论文标题
统一耦合簇的随机方法
A Stochastic Approach to Unitary Coupled Cluster
论文作者
论文摘要
最初作为流行传统耦合群集方法的变异替代品开发的单一耦合群集(UCC)已将复兴视为用于量子计算机上的功能形式。但是,安萨兹(Ansatz)中存在的激励次数通常会在量子计算机上实现障碍。鉴于从量子蒙特卡洛方法获得的波形的自然稀疏性,我们在这里考虑了UCC问题的随机解决方案。使用耦合的群集蒙特卡洛框架,我们开发了群集选择方案,这些方案捕获了UCC波函数的结构及其Trottertterter的近似,并使用它们来求解相应的投影方程。由于方程在群集扩展中的顺序快速收敛,因此该方法随系统的大小而多样地缩放。与传统的UCC实施不同,我们的方法自然会以预计能量的形式产生对能量的非变化估计器。对于小型系统中的UCCSD,我们发现这与能量的期望值非常吻合,并且在两个电子的情况下,结果具有完整的配置相互作用。对于较大的n $ _2 $系统,两个估计器的分歧,预计的能量接近耦合群集结果,而期望值接近传统UCCSD的结果。
Unitary coupled cluster (UCC), originally developed as a variational alternative to the popular traditional coupled cluster method, has seen a resurgence as a functional form for use on quantum computers. However, the number of excitors present in the ansatz often presents a barrier to implementation on quantum computers. Given the natural sparsity of wavefunctions obtained from Quantum Monte Carlo methods, we consider here a stochastic solution to the UCC problem. Using the Coupled Cluster Monte Carlo framework, we develop cluster selection schemes that capture the structure of the UCC wavefunction, as well as its Trotterized approximation, and use these to solve the corresponding projected equations. Due to the fast convergence of the equations with order in the cluster expansion, this approach scales polynomially with the size of the system. Unlike traditional UCC implementations, our approach naturally produces a non-variational estimator for the energy in the form of the projected energy. For UCCSD in small systems, we find this agrees well with the expectation value of the energy and, in the case of two electrons, with full configuration interaction results. For the larger N$_2$ system, the two estimators diverge, with the projected energy approaching the coupled cluster result, while the expectation value is close to results from traditional UCCSD.