We have used the Hanbury Brown-Twiss effect to directly compare the density correlations of a pulsed atom laser and a pulsed ultracold thermal source of metastable helium. It was found that the isotropic RF outcoupling of atoms from a Bose-Einstein condensate does not result in decoherence, while the 'bunching' typical of incoherent sources was observed for thermal atoms. This new method significantly increases data acquisition rates compared to previous measurements, and also permits future novel experiments which may allow us to probe processes such as the birth and death of a condensate by monitoring correlation effects.
We have used the Hanbury Brown-Twiss effect to directly compare the density correlations of a pulsed atom laser and a pulsed ultracold thermal source of metastable helium. It was found that the isotropic RF outcoupling of atoms from a Bose-Einstein condensate does not result in decoherence, while the 'bunching' typical of incoherent sources was observed for thermal atoms. This new method significantly increases data acquisition rates compared to previous measurements, and also permits future novel experiments which may allow us to probe processes such as the birth and death of a condensate by monitoring correlation effects.
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.
This thesis deals with the measurement of the quantum intensity correlations in gases of metastable Helium. The measurement has been performed on thermal gases of bosonic 4He and fermionic 3He, as well as on Bose-Einstein condensates. In 1956, Robert Hanbury Brown et Richard Twiss measured the correlation between photons emitted from a single thermal source. The consequently demonstrated that the photons emitted by such a source tend to arrive grouped on a detector. This bunching characterises bosons from a non-coherent source. Fermions show an anti-bunching behaviour in the same conditions. By using metastable Helium atoms, that can be detected individually through the use of micro-channel plates, we have been able to show a similar bunching of bosons 4He from thermal sources around the microkelvin. As expected, the coherence of the Bose-Einstein condensates did not produce a particular correlation. The measurement on thermal gases of fermionic 3He has demonstrated the anti-bunching. Particular effort has been employed in describing the micro-channel plates based delay-line detector, the key to the experiment.
This thesis deals with the measurement of the quantum intensity correlations in gases of métastable Helium. The measurement has been performed on thermal gases of bosonic 4He and fermionic 3He, as well as on Bose-Einstein condensates. In 1956, Robert Hanbury Brown et Richard Twiss measured the correlation between photons emitted from a single thermal source. The consequently demonstrated that the photons emitted by such a source tend to arrive grouped on a detector. This bunching characterises bosons from a non-coherent source. Fermions show an anti-bunching behaviour in the same conditions. By using metastable Helium atoms, that can be detected individually through the use of micro-channel plates, we have been able to show a similar bunching of bosons 4He from thermal sources around the microkelvin. As expected, the coherence of the Bose-Einstein condensates did not produce a particular correlation. The measurement on thermal gases of fermionic 3He has demonstrated the anti-bunching. Particular effort has been employed in describing the micro-channel plates based delay-line detector, the key to the experiment.
We report the realization of a Hanbury Brown and Twiss (HBT)-like experiment with a gas of interacting bosons at low temperatures. The low-temperature regime is reached in a three-dimensional optical lattice and atom-atom correlations are extracted from the detection of individual metastable helium atoms after a long free fall. We observe, in the noncondensed fraction of the gas, a HBT bunching whose properties strongly deviate from the HBT signals expected for noninteracting bosons. In addition, we show that the measured correlations reflect the peculiar quantum statistics of atoms belonging to the quantum depletion and of the Bogoliubov phonons, i.e., of collective excitations of the many-body quantum state. Our results demonstrate that atom-atom correlations provide information about the quantum state of interacting particles, extending the interest of HBT-like experiments beyond the case of noninteracting particles.
The field of atom optics has progressed rapidly over the past 20 years since the realisation of Bose-Einstein condensation, such that the wave behaviour of atomic gases is now routinely demonstrated. Furthermore, the study of quantum atom optics, which goes beyond a ‘mean-field’ description of quantum systems to consider the behaviour of single particles, has demonstrated both the similarities between photons and massive species, and their differences as a result of the internal structure and external interactions of atoms. An important class of observable quantities which allow such effects to be measured are nth order correlation functions, which can be interpreted as a result of either particle or wave behaviour. These functions provide a statistical description of fluctuations in n-tuples of particles in a source, which rigorously defines concepts such as coherence. The quantum statistics of a Bose-Einstein condensate should be the same as that for an optical laser, while an ideal thermal Bose gas matches the behaviour of incoherent light. However, correlation measurements can also be used to quantify the influence of interactions, dimensionality, confining potentials and waveguides, and the difference in quantum statistics between fermions and bosons, which illustrates the rich range of behaviour exhibited by atomic gases. In this thesis, several aspects of quantum atom optics are explored with experiments using ultracold metastable helium, a species with the unique advantage of facilitating simple single-atom detection with high resolution, while still allowing Bose-Einstein condensates to be formed. The coherence of atomic systems is shown to be maintained when outcoupled as pulsed atom lasers, and the long-range order characteristic of Bose-Einstein condensates is demonstrated to third order for the first time. Conversely, thermal bunching is observed for a variety of atomic systems, including the measurement of correlation functions up to sixth order with near-ideal interference contrast. These results clearly demonstrate the correspondence between the quantum statistics of photons and atoms as was formalised by Glauber, as well as confirming the validity of applying Wick’s theorem to simplify the statistics of atomic gases. Correlation functions are also shown to be an ideal tool to probe the quantum state of an ultracold gas, and were used to observe the phenomenon of transverse condensation in an elongated Bose gas, as well as characterise the mode occupancy of matter waves guided by an optical potential. Ultracold metastable helium is also suitable for exploring other fundamental topics in quantum optics such particle/wave duality. The notion of complementarity stimulated long running philosophical discussions about how apparently mutually exclusive behaviours can coexist, which culminated in Wheeler devising his famous ‘delayed choice’ gedankenexperiment. A proposed experimental method to realise Wheeler’s experiment with ultracold atoms is discussed, and preliminary measurements presented which indicate that the completion of this experiment could be achieved in the near future. Not only is this of interest in its own right, but the implementation of this experiment has also developed techniques which may enable further studies in quantum atom optics such as investigations of the Hong-Ou-Mandel effect and quantum entanglement with massive particles.