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Physical Review Letters

Observation of Cavity-Enhanced Single-Atom Spontaneous Emission

P. Goy, J. M. Raimond, M. Groß, S. Haroche

Published Jun 13, 1983• 870 citations
Publisher

Abstract

It has been observed that the spontaneous-emission lifetime of Rydberg atoms is shortened by a large ratio when these atoms are crossing a high-$Q$ superconducting cavity tuned to resonance with a millimeter-wave transition between adjacent Rydberg states.

DOI
10.1103/physrevlett.50.1903

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A small number N (105≥N≥1) of sodium atoms in an atomic beam are prepared by pulsed laser excitation in a nS Rydberg level (n≂30) while transversing a millimeter wave Fabry–Perot cavity tuned to a transition connecting the nS state to a nearby n′P level. The atoms absorb the radiation in the cavity (nS→nP upward transition), or amplify it in a maser process [nS→(n−1)P, nS→(n−2)P... downward transition]. Due to the huge size of the electric dipole matrix element between nearby Rydberg levels, the system is extremely sensitive to the millimeter wave radiation field (enhanced by the tuned cavity) in both absorption and emission processes. Simple numerical expressions are given. The experiments demonstrate that the system can be used as an absolute power meter, an ultrasensitive millimeter photon counter, an absolute quantum thermometer, and a transient millimeter-wave amplifier.

Journal of Applied Physics•Aug 1, 1984•4 citations
It has been observed that the spontaneous-emission lifetime of Rydberg atoms is shortened by a large ratio when these atoms are crossing a high-QQQ superconducting cavity tuned to resonance with a millimeter-wave transition between adjacent Rydberg states.
Rydberg atoms excited in microwave resonators are ideal systems to test quantum electrodynamics on an unusual scale.1 Owing to their very large electric dipole sizes, these atoms are indeed coupled so strongly to infrared and millimeter-wave radiation that the field of a few photons—ultimately the one of a single photon—stored in a cavity is large enough to induce important transition rates during the atomic time of flight across the cavity. As a result quantum radiative effects are observed in a range of frequencies where usual atomic systems behave as purely classical radiators. The enhancement of the spontaneous emission rate of a single atom prepared in a tuned cavity has already been observed.2 For a resonant cavity with very small losses, a point should be reached where the atom can no longer decay but instead reversibly oscillates between the two states involved in the transition resonant with the cavity mode. Experiments aimed at the observation of this new regime of quantum electrodynamics evolution are under way
As Casimir pointed out many years ago, conducting surfaces and cavities alter the structure of the vacuum states, and these alterations can have physical effects. This is the underlying principle of the Casimir force and also the point of departure for a series of recent studies on atom-vacuum interactions in the microwave and millimeter wave regimes. The natural scale for such effects is set by the spontaneous emission rate. At microwave wavelengths this rate is normally too small for spontaneous emission to be observable, In Rydberg atoms, however, the rate is enhanced by a factor n4 (n is the principal quantum number, typically 20-40). Advances in experimental techniques for Rydberg atoms have opened the way to the study of the atom-vacuum interaction at long wavelengths. The result has been a renewed interest in physical effects due to the vacuum, essentially a new area of macroscopic quantum phenomena. Recent experiments with Rydberg atoms and also with free electrons are described. I will attempt to set the stage by reexamining some of the physical effects of the vacuum and illustrating the ideas with an experiment in which spontaneous emission was effectively turned off by tuning below cutoff a waveguidelike structure that surrounded the atom.1 (Invited paper, 25 min)
A small number N (105≥N≥1) of sodium atoms in an atomic beam are prepared by pulsed laser excitation in a nS Rydberg level (n≂30) while transversing a millimeter wave Fabry–Perot cavity tuned to a transition connecting the nS state to a nearby n′P level. The atoms absorb the radiation in the cavity (nS→nP upward transition), or amplify it in a maser process [nS→(n−1)P, nS→(n−2)P... downward transition]. Due to the huge size of the electric dipole matrix element between nearby Rydberg levels, the system is extremely sensitive to the millimeter wave radiation field (enhanced by the tuned cavity) in both absorption and emission processes. Simple numerical expressions are given. The experiments demonstrate that the system can be used as an absolute power meter, an ultrasensitive millimeter photon counter, an absolute quantum thermometer, and a transient millimeter-wave amplifier.