Measurements on a quantum particle unavoidably affect its state, since the otherwise unitary evolution of the system is interrupted by a nonunitary projection operation. To probe measurement-induced effects in the state dynamics using a quantum simulator, the challenge is to implement controlled measurements on a small subspace of the system and continue the evolution from the complementary subspace. A powerful platform for versatile quantum evolution is represented by photonic quantum walks because of their high control over all relevant parameters. However, measurement-induced dynamics in such a platform have not yet been realized. We implement controlled measurements in a discrete-time quantum walk based on time-multiplexing. This is achieved by adding a deterministic outcoupling of the optical signal to include measurements constrained to specific positions resulting in the projection of the walker's state on the remaining ones. With this platform and coherent input light, we experimentally simulate measurement-induced single-particle quantum dynamics. We demonstrate the difference between dynamics with only a single measurement at the final step and those including measurements during the evolution. To this aim, we study recurrence as a figure of merit, that is, the return probability to the walker's starting position, which is measured in the two cases. We track the development of the return probability over 36 time steps and observe the onset of both recurrent and transient evolution as an effect of the different measurement schemes, a signature which only emerges for quantum systems. Our simulation of the observed one-particle conditional quantum dynamics does not require a genuine quantum particle but is demonstrated with coherent light.

中文翻译：

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通过重复探测量子游走中的测量诱导效应。

量子粒子的测量不可避免地会影响其状态，因为否则系统的整体演化会被非整体投影操作中断。为了使用量子模拟器探测状态动力学中测量引起的影响，挑战在于在系统的一个较小子空间上实施受控测量，并继续从互补子空间演进。光子量子行走代表了通用量子进化的强大平台，因为它们对所有相关参数的高度控制。但是，尚未实现在这种平台上由测量引起的动力学。我们在基于时间复用的离散时间量子漫游中实现受控测量。这是通过添加确定性的光信号输出耦合来实现的，其中包括限制在特定位置的测量，从而导致步行者的状态投射在其余位置上。有了这个平台和相干的输入光，我们就可以实验性地模拟测量引起的单粒子量子动力学。我们展示了在最后一步仅进行一次测量与在演化过程中进行包括测量在内的动力学之间的差异。为此，我们将循环作为优劣指标进行研究，也就是将步行者返回起始位置的概率（在这两种情况下进行测量）。我们跟踪了36个时间步长上返回概率的发展，并观察了周期性和短暂演变的开始，这是不同测量方案的影响，仅在量子系统中出现的签名。我们对观察到的单粒子条件量子动力学的模拟不需要真正的量子粒子，但是用相干光进行了演示。