Emeritus Experiments

Cold Chemistry

Molecules can undergo 2-body chemical reactions, but such reactions often have an associated energy barrier. According to classical thermodynamics, the consequence of the energy barrier is that as the temperature goes to zero, the reaction rate will go to zero exponentially quickly. But according to quantum mechanics (and the Wigner threshold law) as the temperature goes to zero, the reaction rate goes to a nonzero constant for any exothermic reaction. This is because the reaction can proceed not only by thermal activation over the energy barrier, but also by quantum-mechanical tunneling through it.

We measure chemical reactions in the gas phase at low temperatures, to gain understanding of the fundamental physics underlying chemical reactions, for future ultracold molecule work, and to open up the prospect of controlling cold chemical reactions with applied fields. We plan to investigate the role of quantum-mechanical tunneling in such reactions. In addition, will use optical pumping techniques to prepare reactants in pure spin states and investigate controlling chemical reactions with electron spin symmetry.

We also use cold chemical reactions to synthesize never-before-seen helium complexes. These extremely weakly-bound “van der Waals” molecules exhibit structure very different than “normal” molecules, and allow us to explore chemistry in a new regime.

Selected publications:

Cold collisions

Anisotropic atom collisions

Atoms without orbital angular momentum have isotropic electrostatic interaction potentials, resulting in weak coupling between their internal quantum state and their motion. We study atoms with nonzero orbital angular momentum, whose highly anisotropic interactions give rise to richer collisional physics and larger inelastic cross-sections.

While the collisional properties of these atoms have been studied theoretically for decades, there is little experimental data at cold temperatures. New measurements are important because of the wide variety of systems in which anisotropically-interacting atoms play a role. For example, in astrophysics, inelastic fine-structure-changing collisions provide an important cooling process for cold interstellar molecular clouds, which collapse to form stars and planets.

We study inelastic collisions in cryogenic atomic gases by using optical pumping techniques to prepare the quantum states of our atoms, and use laser spectroscopy to measure their subsequent return to equilibrium through elastic collisions.

Using this technique, we study fine-structure-changing, hyperfine-level-changing, and spin-relaxation collisions in anisotropically-interacting atoms.

Cold molecule collisions

While the collisional physics of ultracold atoms has been extensively explored over the last few decades, there is comparatively little data on ultracold molecule interactions. We use optical pumping and laser spectroscopy to measure molecule-atom collisions: elastic, vibrational, rotational, spin-relaxation, and spin-decoherence. These measurements are important for understanding ultracold molecular physics, and are important for future cold molecule research, including fundamental physics measurements.

Selected publications:

Nuclear-spin based quantum information with cryogenic atomic ytterbium

For quantum communication and cryptography, for networking quantum computers, and for any other applications which require quantum information to be transferred, it is essential to build a quantum network. Photons are the most efficient carriers of information, but are difficult to store for long periods of time. Atoms work well for storing quantum information, but are difficult to transfer long distances.

Techniques have been developed to transfer information back and forth between photons and ensembles of atoms. In prior work, the information was stored in the electron wavefunction of the atom. Unfortunately, the electron wavefunction is highly susceptible to decoherence from collisions and magnetic fields, which can destroy the information stored. We have applied these same techniques to the Yb-173 isotope of atomic ytterbium, enabling the storage of information in the nuclear wavefunction of the atom. The nucleus is highly isolated from the environment, and thus a favorable place to store quantum information. We have demonstrated “stopped light” using Yb-173. We achieve light storage times competitive with the world state-of-the-art, limited only by the lifetime of the Yb in our cryogenic cell. 

In addition, we have investigating other nonlinear optics techniques using Yb, demonstrating an enhancement of four-wave-mixing due to the nuclear spin structure of Yb-171.

Selected publications:

Cryogenic buffer-gas beam source

In a collaboration with the Lewandowski Group at JILA, University of Colorado Boulder, we have developed a cryogenic buffer-gas beam to explore the physics of cold ion-radical collisions and chemical reactions. We have investigated the effects of shaped nozzles on the production of cryogenic buffer-gas beams, and discovered an improvement in molecular fluxes at high flow rates. We have produced cryogenic beams of methyidyne (CH) radicals and demonstrated electrostatic guiding of the cold molecular radicals produced. The apparatus moved from Reno to Boulder in 2019 to be combined with their electrostatic decelerator and ion trap, and the research is ongoing at Boulder.

Selected publications: