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Rydberg atoms and radiation. Report for January-December 1986

Daniel Kleppner, Barbara J. Hughey, T. Gentile, Eric S. Hilfer, S. S. Vianna

Published Jan 1, 1987• 0 citations
OpenAlex

Abstract

The authors achieved their initial goal in the study of basic radioactive processes using Rydberg atoms - the observation of inhibited spontaneous emission. An experiment was carried out demonstrating that spontaneous emission can be effectively turned off. The underlying principle is that spontaneous emission results from the radioactive coupling between matter and a continuum of vacuum states. However, the assumptions underlying the conventional calculation of the density of modes in free space are not always valid. In particular, cavities can dramatically effect spontaneous emission. Such effects are difficult to observe in the optical regime because of the problem of making good fundamental mode cavities at short wavelengths. Excellent cavities can be made at microwave wavelengths, but spontaneous emission is enhanced by a factor of n4 where n is the principal quantum number, allowing such effects to be studied.

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Journal of the Optical Society of America B•Feb 1, 1987•6 citations
The authors achieved their initial goal in the study of basic radioactive processes using Rydberg atoms - the observation of inhibited spontaneous emission. An experiment was carried out demonstrating that spontaneous emission can be effectively turned off. The underlying principle is that spontaneous emission results from the radioactive coupling between matter and a continuum of vacuum states. However, the assumptions underlying the conventional calculation of the density of modes in free space are not always valid. In particular, cavities can dramatically effect spontaneous emission. Such effects are difficult to observe in the optical regime because of the problem of making good fundamental mode cavities at short wavelengths. Excellent cavities can be made at microwave wavelengths, but spontaneous emission is enhanced by a factor of n4 where n is the principal quantum number, allowing such effects to be studied.
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)
Spontaneous emission plays a fundamental role in all radiative processes. It limits the lifetime of excited atoms, causing basic uncertainty in their energies. It sets the scale for absorption and stimulated emission, providing a natural scale for the coupling of matter and radiation. It is the ultimate source of noise in nature. Spontaneous emission is such a ubiquitous process that its occurrence is generally taken for granted. In principle, however, spontaneous emission can be altered, either enhanced or inhibited, allowing one to modify primary radiative processes. Experimental techniques are available for realizing these possibilities, and a new area of radiative phenomena is opening for study.
Spontaneous emission from an excited atomic state reflects the properties of the surrounding vacuum field. By placing the atom near a metallic surface or in a cavity, one can modify the spectral density of the vacuum modes coupled to it and either enhance1 or inhibit2 spontaneous emission. Large enhancement3 or inhibition effects have been obtained recently in microwave experiments involving either Rydberg atoms4 or a trapped electron5 radiating in cavities whose sizes were of the order of one-tenth of a millimeter to a centimeter.