Hanbury Brown-Twiss and higher order correlations in ultracold atomic clouds of metastable Helium

Abstract

Trapped ultracold gases of metastable helium atoms provide a unique experimental system. The high internal energy enables electronic detection of single atoms. This thesis presents a number of experimental studies using metastable helium. Ionization rates in a mixture of metastable helium and rubidium were measured. Spin polarisation of the system has been shown to result in suppression of Penning ionization of rubidium by metastable helium atoms, suppressed by at least a hundred fold. This demonstration is a promising step towards the first creation of a binary BEC comprising atoms in both ground and excited states. The other experiments presented are mainly concerned with Hanbury Brown-Twiss and Higher order correlations. Single atom detection for metastable helium empowers experimenters to acquire spatial and temporal information, and to obtain full 3D reconstruction for released atomic clouds. Measurements on incoherent boson sources show bunching of particles. This enhancement to probability of joint detection events for two or more particles within short separations is in contrast to the uniform distribution of a coherent state, as measured for a Bose-Einstein condensate or an atom laser. By manipulating correlation length at the detector we were able to demonstrate the first atomic measurement of spatial three-particle correlation, and the results show significant improvement over previously reported values. Furthermore, by using a different trap geometry, it was possible perform measurement of atomic correlation functions to the 6th order, and have observed near ideal correlation functions to the 5th order, demonstrating enhancement for multi-particle detection probability consistent with the prediction of Wick's theorem. Correlations are then exploited to monitor the evolution of coherence property during the formation of a Bose-Einstein condensate. The experimental results are consistent with the existence of a quasi-condensation stage during the system's evolution from an incoherent thermal cloud towards a condensate with long range order. Two cases of mixing processes leading to the production of paired atom beams were Studied. In one case, correlation measurement provided the first higher order test of coherence of amplified matter waves. Coherence properties of matter waves generated via Bose stimulation are critical for active atom optical devices. Previously, coherence properties of amplified matter waves have only been demonstrated to first order by interference fringes. In the second case, a novel mixing process previously unknown was observed. Single atom counting is used to show number difference squeezing between opposite modes. Both processes studied hold potential of providing simple and elegant ways of producing pairs of coherent, sub-Poissonian number difference, and even entangled modes of atomic beams. Finally, physics of lower dimensions was studied. Observation was made of the phenomenon of two-step condensation in a quasi-1D system, where the gas condenses in the tightly confining transverse directions, yet remain thermal in the longitudinal direction. The lack of long-rang coherence of the system is revealed with correlations. Furthermore, an attempt was made to identify the Berezinskii-Kosterlitz-Thouless (BKT) transition.