Integrated photonics enables continuous-beam electron phase modulation

The ability to tailor laser light on a chip using integrated photonics has allowed for extensive control over fundamental light-matter interactions in manifold quantum systems including atoms, trapped ions, quantum dots, and defect centers. Free electrons, enabling high-resolution microscopy for decades, are increasingly becoming the subject of laser-based quantum manipulation. Using free-space optical excitation and intense laser pulses, this has led to the observation of free-electron quantum walks, attosecond electron pulses, and imaging of electromagnetic fields. Enhancing the interaction with electron beams through chip-based photonics promises unique applications in nanoscale quantum control and sensing, but has yet to enter electron microscopy. Here, we merge integrated photonics with electron microscopy, demonstrating coherent phase modulation of an electron beam using a silicon nitride microresonator driven by a continuous-wave laser. The high-Q factor (~$10^6$) cavity enhancement and a waveguide designed for phase matching lead to efficient electron-light scattering at unprecedentedly low, few-microwatt optical powers. Specifically, we fully deplete the initial electron state at a cavity-coupled power of 6 $\mu$W and create >500 photon sidebands for only 38 mW in the bus waveguide. Moreover, we demonstrate $\mu$eV electron energy gain spectroscopy (EEGS). Providing simultaneous optical and electronic spectroscopy of the resonant cavity, the fiber-coupled photonic structures feature single-mode electron-light interaction with full control over the input and output channels. This approach establishes a versatile framework for exploring free-electron quantum optics, with future developments in strong coupling, local quantum probing, and electron-photon entanglement. Our results highlight the potential of integrated photonics to efficiently interface free electrons and light.