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

TL;DR

This paper demonstrates how a single two-level emitter coupled to a nanophotonic waveguide can generate and verify genuine photonic entanglement by violating Bell's inequality, advancing low-energy quantum information processing.

Contribution

It introduces a method to produce high-fidelity photonic entanglement using a passive two-level emitter in a nanophotonic waveguide, without requiring advanced spin control.

Findings

Successfully violated Bell inequality with S=2.67(16)

Generated energy-time entanglement via two-photon scattering

Demonstrated a fundamental step towards low-energy quantum photonics

Abstract

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.