Cooling molecules to quantum degeneracy

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Everyone is familiar with the three normal states of matter - solids, liquids and gases. There is also another state, known as a Bose-Einstein condensate (BEC), which is a state of matter that can be described only by quantum mechanics. Due to the uncertainty relation between position and speed, an atom spreads out when it slows down. If a gas of atoms is cooled to very low temperature, the atoms may spread out so much that they all overlap. At this point they coordinate, all gathering together into the quantum state that has the lowest energy, and behaving as a single entity instead of a collection of individuals. This state of matter was predicted in 1924 by Bose and Einstein, and in 1995 researchers created it for the first time by cooling a gas of atoms to less than a microkelvin (a millionth of a degree above absolute zero). Bose-Einstein condensation underlies some extraordinary phenomena such as superfluidity and superconductivity.The study of BECs of atoms has been an immensely fruitful research topic for the last 25 years, and there are strong motivations to extend this to molecules. Importantly, molecules can be polar, having a positive end and a negative end. Due to these electric dipoles, molecules can interact with one another far more strongly than atoms and over much larger distances. In fact, in a BEC of polar molecules, every molecule interacts with every other molecule, creating a strongly interacting quantum system. From these interacting systems emerge new and remarkable phenomena that could not be predicted from the behaviour of the constituents and are far too complex to simulate on a normal computer. Examples include magnetism and high-temperature superconductivity. A molecular BEC would be an ideal, highly controllable system for studying these interacting quantum systems. It may also contribute to the development of quantum computers and improve our understanding of collisions and chemistry at low temperatures. Finally, such low temperatures would hugely improve the precision of ongoing experiments that use molecules to test fundamental physics, such as measurements that search for the origins of matter-antimatter asymmetry.Despite all this motivation, molecules have not yet been cooled to the low temperatures needed for BEC. We aim to do that in this project. We will first use laser cooling, which is a method we have pioneered for molecules over the last few years. Then we will trap the molecules and use collisions to cool them further. Here, there are two approaches. In the first - evaporative cooling - the highest-energy molecules are removed from the trap and the remaining molecules collide and re-distribute the reduced energy, thereby cooling to lower temperatures. In the second - sympathetic cooling - the molecules cool as they collide with atoms at lower temperature. In addition to these crucial temperature-lowering collisions, there can also be bad collisions that cause molecules to change their state, react, or be ejected from the trap. The key to success is to control these collisions, enhancing the good ones and suppressing the bad ones. Our combination of theoretical and experimental expertise will be our guide. The BEC we produce will be a completely new type of quantum matter, whose nature is governed by the strong, long-range dipole-dipole interactions. We will study its behaviour and learn how to control it using electric and magnetic fields, opening up a rich new field of strongly-interacting dipolar matter.