February 2009
Academic Affairs
New Faculty
Faculty and Staff Honors
Current Research Highlights
Atomic slower uses magnets
Comprehensive Control of Atomic and Molecular Motion
Trapping and cooling of atoms in the gas phase has been a major area of research for over thirty years. The advances in this field were enabled by laser cooling, which was recognized by a Nobel Prize in Physics in 1997. Despite the enormous success of this method, it has been limited to a small set of atoms in the periodic table. The reason for the limited applicability of laser cooling is that it requires a two-level cycling transition and one that is accessible with stabilized lasers. These constraints have excluded most of the periodic table as well as any molecules. The potential impact of general trapping and cooling methods is great, ranging from fundamental physics and chemistry to important technological applications.
In the past few years, my group tackled this important problem, and just in the past year have succeeded in our goal with an extremely general two-step solution. Our starting point is the supersonic molecular beam which has been the workhorse of physical chemistry for many years, creating a very monochromatic but fast beam of atoms or molecules. These beams are typically operated with a high-pressure noble gas carrier gas that is “seeded” with another gas. Alternatively, atoms or molecules are entrained into the flow near the output of the supersonic valve. We recently proposed that paramagnetic atoms in the supersonic beam could be stopped using a series of pulsed electromagnetic coils. In an interesting reversal of the norm, this idea was inspired by an advanced weapon technology, the coilgun, which launches ferromagnetic bullets with pulsed magnetic fields. The atomic coilgun works on a similar principle, except that the bullets are single atoms and the gun is operated in reverse (fast projectiles brought to rest). Nearly all atoms in the periodic table have a magnetic moment in their ground state or in a long-lived metastable state (over 90%) allowing the control of the atomic motion using magnetic fields. After stopping the atoms, they can be confined in a magnetic quadrupole trap. In a series of experiments, we were able to use this method to stop a beam of metastable atomic neon as well as a beam of molecular oxygen.
Atoms that are magnetically trapped can be further cooled near the absolute zero of temperature by the method of single-photon cooling as proposed by us in 2005 and based on a one-way wall for atoms. We proposed the equivalent of a cellular membrane for atoms, with those moving in one direction being reflected back, while those moving in the other direction being transmitted. The experiment was performed with trapped rubidium atoms but is completely general and can be applied to any species that can be magnetically trapped. We have also shown that this cooling method is a physical realization of a proposal due to Leo Szilard from 1929 in an effort to explain Maxwell's demon in terms of information entropy.
We are now applying these methods towards trapping and cooling of hydrogen isotopes. These include hydrogen, deuterium and tritium. Our ultimate goal is to perform experiments with trapped atomic tritium for precision measurement of beta decay and possible determination of the neutrino rest mass. More generally, many other groups are now following our lead to trap and cool atoms and molecules that are not amenable to laser cooling. We are therefore entering a very exciting time, with many future directions in physics and chemistry.
