A central challenge in the emerging field of free-electron
quantum
optics is to achieve strong quantum interaction and single-photon
nonlinearity between a flying free electron and a photonic mode. Existing
schemes are intrinsically limited by electron diffraction, which puts
an upper bound on the interaction length and, therefore, on the strength
of quantum coupling and nonlinearity. Here, we propose “free-electron
fibers”: effectively one-dimensional photonic systems where
free electrons copropagate with two guided modes. The first mode applies
a ponderomotive trap to the free electron, removing the limitations
due to electron diffraction. The second mode strongly couples to the
guided free electron with an enhanced coupling that is orders of magnitude
larger than previous designs. The extended interaction lengths enabled
by our scheme allow for strong single-photon nonlinearities mediated
by free electrons. We predict novel quantum effects in our system
such as deterministic single-photon emission and nonlinear multimode
dynamics. Our proposal paves the way toward the realization of heralded
macroscopic nonclassical light generation, deterministic single-photon
sources, and quantum gates controlled by free-electron–photon
interactions.