Publication date: 8th June 2021
Recent theoretical works on the interaction of free electrons with light[1],[2] predicted how the interaction is altered by the quantum properties of light, leading to new exciting application such as electron-electron entanglement[2], new types of quantum light source[3], novel ideas for electron shaping[4], and quantum state tomography of light[5]. Yet, to this day – in all experiments involving the interaction between free electrons and light – the light acted as a classical wave, disregarding its quantum nature.
We observe the effect of the quantum statistics of photons on free-electron interactions with light. Our study shows interactions for different statistics passing continuously from Poissonian up to thermal statistics, unveiling a surprising manifestation of Bohr's Correspondence Principle: the continuous transition from quantum walk[6],[7] to classical random walk of a free electron on the energy ladder. The electron walker serves as the probe in non-destructive quantum detection experiments, measuring the photon statistics as well as degrees of coherence g2(0) and higher-orders gn(0). Unlike conventional quantum-optical detectors, the electron can perform both quantum weak measurements[8] and projective measurements of light by evolving into an entangled joint-state with the photons.
We achieve this free-electron–quantum-light interaction using the inverse design of silicon-photonic nanostructures recently used for miniaturizing particle accelerators[10]. We use these nanostructures in a transmission electron microscope (TEM) to facilitate a strong free-electron–light interaction, i.e., each electron exchanges multiple photons with the light field. Such strong interactions were previously only realized with intense laser pulses in photon-induced nearfield electron microscopy (PINEM). In contrast with such intense pulses that can be considered as coherent states, all other states of light are usually not so intense – necessitating an optimized interaction in specialized nanostructures.
The interaction efficiency is high enough to enable the use of continuous-wave (CW) light, while still maintaining the strong interaction. Recent experimental works have shown that even weak CW electron–light interactions (up to one photon absorbed/emitted by the electron) have intriguing applications[10],[11] Our experiment offers an avenue for taking these ideas forward to regimes of stronger interactions with CW light. Our findings suggest hitherto inaccessible concepts in quantum optics: free-electron-based non-destructive quantum tomography of light, even with ultrafast modulation of the photon statistics or the field quadratures[5]. The study of high-efficiency electron–light coupling constitutes an important step towards combined attosecond-temporal and sub-Å-spatial resolution microscopy.