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Laser Physics Letters

Polarization properties of Raman micromaser

Решетов Владимир Александрович, И. Н. Пигалева, Igor V. Yevseyev

Published Dec 12, 2008• 0 citations
Publisher

Abstract

The polarization properties of one-atom maser pumped by the coherent field via Raman transitions are studied theoretically. The three-level Λ-type atoms with degenerate Rydberg levels E b < E a < E c enter the micromaser cavity being excited to the lower level E b . The coherent microwave field drives the transition b → c , while the maser transition a → c is coupled to the cavity quantum field. The dependence of the maser field parameters, such as the mean photon number and the variance, as well as of the correlation functions of the atoms, emerging from the cavity, versus the angle ψ between the polarizations of the maser and driving fields are investigated. The case of transitions J b = 3/2 → J c = 3/2 and J a = 5/2 → J c = 3/2 in the rubidium atoms, which are exploited in micromaser experiments (see, e.g., the review [1]), is analyzed numerically. The possibility of monitoring the state of the maser field by means of driving field polarization is discussed.

DOI
10.1002/lapl.200810133

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Laser Physics•Jul 1, 2007•2 citations
The polarization properties of one-atom maser pumped by the coherent field via Raman transitions are studied theoretically. The three-level Λ-type atoms with degenerate Rydberg levels E b < E a < E c enter the micromaser cavity being excited to the lower level E b . The coherent microwave field drives the transition b → c , while the maser transition a → c is coupled to the cavity quantum field. The dependence of the maser field parameters, such as the mean photon number and the variance, as well as of the correlation functions of the atoms, emerging from the cavity, versus the angle ψ between the polarizations of the maser and driving fields are investigated. The case of transitions J b = 3/2 → J c = 3/2 and J a = 5/2 → J c = 3/2 in the rubidium atoms, which are exploited in micromaser experiments (see, e.g., the review [1]), is analyzed numerically. The possibility of monitoring the state of the maser field by means of driving field polarization is discussed.
The operation of one atom-maser is considered under the conditions, when the atoms enter the cavity being excited to the state, which is the mixture of Zeeman sublevels of both the upper and lower resonant atomic levels. The analytic expression for the steady state photon distribution in the cavity under such conditions is obtained. It is shown, that the micromaser parameters, such as the atomic flux and velocity of atoms, the portion of atoms at the lower maser level, the polarizations of laser excitation fields may be chosen in such a way as to obtain the sharp maximum of photon distribution function at a given number of photons, providing the possibility of creation of pure Fock states.
The polarization properties of the micromaser field pumped by the atoms with the resonant levels, which are degenerate in the projections of the total angular momentum on the quantization axis, are studied numerically. The standard micromaser operation, when the atoms enter the cavity excited to the upper resonant level, and the micromaser operation under the coherent pump, when the atoms enter the cavity in a superposition of resonant atomic levels, are considered. The treatment is based on the master equation for the density matrix of the micromaser field, which takes into account the degeneracy of atomic levels.
We describe the dynamics of a micromaser pumped by atoms in superposition states and driven by a resonant coherent field. By a quantum jump approach to the master equation solution, we show how the coherences excited in the cavity mode affect the nonclassical steady-state behaviour of the system, in a way that depends on the relative phase between the polarized atoms and the driving field. Both the mean photon number and the phase of the cavity field can be controlled via the atomic transit time and pump rate, or the driving field intensity. The predicted coherence and phase competition effects can show up in the statistics of the populations of outgoing atoms.
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 parameters of the micromaser field in experiments are measured by means of detection of the state of the atoms emerging from the cavity. The employment of the atoms with degenerate levels makes it possible to obtain more information about the micromaser field than in the case of two-level atoms. A way to obtain such information recorded in various Zeeman sublevels of resonant atomic levels is suggested in the present paper. This information is provided by the average quantities , where θμ are Rabi angles related to the transitions between corresponding Zeeman sublevels. At small values of the pump parameter the combinations of these quantities Fμν = zμμzνν−zμν2 are proportional to the Fano factor of the micromaser field. The examples of atomic transitions with the angular momentum changes and are considered.
The theory of a one-atom maser, when the atoms enter the microresonator being excited to the upper maser level from its ground state by the bichromatic laser field resonant with the two adjacent optically allowed transitions with arbitrary angular momenta, is built. This theory is applied to the investigation of the polarization properties of a one-atom maser, i.e., the dependence of the maser field characteristics on the mutual orientations of the polarization vectors of the maser field mode and of the pump laser field. New experiments are proposed to study such polarization properties, which may be performed within the framework of the experimental setup used by Professor H. Walther and collaborators [1–3].