Trapping and cooling of atoms in the gas phase has been a major area of research for many years, dominated by the method of laser cooling. Although very successful, laser cooling has been limited to a small set of atoms in the periodic table. In a recent breakthrough, a general two-step approach to trapping and cooling was developed. These new methods will be further optimized, and applied towards trapping and cooling of atomic hydrogen and deuterium in a simple room temperature apparatus.
Development of general methods for trapping and cooling of almost any atom in the periodic table will open new directions in physics. In particular, trapping and cooling of hydrogenic atoms will enable in the future more precise spectroscopy, important for atomic and nuclear physics. The same methods can be applied to cooling of anti-hydrogen, enabling a more precise test of fundamental symmetries. This work will enable in the future precision measurement of beta decay of trapped ultracold atomic tritium, relevant to the question of neutrino mass. The broader impact on education and diversity is training and support of two graduate students from underrepresented minorities.
During the funded period of this grant, we developed general methods for trapping and cooling of atoms near the absolute zero of temperature. This was motivated by the inherent limitations of the existing method of laser cooling which only works on a small fraction of the periodic table. To illustrate this point, the simplest atom in the periodic table, hydrogen, is not amenable to laser cooling. Our goal was to find methods that are simple, robust, and operate in a room-temperature apparatus. In particular, the goal of this project was to develop methods for cooling and trapping of hydrogen and its two isotopes, deuterium and tritium. Hydrogen is the Rosetta-stone of physics, and still holds the key to many questions. For example, the rate of three-body association of atomic hydrogen has not yet been measured, and it is a crucial parameter in astrophysics modeling of early star formation. The measurement of atomic parity violation was recently analyzed and proposed in atomic deuterium, and would provide complementary tests of the Standard Model of elementary particle physics. Atomic tritium is the least studied of the hydrogen isotopes. Precision spectroscopy of tritium would provide information on the triton charge radius, an important test of nuclear physics. The study of beta decay of atomic tritium is especially important, since the decay energy is so small, and could hold the key to determining the neutrino mass, one of the most pressing questions in physics. The starting point for our work is the supersonic molecular beam which has been the workhorse of physical chemistry for many years, creating a nearly mono-energetic beam of atoms or molecules. These beams are typically operated with a high-pressure noble gas carrier gas that is "seeded" with another gas. We proposed that paramagnetic atoms in the supersonic beam could be stopped using a series of pulsed electromagnetic coils. Most atoms in the periodic table have a magnetic moment in their ground state or in a long-lived metastable state, allowing the control of the atomic motion using magnetic fields alone. The operation of this device is similar to a device called a coilgun, an electromagnetic launch method for magnetic projectiles. In our case, the projectiles are individual atoms or molecules. After the atoms are stopped, we developed another method to cool their translational motion. We called this method "single-photon cooling" and it is based on the concept of a one-way wall for atoms as first proposed by us in 2005. This is actually the first experimental realization of a famous thought experiment, proposed by James Clerk Maxwell in 1871, and known as Maxwell's Demon. In the original thought experiment, an intelligent being (the Demon) controls the opening and closing of a gate in order to lower the entropy of the gas. In 1929, Leo Szilard showed that the Demon must collect information which carries away the entropy, saving the Second Law. In our experimental realization, the gate is 'self acting' and relies on an irreversible change of the internal state of the atom, accompanied by the emission of a photon which carries away the entropy. Following the conceptual development and detailed numerical simulations, we developed a multi-stage "atomic coilgun" consisting of miniature electromagnets with each one generating a short magnetic pulse. As a first accomplishment, we were able to use this device to slow and then stop a supersonic beam of metastable neon atoms. The same apparatus was next used to stop a beam of molecular oxygen. Our method only depends on the ratio of mass of a particle to its magnetic moment, and therefore can be applied to any paramagnetic species (atoms, molecules or clusters). In parallel, we built an experiment to demonstrate single-photon cooling. Magnetically trapped rubidium atoms were loaded into a magnetic trap. We constructed an optical "cup" with laser beams. The cup was placed under the magnetic trap and we were able to accumulate ultra-cold atoms in this fashion, relying on the one-way wall concept. The combination of the atomic and molecular coilgun together with single-photon cooling provide a two-step approach to trapping and cooling that will work on most of the periodic table (any paramagnetic atom in the ground or meta-stable state) as well as many molecules.
