Comprehensive Control of Atomic and Molecular Motion

Head-on view of atomic coilgun. Photo by Adam Libson.

The ability to observe and control matter has been one of the holy grails of modern science. This is a very broad goal, ranging from sub-atomic particles, to atoms, molecules, nanoparticles, and ultimately materials science. For atoms in gas phase, laser cooling has been the standard method of control for over thirty years. While it is an extremely successful method, laser cooling requires a two-level cycling transition, and has thus been limited to a small fraction of the periodic table. Laser cooling has also saturated in terms of the number of ultra-cold atoms produced per second, and their phase space density. It is natural to ask whether there could be an alternative approach to laser cooling that could break these barriers.

In response to this challenge, we have been developing new methods of cooling and trapping atoms. The starting point for our work is the supersonic beam which is a standard tool in Physical Chemistry. The supersonic beam serves as a universal platform for cold but fast atoms that are seeded or entrained into the 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 atoms, as well as a beam of molecular oxygen. Most atoms are 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. We proposed that the atomic coilgun could be operated as an adiabatic decelerator, conserving phase space. This was recently demonstrated by the Narevicius group at the Weizmann Institute. The combination of the supersonic beam and the adiabatic coilgun already has the promise to surpass laser cooling in terms of generality and flux, without using any lasers at all! In order to increase the phase-space density, we proposed a new method of cooling that requires lasers, but for a different purpose than laser cooling.

The method is based on a self-acting one-way wall for atoms that we proposed, and realized experimentally. The basic ingredients for the operation of this one-way wall are laser optical pumping of atoms, and magnetic forces. An interesting historical side-note: In 1871, James Clerk Maxwell proposed a thought experiment known as Maxwell's Demon. In one letter, to Lord Kelvin, Maxwell said that the demon is "impossible to us." In a second letter, to Lord Rayleigh, Maxwell wrote that the demon could be a self-acting one-way wall, predicting the ultimate experimental realization in our laboratory. The cooling in our case is due to the increase of the photon entropy as it is scattered from the laser beam, which compensates for the decrease in the entropy of the gas. This concept was first proposed by Leo Szilard in 1929 in an effort to resolve the paradox of Maxwell's Demon. Our method can work on any atomic element or molecule that can be optically pumped. We are now combining these methods into one system, and predict that they will far exceed laser cooling, in terms of flux of ultra-cold atoms, and phase space density.

Trapping and Cooling of Atomic Hydrogen Isotopes

We are working to apply the methods described above to trapping and cooling of atomic hydrogen and its isotopes, not amenable to laser cooling. Atomic hydrogen has been the "Rosetta Stone" of physics for many years and is the simplest and most abundant atom in the universe. One important case, not yet measured, is three-body association of hydrogen, the basic mechanism for early star formation 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.

The first stage of our research will concentrate on magnetic stopping and 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. One of our first goals is to measure three-body association in hydrogen. 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

Magnetic mirror for atom beam manipulation.

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 an aberration-corrected magnetic lens, atoms can be imaged to spot sizes well-below 10 nm. This process can be massively parallel, and can be used to produce quantum dots and quantum wires, plasmonic devices, etc.. This direction will be a new paradigm in Nanoscience, a bottom-up approach that we call Atomoscience. The same method will enable the development of the world's first neutral-atom microscope that is chemically sensitive and surface specific. The microscope would use metastable noble-gas atoms, which deactivate on impact with a surface, in a process of Penning ionization. The inelastic interaction with surface states results in a unique molecular spectral signature of the emitted electrons, which has been extensively studied. Our microscope will be able to probe local features at the nm level, providing details of chemical composition and reactivity.

Optical Trapping of Glass Microspheres

Over the past few years we have built a new experiment to trap micron-sized glass beads in liquid, air, and vacuum. We have also built a novel detecting system to monitor the real-time position of a trapped bead on very short time and length scales.

A 4.7-micrometer diameter glass microsphere trapped inside of vacuum chamber at 2.5 Pa.

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 recently conducted similar measurements in liquid, and resolved the instantaneous velocity. The physics in the case of liquid is much richer, and therefore more interesting, than air. This system will enable a precise test of the equipartition theorem for a classical liquid. We plan to use this system to study thermodynamics and statistical mechanics of small systems out of equilibrium, and the emergence of the "arrow of time."