  Entanglement, the non-local correlations present in multipartite quantum
systems, is a curious feature of quantum mechanics and the fuel of quantum
technology. It is therefore a major priority to develop energy-conserving and
simple methods for generating high-fidelity entangled states. In the case of
light, entanglement can be realized by interactions with matter, although the
required nonlinear interaction is typically weak, thereby limiting its
applicability. Here, we show how a single two-level emitter deterministically
coupled to light in a nanophotonic waveguide is used to realize genuine
photonic quantum entanglement for excitation at the single photon level. By
virtue of the efficient optical coupling, two-photon interactions are strongly
mediated by the emitter realizing a giant nonlinearity that leads to
entanglement. We experimentally generate and verify energy-time entanglement by
violating a Bell inequality (Clauder-Horne-Shimony-Holt Bell parameter of
$S=2.67(16)>2$) in an interferometric measurement of the two-photon scattering
response. As an attractive feature of this approach, the two-level emitter acts
as a passive scatterer initially prepared in the ground state, i.e., no
advanced spin control is required. This experiment is a fundamental advancement
that may pave a new route for ultra-low energy-consuming synthesis of photonic
entangled states for quantum simulators or metrology.
