Quantum-coherent light-electron interaction in an SEM

The last two decades experimentally affirmed the quantum nature of free electron wavepackets by the rapid development of transmission electron microscopes into ultrafast, quantum-coherent systems. In particular, ultrafast electron pulses can be generated and timed to interact with optical near-fields, yielding coherent exchange of the quantized photon energy between the relativistic electron wavepacket and the light field. So far, all experiments have been restricted to the physically-confining bounds of transmission electron microscopes, with their small, millimeter-sized sample chambers. In this work, we show the quantum coherent coupling between electrons and light in a scanning electron microscope, at unprecedentedly low electron energies down to 10.4 keV, so with sub-relativistic electrons. Scanning electron microscopes not only afford the yet-unexplored electron energies from ~0.5 to 30 keV providing optimum light-coupling efficiencies, but they also offer spacious and easily-configurable experimental chambers for extended and cascaded optical set-ups, potentially boasting thousands of photon-electron interaction zones. Our results unleashes the full potential of quantum experiments including electron wavepacket shaping and quantum computing with multiple arithmetic operations and will allow imaging with low-energy electrons and attosecond time resolution

Demonstration of weak measurements, projective measurements, and quantum-to-classical transitions in ultrafast free electron-photon interactions

How does the quantum-to-classical transition of measurement occur? This question is vital for both foundations and applications of quantum mechanics. Here, we develop a new measurement-based framework for characterizing the classical and quantum free electron-photon interactions and then experimentally test it. We first analyze the transition from projective to weak measurement in generic light-matter interactions and show that any classical electron-laser-beam interaction can be represented as an outcome of a weak measurement. In particular, the appearance of classical point-particle acceleration is an example of an amplified weak value resulting from weak measurement. A universal factor quantifies the measurement regimes and their transition from quantum to classical, where Gamma corresponds to the ratio between the electron wavepacket size and the optical wavelength. This measurement-based formulation is experimentally verified in both limits of photon-induced near-field electron microscopy and the classical acceleration regime using a dielectric laser accelerator. Our results shed new light on the transition from quantum to classical electrodynamics, enabling to employ the essence of wave-particle duality of both light and electrons in quantum measurement for exploring and applying many quantum and classical light-matter interactions