Efficient Sympathetic Cooling of Neutral and Ionic Molecules for Quantum Information Processing

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The objective of quantum information technology is to build a computer processor that does not operate with zeros and ones. Instead, it will save data in the energy levels of quantum systems such as atoms, ions, or molecules. Relatively simple molecules are favorable candidates because such molecules have enough levels to encode all the information, but not so many as to make them difficult to control. To take advantage of this approach, however, molecules must be held in one place for long enough to prepare them in an energy level, wait for a computation to occur, and then read out the information again. Holding molecules so still, nearly at rest, is challenging because molecules at room temperature tend to vigorously move, vibrate, and rotate. Taming all of that movement requires that molecules to be chilled to a low temperature, just a fraction of a degree above absolute zero. This project will explore ways to cool molecules, and thereby prepare their external (motional) and internal (rotational and vibrational) states, by using collisions with slow ("laser cooled") atoms that can act as a refrigerator to "sympathetically cool" the molecules. To inform this approach, part of the project will be devoted to calculating the energy landscapes for atoms and molecules as a function of their separation, their orientation, and their internal energy states. The probabilities of "inelastic" (internal-state-changing) collisions will also be calculated. Together, these calculations will help this research team analyze the feasibility of different ways to prepare ultra-cold molecules. As part of this project, students will also receive training on how to perform these calculations and how to evaluate the benefits of such ultra-cold molecules for quantum technologies, important preparation for the future workforce in this emerging area. For several decades now, physicists have been successful in cooling and trapping simple atoms with laser-generated forces to get gasses down to temperatures near absolute zero, for example using optical molasses and magneto-optical traps. These approaches, however, have limited applications for molecules, because molecules lack a cycling transition. In this project, the principal investigator will theoretically study a novel cooling technique whereby molecules are manipulated by collisions with laser cooled atoms. In particular, in inelastic collisions, atoms steal energy from the molecule and then fly away leaving the molecule colder, both internally and externally. This project will rely on quantum dynamics calculations and hybrid semi-classical scattering models to understand the collision processes for atoms and molecules. Of particular interest are molecules with a chemical bond that is dominated by a single-valence electron. They are both polar and paramagnetic, and thus can be manipulated by electric and magnetic fields. An optically-trapped array of such molecules is a candidate system for error-free quantum computation.