At the intersection between electron microscopy and ultrafast optics, ultrafast electron microscopy has emerged as a research frontier aiming to investigate material excitations with an unprecedented combination of spatiotemporal resolution. In this context, we will discuss the fundamental principles ruling the interactions between free electrons, light, and photonic nanostructures, with an emphasis on exploring quantum aspects that include electron decoherence caused by coupling to radiative modes and the generation of quantum states of light. In particular, radiative decoherence could be potentially useful to sense the presence of distant objects and measure the vacuum temperature, while the study of quantum correlations between electrons and surface polaritons enables the generation of single and entangled photons heralded by the detection of electrons that have experienced specific amounts of energy losses and angular deflections.
