Applied Physics, University of Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany.
Department of Physics, Lancaster University, Lancaster LA1 4YB, United Kingdom.
Phys Rev Lett. 2018 Dec 28;121(26):260501. doi: 10.1103/PhysRevLett.121.260501.
Quantum anomalies lead to finite expectation values that defy the apparent symmetries of a system. These anomalies are at the heart of topological effects in electronic, photonic, and atomic systems, where they result in a unique response to external fields but generally escape a more direct observation. Here, we implement an optical-network realization of a discrete-time quantum walk, where such an anomaly can be observed directly in the unique circular polarization of a topological midgap state. We base the system on a single-step protocol overcoming the experimental infeasibility of earlier multistep protocols. The evolution combines a chiral symmetry with a previously unexplored unitary version of supersymmetry. Having experimental access to the position and the coin state of the walker, we perform a full polarization tomography and provide evidence for the predicted anomaly of the midgap states. This approach opens the prospect to dynamically distill topological states for quantum information applications.
量子异常导致有限的期望值，这违背了系统的明显对称性。这些异常是电子、光子和原子系统中拓扑效应的核心，它们导致对外部场的独特响应，但通常无法进行更直接的观察。在这里，我们实现了离散时间量子行走的光学网络，其中这种异常可以直接在拓扑带隙态的独特圆偏振中观察到。我们基于单个步骤协议，克服了早期多步骤协议的实验不可行性。该演化结合了手性对称性和以前未探索过的超对称的幺正版本。通过对步行者的位置和硬币状态进行实验访问，我们进行了完整的极化层析成像，并为带隙态的预测异常提供了证据。这种方法为量子信息应用动态提取拓扑态开辟了前景。
