Integrated photonics enables continuous-beam electron phase modulation

Integrated photonics facilitates extensive control over fundamental light-matter interactions in manifold quantum systems including atoms¹, trapped ions^2,3, quantum dots⁴ and defect centres⁵. Ultrafast electron microscopy has recently made free-electron beams the subject of laser-based quantum manipulation and characterization^6-11, enabling the observation of free-electron quantum walks^12-14, attosecond electron pulses^10,15-17 and holographic electromagnetic imaging¹⁸. Chip-based photonics^19,20 promises unique applications in nanoscale quantum control and sensing but remains to be realized in electron microscopy. Here we merge integrated photonics with electron microscopy, demonstrating coherent phase modulation of a continuous electron beam using a silicon nitride microresonator. The high-finesse (Q₀ ≈ 10⁶) cavity enhancement and a waveguide designed for phase matching lead to efficient electron-light scattering at extremely low, continuous-wave optical powers. Specifically, we fully deplete the initial electron state at a cavity-coupled power of only 5.35 microwatts and generate >500 electron energy sidebands for several milliwatts. Moreover, we probe unidirectional intracavity fields with microelectronvolt resolution in electron-energy-gain spectroscopy²¹. The fibre-coupled photonic structures feature single-optical-mode electron-light interaction with full control over the input and output light. This approach establishes a versatile and highly efficient framework for enhanced electron beam control in the context of laser phase plates²², beam modulators and continuous-wave attosecond pulse trains²³, resonantly enhanced spectroscopy^24-26 and dielectric laser acceleration^19,20,27. Our work introduces a universal platform for exploring free-electron quantum optics^28-31, with potential future developments in strong coupling, local quantum probing and electron-photon entanglement.