Quantum Correlations of Spin-wave-photon Pairs Produced from an Atomic Ensemble Coupled to a Ring Cavity

The quantum interface (QI) that generates non-classically-correlated pairs of spin-wave memories and photons is a building block (node) for DLCZ-like quantum repeater. The cross-correlation function, which characterizes quantum correlation of the pairs, and retrieval efficiency together with lifetime are important figures of merits of QIs. In this presented work, we generate highly retrievable spin-wave-photon pairs via cavity-enhanced DLCZ-like interaction between light and a cloud of cold atoms, where, the intrinsic retrieval efficiency reaches to similar to 77.1% together with a memory lifetime of 0.65 ms. The cross-correlation functions of the pairs as a function of the storage time and excitation probability are measured, respectively. More important, we develop a theoretical model to describe the measured results in quantity. The theoretical model includes the cavity-enhanced noise coming from the imperfect readouts and is in agreement with the measured dependence of the cross-correlation function on the storage time and excitation probability.