Experiments with Supersonic Beams as a Source of Cold Atoms

producing a cold sample. Alternatively, by changing the conditions under which the
beam is created, a supersonic beam can be produced. Supersonic beams are signif-
icantly colder and more directional than effusive beams, and temperatures as cold
as a few tens of millikelvin have been achieved in the co-moving frame. Supersonic
beams are also very general sources of atoms, as most species can be entrained into
the beam. Since the supersonic beam provides a significant degree of cooling, the
development of methods to control the atoms in the beam is an exciting field which
holds the promise of fulfilling the goal of general control of atomic motion.
1.1 Cooling an Effusive Beam
Starting with a hot beam and using interactions with external fields or col-
lisions with other atoms is the typical method of producing cold samples of atoms.
The methods described here are some of the techniques used for cooling and trapping
the atomic beams created by effusive sources.
1.1.1 Laser Cooling
The use of laser induced transitions to cool atomic samples has developed
into a mature and widely used technique [1]. This cooling method uses momentum
transfer from photons to atoms to create a dissipative force that is cools the sample.
While this method has found great success, and is used in many labs in the form of the
Zeeman slower [2], and the Magneto-Optical Trap (MOT) [3], there are fundamental
limits to its applicability. Because the momentum of a photon is typically less than
that of an atom, many scattering events are required to cool each atom, usually on
the order of a few thousand. Each scattering event consists of an atom absorbing a
photon and transitioning from a lower state into an excited state, and then undergoing
spontaneous decay back to the original lower state.
2