- Comprehensive Control of Atomic and Molecular Motion
- Trapping of Atomic Hydrogen Isotopes
- Applications to New Materials and Processes
- Optical Trapping and Cooling of Glass Microspheres
Comprehensive Control of Atomic and Molecular Motion
We have developed new methods of cooling and trapping that are applicable to any atom in the periodic table as well as many molecules. The starting point for our work is the supersonic molecular beam which is an standard tool in Physical Chemistry. The supersonic beam serves as a universal platform for cold but fast atoms and molecules that are seeded or entrained into the supersonic flow. We proposed that a series of pulsed magnetic field coils could stop and trap any paramagnetic species. This "atomic coilgun" was inspired by the coilgun which is used to launch large projectiles. Using this device, we have stopped a beam of metastable neon as well as a beam of molecular oxygen. All atoms are either paramagnetic in their ground state, or can be excited to a metastable state that is paramagnetic. This approach is therefore extremely general, depending only on the ratio of magnetic moment to mass.
In parallel, we developed a general method of cooling atoms or molecules after they are trapped. The method is based on a "one-way wall of light" that we proposed, and realized experimentally. Single-photon cooling works by having each atom scatter, on average, only one photon in order to change its internal state irreversibly. We showed that our method is an experimental realization of Maxwell's Demon, first proposed by James Clerk Maxwell in 1871. The cooling in our case is due to the increase of the photon entropy as it is scattered from the laser beam, which compensates the decrease of the entropy of the gas. In that sense, our cooling method is due to information entropy, as proposed by Leo Szilard in 1929 in an effort to resolve the paradox of Maxwell's Demon. Our method can work on any multi-level atom or molecule, and the only case excluded is a two-level atom. We are now combining these methods into one system, and predict very efficient production of ultra-cold atoms at high phase space density.
Trapping of Atomic Hydrogen Isotopes
We are working to apply the methods described above to trapping and cooling of atomic hydrogen isotopes. Atomic hydrogen has been the "Rosetta Stone" of physics for many years and is the simplest and most abundant atom in the universe. The two isotopes of hydrogen are deuterium with one neutron, and tritium with two neutrons. Precision spectroscopy of these isotopes continues to be of great interest to atomic physics and nuclear physics. Tritium is the simplest radioactive element and serves as an ideal system for the study of beta decay. The latter may be the only way to determine the electron neutrino rest mass, one of the most pressing questions in physics and astronomy. Despite these very important features, hydrogen isotopes have remained very difficult to trap and cool.
The first stage of our research will concentrate on magnetic stopping and single-photon cooling of atomic hydrogen. The difficulty with direct cooling of hydrogen is that the photon recoil velocity is so large. Instead, we will co-trap atomic lithium, using the above cooling methods, and cool the hydrogenic atoms by sympathetic evaporation. We are also looking into the prospects of trapping and cooling of atomic tritium for precision measurement of beta decay.
Applications to New Materials and Processes
We are applying our methods of controlling atoms in gas phase as an intermediate step before depositing them back to solid phase. One avenue of research is atomic lithography. We predict that using transmission masks and electromagnetic lensing, atoms can be imaged to spot sizes below 10 nm. This process can be massively parallel, and is capable of producing quantum dots and nanostructured arrays (e.g., plasmonic and photonic devices) with unprecedented precision. This direction will be a new paradigm in Nanoscience, a bottom-up approach that we call Atomoscience. Magnetic focusing can also be combined with existing spectroscopy techniques in the construction of a high-resolution atom microscope. Such a device would provide high sensitivity to surface structure and composition while causing minimal sample damage. Another direction is an efficient method of isotope separation. An experiment is in progress in the lab, and the method is also being commercialized for medical applications.
Optical Trapping and Cooling of Glass Microspheres
Over the past few years we have built a new experiment to trap micron-sized glass beads in air and vacuum. We have also built a novel detecting system to monitor the real time position of a trapped bead with Ångstrom spatial resolution and microsecond temporal resolution.
One motivation for this work was a long-standing prediction in physics: In 1907, Albert Einstein published a paper where he considered the instantaneous velocity of a Brownian particle, and showed that it could be used to test the equipartition theorem, one of the basic tenets of statistical mechanics. However, Einstein concluded that it would be impossible to measure instantaneous velocity in practice due to the very rapid randomization of the motion.
We have now measured the instantaneous velocity of a Brownian particle, over 100 years since the prediction by Einstein. Our velocity data was used to verify the Maxwell-Boltzmann velocity distribution, and the equipartition theorem for a Brownian particle. We are now repeating these measurements for a particle trapped in a fluid, and expect to reach the same goal in the near future. This system will then be used to study thermodynamics and statistical mechanics of small systems out of equilibrium.