Upper Bound for the Quantum Coupling between Free Electrons and Photons, published in PRL
The fundamental limit of free-electron—light coupling is an important question relevant to frontier studies to reach the strong coupling regime, which is crucial for applications such as free-electron based photon sources, quantum sensing and quantum computing using free electrons. This study rigorously establishes the fundamental upper bound of free-electron—photon coupling. The upper bound has a simple expression and depends on the optical medium, the electron velocity, the separation distance, and the interaction length. The analytical upper bound can be applied to a wide range of optical materials and arbitrary design region. Furthermore, the numerically calculated coupling coefficient can almost approach the analytical upper bound, in a realistic system where the free electron interacts with the surface plasmon polariton mode in a metallic hole. Therefore, it can also provide simple and valuable guidance to the choice of system parameters to reach the strong coupling regime of free-electron—photon interaction.