Quantum Monte-Carlo Method for Nonlinear Master Equations

Yvan Castin, Klaus Mølmer

The Monte Carlo wave-function method has recently proved to be an efficient tool in the analysis of linear dissipative quantum systems, i.e., systems with linear equations of motion for their density matrix. We generalize this method to systems with nonlinear master equations of a parametrized Lindblad form, which includes master equations obtained by Hartree-Fock approximations. Convergence properties of the algorithm are discussed in detail. The method is illustrated by a numerical analysis of the bosonic enhancement of laser cooling of trapped particles. \textcopyright{} 1996 The American Physical Society.

Stephen Fahy

R. Härtle, Guy Cohen, David R. Reichman +1 more

We give a detailed comparison of the hierarchical quantum master equation (HQME) method to a continuous-time quantum Monte Carlo (CT-QMC) approach, assessing the usability of these numerically exact schemes as impurity solvers in practical nonequilibrium calculations. We review the main characteristics of the methods and discuss the scaling of the associated numerical effort. We substantiate our discussion with explicit numerical results for the nonequilibrium transport properties of a single-site Anderson impurity. The numerical effort of the HQME scheme scales linearly with the simulation time but increases (at worst exponentially) with decreasing temperature. In contrast, CT-QMC is less restricted by temperature at short times, but in general the cost of going to longer times is also exponential. After establishing the numerical exactness of the HQME scheme, we use it to elucidate the influence of different ways to induce transport through the impurity on the initial dynamics, discuss the phenomenon of coherent current oscillations, known as current ringing, and explain the nonmonotonic temperature dependence of the steady-state magnetization as a result of competing broadening effects. We also elucidate the pronounced nonlinear magnetization dynamics, which appears on intermediate time scales in the presence of an asymmetric coupling to the electrodes.

Michael Dzierzawa, X. Zotos

Kurt A. Jacobs

The "standard" Brownian motion master equation, used to describe thermal damping, is not completely positive, and does not admit a Monte Carlo method, important in numerical simulations. To eliminate both these problems one must add a term that generates additional position diffusion. He we show that one can obtain a completely positive simple quantum Brownian motion, efficiently solvable, without any extra diffusion. This is achieved by using a stochastic Schrödinger equation (SSE), closely analogous to Langevin's equation, that has no equivalent Markovian master equation. Considering a specific example, we show that this SSE is sensitive to nonlinearities in situations in which the master equation is not, and may therefore be a better model of damping for nonlinear systems.

Malvin H. Kalos, G.A. Mikhailov