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Journal of Optics B Quantum and Semiclassical Optics

Micromaser spectrum and phase diffusion dynamics

Federico Casagrande, Alessandro Ferraro, ALFREDO LULLI, R. Bonifacio, E. Solano, HERBERT WALTHER

Published Aug 1, 2003• 3 citations
Publisher

Abstract

We discuss a scheme to relate the phase diffusion dynamics of the micromaser field to the measured atomic population statistics. This can allow us to measure the linewidth of the micromaser spectrum and to solve a relevant decoherence problem in cavity quantum electrodynamics. The main steps are (i) a suitable preparation of the cavity field state to generate coherences, (ii) the transfer of information on the dynamics of field coherences to probe atoms by the action of an external resonant coherent field and (iii) the derivation of the phase diffusion rate, hence the micromaser linewidth, from the measured population statistics of the probe atoms. The method can be applied even in the presence of trapping states, where peculiar linewidth oscillations are expected for increasing pump rate, due to the quantum nature of the micromaser field.

DOI
10.1088/1464-4266/5/4/351

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We discuss a scheme to relate the phase diffusion dynamics of the micromaser field to the measured atomic population statistics. This can allow us to measure the linewidth of the micromaser spectrum and to solve a relevant decoherence problem in cavity quantum electrodynamics. The main steps are (i) a suitable preparation of the cavity field state to generate coherences, (ii) the transfer of information on the dynamics of field coherences to probe atoms by the action of an external resonant coherent field and (iii) the derivation of the phase diffusion rate, hence the micromaser linewidth, from the measured population statistics of the probe atoms. The method can be applied even in the presence of trapping states, where peculiar linewidth oscillations are expected for increasing pump rate, due to the quantum nature of the micromaser field.
We show how our recently proposed scheme for the measurement of the micromaser linewidth, which relates the phase diffusion dynamics of the cavity field to the population statistics of probe atoms, can be applied in the presence of trapping states, where the phase diffusion approximation does not strictly hold. This should allow the observation of the peculiar linewidth oscillations versus atomic pumping which are expected in this regime, and whose origin lies in the quantum nature of the cavity field.
We propose a realistic scheme for measuring the micromaser linewidth by monitoring the phase diffusion dynamics of the cavity field. Our strategy consists of exciting an initial coherent state with the same photon number distribution as the micromaser steady-state field, singling out a purely diffusive process in the system dynamics. After the injection of a counterfield, measurements of the population statistics of a probe atom allow us to derive the micromaser linewidth in all ranges of the relevant parameters, establishing experimentally the distinctive features of the micromaser spectrum due to the discreteness of the electromagnetic field.
We propose a method to investigate the phase dynamics of the radiation field of a micromaser in steady-state operation. The phase-diffusion information is deduced from the population statistics of the atoms leaving a classical coherent field placed after the micromaser cavity. By using the classical coherent field, one gains an additional degree of freedom which allows one to probe the polarization component of the atomic dipole which is directly correlated to the cavity-field amplitude. As an illustration we present the results of a computer experiment, modeling the complete dynamics of the proposed experiment.
In earlier papers, the linewidth of a micromaser was approximated by a phase diffusion rate that was obtained using a single decay rate assumption to model the decay of the electric field in the micromaser cavity. These methods gave analytic but approximate answers. Exact methods calculated the spectrum via the two-time correlation function, but required extensive numerical calculations. In this paper, we present an approximate analytic derivation of the micromaser spectrum via the two-time correlation function that is more accurate than other approximate analytic methods yet requires much less numerical calculation than the exact methods. We also review the other methods, both analytical and exact, and present numerical comparisons.
In this paper we show how different measurement schemes for a micromaser system may be used to investigate different aspects of the maser dynamics. The features which are investigated include a measurement of the phase diffusion in steady-state, transient states with several orders of coherence, and an interesting connection to questions of the Einstein-Podolsky-Rosen paradox. In addition, it is shown rigorously that different measurement schemes acting only on the atomic part of the system after the cavity have no influence on the ensemble properties of the cavity field.
In a micromaser where a classical field strongly drives the atoms while they cross the cavity, remarkable atom–atom correlations show up at steady state, which vanish much faster than dissipative decay. Hence we consider atom pair correlation measurements in which the detection of the first probe atom prepares a mesoscopic superposition state of the cavity field, that entangles with a second probe atom. The conditional probabilities for the latter atomic detection provide a description of the decoherence of the superposition state, occurring in an open system in the presence of pumping, driving, dissipative, and thermal effects. The decoherence rate scales as the squared interaction time, that sets the separation in phase space between the superposition components, whereas the quantum coherence is unaffected by the atomic pumping. Hence we further investigate the system when the cavity is not pumped. Starting the correlation measurements from a thermal state, we can describe the effect of temperature on decoherence. Starting from a vacuum state, the superposition states are maximally separated Schrodinger cat states, whose decoherence can thus be monitored.