BOSEFERMIHE · Ultracold Bose-Fermi Mixtures of Metastable Helium

Ultracold Bose-Fermi mixtures of metastable helium

The BOSEFERMIHE project launched research on ultracold metastable helium atoms in order to assist the realisation of strongly interacting systems of ultracold atoms which would lend themselves to detection on a single atom basis. The project permitted a brilliant PhD student, Guthrie Partridge, who during his PhD work acquired extensive experience in the physics of correlated ultracold Fermionic atoms, to work in a leading European laboratory, the Laboratoire Charles Fabry de l'Institut d'Optique in Palaiseau, France, specialising in the physics of degenerate gases of metastable helium. The combination of these two areas of expertise led to several important publications concerning correlations in degenerate quantum gases and, even though the lifetime of the project had officially ceased, several more results were in the analysis stage. Metastable helium atoms have the unique feature that they can be brought to quantum degeneracy and can be detected using microchannel plates and electron multipliers. This feature implies that single atoms are easily detected. Micro channel plates, which have excellent intrinsic time resolution, can also be coupled with position sensitive anodes to acquire a true three-dimensional imaging capability. Chris Westbrook's group in Palaiseau had, during the past six years, adapted this technology to the detection of metastable atoms and, before the arrival of Dr Partridge, had already achieved several firsts in the competitive field of degenerate quantum gas physics, most of which involved the detection of various types of two particle correlations. This work was continued, and even accelerated, during the tenure of Dr Partridge. Firstly, the group undertook a careful study of the atom pair production process in the four wave mixing process of matter waves. Building on an experiment performed in 2007, the group modified the geometry of the experimental configuration in order to better reveal the anisotropies of the atom pair formation process. The most surprising result concerned an anisotropy observed in the relative momenta of the atom pairs. Instead of being distributed on a perfect sphere, as was dictated by elementary phase matching considerations, the momentum of the atoms showed a periodic variation of about 5 % as a function of the angle of emission. This measurement was only possible because of the extremely accurate imaging that became possible using the delay line anode detector. After extensive analysis and modelling, this anisotropy was shown to happen because of a complicated interplay between mean field and many body effects in the condensates giving rise to the pairs. The result was published in Physical Review Letters in April 2010. Moreover, less surprising effects related to the collision anisotropy were also observed and were still under analysis by the time of the project completion. A further paper on these results was planned. Secondly, the group led by Dr Partridge introduced the world’s first laser trap for metastable helium and used in to cool atoms to quantum degeneracy. The ability to optically trap atoms promised to be important for further studies of Bose-Fermi mixtures because of its robustness and the fact that it could trap atoms regardless of their spin state. The group used this capability to perform measurements of inelastic rate constants among different spin states, confirming recent theoretical calculations which showed that different combinations could have very different inelastic loss rates. This was the first measurement of spin dependent losses for metastable helium. A paper on this research was published in the Physical Review in May 2010. Having perfected the laser trap, the group went on to study relative number squeezing, also via utilisation of the four wave mixing process. The high shot to shot stability of the laser trap proved extremely valuable in this study, because it allowed us to average our measurements over 3 600 repetitions with little technical noise. A straight forward analysis showed that the four wave mixing process did indeed produce relative number squeezing at the level of about 10 % in the variance. Finally, the group used the laser trap and correlation techniques to acquire data on several other processes in trapped metastable helium.

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