  Entanglement of photons is a fundamental feature of quantum mechanics, which
stands at the core of quantum technologies such as photonic quantum computing,
communication, and sensing. An ongoing challenge in all these is finding an
efficient and controllable mechanism to entangle photons. Recent experimental
developments in electron microscopy enable to control the quantum interaction
between free electrons and light. Here, we show that free electrons can create
entanglement and bunching of light. Free electrons can control the second-order
coherence of initially independent photonic states, even in spatially separated
cavities that cannot directly interact. Free electrons thus provide a type of
optical nonlinearity that acts in a nonlocal manner, offering a new way of
heralding the creation of entanglement. Intriguingly, pre-shaping the
electron's wavefunction provides the knob for tuning the photonic quantum
correlations. The concept can be generalized to entangle not only photons but
also photonic quasiparticles such as plasmon-polaritons and phonons.
