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Quantum Electronics

On stimulated VUV emission of atomic helium in a Bose—Einstein condensate: II

Лев Абрамович Ривлин

Published Apr 30, 2006• 1 citation
Publisher

Abstract

The scenario of the experiment on observation of stimulated VUV emission at a wavelength of 62 nm from metastable states of helium atoms in a Bose—Einstein condensate moving along an extended quantum-well trap is further considered taking into account the asymptotic behaviour of the current value of the stimulated emission cross section. Quantitative estimates are presented.

DOI
10.1070/qe2006v036n04abeh013145

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The scenario of the experiment on observation of stimulated VUV emission at a wavelength of 62 nm from metastable states of helium atoms in a Bose—Einstein condensate moving along an extended quantum-well trap is further considered taking into account the asymptotic behaviour of the current value of the stimulated emission cross section. Quantitative estimates are presented.
This paper examines the feasibility of constructing an experiment that detects the atomic stimulation of a photon-emission process. A beam of atoms (bosons) in an excited state is passed through an atomic trap that traps the atoms when they are in their internal ground state. When the trap contains a Bose-Einstein condensate, the cross section for absorption of the atomic beam is increased. We examine a particular model in which this atom stimulation is observable and is also characterized by the emission of photons in a narrow cone in the direction of the atomic beam. \textcopyright{} 1996 The American Physical Society.
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.
We have observed a Bose-Einstein condensate in a dilute gas of 4He{}^{4}\mathrm{He}4He in the 32S1^{3}2S_{1}32S1​ metastable state. We find a critical temperature of $(4.7\ifmmode\pm\else\textpm\fi{}0.5)\ensuremath{\mu}\mathrm{K}$ and a typical number of atoms at the threshold of $8\ifmmode\times\else\texttimes\fi{}{10}^{6}$. The maximum number of atoms in our condensate is about $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$. An approximate value for the scattering length $a\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(16\ifmmode\pm\else\textpm\fi{}8)\mathrm{nm}$ is measured. The mean elastic collision rate at threshold is then estimated to be about $2\ifmmode\times\else\texttimes\fi{}{10}^{4}{\mathrm{s}}^{\ensuremath{-}1}$, indicating that we are deeply in the hydrodynamic regime. The typical decay time of the condensate is 2s2\mathrm{s}2s, which places an upper bound on the rate constants for two-body and three-body inelastic collisions.