Experiments with Supersonic Beams as a Source of Cold Atoms

at the same field strength. This change in behavior is due to an avoided crossing of
the levels. By sweeping the RF frequency, hydrogen atoms can be pumped at their
classical turning point from the |−〉 dressed state to the 2S manifold by 243 nm light.
The lifetime of the 2S state is 0.122 s, which is too long for single photon cooling to
operate effectively. The 2S state can, however, be Stark mixed with the 2P state,
resulting in rapid decay back to the 1S manifold via spontaneous emission of a Lyman
α photon [154]. The branching ratio determines the probablity of the atom decaying
to the |+〉 dressed state where it is trapped with nearly zero kinetic energy. The
process is irreversible because the 243 nm light will not be on resonance for an atom
in the |+〉 dressed state. The scattered Lyman α photon sets the fundamental limit of
the cooling, since this scattered photon gives a random momentum kick. With a mass
of 1.67 × 10 −27 kg for hydrogen, and the Lyman α photon having 5.5 × 10 −27 kg m/s
momentum, the recoil limit works out to be 1.3mK.
Once the atoms have been cooled to the recoil limit, there are several avenues
of research which can be pursued. Spectroscopy of the 1S − 2S transition in tritium
would provide information on the charge radius of the triton [120]. Cold trapped
tritium has been proposed as a source for studies of bound state beta-decay [155, 156],
and possible determination of the neutrino rest mass [157]. It has also been predicted
that a degenerate gas of tritium may exhibit novel liquid like behavior [158–161].
Finally, cold trapped samples at high phase space density may enable the study of
the quantum nature of chemical reactions at cold temperatures [162].
163