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arXiv:cond-mat/0211112 · Journal of Optics B Quantum and Semiclassical Optics

Using ion production to monitor the birth and death of a metastable helium Bose Einstein condensate

Signe Louise Seidelin, O. Sirjean, José Viana‐Gomes, Denis Boiron, Christoph Ira Westbrook, Alain Aspect

Published Apr 1, 2003• 15 citations
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Abstract

We discuss observations of the ion flux from a cloud of trapped 2 3 S 1 metastable helium atoms. Both Bose–Einstein condensates (BEC) and thermal clouds were investigated. The ion flux is compared with time-of-flight observations of the expanded cloud. We show data concerning BEC formation and decay, as well as measurements of two-and three-body ionization rate constants. We also discuss possible improvements and extensions of our results.

DOI
10.1088/1464-4266/5/2/367

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We have studied the decay of a Bose-Einstein condensate (BEC) of metastable helium atoms in an optical dipole trap. In the regime where two- and three-body losses can be neglected we show that the Bose-Einstein condensate and the thermal cloud show fundamentally different decay characteristics. The total number of atoms decays exponentially with time constant $\ensuremath{\tau}$; however, the thermal cloud decays exponentially with time constant $\frac{4}{3}\ensuremath{\tau}$ and the condensate decays much faster, and nonexponentially. We show that this behavior, which should be present for all BECs in thermal equilibrium with a considerable thermal fraction, is due to a transfer of atoms from the condensate to the thermal cloud during its decay.
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.
We have produced a Bose-Einstein condensate of metastable helium (4He∗)(^{4}\mathrm{He}^{*})(4He∗) 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 (3He∗)(^{3}\mathrm{He}^{*})(3He∗), 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.
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.