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Cavity quantum electrodynamics

Daniel Kleppner

Published Jun 9, 1986• 2 citations
OpenAlex

Abstract

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)

Related papers

Rydberg Atoms and Radiation in Cavities–Quantum and Collective Effects

S. Haroche, P. Goy, M. Groß +1 more

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

•Jun 18, 1984•1 citation

Atoms in micron-sized metallic and dielectric waveguides

Edward A. Hinds, K. S. Lai, M. Schnell

Over the past ten years, our group has investigated the effects of confinement on atoms inside metallic micron–sized cavities in order to elucidate some basic phenomena in the field of cavity quantum electrodynamics (QED). The first of these was the inhibition of spontaneous emission from an atom inside a cavity. This was followed by a laser spectroscopic measurement of the van der Waals interaction between a single Rydberg atom and a gold cavity, which showed that a simple electrostatic model of the atom–cavity interaction is correct when the cavity is small enough. More recently, the retarded Casimir–Polder force was measured between a ground state sodium atom and a large cavity, demonstrating that the van der Waals potential fails at long enough range and that the vacuum fluctuations of the field then have an important role in the interaction of the atom with the cavity. Our group is now pushing forward these investigations to study cavities whose walls have losses and dispersion, where the theory of cavity QED is significantly more complicated. With real surfaces, we have to deal with the complex dielectric response ε(ω) of the material, which exhibits frequency–dependent absorption and dispersion. One particularly interesting case is when a downward transition in the atom is resonant with an excitation of the cavity walls. This opens a new branch for the atomic decay: as an alternative to creating a photon within the space surrounded by the cavity walls the atomic decay can now create an electromagnetic excitation of the walls themselves. Another novel feature of our experiments is that the Bohr frequencies of the atom are close to the kT/h, where T is room temperature. We therefore expect to be able to measure effects associated with QED at finite temperature; in other words, to study how the blackbody radiation affects our experiments. By conducting experiments with real surfaces, we hope to elucidate and perhaps simplify the theoretical models used to describe these systems.

Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences•Dec 15, 1997•22 citations

Cavity quantum electrodynamics

Partha Ghose

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.

Cambridge University Press eBooks•Apr 29, 1999•0 citations

Atomic physics experiments in micron-sized cavities: suppressing spontaneous decay at optical frequency

A. Anderson, S. Haroche, E. A. Hinds +3 more

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.

•Apr 26, 1987•1 citation

Rydberg atoms and radiation. Report for January-December 1986

Daniel Kleppner, Barbara J. Hughey, T. Gentile +2 more

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.

•Jan 1, 1987•0 citations

Inhibited Spontaneous Emission by a Rydberg Atom

Randall G. Hulet, Eric S. Hilfer, Daniel Kleppner

Spontaneous radiation by an atom in a Rydberg state has been inhibited by use of parallel conducting planes to eliminate the vacuum modes at the transition frequency. Spontaneous emission is observed to "turn off" abruptly at the cutoff frequency of the waveguidelike structure and the natural lifetime is measured to increase by a factor of at least 20.

Physical Review Letters•Nov 11, 1985•657 citations
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)
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
Over the past ten years, our group has investigated the effects of confinement on atoms inside metallic micron–sized cavities in order to elucidate some basic phenomena in the field of cavity quantum electrodynamics (QED). The first of these was the inhibition of spontaneous emission from an atom inside a cavity. This was followed by a laser spectroscopic measurement of the van der Waals interaction between a single Rydberg atom and a gold cavity, which showed that a simple electrostatic model of the atom–cavity interaction is correct when the cavity is small enough. More recently, the retarded Casimir–Polder force was measured between a ground state sodium atom and a large cavity, demonstrating that the van der Waals potential fails at long enough range and that the vacuum fluctuations of the field then have an important role in the interaction of the atom with the cavity. Our group is now pushing forward these investigations to study cavities whose walls have losses and dispersion, where the theory of cavity QED is significantly more complicated. With real surfaces, we have to deal with the complex dielectric response ε(ω) of the material, which exhibits frequency–dependent absorption and dispersion. One particularly interesting case is when a downward transition in the atom is resonant with an excitation of the cavity walls. This opens a new branch for the atomic decay: as an alternative to creating a photon within the space surrounded by the cavity walls the atomic decay can now create an electromagnetic excitation of the walls themselves. Another novel feature of our experiments is that the Bohr frequencies of the atom are close to the kT/h, where T is room temperature. We therefore expect to be able to measure effects associated with QED at finite temperature; in other words, to study how the blackbody radiation affects our experiments. By conducting experiments with real surfaces, we hope to elucidate and perhaps simplify the theoretical models used to describe these systems.
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.
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.
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.