used to create a one-way wall is a laser that
switches the internal state of atoms that encounter it, such that the trapping po-
tential in the new state is changed. One important point though is that the state
change must involve spontaneous emission to make the process irreversible. Single-
photon cooling takes its name from the fact that changing the state of the atoms,
and thus the potential landscape they see, requires each atom to scatter just one
photon. A closed two level system, as is required for traditional laser cooling, is not
required for single-photon cooling, making the technique broadly applicable. In fact,
single-photon cooling will not work in a true two level system, since the atom will
return to its orignal state after scattering the photon, and the process will not be
irreversible.
Single-photon cooling is a physical realization of the “Maxwell’s Demon” thought
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