Polarization-entangled photon pairs from a single molecule

Abstract

Photonic entanglement is one of the key resources in modern quantum optics. It opens the door to schemes such as quantum communication, quantum teleportation, and quantum-enhanced precision sensing. Sources based on parametric down-conversion or cascaded decays in atomic and atom-like emitters are limited because of their weak interaction with stationary qubits. This is due to their commonly broadband emission. Furthermore, these sources are commonly in the near-infrared such that quantum emitters in the blue spectral region, such as ions or many defect centers, cannot be addressed. Here, we present a sodium-resonant (589.0 nm) and narrow-band (14 MHz) degenerate entanglement source based on a single molecule. A beam-splitter renders two independently emitted photons into a polarization-entangled state. The quality of the entangled photon pairs is verified by the violation of Bell’s inequality. We measure a Bell parameter of S=2.26±0.05. This attests that the detected photon pairs exceed the classical limit; it is reconfirmed by quantum-state tomography and an analysis of the raw detector counts, which result in a value of S=2.24±0.12. The tomography shows fidelity of 82% to a maximally entangled Bell state. This work opens the route to background-free solid-state entanglement sources which surpass the probabilistic nature of the commonly used sources and are free from unwanted multi-photon events. The source is ideal for combination with stationary qubits such as atoms, ions, quantum dots, or defect centers.