Violation of Bell inequality by photon scattering on a two-level emitter

Entanglement, the non-local correlations present in multipartite quantum systems, is a key resource for quantum technologies. It is therefore a major priority to develop simple and energy-efficient methods for generating high-fidelity entangled states. In the case of light, entanglement can be realized by interactions with matter but the required nonlinear interaction is often impractically weak. Here we show how a single two-level emitter deterministically coupled to light in a nanophotonic waveguide can be used to realize photonic quantum entanglement by excitation at the single-photon level. Efficient optical coupling enables mediation of two-photon interactions by the emitter, creating a strong nonlinearity that leads to entanglement. We experimentally verify energy-time entanglement by violating a Bell inequality in an interferometric measurement of the two-photon scattering response. The on-chip two-level emitter acts as a passive scatterer, so that no advanced spin control is required. As such, our method may provide a more efficient approach to synthesizing photonic entangled states for quantum simulators or metrology.

Shikai Liu, Oliver August Dall’Alba Sandberg, Ming Lai Chan, Björn Schrinski, Yiouli Anyfantaki, Rasmus B. Nielsen, Robert G. Larsen, Andrei Skalkin, Ying Wang, Leonardo Midolo, Sven Scholz, Andreas D. Wieck, Arne Ludwig, Anders S. Sørensen, Alexey Tiranov, Peter Lodahl