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Philosophical Transactions of the Royal Society of London Series A Mathematical and Physical Sciences

Exploration of radiative properties of very excited atoms

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

Published Nov 12, 1982• 25 citations
Publisher

Abstract

Abstract Various experiments involving very excited atoms and millimetre-wave photons are described, including high-resolution spectroscopy of Rydberg transitions, Rabi nutation and transient maser action between Rydberg levels. The large number of extremely narrow Rydberg spectral lines observed throughout the millimetre-wave spectrum provide a very good ‘calibration' of the scale of Rydberg levels of Na and Cs near the ionization limit, which could be used for the realization of precise infrared frequency markers. The observation of Rabi nutation and maser signals involving an unusually small absolute number of atoms and photons reveals the very great strength of the Rydberg atom to radiation coupling and is a clear indication that these atoms could be used as extremely sensitive millimetre-wave detectors. At a more fundamental level, the study of transient Rydberg masers provides a test of superradiance theory. Extrapolation of these maser experiments to the investigation of a situation where a single atom would exchange a single millimetre-wave photon with a resonant cavity is considered.

DOI
10.1098/rsta.1982.0137

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•Jan 1, 2009•0 citations
Abstract Various experiments involving very excited atoms and millimetre-wave photons are described, including high-resolution spectroscopy of Rydberg transitions, Rabi nutation and transient maser action between Rydberg levels. The large number of extremely narrow Rydberg spectral lines observed throughout the millimetre-wave spectrum provide a very good ‘calibration' of the scale of Rydberg levels of Na and Cs near the ionization limit, which could be used for the realization of precise infrared frequency markers. The observation of Rabi nutation and maser signals involving an unusually small absolute number of atoms and photons reveals the very great strength of the Rydberg atom to radiation coupling and is a clear indication that these atoms could be used as extremely sensitive millimetre-wave detectors. At a more fundamental level, the study of transient Rydberg masers provides a test of superradiance theory. Extrapolation of these maser experiments to the investigation of a situation where a single atom would exchange a single millimetre-wave photon with a resonant cavity is considered.
We present various experiments involving Rydberg atoms interacting with resonant microwave electromagnetic fields inside a cavity, allowing to check different features of the theory presented in the previous paper. We first describe experiments about superradiant emission by Rydberg states in the cavity, also called transient Rydberg atoms maser ; the evolution of the system is detected either on the populations by selective field ionization technique or on the microwave field itself by heterodyne detection. Very low thresholds, down to 100 atoms, for the maser emission have been observed. We then describe various aspects of collective absorption of radiation by very excited atoms including the observation of Rabi nutation in a small applied resonant field, the study of the quantum efficiency of this set-up considered as a microwave photon counter, and finally the unexpected characteristics of the blackbody radiation absorption in the cavity.
The operation of maser devices using highly excited Rydberg atoms as active medium is investigated. These systems, working in a transient regime reminiscent of super-radiance generate few-hundred-ns-long bursts of mm-wave radiation. They are characterized by extremely low inversion density thresholds (${N}_{0}\ensuremath{\sim}{10}^{4}$ atoms typically) and very small peak power outputs (in the ${10}^{\ensuremath{-}11}$- to ${10}^{\ensuremath{-}13}$-watt range). This study is interesting in the context of new tests of super-radiance and electrodynamics theory. It also opens new possibilities in the important domain of mm-wave amplification and detection. In this paper, we present a simple semiclassical theory of transient Rydberg maser operation and we report the experimental observation of a large number of maser emission lines on Na atoms, spanning a wide frequency interval ($60<\ensuremath{\nu}<950$ GHz). The technique used to monitor the emission is the indirect field ionization procedure which reveals fast radiative transfers occurring between Rydberg levels. Another method involving the direct detection of the maser radiation by a heterodyne receiver is described in a subsequent paper, along with a report on the investigation of various effects related to the triggering of the transient maser emission by external radiation.
Superradiance and transient maser action have been obtained at millimeter wavelengths with alkali Rydberg atoms as the active medium. The emission was detected by monitoring the evolution of Rydberg-state populations using field ionization. Because of the strong Rydberg-atom-radiation-field coupling, the number of radiators at threshold is far smaller than in conventional masers. The estimated radiated energy is extremely low, 10 to 103{10}^{3}103 eV. Applications for spectroscopy and microwave detectors are considered.
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.
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
There are only fistful of systems in physics that can be described by an exact theory from first principles and, at the same time, can be investigated under experimental conditions approaching the idealized theoretical ones. The one-atom-maser or micromaser provides such an example, making a detailed study of the fundamental properties of the atom-field interaction possible. The situation realized in a micromaser is very close to the ideal case of a single two-level atom interacting with a single quantized mode of a cavity field. In the micromaser the atoms have a dual purpose of both pumping the field and also probing it via measurements of the outgoing atoms. Any other means of measuring the field inside the resonator has the detrimental effect of lowering its Q-factor and thereby the photon storage time. The long photon storage time of the resonator allows for the decay of the field to be negligible during the passage of an atom and its interaction with the field. The behavior of atoms in a cavity is governed by the oscillatory exchange of energy between the atoms and the field, which is called Rabi oscillation. For cavity fields in the vacuum and few photon number states (Fock states), Rabi oscillations have been measured in the past. To prevent thermal effects from polluting the pure number states these experiments were performed at low temperatures (below 1 K). However, the observed resolution of the Rabi oscillations measurements was quite disappointing (only about 2% of the predicted theoretical value) and precluded more ambitious experiments like atom-atom correlations or time resolved investigations of the maser field dynamics. The observed contrast of Rabi oscillations can be considered as a figure of merit for the experimental resolution of quantum features of a maser field. Therefore, a high resolution in the measurements of Rabi oscillations is crucial. This thesis describes the methods and improvements applied to enhance the resolution of Rabi oscillations by a factor of 8. The following upgrades and developments proved to be crucial in this work: 1) The base temperature of the cryogenic system was lowered to 0.3 K, i.e. one third of the original value. At the same time, the time during which the system remains at the base temperature was improved by almost of an order of magnitude (to more than 12 h). 2) For the promotion of the atoms to the Rydberg regime new three-step diode laser setup was constructed. The excitation efficiency compared to the old dye laser was increased by two orders. Using top-of-fringe stabilization with new spectroscopic methods, specially designed error detection and feedback schemes as well as state-of-the-art PID regulators the continuous locking of lasers is about 8 h (compared to about 20 min with the old dye laser system). 3) The flexibility of using the three step excitation allows to excite different maser states and for the first time to investigate pure maser transition. 4) Various improvements on the atomic oven and the channeltron detection unit led to the reliable and stable production and detection of the atomic beam. Observed distribution of the atomic beam statistics reaches to within 6% the Poisson limit which is expected in the absence of experimental imperfections. 5) New developed techniques in magnetic field compensation allow to perform such measurements with two orders of magnitude better accuracy compared to the previous experiments. To demonstrate the new capabilities of the apparatus and to apply its improved resolution of quantum fields we performed a previously impossible in-depth analysis of mean photon number and the atomic inversion produced by a micromaser for various detunings between the cavity- and atomic resonance frequency.