On spatial electron-photon entanglement

Free electron beams and their quantum coupling with photons is attracting a rising interest due to the basic questions it addresses and the cutting-edge technology these particles are involved in, such as microscopy, spectroscopy, and quantum computation. This work investigates theoretically the concept of electron-photon coupling in the spatial domain. Their interaction is discussed as a thought experiment of spontaneous photon emission from a dual-path free-electron (free-e) beam. We discuss a retro-causal paradox that may emerge from naively extending perceptions of single-path e-photon coupling to transversely separated paths, and its resolution through the physics of two-particle interference. The precise spatial control of electrons and photons within e-microscopes enables manipulation of their respective states, thus, such instruments can harness position-encoded free-e qubits for novel quantum sensing and the transfer of quantum information.