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Conference on Lasers and Electro-Optics

Evaporative cooling of metastable helium atoms

Paul J. J. Tol, Norbert Herschbach, W. Hogervorst, W. Vassen

Published Aug 29, 2005• 0 citations
Publisher

Abstract

Summary form only given. The importance of helium atoms in the metastable state 2/sup 3/S/sub 1/ (He*, lifetime 8000 s) for studies of ultracold atomic clouds in the quantum degeneracy regime has been pointed out. For /sup 4/He* Bose-Einstein condensation (BEC) is expected, whereas the /sup 3/He isotope allows the study of a fermionic system, if the technique of sympathetic cooling is feasible. We plan to reach the BEC transition using forced evaporative cooling in our magnetostatic trap (MST) of cloverleaf geometry.

DOI
10.1109/iqec.2000.907928

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Summary form only given. The importance of helium atoms in the metastable state 2/sup 3/S/sub 1/ (He*, lifetime 8000 s) for studies of ultracold atomic clouds in the quantum degeneracy regime has been pointed out. For /sup 4/He* Bose-Einstein condensation (BEC) is expected, whereas the /sup 3/He isotope allows the study of a fermionic system, if the technique of sympathetic cooling is feasible. We plan to reach the BEC transition using forced evaporative cooling in our magnetostatic trap (MST) of cloverleaf geometry.
We give an overview of the experiments at the Laser Centre of the Vrije Universiteit in Amsterdam on ultracold gases of metastable helium-4 and helium-3 as well as mixtures of both isotopes. We describe our experimental setup and discuss our experiments on Bose-Einstein condensation of 4He* (more than 10 million atoms in a BEC) and sympathetic cooling of 3He* towards Fermi degeneracy (more than 1 million atoms below the Fermi temperature as well as realization of a degenerate boson-fermion mixture). We also present our results on Penning Ionization in ~1 mK clouds containing a single isotope (homonuclear losses) or a mixture of both isotopes (heteronuclear losses).
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.
Summary form only given. The last five years has seen an explosive growth in activity in the field of quantum degenerate gases, and there are now many realizations of Bose-Einstein condensation (BEC) of trapped atomic gasses. However, only one experiment has produced a quantum degenerate Fermi gas. The ultimate goal of the experiment discussed here is to achieve a BCS phase transition in a trapped gas of fermionic /sup 6/Li atoms. This requires a phase-space density that is much higher than that needed for BEC, as the temperature must be made much less than the Fermi temperature.BEC in trapped gases has only been achieved using forced evaporative cooling. Since this method depends on elastic collisions, however, it fails for an ultracold Fermi gas, as the Pauli principle forbids s-wave collisions between identical fermions. To circumvent this obstacle, we simultaneously trap a /sup 7/Li-/sup 6/Li mixture, and use rf-induced evaporation to cool /sup 7/Li. /sup 6/Li is then cooled via thermalizing elastic collisions with the /sup 7/Li. The clover-leaf-type magnetic trap is loaded from a two-species magneto-optical trap (MOT).
We discuss our work to obtain a condensate containing more than 107 atoms and the first degenerate Fermi gas in a metastable state. Sympathetic cooling with Helium-4 is used to cool 106 Helium-3 atoms to a temperature T/TF < 0.5. The ultracold bosonic and fermionic gases have been used to observe the Hanbury Brown and Twiss effect for both isotopes, showing bunching for the bosons and antibunching for the fermions. A proposal for high resolution spectroscopy at 1.557 μm, connecting both metastable states directly, is discussed at the end.
Photoassociation of metastable He (2/sup 3/S/sub 1/) atoms at ultra low temperatures close to the BEC phase transition is studied, which was achieved previously in an Ioffe-Pritchard magnetic trap and detected optically through absorption imaging of the optical density profile. In the present experiment, the cloud is brought close to the BEC transition by evaporation and then illuminated by laser pulse tunable around the three (2/sup 3/S/sub 1/-2/sup 3/P/sub 0,1,2/) resonance lines at 1083 nm. A subsequent optical density measurement is used to detect any increases in the trap loss rate that may have been caused by the pulse. The experiment is particularly interested in detecting losses due to the production of ultracold molecules by photoassociation (PA), and in finding several weakly bound S-P states that have not previously been detected.