Understanding Collisions of Ultracold Polar Molecules

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At temperatures about a millionth of a degree above absolute zero, atoms and molecules enter a new regime where all their motions are governed by the laws of quantum mechanics. Over the last 20 years, there has been a renaissance in atomic physics based on the new phenomena that emerge in this regime. Experiments with atomic Bose-Einstein condensates (recognised by the 2001 Nobel Prize in Physics) have explored a wide range of topics including nonlinear atom optics, quantum vortices, and phase-transitions in optical lattices. At the same time, degenerate Fermi gases have been used, for example, to improve our understanding of Fermionic superfluidity and the physics of polarons. At the heart of all these advances is a deep and detailed understanding of ultracold atomic collisions that has developed over the last two decades through the close interplay of theory and experiment.Attention in this field is now turning to the new possibilities offered by ultracold molecules. Unlike atoms, molecules can possess an electric dipole moment, with one end positively charged and the other negatively charged. These dipoles mean that molecules can interact with one another more strongly than atoms, and crucially at longer range. Molecules also have more complicated internal structure than atoms: they have multiple spinning nuclei, and they can rotate and be oriented by external fields. Because of this, ultracold molecules offer many new possibilities for the study of novel physical phenomena and the development of new quantum technologies. Examples include the study of exotic forms of quantum magnetism and the potential to design new material properties using so-called "quantum simulators" based on arrays of molecules confined in optical lattices. We have recently become only the third group in the world to succeed in forming a sample of ultracold polar molecules. In our case, this was done by cooling gases of rubidium and cesium atoms to ultracold temperatures, and then pairing up the atoms to form molecules. The molecules we formed were initially nonpolar and only very weakly bound, but we succeeded in transferring them to deeply bound, polar states using a two-photon optical transfer process, known as stimulated Raman adiabatic passage (STIRAP). The entire process occurs without heating, so that the temperature of the resulting molecular gas mirrors that of the atomic mixture.Before our molecules can be used in new quantum devices, we need to understand a lot more about their interactions and collisions. A key question, which does not arise for atoms, is whether pairs of molecules "stick together" for a long time when they collide. If they do, then a third molecule may come along and destroy the first two, shortening the lifetime of the molecular sample. The objective of this project is to investigate collisions of ultracold molecules, both experimentally and theoretically, in order to understand the processes involved. We will investigate both 2-body and 3-body collisions, and distinguish between them by loading our molecules into optical lattices formed by standing waves of laser light. These can confine the molecules in stacks of flat pancakes, bundles of tubes, or even individual boxes.Even if molecular collisions are "sticky", we expect to be able to find ways to control them and thereby preserve the molecular sample for sufficient time to perform interesting experiments. We will investigate orienting the confined molecules with applied electric fields and dressing them with microwave photons, allowing us to create forces that will hold the molecules apart and prevent collisions.This ambitious project, combining state-of-the-art experiments with world-leading theory, will cement the UK's position at the forefront of an exciting international field.