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arXiv 2002.07700quant-phcond-mat.stat-mech
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耗散环境下自旋玻色子模型的精确热量子动力学

Exactly Thermalised Quantum Dynamics of the Spin-Boson Model coupled to a Dissipative Environment

M. A. Lane, D. Matos, I. J. Ford, L. Kantorovich

摘要:

我们提出了早期引入的扩展随机Liouville-von Neumann方程(ESLN)方法的应用[PRB 95,125124(2017); PRB 97,224310(2018)],该方法描述了耦合到非马尔可夫谐波环境的精确热化开放量子系统约化密度矩阵的动力学。关键的是,完全耦合到其环境的开放系统的组合系统在有限温度下热化,在应用实时演化之前使用假想的时间演化过程。这将组合系统初始化为正确的正则平衡态,而不是初始解耦。我们将我们的理论应用到自旋玻色子哈密顿量,并开发了一些竞争的ESLN变体,旨在减少开放系统密度矩阵的迹的数值发散。我们发现,一个谨慎的选择的驱动噪声是必不可少的,以提高数值稳定性。我们还研究了应用高阶数值方案求解随机微分方程的效果,如Stratonovich-Heun方案,并得出结论,随机采样占主导地位的收敛性与数值方案的改善是不太重要的短时间,但需要较晚的时间。为了验证该方法及其数值实现,我们考虑在一个固定的哈密顿量下的演化,并表明系统在很长一段时间内保持或接近正确的正则平衡态。此外,考虑了开放系统在非平衡Landau-Zener(LZ)驱动下的演化,并在系统-环境耦合和温度为零的情况下,观察到系统渐近收敛于LZ极限.当耦合和温度是非零的,最初热化的组合系统在有限的时间在过去被发现是一个更好的近似真实的LZ初始状态比纯状态。

英文摘要:

We present an application of the Extended Stochastic Liouville-von Neumann equations (ESLN) method introduced earlier [PRB 95, 125124 (2017); PRB 97, 224310 (2018)] which describes the dynamics of an exactly thermalised open quantum system reduced density matrix coupled to a non-Markovian harmonic environment. Critically, the combined system of the open system fully coupled to its environment is thermalised at finite temperature using an imaginary time evolution procedure before the application of real time evolution. This initialises the combined system in the correct canonical equilibrium state rather than being initially decoupled. We apply our theory to the spin-boson Hamiltonian and develop a number of competing ESLN variants designed to reduce the numerical divergence of the trace of the open system density matrix. We find that a careful choice of the driving noises is essential for improving numerical stability. We also investigate the effect of applying higher order numerical schemes for solving stochastic differential equations, such as the Stratonovich-Heun scheme, and conclude that stochastic sampling dominates convergence with the improvement associated with the numerical scheme being less important for short times but required for late times. To verify the method and its numerical implementation, we consider evolution under a fixed Hamiltonian and show that the system either remains in, or approaches, the correct canonical equilibrium state at long times. Additionally, evolution of the open system under non-equilibrium Landau-Zener (LZ) driving is considered and the asymptotic convergence to the LZ limit was observed for vanishing system-environment coupling and temperature. When coupling and temperature are non-zero, initially thermalising the combined system at a finite time in the past was found to be a better approximation of the true LZ initial state than a pure state.

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