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arXiv 1808.09707cond-mat.stat-mechquant-ph
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非静态近似下平均场量子自旋系统配分函数的精确解

An exact solution of the partition function for mean-field quantum spin systems without the static approximation

Manaka Okuyama, Masayuki Ohzeki

摘要:

Suzuki-Trotter分解是计算量子自旋系统配分函数的一种常用方法,但在该方法中不可避免地会出现配分函数的时间依赖性。由于很难明确地处理配分函数的时间-时间依赖性,我们通常忽略时间-时间动力学效应,这被称为静态近似。虽然静态近似是第一种方法,但对于平均场量子自旋系统,静态近似何时成立还不清楚,也就是说,迄今为止,平均场量子自旋系统还没有被精确地求解。在这项研究中,我们精确地解决了配分函数的一类特殊的平均场量子自旋系统的随机性,没有静态近似。配分函数可以看作是时-时薛定谔方程随时间演化的结果,求解配分函数的精确解等价于求解时-时薛定谔方程的最优控制问题.其结果是,最优控制问题的解决方案完全符合静态近似解的配分函数,因此,静态近似是准确的特定类的平均场量子自旋系统,包括一般的随机性。此外,我们证明了以前的量子退火研究的分析是准确的,其中的非随机相互作用和非均匀横向场加速计算时间指数平均场量子自旋系统。

英文摘要:

Suzuki-Trotter decomposition is a well-known technique used to calculate the partition function of quantum spin systems, in which the imaginary-time dependence of the partition function occurs inevitably. Since it is very difficult to explicitly treat the imaginary-time dependence of the partition function, we usually neglect the imaginary-time dynamical effect, which is called the static approximation. Although the static approximation is the first approach, it is not even clear when the static approximation is justified for mean-field quantum spin systems, that is, mean-field quantum spin systems have not been solved exactly so far. In this study, we solve exactly the partition function for a particular class of mean-field quantum spin systems including randomness without the static approximation. The partition function can be regarded as a result of time evolution in the imaginary-time Schrödinger equation, and solving the exact solution of the partition function is equivalent to solving the optimal control problem in the imaginary-time Schrödinger equation. As the result, the solution of the optimal control problem coincides exactly with the static approximate solution of the partition function and, therefore, the static approximation is exact for the particular class of mean-field quantum spin systems including randomness in general. Furthermore, we prove that the analysis of the previous study in quantum annealing is exact where the non-stoquastic interaction and the inhomogeneous transverse field accelerate the computational time exponentially for mean-field quantum spin systems.

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