Free-electron quantum optics

Recent theoretical and experimental breakthroughs have given rise to the emerging field of free-electron quantum optics, reshaping the understanding of free-electron physics. Traditionally rooted in classical electrodynamics, this field now reveals quantum-mechanical features that necessitate the frameworks of quantum electrodynamics and quantum optics. This shift compels a re-evaluation of well-established areas, bringing quantum-mechanical corrections to accelerator science and to electron-radiation phenomena. Simultaneously, the ability to shape single-electron wavefunctions opens new possibilities in microscopy and spectroscopy. These developments are primarily driven by innovations in electron microscopy and its intersection with laser science, where laser-driven electron modulation substantially influences quantum electron interactions with light and matter. In this Perspective, we review these developments, highlighting the current challenges and future opportunities. We explore the role of the free electron as a quantum resource, complementing conventional two-level systems and harmonic oscillators. In the coming years, free electrons may offer new modalities for reading and writing quantum information on ultrafast timescales, performing quantum-state tomography, and ultrafast quantum gates on the atomic scale.