Imprinting the quantum statistics of photons on free electrons

TL;DR

This paper demonstrates that free electrons can detect and measure the quantum statistical properties of photons, revealing a transition from quantum to classical behavior and enabling non-destructive quantum light tomography.

Contribution

It introduces a novel method where free electrons serve as probes for quantum photon statistics, bridging classical and quantum light interactions.

Findings

Electrons can measure photon correlation functions like g^{(2)}(0) and higher orders.

Photon statistics can be continuously tuned from Poissonian to thermal.

The approach enables non-destructive quantum tomography of light.

Abstract

The fundamental 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, disregarding its quantum nature. 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, unveiling a surprising manifestation of Bohr's Correspondence Principle: the 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 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. Our findings suggest free-electron-based non-destructive quantum tomography of light, and constitute an important step towards combined attosecond-temporal and sub-A-spatial resolution microscopy.