  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. Here we report the experimental realization of memory-enhanced
quantum communication. We use a single solid-state spin memory integrated in a
nanophotonic diamond resonator to implement asynchronous Bell-state
measurements. 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.
