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Physical Review Letters

Fine Structure of Thresholds in a Micromaser Pumped with Atom Clusters

Giacomo Mauro D’Ariano, Nicoletta Sterpi, A. Zucchetti

Published Feb 6, 1995• 12 citations
Publisher

Abstract

We present the first quantum microscopic study of a micromaser with many atoms in the cavity. Excited atoms are injected in clusters with average number $\overline{N}$ up to 100. The stationary state of radiation is evaluated by means of a novel Green-function Monte Carlo technique. For $\overline{N}<{N}_{\mathrm{ex}}$ (${N}_{\mathrm{ex}}$ being the number of excited atoms entering the cavity in a photon lifetime) the system behaves similarly to the one-atom maser. At $\overline{N}\ensuremath{\simeq}{N}_{\mathrm{ex}}$ a transition occurs to a novel cluster behavior, where a fine structure of thresholds emerges.

DOI
10.1103/physrevlett.74.900

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We present the theory of a truly microscopic maser consisting of a single-mode high-Q resonator in which a monoenergetic beam of excited two-level atoms is injected at such a low flux that at most one atom at a time is present inside the cavity. Both a microscopic theory and a heuristic Fokker-Planck approach are presented. We show that the micromaser exhibits a number of novel features that are averaged out in usual masers and lasers. First, the field is in general sub-Poissonian, which reflects the quantization of both the field and its sources. Second, the onset of maser oscillations may be followed by a succession of abrupt transitions in the state of the field. Finally, as the atomic flux through the resonator is increased, the maser threshold acquires characteristics of a continuous phase transition, whereas the subsequent changes in the field distribution become analogous to first-order phase transitions.
Phase diagrams of the micromaser system are mapped out in terms of the physical parameters at hand like the atom cavity transit time, the atom-photon frequency detuning, the number of thermal photons and the probability for a pump atom to be in its excited state. Critical fluctuations are studied in terms of correlation measurements on atoms having passed through the micromaser or on the microcavity photons themselves. At sufficiently large values of the detuning we find a ``twinkling'' mode of the micromaser system. Detailed properties of the trapping states are also presented.
The long lifetime of photons in a high-QQQ micromaser cavity is used to generate a ∣n〉|n〉∣n〉-state maser field by injecting atoms, one at a time. Every atom is probed for its excitation after leaving the cavity so that the precise number of photons in the field is known. We calculate the probability of obtaining nnn photons after mmm atoms have passed the cavity.
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.
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.