Entanglement swapping with spatially multiplexed modes in an atomic-ensemble quantum memory in a single excitation regime

Entanglement swapping (ES) between memory repeater links is critical for establishing quantum networks via quantum repeaters. So far, ES with atomic-ensemble-based memories has not been achieved. Here, we experimentally demonstrate ES between different spatial modes for a cold-atom-ensemble quantum memory via the Duan-Lukin-Cirac-Zoller scheme. With a cloud of cold atoms inserted in a cavity, we produce non-classically correlated spin-wave-photon pairs in 12 spatial modes. We then prepare two entangled pairs of spin-wave modes through memory multiplexing. Via single-photon Bell-state measurement on retrieved fields from two spin-wave modes, we project the two remaining modes never entangled previously into an entangled state with a measured concurrence of C =0.0124±0.0030. The successful probability of ES in our scheme is increased by three times, compared with that in the scheme without memory multiplexing. Our presented work shows that the generation of entanglement ( C >0) between the remaining spin-wave modes requires the average cross-correlation function of the spin-wave-photon pairs to be ≥30. This presented work represents a significant step toward practical quantum repeaters.