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Optics and Spectroscopy

Sub-Poisson statistics of subensembles of a micromaser field mode

George P. Miroshnichenko

Published Apr 23, 2004• 2 citations
Publisher

Abstract

It is proposed to use a result of indirect measurements—the average relative frequency of counts of a detector selective for the states of atoms escaping from a micromaser—for time selection of subensembles of a field mode. Analytical expressions for the reduced density matrix, average number of photons, and Mandel Q parameter for mode subensembles were obtained by the method of periodic paths. The generation of a random sequence of results of indirect quantum measurements was performed by the Monte Carlo method. The results of the analytical and numerical calculations are compared. It is found that the statistics of subensembles is sub-Poisson even when the calculation based on the matrix for the entire ensemble (in the absence of quantum measurements) gives super-Poisson statistics. It is noted that the time selection of subensembles is possible within a limited range of variation in the pumping parameter Θ, when the probability of quantum jumps is relatively low.

DOI
10.1134/1.1719145

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arXiv:quant-ph/0008082•Physical Review A•Feb 9, 2001•7 citations
It is proposed to use a result of indirect measurements—the average relative frequency of counts of a detector selective for the states of atoms escaping from a micromaser—for time selection of subensembles of a field mode. Analytical expressions for the reduced density matrix, average number of photons, and Mandel Q parameter for mode subensembles were obtained by the method of periodic paths. The generation of a random sequence of results of indirect quantum measurements was performed by the Monte Carlo method. The results of the analytical and numerical calculations are compared. It is found that the statistics of subensembles is sub-Poisson even when the calculation based on the matrix for the entire ensemble (in the absence of quantum measurements) gives super-Poisson statistics. It is noted that the time selection of subensembles is possible within a limited range of variation in the pumping parameter Θ, when the probability of quantum jumps is relatively low.
The method of stochastic recurrent relation is used for simulating a random sequence (trajectory) of the counts of a detector of atomic states at the output of a single-atom micromaser. A random sequence of the relative frequencies of counts of a detector during a fixed time interval is calculated. The frequencies of counts vary randomly near the average time-independent level. It is assumed that these average levels are reproducible observables for each random trajectory. A micromaser can jumpwise transfer from one average level of the relative frequencies of counts to another. It is assumed that a certain subensemble of the states of a field mode corresponds to each measured average level of the frequencies of counts. A method is proposed for calculating possible average relative frequencies of counts and corresponding (mean) reduced density matrices of the mode ρ st . The matrix ρ st characterizes a micromaser during its development along a specified periodic trajectory. It is found by solving the eigenvalue problem for the evolution operator on the period. An analytic method for solving this problem is developed. The matrix ρ st is the solution to the inverse problem of the reconstruction of the statistics of a field mode from the statistics of a random trajectory. The procedure of selecting the parameters of the evolution operator on the period is discussed through a numerical example.
Explicit expressions are derived for calculating the two-time coincidence probability density, the Mandel Q parameter of the counting statistics, and the waiting-time distribution between detector clicks for the detection of de-excited atoms leaving a micromaser with Poissonian pumping. The procedure is illustrated by applying it to the simplest trapped micromaser state for which all results can be obtained analytically. Moreover, by making use of a symmetry relation which determines the properties of the time-dependent micromaser master equation solution, it is shown that in the stationary regime for negligible thermal photon number the normalized two-time intensity correlation function of the cavity field with respect to a given time difference is equal to the normalized two-time coincidence probability density of detecting two de-excited atoms with the same time difference at the exit of the micromaser.
The differences between macroscopic (all time or entire ensemble average) and submacroscopic (partial time or partial ensemble average) fields of micromasers with non-Poissonian pumping are discussed. A formula to calculate the steady-state density matrix of the macroscopic field is derived. The photon statistics, average photon numbers, and photon-number variances of the macroscopic field, together with those of the submacroscopic fields, are obtained and compared. The photon statistics of a special submacroscopic field are almost the same as those of the macroscopic field.
A method is presented for theoretically investigating the properties of a one-atom micromaser with stationary non-Poissonian pumping. The method is based on treating the statistics of the arrival times of the individual pump atoms with the help of the theory of stochastic point processes. Considerable simplification is achieved by assuming the pump statistics to be described by a stationary renewal process. Thus the influence of super- as well as sub-Poissonian pumping with different strengths of correlation between the pump atoms and different correlation decay times may be studied quantitatively. The level-selective statistics of the atoms leaving the cavity is investigated as well as the photon statistics of the cavity field. Moreover, the relation to the other models used in the literature for describing the micromaser pump statistics is discussed. It is found that for sub- (super-)Poissonian pumping, the stationary expectation value of the cavity photon number (which corresponds to its time-averaged value) is larger (smaller) than the conditioned mean photon number that would result from averaging over the number of photons present in the cavity at the time instants immediately before the injection of the individual pump atoms. For Poissonian pumping, both quantities are shown to be equal. The relative standard deviation of the stationary cavity photon number is decreased (increased) for sub- (super-) Poissonian pumping, in comparison to the corresponding values that would result from Poissonian pumping. Moreover, it turns out that for sub- (super-)Poissonian pumping, the normalized coincidence probability density for the detection of the outgoing deexcited atoms is smaller (larger) than the normalized cavity field intensity correlation function at zero time delay. The difference decreases with increasing lifetime of the pump-atom correlations. Finally, the level-selective delayed coincidence probability densities of the outgoing atoms and their waiting-time distributions are found to be affected greatly by the correlation strength as well as the correlation decay time of the incoming pump atoms.
We describe the first investigation of the nonclassical radiation field of a micromaser. The maser cavity is cooled to 0.5 K to exclude thermal photons and has a quality factor 3\ifmmode\times\else\texttimes\fi{}1010{10}^{10}1010. Velocity-selected Rydberg atoms pump the maser. The photon statistics in the cavity are measured via the statistics of the atoms leaving the cavity in the lower maser level. Taking into account a detection efficiency of 10%, their fluctuations are reduced up to 40% below the Poisson level. This corresponds to photon-number fluctuations in the cavity of up to 70% below the vacuum-state limit.
We present a general method for the derivation of various statistical quantities describing the detection of a beam of atoms emerging from a micromaser. The use of non-normalized conditioned density operators and a linear master equation for the dynamics between detection events is discussed as are the counting statistics, sequence statistics, and waiting time statistics. In particular, we derive expressions for the mean number of successive detections of atoms in one of any two orthogonal states of the two-level atom. We also derive expressions for the mean waiting times between detections. We show that the mean waiting times between detections of atoms in like states are equivalent to the mean waiting times calculated from the uncorrelated steady state detection rates, although like atoms are indeed correlated. The mean waiting times between detections of atoms in unlike states exhibit correlations. We evaluate the expressions for various detector efficiencies using numerical integration, reporting results for the standard micromaser arrangement in which the cavity is pumped by excited atoms and the excitation levels of the emerging atoms are measured. In addition, the atomic inversion and the Fano-Mandel function for the detection of deexcited atoms are calculated for comparison to the experimental results of Weidinger et al. [Phys. Rev. Lett. 82, 3795 (1999)], who report the observation of trapping states.