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arXiv:atom-ph/9602002 · Physical Review A

Stimulated enhancement of cross section by a Bose-Einstein condensate

J. J. Hope, Craig M. Savage

Published Oct 1, 1996• 14 citations
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Abstract

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.

DOI
10.1103/physreva.54.3177

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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.
Nuclear processes can be stimulated by the presence of a macroscopic number of bosons in one of the final states. We describe the conditions necessary to observe the atom-stimulation of a beta decay process. The stimulation may be observable if it becomes possible to produce a Bose-Einstein condensate with the order of 10 14 atoms in a trap. It has recently been noted that the rate of gamma ray emission from a nuclear process can be altered by the presence of a Bose-Einstein condensate (BEC) [1]. This process of atom-stimulated photon emission is similar to other proposals involving electronic transitions [2–5], except that it emphasises the fact that any process, including nuclear transtions, may be enhanced. We calculate the stimulation of the emission of a massive particle, in particular a beta particle, and discover that the necessary experimental conditions are less restrictive. The origin of this improvement is that the total momentum kick due to the neutrino and the beta particle may be zero.
We consider the behavior of an atomic Bose-Einstein condensate in the presence of an atom in an (internal) excited electronic level. We analyze the boson-accumulation regime, defined by the relation N0{\mathit{N}}_{0}N0​\ensuremath{\gg}a,N-N0{\mathit{N}}_{0}N0​, where N is the total number of atoms in the ground electronic state, N0{\mathit{N}}_{0}N0​ is the number of atoms in the condensate, and a is the number of levels to which the excited atom can decay. In this regime, quantum statistical effects related to the boson nature of the atoms are predominant in the process of spontaneous emission. Simple arguments suggest that the proportion of atoms in the condensate decreases after the spontaneous decay. A more elaborated model based on a master equation description shows that, in general, these arguments may lead to erroneous conclusions. Under certain conditions it predicts that the proportion of atoms in the condensate increases. We give an interpretation of this phenomenon in terms of quantum interferences between processes that include reabsorption of the emitted photons, and dipole-dipole interactions between the atoms. The physical effect considered here may help to pump atoms into a condensate and to compensate for atomic losses in the atomic trap. \textcopyright{} 1996 The American Physical Society.
We show that the phase difference of two overlapping ground-state Bose-Einstein condensates can affect the optical spontaneous emission rate of excited atoms. Depending on the phase difference the atom-stimulated spontaneous emission rate can vary between zero and the rate corresponding to all the ground-state atoms in a single condensate. Besides giving control over spontaneous emission this provides an optical method for detecting the condensate phase difference. It differs from previous methods in that no light fields are applied. Instead the light is spontaneously emitted when excited atoms make a transition into either condensate.
Impurity atoms propagating at variable velocities through a trapped Bose-Einstein condensate were produced using a stimulated Raman transition. The redistribution of momentum by collisions between the impurity atoms and the stationary condensate was observed in a time-of-flight analysis. The collisional cross section was dramatically reduced when the impurity velocity was reduced below the condensate speed of sound, in agreement with the Landau criterion for superfluidity. For large numbers of impurity atoms, we observed an enhancement of atomic collisions due to bosonic stimulation. This enhancement is analogous to optical super-radiance.