论文标题

通过连续时间量子步行进行反馈辅助量子搜索

Feedback-assisted quantum search by continuous-time quantum walks

论文作者

Candeloro, Alessandro, Benedetti, Claudia, Genoni, Marco G., Paris, Matteo G. A.

论文摘要

我们通过连续测量和反馈辅助的量子步行在周期图上对目标节点的量子搜索。与以前的空间搜索方法不同,甲骨文被描述为目标状态上的投影仪,而是考虑通过反馈哈密顿式实施的动态甲骨文。特别是,我们的协议能够将步行者驱动到所需的目标节点。该想法是基于不断监视量子步行者在图表上的位置,然后根据从测量中获得的信息应用单一反馈操作。反馈更改节点之间的耦合,并通过数值过程在每个时间进行优化。我们在数值上模拟了最高$ n = 15 $的尺寸图的演变的随机轨迹,并通过沃克状态和目标节点之间的平均保真度来量化协议的性能。我们讨论了对控制策略的不同限制,尤其是关于反馈耦合可能采取的可能值的限制。我们发现,对于无界控件,该协议能够快速将步行者定位在目标节点上。然后,我们讨论如何通过在控制耦合上放置上限来降低性能,但仍允许在所需节点中稳定步行者。最后,我们展示了一个数字反馈协议如何仅从离散集中获取值的数字反馈协议似乎与连续有界的数字一样有效。

We address the quantum search of a target node on a cycle graph by means of a quantum walk assisted by continuous measurement and feedback. Unlike previous spatial search approaches, where the oracle is described as a projector on the target state, we instead consider a dynamical oracle implemented through a feedback Hamiltonian. In particular, our protocol is able to drive the walker to a desired target node. The idea is based on continuously monitoring the position of the quantum walker on the graph and then to apply a unitary feedback operation based on the information obtained from measurement. The feedback changes the couplings between the nodes and it is optimized at each time via a numerical procedure. We numerically simulate the stochastic trajectories describing the evolution for graphs of dimensions up to $N=15$, and quantify the performance of the protocol via the average fidelity between the state of the walker and the target node. We discuss different constraints on the control strategy, in particular on the possible values that the feedback couplings can take. We find that for unbounded controls, the protocol is able to quickly localize the walker on the target node. We then discuss how the performance is lowered by posing an upper bound on the control couplings, but still allowing to stabilize the walker in the desired node. Finally, we show how a digital feedback protocol, where the couplings can take values only from a discrete set, seems in general as efficient as the continuous bounded one.

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