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arXiv 0810.0312quant-ph
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关于连续时间和离散时间量子行走之间的关系

On the relationship between continuous- and discrete-time quantum walk

Andrew M. Childs

摘要:

量子行走是量子算法的主要工具之一。通过类比经典的随机游走定义,量子游走是图上的时间齐次量子过程。随机游走和量子游走都可以在连续时间或离散时间中定义。但是,尽管连续时间随机游走可以作为离散时间随机游走序列的极限而得到,但由于在离散时间情况下需要额外的自由度,这两种类型的量子游走看起来根本不同。在本文中,我描述了任意图上连续时间和离散时间量子游走之间的精确对应关系。利用这种对应关系,我们证明了连续时间量子行走可以作为离散时间量子行走的一个适当的极限。对应关系也导致了一种新的技术,用于模拟哈密顿动力学,即使在哈密顿量不稀疏的情况下,也能进行有效的模拟。模拟的复杂性是线性的总进化时间,一个改进的模拟的基础上高阶近似的李积公式。作为应用,我描述了一个连续时间量子行走算法的元素的独特性,并显示如何最佳地模拟连续时间查询算法的某种形式在传统的量子查询模型。最后,我讨论了模拟哈密顿与负矩阵元素的方法的局限性,并提出了两个问题,激励试图规避这些限制。

英文摘要:

Quantum walk is one of the main tools for quantum algorithms. Defined by analogy to classical random walk, a quantum walk is a time-homogeneous quantum process on a graph. Both random and quantum walks can be defined either in continuous or discrete time. But whereas a continuous-time random walk can be obtained as the limit of a sequence of discrete-time random walks, the two types of quantum walk appear fundamentally different, owing to the need for extra degrees of freedom in the discrete-time case. In this article, I describe a precise correspondence between continuous- and discrete-time quantum walks on arbitrary graphs. Using this correspondence, I show that continuous-time quantum walk can be obtained as an appropriate limit of discrete-time quantum walks. The correspondence also leads to a new technique for simulating Hamiltonian dynamics, giving efficient simulations even in cases where the Hamiltonian is not sparse. The complexity of the simulation is linear in the total evolution time, an improvement over simulations based on high-order approximations of the Lie product formula. As applications, I describe a continuous-time quantum walk algorithm for element distinctness and show how to optimally simulate continuous-time query algorithms of a certain form in the conventional quantum query model. Finally, I discuss limitations of the method for simulating Hamiltonians with negative matrix elements, and present two problems that motivate attempting to circumvent these limitations.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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