The ability to communicate quantum information over long distances is of central importance in quantum science and engineering. For example, it enables secure quantum key distribution (QKD) relying on fundamental principles that prohibit the "cloning" of unknown quantum states. While QKD is being successfully deployed, its range is currently limited by photon losses and cannot be extended using straightforward measure-and-repeat strategies without compromising its unconditional security. Alternatively, quantum repeaters, which utilize intermediate quantum memory nodes and error correction techniques, can extend the range of quantum channels. However, their implementation remains an outstanding challenge, requiring a combination of efficient and high-fidelity quantum memories, gate operations, and measurements. In this talk, I will describe the experimental realization of memory-enhanced quantum communication . Using the spin quantum memory of a single silicon-vacancy color-center integrated into a nanophotonic diamond resonator [2, 3], we implement asynchronous Bell-state measurements between pairs of incoming photons. This enables a four-fold increase in the secret key rate of measurement device-independent (MDI)-QKD over the loss-equivalent direct-transmission method while operating megahertz clock rates. Our results represent a significant step towards practical quantum repeaters and large-scale quantum networks.
 M. K. Bhaskar, R. Riedinger, B. Machielse, D. S. Levonian, C. T. Nguyen et al, "Experimental demonstration of memory-enhanced quantum communication," arXiv:1909.01323 (2019)
 C. T. Nguyen, D. D. Sukachev, M. K. Bhaskar, B. Machielse et al, "Quantum network nodes based on diamond qubits with an efficient nanophotonic interface," PRL (in press), arXiv:1909.13199 (2019)
 C. T. Nguyen, D. D. Sukachev, M. K. Bhaskar, B. Machielse, D. S. Levonian et al, "An integrated nanophotonic quantum register based on silicon-vacancy spins in diamond," PRB (in press), arXiv:1909.13200 (2019)