Physics Today Article Features Fundamental Physics Research

A 'Search and Discovery' feature story in the October issue of the Physics Today (Phys. Today 56, 18 (2003)) magazine, distributed to all members of the American Physical Society, describes the research eing performed in the Fundamental Physics program to study ultracold fermionic atoms. Written by Barbara Gross Levi, the article, titled "Ultracold Fermionic Atoms Team up as Molecules: Can They Form Cooper Pairs as Well?", points to the difficulty for cooling fermions that don't interact as much as bosons. The expertise being developed in several groups around the world to cool fermions into degeneracy at sub-microkelvin temperatures using sympathetic cooling by cold bosons includes four groups in the Fundamental Physics research program subdiscipline of Laser Cooled Atomic Physics. Many stimulating quantum phenomena can be observe in these systems, but the pot of gold for these researchers is to observe superfluidity in this system, much as superconductivity occurs in fermionic electrons by forming Cooper pairs.

Theorists have predicted that superfluidity may well occur if attractive forces exist between the fermions. There is a region, called a Feshbach resonance region, where the collision energies of approaching atoms equals the energy of an excited state of the diatomic molecule, where such strong interactions do occur. The experimenter has a knob to turn, the applied magnetic field, that controls the sign and strength of such interactions. PI Jason Ho (Ohio State Univ.) has pointed out that studies in these regions of large interactions, where perturbation approximations no longer apply, can lead to answers to some very difficult problems in physics. What's more, the experiments to test theories of these problems are essentially 'tabletop physics.'

Jason Ho Randy Hulet Wolfgang Ketterle John Thomas
Jason Ho Randy Hulet Wolfgang Ketterle John Thomas

The article cites several recent experimental results showing the enhanced scattering near these resonances. PI John Thomas and his group at Duke are cited as having seen indications of strong, anisotropic universal forces near a Feshbach resonance. Another PI, Wolfgang Ketterle at MIT, is quoted to state, "I never thought that we would so soon be in a regime where we need new many-body theory." The group of PI Randy Hulet at Rice has reported forming molecules near a Feshbach resonance in an ultracold 6Li system of fermions.

The article raises the question of how superfluidity in these systems will be recognized, and hints that some experiments may already have seen indications of superfluidity in the ultracold fermions. Hulet suggests that several pieces of data may be required: for example, evidence for an energy gap opening up in the spectrum, plus flow without resistance, may provide the required manifestations.