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Journal of Modern Optics

Master equation and its Fokker–Planck representation in two quantum interactions of radiators with squeezed field

N. A. Enaki

Published Apr 15, 2010• 3 citations
Publisher

Abstract

A new method of elimination of squeezed field operators in nonlinear interaction with atomic subsystem is proposed. This method can be used for the opposite laser situation, in which the operators of an external coherent atomic source are eliminated from the density matrix equation. New types of master equations, belonging to su(1, 1) and su(2) symmetries are obtained. The solution of this equation is obtained using the generalized non-diagonal P and Q representations for su(1, 1) and su(2) symmetries. These master equations and their solutions can be used for further studies on the behavior of atomic polarization or quantum properties of the cavity field.

DOI
10.1080/09500341003706813

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A new method of elimination of squeezed field operators in nonlinear interaction with atomic subsystem is proposed. This method can be used for the opposite laser situation, in which the operators of an external coherent atomic source are eliminated from the density matrix equation. New types of master equations, belonging to su(1, 1) and su(2) symmetries are obtained. The solution of this equation is obtained using the generalized non-diagonal P and Q representations for su(1, 1) and su(2) symmetries. These master equations and their solutions can be used for further studies on the behavior of atomic polarization or quantum properties of the cavity field.
In Chapter 9, we derived the master equation for a reduced density operator of a two-level atom coupled to a multi-mode vacuum field. We have seen that the master equation is an operator type equation that, in general, is not easy to solve. We have illustrated the solution of the master equation by representing the density operator in the basis of the two states of the two-level atom. The purpose of the representation of the master equation was to obtain c-number differential equations that are equivalent to the operator equations, but are more readily soluble.
The basic tool in quantum optics for the description of atomic systems interacting with coherent and incoherent radiation fields is the atomic density matrix governed by the master equation. In the present paper we apply standard condensed-matter nonequilibrium many-body methods to the problem of a two-level atom driven by an external laser field and relaxing by interaction with the radiation field. We recover the well-known dressed-atom picture and, in particular, we present an alternative derivation of the master equation for the atomic density matrix which allows simple generalizations.
Starting from the master equation for density operator of the atoms and the field mode and applying the operator method of adiabatic elimination of the atomio variables developed by Haake and Lewenstein, we derived the Fokker-Planck Equation for Wigner function of the lightfield and its steady-state solution. With a Gaussian approximation to the solution, analytical and numerical results on the photon statistics for various pumping strengths are obtained. Comparisons with previous works are made.
The dissipative and decoherence properties of a two-level atom interacting with a squeezed vacuum field reservoir are investigated based on the nonautonomous master equation of the atomic density matrix in the framework of algebraic dynamics. The nonautonomous master equation is converted into a Schrödinger-like equation and its dynamical symmetry is found based on the left and right representations of the relevant algebra. The time-dependent solutions and the steady solutions are obtained analytically. The asymptotic behaviour of the solutions is examined and the approach to the equilibrium state is proved. Based on the analytic solution the response of the system to the squeezed vacuum field reservoir is studied numerically.
We generalize the method of deriving a kinetic equation for a gas laser developed in the first paper of this series to include radiation-matter correlations. As a consequence, we are able to show that the first Born approximation with asymptotic conditions which contain radiation-matter correlations is sufficient to explain the nonthermal photon distribution observed in photon-counting experiments. Our derivation includes the spatial and velocity effects of the motion of the gas atoms. After adiabatically eliminating the matter variables, we obtain a master equation for the radiation density matrix RRR with nonlinear coefficients. Our radiation master equation for stationary atoms without spatial effects is the same as the generalized Fokker-Planck equation of the Langevin-noise-equation treatment of lasers. We show that the generalized Fokker-Planck equation is the result of the first-Born-approximation treatment of the j\ifmmode\cdot\else\textperiodcentered\fi{}A interaction and is not dependent upon the microscopic structure of the reservoirs. In the first Born approximation, the diagonal and off-diagonal matrix elements of RRR do not interact with each other. Consequently, an initially diagonal radiation density matrix remains diagonal. However, we show that even though the stationary state is diagonal it is necessary to know the propagator of the off-diagonal part of RRR to answer time-dependent questions about quantities, such as the line shape, that depend on phase.
The nonlinear interaction of light with matter is described from a quantum-statistical point of view. The phenomena of two-photon emission and two-photon absorption including both the single- and two-mode cases and the Raman effect are discussed in detail. A master equation for the density operator of the light fields alone is derived. This operator equation is converted to a c number equation and analytic solutions are obtained for the diagonal matrix elements of the density operator in the Fock representation. No linearizing approximation is introduced. These solutions allow one to compute the moments of the photon distribution for the above nonlinear processes.