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Annual Meeting Optical Society of America

Number state preparation in a micromaser

HERBERT WALTHER

Published Jan 1, 1989• 0 citations
Publisher

Abstract

Experiments with the one-atom maser1 have demonstrated that it is possible to study the interaction of a single atom with a single-mode of a resonant electromagnetic field in a cavity. The atoms used in these experiments are Rydberg atoms with a very large principal quantum number. Another important ingredient is the superconducting cavity: The quality factor is high enough for a periodic energy exchange between atom and cavity field to be observed; i.e., the relaxation time of the cavity field is larger than the characteristic time of the atom-field interaction, which is given by the reciprocal of the Rabi frequency. It is therefore possible to perform experiments on the dynamics of the atom-field interaction. Some of the features are explicitly a consequence of the quantum nature of the electromagnetic field.2 With a new cavity (quality factor 3 × 1010) it could be demonstrated that the statistics of the photons in the cavity is sub-Poissonian; in suitable conditions it is even possible to generate number states. Using number states of the single-atom maser novel quantum interference experiments can be performed giving new insight in the wave particle duality.

DOI
10.1364/oam.1989.wdd1

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Experiments with the one-atom maser1 have demonstrated that it is possible to study the interaction of a single atom with a single-mode of a resonant electromagnetic field in a cavity. The atoms used in these experiments are Rydberg atoms with a very large principal quantum number. Another important ingredient is the superconducting cavity: The quality factor is high enough for a periodic energy exchange between atom and cavity field to be observed; i.e., the relaxation time of the cavity field is larger than the characteristic time of the atom-field interaction, which is given by the reciprocal of the Rabi frequency. It is therefore possible to perform experiments on the dynamics of the atom-field interaction. Some of the features are explicitly a consequence of the quantum nature of the electromagnetic field.2 With a new cavity (quality factor 3 × 1010) it could be demonstrated that the statistics of the photons in the cavity is sub-Poissonian; in suitable conditions it is even possible to generate number states. Using number states of the single-atom maser novel quantum interference experiments can be performed giving new insight in the wave particle duality.
Experiments on the interaction of a single atom with a single mode of a resonant electromagnetic field in a cavity are reviewed. The atoms used in these experiments were Rydberg atoms with a very large principal quantum number. Another important ingredient is a superconductivity cavity, the quality factor of which is high enough for a periodic energy exchange between atom and cavity field to be observed. The dynamics of the atom-field interaction predicted by theory was measured. Some of the features are explicitly a consequence of the quantum nature of the electromagnetic field: The statistical and discrete nature of the photon field leads to new dynamic characteristics such as collapse and revivals in the Rabi nutation.
Recent experiments with the one-atom maser /1/ have demonstrated that it is possible to study the interaction of single Rydberg atoms with a single quantized mode /2/ of a resonant electromagnetic field in a superconducting cavity. The quality factor is high enough to observe the periodic energy exchange between the atom and the field. The statistical and discrete nature of the photon field leads to new dynamic characteristics such as collapse and revial in the Rabi nutation /3/. Most of the observed effects are purely quantum features /4/.
Recent experiments with the one-atom maser1 have demonstrated that it is possible to study the interaction of single Rydberg atoms with a single quantized mode2 of a resonant electromagnetic field in a super conducting cavity. The quality factor is high enough to observe the periodic energy exchange between the atom and field. The statistical and discrete nature of the photon field leads to new dynamic characteristics such as collapse and revival in the Rabi nu tation.3 Most of the observed effects are purely quantum features.4
The development of a single-atom maser or a micromaser allows a detailed study of the atom–field interaction. The situation realized is very close to the ideal case of a single two-level atom interacting with a single-mode quantized field as treated in Section 6.2. In a micromaser a stream of two-level atoms is injected into a superconducting cavity with a high quality factor. The injection rate can be such that only one atom is present inside the resonator at any time. Due to the high quality factor of the cavity, the radiation decay time is much larger than the characteristic time of the atom–field interaction, which is given by the inverse of the single-photon Rabi frequency. Therefore, a field is built up inside the cavity when the mean time between the atoms injected into the cavity is shorter than the cavity decay time. A micromaser, therefore, allows sustained oscillations with less than one atom on the average in the cavity. The realization of a single-atom maser or a micromaser has been made possible due to the enormous progress in the construction of superconducting cavities together with the laser preparation of highly excited atoms called Rydberg atoms. The quality factor of the superconducting cavities is high enough for periodic energy exchanges between atom and cavity field to be observed. The interesting properties of the Rydberg atoms make them ideal for micromasers. In Rydberg atoms the probability of induced transitions between adjacent states becomes very large and scales as n 4 , where n denotes the principle quantum number.
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.
The work on the one-atom maser is reviewed. The superconducting maser cavity made of niobium and cooled to 0.5 K to exclude thermal photons has a quality factor of 3 X 1010. Velocity-selected Rydberg atoms pump the maser. The field inside the cavity is investigated by probing the atoms leaving the cavity. The dynamics of the Rabi nutation were investigated. Furthermore, the statistics of the photons in the cavity were determined to be sub-Poissonian corresponding to photon-number fluctuations of 70% below the vacuum-state limit. The one-atom maser is the only maser system which leads to nonclassical radiation even when pumping processes with Poissonian statistics are used. Some applications of the one- atom maser to study the quantum measurement process also are discussed.