Correlations in Amplified Four-Wave Mixing of Matter Waves

The coherence properties of amplified matter waves generated by four-wave mixing (FWM) are studied using the Hanbury-Brown-Twiss method. We examine two limits. In the first case stimulated processes lead to the selective excitation of a pair of spatiall

Observation of transverse Bose-Einstein condensation via hanbury brown-twiss correlations

A fundamental property of a three-dimensional Bose-Einstein condensate is long-range coherence; however, in systems of lower dimensionality, not only is the long-range coherence destroyed but additional states of matter are predicted to exist. One such s

Many-body physics in two-component Bose-Einstein condensates in a cavity: fragmented superradiance and polarization

We consider laser-pumped one-dimensional two-component bosons in a parabolic trap embedded in a high-finesse optical cavity. Above a threshold pump power, the photons that populate the cavity modify the effective atom trap and mediate a coupling between the two components of the Bose-Einstein condensate. We calculate the ground state of the laser-pumped system and find different stages of self-organization depending on the power of the laser. The modified potential and the laser-mediated coupling between the atomic components give rise to rich many-body physics: an increase of the pump power triggers a self-organization of the atoms while an even larger pump power causes correlations between the self-organized atoms -- the BEC becomes fragmented and the reduced density matrix acquires multiple macroscopic eigenvalues. In this fragmented superradiant state, the atoms can no longer be described as two-level systems and the mapping of the system to the Dicke model breaks down.Comment: 8 pages, 3 figures, software available at http://ultracold.or

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

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

Suppression of Penning ionization in a spin-polarized mixture of rubidium and He

This paper presents the first study of the collision dynamics of an ultra-cold spin-polarized mixture of rubidium and metastable helium (He*) atoms. Our experiment monitors ion production from the mixture for both magnetically polarized and unpolarized cases. In the unpolarized case, we observe an increase in our background ion rate. However, in the completely polarized sample the ion production is below the sensitivity of our experiment. Nonetheless, we determine an upper limit of 5 × 10-12 cm 3 s-1 for the polarized rate constant (β Rb-He*), which is two orders of magnitude below the unpolarized rate constant. Such a suppression of the He*-87Rb polarized rate was not apparent a priori and opens the intriguing possibility of creating a dual Bose-Einstein condensate comprising an alkali ground-state atom and an excited-state noblegas atom

Third-order spatial correlations for ultracold atoms

We present here the first measurement of the third-order spatial correlation function for atoms, made possible by cooling a metastable helium cloud to create an ultracold thermal ensemble just above the Bose-Einstein condensation point. The resulting large correlation length well exceeds the spatial resolution limit of the single-atom detection system, and enables extension of our earlier temporal measurements to evaluate the third-order correlation function in the spatial plane of the detector. The enhancement of the spatial third-order correlation function above a value of unity demonstrates the presence of spatial three-atom bunching, as expected for an incoherent source

Higher order correlations in ultracold quantum gases

One of the seminal advances in quantum optics was the understanding that a quantised description of ensembles of photons is best characterised by correlation functions. Correlations are also a fundamental property of matter waves, and the single wavefunction that describes a Bose-Einstein condensate (BEC) is in principle characterised by long range coherence to all orders (i.e. a universal correlation value of unity). Here we measure the higher order correlation properties of ultra-cold matter waves and use them to probe the coherence of the gas