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From few-body interactions to novel quantum phases of ultracold gases

Final Report Summary - QUANTUMPHASES (From few-body interactions to novel quantum phases of ultracold gases)

Since the experimental realisation of Bose-Einstein condensation (BEC) in 1995, the field of ultracold atomic and molecular optics has been developing very rapidly. More recently, the study of fermionic gases in the quantum degenerate regime has led to the realisation of a BEC from diatomic molecules in 2003, while the BEC to Bardeen-Cooper-Schrieffer (BCS) superfluidity) crossover was observed in 2005. One central theme of current research is quantum simulation and realisation of novel strongly correlated quantum phases. The realisation of such phases is in part made achievable by a unique control of dimensionality (by the use of laser light), strength of interparticle interactions (from Feshbach resonances), and relative atom numbers, possible in ultracold atomic gases. This allows the precise control of few-atom interactions which are of fundamental importance to the properties of the many-particle system.

The aim of the present project was to utilise the possibility to engineer few-body interactions in order to obtain interesting quantum phases of matter. The uniqueness of the present approach is that it combines methods of few-body physics (traditionally in the realm of atomic physics) with those of many-body physics (traditionally in the realm of solid state). In this theoretical study, which was aided by several collaborators, two areas under current intense experimental and theoretical scrutiny were investigated. In the first, an ultracold gas of potassium and lithium atoms, it was demonstrated how a peculiar interference pattern can emerge in the collision of a cloud of potassium atoms and potassium-lithium diatomic molecules. Another question which has received enormous attention is the possibility of a magnetic instability in such a gas. This had been previously ruled out in a three-dimensional geometry, but due to increased quantum fluctuations in two dimensions there was reason to believe that two dimensions might favour magnetism. However, investigating this system we found ferromagnetism to be precluded by a fast decay mechanism.

In parallel, the superfluid properties of fermionic polar molecules (akin to tiny magnetic rods) in two dimensions dressed by a microwave field was investigated. The result was a realistic proposal for obtaining the elusive px + ipy superfluid phase, which has emerged as a major candidate for topologically protected quantum computation.

An important goal of this project was the dissemination of knowledge to researchers at the host institution, the University of Cambridge, and to the broader European community. This was achieved through setting up new collaborations with both local researchers, and researchers in Spain, France, Austria, as well as other locations in the United Kingdom.

In summary, this project has provided an important understanding of fundamental interactions in atomic gases, and the impact on the bulk properties of the system. A deep understanding of such properties could have a major impact on society: for instance, the achievement of topologically protected quantum computation could revolutionise encryption technology.