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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Experimental detection of photons emitted during inhibited spontaneous emission

David A. Branning, Alan L. Migdall, Paul G. Kwiat

Published Aug 26, 2007• 1 citation
Publisher

Abstract

We present an experimental realization of a "sudden mirror replacement" thought experiment, in which a mirror that is inhibiting spontaneous emission is quickly replaced by a photodetector. The question is, can photons be counted immediately, or only after a retardation time that allows the emitter to couple to the changed modes of the cavity, and for light to propagate to the detector? Our results, obtained with a parametric downconverter, are consistent with the cavity QED prediction that photons can be counted immediately, and are in conflict with the retardation time prediction.

DOI
10.1117/12.734807

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We present an experimental realization of a "sudden mirror replacement" thought experiment, in which a mirror that is inhibiting spontaneous emission is quickly replaced by a photodetector. The question is, can photons be counted immediately, or only after a retardation time that allows the emitter to couple to the changed modes of the cavity, and for light to propagate to the detector? Our results, obtained with a parametric downconverter, are consistent with the cavity QED prediction that photons can be counted immediately, and are in conflict with the retardation time prediction.
We consider an excited atom in a cavity such that spontaneous emission is inhibited, and address the question of whether a sudden replacement of one of the cavity mirrors by a detector can result in a photon count immediately or only after some retardation time. The feasibility of an experiment of this type has led to considerable discussion as to its outcome. Following a brief summary of the conflicting arguments, we show that it is possible to count a photon immediately following the substitution of a photodetector for a mirror.
In the process of parametric downconversion, a high‐energy photon is annihilated to create a pair of lower‐energy photons within a nonlinear dielectric material. It was shown some years ago that this process can be modified by quantum interference, by reflecting the high‐energy and low‐energy light back into the dielectric. The suppression or enhancement of the downconversion rate is analogous to the phenomenon of inhibited spontaneous emission that occurs when an excited atom is placed in a cavity whose allowed modes cannot support the spontaneous emission. In this case, the atom remains excited—but then, how does it interact with the cavity? If the cavity mirror were replaced by a detector, could photons be counted immediately, or only after some time delay? Our experiment uses spontaneous downconversion, a 2‐m long cavity, and a fast optical switch to obtain the answer.
One of the most curious predictions of quantum electrodynamics (QED) is the suppression of spontaneous emission from an excited atom sitting between two parallel mirrors. Although this effect has been measured experimentally and calculated from full QED theory, there are two different interpretations of what actually happens – a partially quantum mechanical explanation in which both the atom and the radiation field are quantized, and a semiclassical interpretation in which the radiation field is treated classically. While both viewpoints provide an intuitive explanation of the effect, they seem to yield conflicting answers to the question: what happens if one of the mirrors is suddenly replaced by a photodetector?
We revisit the problem of spontaneous emission by a two-level system in a cavity, concentrating in particular on those aspects associated with the finite propagation time of light between the atom and the cavity boundaries. We find that spontaneous emission always occurs first at the free space rate and there is then an abrupt change in these dynamics when light reflected by the cavity mirrors returns to the atom, leading to interference effects that result either in an enhancement or a reduction of this rate or in a periodic exchange of excitation between the atom and the cavity field. From this point of view, reversible spontaneous emission in terms of single-mode ``vacuum Rabi oscillations'' can be interpreted as resulting from the interference between the multimode partial waves reflected at the cavity mirrors and the instantaneously radiated light. \textcopyright{} 1996 The American Physical Society.
Introduction Ever since 1916/17 when Einstein argued [45] [46] that spontaneous emission must occur if matter and radiation are to achieve thermal equilibrium, physicists have believed that spontaneous emission is an inherent quantum mechanical property of atoms and that excited atoms inevitably radiate. This view, however, overlooks the fact that spontaneous emission is a consequence of the coupling of quantized energy states of atoms with the quantized radiation field, and is a manifestation of quantum noise or of emission ‘stimulated’ by ‘vacuum fluctuations’. An infinity of vacuum states is available to the photon radiated by an excited atom placed in free space, leading to the effective irreversibility of such emissions. If these vacuum states are modified, as for example by placing an excited atom between closely spaced mirrors or in a small cavity (essentially the Casimir effect), spontaneous emission can be greatly inhibited or enhanced or even made reversible. Recent advances in atomic and optical techniques have made it possible to control and manipulate spontaneous emission. A whole new branch of quantum optics called ‘cavity QED’ has developed since 1987 utilizing these dramatic changes in spontaneous emission rates in cavities to construct new kinds of microscopic masers or micromasers that operate with a single atom and a few photons or with photons emitted in pairs in a two-photon transition.