论文标题
通过漂移实时演化,有效的量子假想时间演变:低门和测量复杂性的方法
Efficient quantum imaginary time evolution by drifting real time evolution: an approach with low gate and measurement complexity
论文作者
论文摘要
量子假想的时间演化(Qite)是找到哈密顿量的特征值和特征状态的有前途的候选者之一。但是,原始的Qite提案[Nat。物理。 16,205-210(2020)],它通过实时演变近似于假想的时间演变,由于Pauli操作员池的大小和Trotterterization的大小而受到较大的电路深度和测量。为了减轻对深度电路的要求,我们提出了一个受QDRIFT算法启发的时间依赖的漂移方案[Phys。 Rev. Lett 123,070503(2019)],该词根据Pauli条款的总强度,从Qite的近似统一操作发电机中随机绘制了一个Pauli术语。我们表明,这种漂移方案消除了对操作员池大小的深度依赖性,并将逆向线性收敛到步骤数。我们进一步提出了一种确定性算法,该算法选择主要的保利术语来减少基态制备的波动。同时,我们在跨托特步骤中引入了一个有效的测量减少方案,该方案消除了其成本依赖性对迭代次数的依赖性,并在每个时间步骤内针对不同可观察到的测量分布协议。我们还在理论和数值上分析了我们方案的主要误差源。我们在数值上测试了深度降低,收敛性能和忠诚度的有效性,以及我们对LIH,BEH $ _2 $和n $ _2 $分子的算法的测量减少近似。特别是,LIH分子的结果给出的电路深度与高级自适应变分的量子质量〜(VQE)方法相当,同时需要更少的测量值。
Quantum imaginary time evolution (QITE) is one of the promising candidates for finding eigenvalues and eigenstates of a Hamiltonian. However, the original QITE proposal [Nat. Phys. 16, 205-210 (2020)], which approximates the imaginary time evolution by real time evolution, suffers from large circuit depth and measurements due to the size of the Pauli operator pool and Trotterization. To alleviate the requirement for deep circuits, we propose a time-dependent drifting scheme inspired by the qDRIFT algorithm [Phys. Rev. Lett 123, 070503 (2019)], which randomly draws a Pauli term out of the approximated unitary operation generators of QITE according to the strength and rescales that term by the total strength of the Pauli terms. We show that this drifting scheme removes the depth dependency on size of the operator pool and converges inverse linearly to the number of steps. We further propose a deterministic algorithm that selects the dominant Pauli term to reduce the fluctuation for the ground state preparation. Meanwhile, we introduce an efficient measurement reduction scheme across Trotter steps, which removes its cost dependence on the number of iterations, and a measurement distribution protocol for different observables within each time step. We also analyze the main source of error for our scheme both theoretically and numerically. We numerically test the validity of depth reduction, convergence performance, and faithfulness of measurement reduction approximation of our algorithms on LiH, BeH$_2$ and N$_2$ molecules. In particular, the results on LiH molecule give circuit depths comparable to that of the advanced adaptive variational quantum eigensolver~(VQE) methods while requiring much fewer measurements.