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Physical Review A

Pump-coupled masers: Transient regime

János A. Bergou, Mark S. Hillery

Published Dec 1, 1991• 8 citations
Publisher

Abstract

We consider a system of two masers that are pumped by the same beam of excited atoms. The atoms pass first through one maser and then through the other. As a result, the first maser modifies the pump going into the second and an effective coupling results. We examine this system in the amplifier configuration, i.e., we do not consider saturation effects. The fields in the two masers are mutually coherent and can, under the proper circumstances, be free of noise in their relative phase. Therefore this system exhibits behavior similar to that of a correlated-emission laser. We also find that the photon number in the second maser grows faster than that in the first. These effects should be observable even if the pump beam has a finite velocity spread.

DOI
10.1103/physreva.44.7502

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We consider a system of two masers that are pumped by the same beam of excited atoms. The atoms pass first through one maser and then through the other. As a result, the first maser modifies the pump going into the second and an effective coupling results. We examine this system in the amplifier configuration, i.e., we do not consider saturation effects. The fields in the two masers are mutually coherent and can, under the proper circumstances, be free of noise in their relative phase. Therefore this system exhibits behavior similar to that of a correlated-emission laser. We also find that the photon number in the second maser grows faster than that in the first. These effects should be observable even if the pump beam has a finite velocity spread.
We study the one-atom maser with a periodic-pump scheme for the injected atoms. The dynamics of the system is solved also in a regime where the losses of the field cannot be neglected while the atom is inside the cavity, as is the situation in an optical maser. The time development of the maser field is described by a general evolution operator, whose matrix representation in the damping basis is known analytically for zero temperature. The numerical evaluation of this matrix yields the stationary as well as the transient properties of the maser. Noise-producing mechanisms of the pump are included explicitly in the evolution operator for the field. These are a finite efficiency of the pump laser, a velocity spread in the atomic beam, and spontaneous transitions between the atomic levels and to the ground state. We find that features such as trapped states and sub-Poissonian photon statistics also occur in the high-loss regime like in an optical resonator. The observation of such effects will, as in the ordinary micromaser experiments, depend very sensitively on the reduction of the pump noise. For highly regular excitation, we find a pronounced oscillation of the field-correlation function for a wide range of the transit time, which leads to a distinct line splitting in the spectrum of the field. The amount of the splitting is determined by the periodicity of the pump. For less regular excitation, and in particular in the Poissonian limit, these oscillations disappear.
Two micromasers are coupled by sharing a common pump beam. A stream of excited atoms first passes through one micromaser and then enters a second one. The steady state of the second micromaser exhibits a second threshold at θ2= 2. The second threshold is accompanied with a jump of π in the phase of the second micromaser. For higher values of the pump parameter θ new branches of the steady-state intensity keep appearing. In certain parameter regions the phase of the second micromaser locks to that of the first and exhibits correlated emission laser-like noise quenching. In other regions the intensity fluctuations exhibit antibunching. The approach to steady state is also highly nonclassical. There are sharp jumps of magnitude π in the phase of the second micromaser and the intensity tends to overshoot before they settle to their steady-state value. In this transient regime the system exhibits phase noise squeezing.
Reaching high cavity population with a coherent pump in the strong-coupling regime of a single-atom laser is impossible due to the photon blockade effect. In this work, we experimentally demonstrate that in a single-atom maser based on a transmon strongly coupled to two resonators it is possible to pump over a dozen of photons into the system. The first high-quality resonator plays a role of usual lasing cavity, and the second one presents a controlled dissipation channel, bolstering population inversion, and modifies the energy level structure to lift the blockade. As a confirmation of lasing action, we observe conventional laser features such as the narrowing of emission linewidth and external signal amplification. Additionally, we report unique single-atom features: self-quenching and several lasing thresholds.
An intense single-mode correlated-spontaneous-emission laser or maser can be realized by driving the active atoms coherently with an injected external field. The scheme involves single-photon transitions unlike the two-photon or quantum-beat correlated-emission lasers that utilize correlations between successive photon-cascade emissions or between simultaneous emissions into modes of the field. Quenching of both the amplitude and phase noise and, in certain cases, squeezing of the amplitude fluctuations are found.
Masers exhibit interesting transient behavior that cannot be completely understood on the basis of the rate equations. The use of the rate equations in most transient analyses is usually justified on a more or less intuitive basis and the implied assumptions are not always clear. In this paper, the macroscopic maser rate equations are derived systematically from the Boltzmann equation for the density matrix of the atomic systems and Maxwell's equations for the radiation fields. When the coherence linewidth (T2−1) of the atomic systems is much larger than the cavity linewidth and the natural linewidth (T1−1) of the atomic emission, and with a WKB approximation, in the lowest order of approximation one obtains the two widely used, coupled first-order nonlinear rate equations of Statz and deMars. On the other hand, if the cavity linewidth is much larger than the atomic linewidths (T1−1 and T2−1), one can use the so-called ``reaction-field principle'' of Anderson and obtain, again, two coupled first-order rate equations; however, only one of the equations is nonlinear. The ranges of validity of both approaches are discussed in some detail.
The coherent microwave emission from an optically thick atomic ensemble in a cavity under coherent population trapping is analyzed. Transient and continuous operations are theoretically examined within the frame of a closed three-level system in the Dicke regime. The effects related to the atomic density and to the propagation in the active medium are examined with particular reference to the subnatural linewidth, the low group velocity and the shifts of the maser emission profile from the unperturbed atomic transition. The case of alkali-metal atoms submitted to a \ensuremath{\Lambda} excitation scheme is addressed in view of applications in the atomic frequency standard field. Experimental observations in agreement with the theoretical predictions are reported for the case of rubidium in a buffer gas. Apparent superluminal propagation is also reported and briefly discussed.