  Advancing quantum information, communication and sensing relies on the
generation and control of quantum correlations in complementary degrees of
freedom. Here, we demonstrate the preparation of electron-photon pair states
using the phase-matched interaction of free electrons with the evanescent
vacuum field of a photonic-chip-based optical microresonator. Spontaneous
inelastic scattering produces intracavity photons coincident with
energy-shifted electrons. Harnessing these pairs for correlation-enhanced
imaging, we achieve a two-orders of magnitude contrast improvement in
cavity-mode mapping by coincidence-gated electron spectroscopy. This parametric
pair-state preparation will underpin the future development of free-electron
quantum optics, providing a pathway to quantum-enhanced imaging,
electron-photon entanglement, and heralded single-electron and Fock-state
photon sources.
