Project title: Towards a quantum gas of ultracold molecules

Supervisors: Ben Sauer and Mike Tarbutt

Project description:
Ultracold molecules can be used for a wide range of applications [1], including the investigation of chemistry in the quantum regime, tests of fundamental physics, simulation of many-body quantum systems, and quantum information processing. For these applications, it is desirable to produce a gas of polar molecules at high phase-space density, preferably in the quantum degenerate regime. The behaviour of a quantum gas of polar molecules will be dominated by strong, long-range dipole-dipole interactions and will be very different from its atomic counterpart where there are only short-range van der Waals interactions. This dipolar quantum gas can be used to explore a wide range of many-body quantum phases and the phase transitions between them.

There has been tremendous progress in cooling molecules to ultracold temperatures recently. We are now able to capture CaF molecules in a magneto-optical trap, cool them to a few microkelvin, and transfer them into pure magnetic or optical traps [2-4]. These advances open a clear path to cooling molecules into the quantum degenerate regime using the techniques of evaporative and sympathetic cooling to further reduce the phase space density of the molecules. In evaporative cooling, the high energy tail of a thermal distribution is ejected from a trap, and the remaining particles re-thermalize at a lower temperature. This will be used to create a very cold atomic gas of Rb. If this gas is trapped simultaneously with Caf molecules in a conservative trap, sympathetic cooling should occur with the molecules thermalizing with the ultracold atoms to reach a lower temperature. This will allow us to cool the molecules without the need to discard any of them.
We have begun by investigating ultracold collisions between CaF molecules and Rb atoms. Elastic collisions between the two species will cause them to thermalize but inelastic collisions will likely lead to loss of molecules from the trap. As a result, the relative rates of elastic and inelastic collisions will determine the viability of sympathetic cooling. Since the experimental capability is only now becoming available, very little is known about collisions between atoms and molecules at ultracold temperatures so we can expect our first investigations to yield a great deal of new information. At first our focus will be on collisions between both species in the magneto-optical trap, before moving on to conservative traps.
[1] L. D. Carr, D. DeMille, R. V. Krems and J. Ye, New J. Phys. 11, 055049 (2009).
[2] S. Truppe et al., Nature Physics 13, 1173 (2017).
[3] H. J. Williams et al., Phys. Rev. Lett. 120, 163201 (2018).
[4] L. Anderegg et al., Nature Physics 14, 890 (2018).