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Tenth International Conference on Information Optics and Photonics

Non-perturbation calculation for the dynamic problem of quantum many-body systems

Xueying Liu, Xuezao Ren, K. Wang, Xiaohong Li, Xiaoming Lin

Published Nov 15, 2018• 1 citation
Publisher

Abstract

We propose a new method to discuss the evolution of physical systems, which has an analytic form. And we systematically introduce this new method by the example of Jaynes-Cummings model without rotation wave approximation. Simultaneously, while we repeat previous work by our method, we also calculate it by adding the time growth factor to the initial state unfolded in the steady state which is based on Fock state and is obtained by solving the time-independent Schrodinger equation (in this article, the traditional method refers to this method). By comparing these two results, we find the drawback of our method and improve it. Finally, we show that our improved method need the smaller Fock space than the traditional method for physical systems with two-mode cavity field and put forward expectations for the follow-up study.

DOI
10.1117/12.2505698

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We propose a new method to discuss the evolution of physical systems, which has an analytic form. And we systematically introduce this new method by the example of Jaynes-Cummings model without rotation wave approximation. Simultaneously, while we repeat previous work by our method, we also calculate it by adding the time growth factor to the initial state unfolded in the steady state which is based on Fock state and is obtained by solving the time-independent Schrodinger equation (in this article, the traditional method refers to this method). By comparing these two results, we find the drawback of our method and improve it. Finally, we show that our improved method need the smaller Fock space than the traditional method for physical systems with two-mode cavity field and put forward expectations for the follow-up study.
The Jaynes-Cummings model (JCM) is studied in the absence of the rotating-wave approximation (RWA) by a coherent-state expansion technique. In comparison with the previous paper in which the coherent-state expansion was performed only to the third order, we carry out in this paper a complete expansion to demonstrate exactly the dynamics of the JCM without the RWA. Our study gives a systematic method to solve the non-RWA problem, which would be useful in various physical systems, e.g., in a system with an ultracold trapped ion experiencing the running waves of lasers.
We develop a systematic method of operator perturbation theory in the Heisenberg picture, which is the formal analog of time-independent stationary-state perturbation theory in the Schr\"odinger picture. However, we use the eigenoperators and operator eigenfrequencies to calculate the time evolution of the dynamical variables of interest. The spectrum of the unperturbed Hamiltonian is assumed to be discrete. The method is especially well-suited for treating systems where the unperturbed Hamiltonian represents noninteracting fermions and/or bosons, and therefore, we illustrate the method using the exactly solvable model of Jaynes and Cummings: a single two-level atom interacting with a single quantized field mode. The time evolution of these operator solutions is shown to be unitary order by order in powers of the interaction strength. We find that the time evolution of exact operator solutions is well approximated for significantly longer times by the time evolution of this method's operator solutions than by the time evolution of the same operator solutions calculated using the Dyson expansion. We also demonstrate that time-dependent operator solutions are very convenient for computing quantities such as correlation functions.
The two-level system interacting with a single-mode boson is studied using the method of coherent approximation. We calculated the occupation probability P i(t) of the system, and compare it with the results of Jaynes-Cummings model under different situations. The results show that this method can be used to deal with the problems when J-C model is not valid. CA(coherent approximation) method can provide better approximate results when the differential physical effect of P i(t) is considered. CA is a non-variational and non-perturbational method that provides a systematic scheme to improve the approximation of the system.
Jaynes–Cummings (JC) models are by far the most widely used to study the dynamics of two-level systems, such as atoms or molecules under suitable conditions, coupled to quantized electromagnetic fields. Nowadays, analytical solutions of JC models are more useful than ever for understanding and controlling radiation-matter interactions relevant to quantum information processing, quantum computing, and other quantum technologies. This study presents the analysis and generalization of a method for the solution of the dynamical problem in the time-independent JC model (Carbonaro et al., 1979) that enables an exact description of the atom-field dynamics in the resonant time-dependent JC model and in a nontrivial class of off-resonance time-dependent JC models. Our analysis produces the ready-to-use forms of the evolution operator in the resonant and non-resonant cases, which are then used to derive the evolution of the atomic population inversion for some types of atom-field coupling. The central idea behind the proposed approach offers opportunities for its extension to other time-dependent cases and its generalization to JC-related models.
Abstract The not necessarily unitary evolution operator of a finite dimensional quantum system is studied with the help of a projection operators technique. Applying this approach to the Schrödinger equation allows the derivation of an alternative expression for the evolution operator, which differs from the traditional chronological exponent. An appropriate choice of projection operators results in the possibility of studying the diagonal and non-diagonal elements of the evolution operator separately. The suggested expression implies a particular form of perturbation expansion, which leads to a new formula for the short time dynamics. The new kind of perturbation expansion can be used to improve the accuracy of the usual chronological exponent significantly. The evolution operator for any arbitrary time can be efficiently recovered using the semigroup properties. The method is illustrated by two examples, namely the dynamics of a three-level system in two nonresonant laser fields and the calculation of the partition function of a finite XY-spin chain.