Physics with New Molecular Systems: Quantum Interactions, Cooling, and Applications

Harvard University, Cambridge MA

Investigators

Abstract

Atoms are the basic building blocks of nature and their behavior is governed by a microscopic theory of matter called quantum mechanics. Our understanding of nearly everything we see in nature relies on quantum mechanics as applied to atoms and their close cousins, molecules (two or more atoms stuck together by a chemical bond). From humans and other living things to computers and the internet, physical systems can be explained only by knowing how atoms and molecules behave in detail. Yet, although quantum mechanics has been very successful in describing very simple atomic systems, we do not yet know how to apply the theory quantitatively to describe all the phenomena that we see. In particular, to invent new technological and biological substances, we need to understand the quantum mechanics of atoms and molecules better. The eventual goal of much of physics (including atomic physics) is to have a complete understanding of all matter and the tools to invent new types of matter. This project is a step toward developing a detailed quantum understanding of the interactions between atoms and molecules in a gas at a very low temperature. When cooled, the quantum nature of atoms and molecules is greatly amplified, exposing its nature to careful study. The essential experimental approach to this work is to use magnetic trapping of atoms and molecules to study their collisions using laser spectroscopy. The technical method will be to use buffer-gas cooling to form a beam of atoms and molecules, which will be optically pumped into magnetically trapped states as they pass through the trapping region. Half the molecules in the beam are originally in the low-field-seeking quantum state. These molecules lose energy as they approach the magnetic field maximum of the trap, where they will be optically pumped into their high-field-seeking state. These molecules then continue to lose energy as they travel toward the trap center. Near the trap center lasers pump the molecules into their trapped state. Only two photons are scattered in this process and this (along with energy loss as the molecules pass through the trap) leads to irreversible trap loading. We will trap molecules of calcium monofluoride and other small molecules into a magnetic trap using this method. Atoms can also be co-loaded with molecules, at high enough density for evaporative cooling. We will co-load lithium and/or potassium atoms and study collisions between them and the trapped molecules, testing molecular theory and investigating a route toward ultracold molecules using sympathetic cooling. Spectroscopy will reveal both the state distribution of the molecules, as well as their number and temperature. Investigation of trap loss can be used to study spin-relaxation collisions (for which there is detailed theory for some atom-molecule pairs). The longer term goal of this work is to enable observation of exchange of energy and other phenomena in increasingly complex atom-molecule collisions, starting with diatomic and triatomic molecules. This will add to our fundamental understanding of nature and help science to design new physical systems and new tools for chemistry and biology.