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Heisenberg Equations of Motion

Z. Ficek, Mohamed Ridza Wahiddin

Published Apr 19, 2016• 0 citations
Publisher

Abstract

In the master equation method, we have already illustrated a powerful technique for the calculation of the dynamics of an atomic system interacting with the vacuum field. Another technique for calculating the dynamics of an atomic system coupled to the elec- tromagnetic (EM) field involves Heisenberg equations of motion for the system’s operators. A difference between the master equation and the Heisenberg equations is that the later involves dynamics of the operators, which allows to analyse the evolution of an atomic system in terms of the field and atomic operators. This creates some problems with handling the Heisenberg equations as, in general, operators do not commute and then in the course of solution of the equations we may face the problem of ordering of the operators. It is usually resolved by putting the operators in the normal order. We have gained some experience with the Heisenberg equations of motion in Chapter 6, Example 6.3, where we studied squeezing generation in the nonlinear degenerate parametric amplifier (DPA) process. Here, we illustrate the technique on the standard model of a two-level atom interacting with a multi-mode field. We then generalize the technique to some specific models such as Lorenz- Maxwell and Langevin equations, the derivation of which involves some approximations that can be applied only in some limited cases. We also present in detail the Floquet approach, which is usually applied to problems determined by differential equations with time- dependent coefficients.

DOI
10.1201/b16875-16

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In the master equation method, we have already illustrated a powerful technique for the calculation of the dynamics of an atomic system interacting with the vacuum field. Another technique for calculating the dynamics of an atomic system coupled to the elec- tromagnetic (EM) field involves Heisenberg equations of motion for the system’s operators. A difference between the master equation and the Heisenberg equations is that the later involves dynamics of the operators, which allows to analyse the evolution of an atomic system in terms of the field and atomic operators. This creates some problems with handling the Heisenberg equations as, in general, operators do not commute and then in the course of solution of the equations we may face the problem of ordering of the operators. It is usually resolved by putting the operators in the normal order. We have gained some experience with the Heisenberg equations of motion in Chapter 6, Example 6.3, where we studied squeezing generation in the nonlinear degenerate parametric amplifier (DPA) process. Here, we illustrate the technique on the standard model of a two-level atom interacting with a multi-mode field. We then generalize the technique to some specific models such as Lorenz- Maxwell and Langevin equations, the derivation of which involves some approximations that can be applied only in some limited cases. We also present in detail the Floquet approach, which is usually applied to problems determined by differential equations with time- dependent coefficients.
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.
Despite many years of research into Raman phenomena, the problem of how to include both spontaneous and stimulated Raman scattering into a unified set of partial differential equations persists. The issue is solved by formulating the quantum dynamics in the Heisenberg picture with a rigorous accounting for both time- and normal-ordering of the operators. It is shown how this can be done in a simple, straightforward way. Firstly, the technique is applied to a two-level Raman system, and comparison of analytical and numerical results verifies the approach. A connection to a fully time-dependent Langevin operator method is made for the spontaneous initiation of stimulated Raman scattering. Secondly, the technique is demonstrated for the much-studied two-level atom both in vacuum and in a lossy dielectric medium. It is shown to be fully consistent with accepted theories: using the rotating wave approximation, the Einstein A coefficient for the rate of spontaneous emission from a two-level atom can be derived in a manner parallel to the Weisskopf-Wigner approximation. The Lamb frequency shift is also calculated. It is shown throughout that field operators corresponding to spontaneous radiative terms do not commute with atomic/molecular operators. The approach may prove useful in many areas, including modeling the propagation of next-generation high-energy, high-intensity ultrafast laser pulses as well as spontaneous radiative processes in lossy media.
On one hand a two-level atom is utilized as a qubit in quantum information technology and on the other hand the light in quantum optics is usually in the squeezed state. These reasons motivated us to explore the dynamic of a two-level atom which is driven by a field in the squeezed state. To this goal, the atom operators and the Heisenberg-Langevin equation have been employed. The master equation which reveals the dynamic of the atom has been derived using some features of the squeezed state and doing some algebra. Finally, the dynamic of the two-level atom which is driven by squeezed state field has been simulated by the derived master equation. Also, the effect of atom parameters on the dynamic has been investigated.
A Brief Survey of Analytical Dynamics, Discovery of Matrix Mechanics, Mathematical Preliminaries Postulates of Quantum Theory Equations of Motion, Hamiltonian Operator and the Commutation Relations Symmetries and Conservation Laws, Classical and Quantum Integrability Classical and Quantum Mechanical Degeneracies Bound State Energies for One-Dimensional Problems Exactly Solvable Potentials, Supersymmetry and Shape Invariance Supersymmetry and Superpotential Shape Invariant Potentials Two-Body Problem Determination of the Angular Momentum Eigenvalues Methods of Integration of the Heisenberg Equations of Motion Discrete-Time Formulation of the Heisenberg's Equations of Motion Quantum Tunneling Using Discrete-Time Formulation Integration of the Operator Differential Equations Perturbation Theory Other Methods of Approximation Heisenberg Correspondence Principle, Quantization of the Equations of Motion with Higher Derivatives Determinantal Method in Potential Scattering Time-Dependent Scattering Theory The Scattering Matrix Impact Parameter Representation of the Scattering Amplitude Transition Probability Quantum Diffraction, Motion of a Charged Particle in Electromagnetic Field The Aharanov-Bohm Effect Neutron Interferometry Quantum Many-Body Problem Hartree and Hartree-Fock Approximations Second Quantization Bogoliubov Transformation for a Many-Boson System Bogoliubov Transformation for Fermions Interacting Through Pairing Forces Quantum Theory of Free Electromagnetic Field Casimir Force Interaction of Radiation with Matter Heisenberg Equations for Interaction of an Atom with Radiation Bell's Inequalities
We have analyzed the squeezing and statistical properties of the cavity light beam produced by a coherently driven degenerate three-level laser with a degenerate parametric amplifier (DPA) in an open cavity and coupled to a vacuum reservoir via a single-port mirror. We have carried out our analysis by putting the noise operators associated with the vacuum reservoir in normal order. Applying the solutions of the equations of evolution for the expectation values of the atomic operators and the quantum Langevin equation for the cavity mode operator, the mean photon number and the quadrature squeezing of the cavity light are calculated. And a large part of the mean photon number is confined in a relatively small frequency interval. Furthermore, we also obtain the antinormally ordered characteristic function defined in the Heisenberg picture. With the aid of the resulting characteristic function, we determine the Q function which is then used to calculate the photon number distribution.
We study the squeezing properties of the light generated by nondegenerate three-level laser in which the three-level atoms are in cascade configuration with degenerate parametric oscillator. We obtain the c-number Langevin equations associated with the normal ordering using the pertinent master equation. Making use of the solutions of the c-number Langevin equations, we calculate the quadrature variances. The results show that the two-mode light produced by the system under consideration is in squeezed state and the squeezing occurs in the minus quadrature. The degree of squeezing increase with the linear gain coefficient and almost perfect squeezing can be obtained for large values of the linear gain coefficient and the atoms initially prepared in lower level slightly greater than in the upper level. Moreover, the presence of nonlinear crystal increases the degree of squeezing for single mode and two-mode lights.