Molecular Asymmetry and Optical Cycling: Laser Cooling Asymmetric Top Molecules

Abstract

We present a practical roadmap to achieve optical cycling and laser cooling of asymmetric top molecules (ATMs). Our theoretical analysis describes how reduced molecular symmetry, as compared to diatomic and symmetric nonlinear molecules, plays a role in photon scattering. We present methods to circumvent limitations on rapid photon cycling in these systems. We calculate vibrational branching ratios for a diverse set of asymmetric top molecules and find that many species within a broad class of molecules can be effectively cooled with a manageable number of lasers. We also describe methods to achieve rotationally closed optical cycles in ATMs. Despite significant structural complexity, laser cooling can be made effective by using extensions of the current techniques for linear molecules. Potential scientific impacts of laser-cooled ATMs span frontiers in controlled chemistry, quantum simulation, and searches for physics beyond the Standard Model.

Alternate abstract:

Plain Language Summary

Asymmetry plays a key role in phenomena ranging from interactions of subatomic particles to biological systems. Such asymmetry is present even in small molecules, which may be bent, twisted, or chiral. These structural features make asymmetric molecules fascinating to study but complicate experimental efforts to tame them. Typical techniques, such as laser cooling and trapping, break down in the face of significant asymmetry. We present a method to laser cool asymmetric molecules and demonstrate how to apply it to a diverse set of molecular species.

Physicists and chemists have already demonstrated full control over some quantum systems (like atoms and simple molecules) by cooling them to the point where they are moving slowly enough to probe for long time periods. This requires molecular species that can scatter thousands of photons from a laser without populating states invisible to laser light. To date, only highly symmetric species have met this bar.

We consider the detailed electronic, vibrational, and rotational structure of the complex asymmetric top molecules and find, surprisingly, that rapid photon cycling is possible in many of these species. We show that for properly chosen species and with accurately tuned lasers, many potential loss channels are suppressed or eliminated. We identify many molecules with these properties and show that laser cooling these species is only marginally more complicated than cooling simple diatomic molecules.

This study shows how recent progress in laser cooling molecules can be pushed toward the vast world of asymmetric molecules. Our ideas pave the way for ultracold asymmetric top molecules to study astronomy, chemistry, quantum information processing, and tests of fundamental physics.