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Bulletin of the American Physical Society

Collisions in a gas of metastable helium near quantum degeneracy

Signe Louise Seidelin

Published May 18, 2005• 0 citations

Abstract

This thesis presents an experimental study of an atomic cloud in the vicinity of Bose-Einstein condensation. The atoms used in our experiment are special in the sense that they are metastable -- we use metastable helium. The metastability opens the door for using new and original detection methods. Due to their internal energy, the atoms can be detected electronically by means of a micro-channel plate. The use of metastable helium is also interesting due of the presence of ionizing collisions in the sample. The He+ ions produced in collisions between the metastable atoms can be detected in real-time, and the ion signal constitutes an alternative and useful diagnostic. In particular, this signal allows one to identify very precisely the Bose-Einstein condensation threshold. One of the advantages of using this signal to probe the cloud, is its ''non-invasive nature: observing the ions does not at all alter the natural evolution of the cloud, in contrast to the ordinary detection technique which consists of releasing the atoms from the trap and then imaging the cloud. In this thesis, by using the new diagnostics based on the ion signal, we have measured some important parameters governing the collisions, and thereby the behavior of the cloud: the ionizing rate constants and the scattering length.

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This thesis presents an experimental study of an atomic cloud in the vicinity of Bose-Einstein condensation. The atoms used in our experiment are special in the sense that they are metastable -- we use metastable helium. The metastability opens the door for using new and original detection methods. Due to their internal energy, the atoms can be detected electronically by means of a micro-channel plate. The use of metastable helium is also interesting due of the presence of ionizing collisions in the sample. The He+ ions produced in collisions between the metastable atoms can be detected in real-time, and the ion signal constitutes an alternative and useful diagnostic. In particular, this signal allows one to identify very precisely the Bose-Einstein condensation threshold. One of the advantages of using this signal to probe the cloud, is its ''non-invasive nature: observing the ions does not at all alter the natural evolution of the cloud, in contrast to the ordinary detection technique which consists of releasing the atoms from the trap and then imaging the cloud. In this thesis, by using the new diagnostics based on the ion signal, we have measured some important parameters governing the collisions, and thereby the behavior of the cloud: the ionizing rate constants and the scattering length.

Two groups, in Orsay and in Paris, have recently obtained experimental evidence for Bose-Einstein condensation on He4 atoms in the metastable state 23S1. The condensates which have been obtained are the first ones where atoms are condensed, not in their electronic ground state, but in an excited state, in this case a metastable state about 20 eV above the ground state. In principle, Penning collisions should very rapidly destroy the cloud of cold metastable atoms and it seems hopeless to observe Bose-Einstein condensation in such a system. But, as suggested by Shlyapnikov et al, the conservation of the total spin during the collision reduces the Penning collision cross-sections by several orders of magnitude and this explains why large enough densities of metastable atoms can be obtained to observe the condensation. We briefly describe the first experimental results which have been obtained in Paris. Atoms are optically detected by optical imaging at 1083 nm (wavelength of the transition connecting the metastable state 23S1 to the triplet state 23P2). The evolution of the anisotropy of the images during a ballistic expansion gives clear evidence for the condensation. Orders of magnitude are also obtained for the scattering length describing the collision between 2 metastable atoms. The large value of this scattering length (of the order of 10 nm) allows one to reach the so called hydrodynamic regime where the mean free path between 2 collisions in the thermal cloud becomes smaller than the size of the cloud. Preliminary results showing the transition between the collisionless regime and the hydrodynamic regime are presented and compared to theoretical predictions.

Cette thèse présente des résultats obtenues par des méthodes originales sur les nuages d'atomes ultra-froids au voisinage de la condensation de Bose-Einstein. Les atomes qui constituent les nuages utilisés dans ces expériences sont particuliers du fait de leur métastabilité. Plus précisément, ce sont des atomes d'hélium métastable. Grâce à l'énergie interne qu'ils possèdent, ces atomes peuvent être détectés électroniquement de façon efficace par une galette de micro-canaux. L'utilisation de l'hélium métastable pour l'étude de la dégénérescence quantique est également particulière du fait de la présence de collisions ionisantes au sein de l'échantillon atomique. Les ions He+ créés lors d'une collision inélastique peuvent être détectés "en temps réel", ce qui constitue un diagnostic nouveau et intéressant. En particulier, il est possible d'identifier très précisément le seuil de condensation de Bose-Einstein, c'est-à-dire le moment où commencent à s'accumuler les atomes dans l'état quantique fondamental. L'avantage d'utiliser le taux d'ions comme outil d'observation est sa nature "non-invasive". Contrairement à la technique d'observation habituelle qui exige que les atomes soient lâchés du piège pour permettre leur observation, la mesure du taux d'ions produits par l'échantillon ne change en rien le comportement naturel du nuage. En utilisant cette méthode de diagnostic inhabituelle, une mesure des paramètres collisionnels régissant le comportement du nuage a été obtenue : constantes de collisions ionisantes et longueur de diffusion de l'atome d'hélium métastable.

Residual ionization in a BEC of 2 3 S1 metastable Helium (He*), can be used to monitor in real time the evolution of a single sample of condensate. We use this method, combined to Time Of Flight measurements, to study the decay of the condensate. It has also allowed us to measure 2-body and 3-body ionizing collisions rate constants. This method should allow for the study of the growth of a condensate, in particular in the hydrodynamic regime. We point out the importance of quantum depletion in He * BEC’s. 1

We report an experiment measuring simultaneously the temperature and the flux of ions produced by a cloud of triplet metastable helium atoms at the Bose-Einstein critical temperature. The onset of condensation is revealed by a sharp increase of the ion flux during evaporative cooling. Combining our measurements with previous measurements of ionization in a pure Bose-Einstein condensate, we extract an improved value of the scattering length a=11.3(+2.5)(-1.0) nm. The analysis includes corrections that take into account the effect of atomic interactions on the critical temperature, and thus an independent measurement of the scattering length would allow a new test of these calculations.

We have produced a Bose-Einstein condensate of metastable helium

(^{4}\mathrm{He}^{*})

containing over $1.5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms, which is a factor of 25 higher than previously achieved. The improved starting conditions for evaporative cooling are obtained by applying one-dimensional Doppler cooling inside a magnetic trap. The same technique is successfully used to cool the spin-polarized fermionic isotope

(^{3}\mathrm{He}^{*})

, for which thermalizing collisions are highly suppressed. Our detection techniques include absorption imaging, time-of-flight measurements on a microchannel plate detector, and ion counting to monitor the formation and decay of the condensate.