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

摘要

Two-electron charged self-assembled quantum dot molecules exhibit adecoherence-avoiding singlet-triplet qubit subspace and an efficientspin-photon interface. We demonstrate quantum entanglement between emittedphotons and the spin-qubit after the emission event. We measure the overlapwith a fully entangled state to be $69.5\pm2.7\,\%$, exceeding the threshold of$50\,\%$ required to prove the non-separability of the density matrix of thesystem. The photonic qubit is encoded in two photon states with an energydifference larger than the timing resolution of existing detectors. We devise anovel heterodyne detection method, enabling projective measurements of suchphotonic color qubits along any direction on the Bloch sphere.