Deterministic entanglement between a propagating photon and a singlet-triplet qubit in an optically active quantum dot molecule

Generation of spin-photon entanglement lies at the heart of quantum repeater protocols. Here, the authors experimentally demonstrate quantum entanglement between a photon and a spin degree of freedom of a quantum dot molecule, by measuring a state overlap with a fully entangled state of about 70%. In contrast to earlier demonstrations of spin-photon entanglement, the system realized here, using single-triplet qubits in a quantum dot molecule, combines potentially long spin-coherence times with a very efficient photonic interface. The authors also develop a novel heterodyne technique that may have wide-ranging applications for characterizing photonic color qubits.