The interaction between free electrons and light stands at the base of both classical and quantum physics, with applications in free-electron acceleration, radiation sources, and electron microscopy. Yet, to this day, all experiments involving free-electron–light interactions are fully explained by describing the light as a classical wave. Here, we observe quantum statistics effects of photons on free-electron–light interactions. We demonstrate interactions passing continuously from Poissonian to super-Poissonian and up to thermal statistics, revealing a transition from quantum walk to classical random walk on the free-electron energy ladder. The electron walker serves as the probe in non-destructive quantum detection, measuring the second order photon-correlation g⁽²⁾(0) and higher-orders g⁽ⁿ⁾(0). Unlike conventional quantum-optical detectors, the electron can perform both quantum weak measurements and projective measurements by evolving into an entangled joint-state with the photons. These findings inspire hitherto inaccessible concepts in quantum optics, including free-electron-based ultrafast quantum tomography of light.

中文翻译：

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将光子的量子统计印在自由电子上

自由电子和光之间的相互作用是经典物理学和量子物理学的基础，在自由电子加速、辐射源和电子显微镜中都有应用。然而，直到今天，所有涉及自由电子-光相互作用的实验都可以通过将光描述为经典波来充分解释。在这里，我们观察到光子对自由电子-光相互作用的量子统计效应。我们展示了从泊松到超泊松以及热统计的相互作用，揭示了在自由电子能量阶梯上从量子游走到经典随机游走的转变。电子步行器作为非破坏性量子检测的探针，测量二阶光子相关性g ⁽²⁾ (0) 和更高阶g ^{( n )} (0)。与传统的量子光学探测器不同，电子可以通过与光子演变成纠缠联合态来执行量子弱测量和投影测量。这些发现激发了量子光学中迄今为止难以理解的概念，包括基于自由电子的超快光量子断层扫描。