  Memory-assisted measurement-device-independent quantum key distribution
(MA-MDI-QKD) is a promising scheme that aims to improve the
rate-versus-distance behavior of a QKD system by using the state-of-the-art
devices. It can be seen as a bridge between current QKD links to quantum
repeater based networks. While, similar to quantum repeaters, MA-MDI-QKD relies
on quantum memory (QM) units, the requirements for such QMs are less demanding
than that of probabilistic quantum repeaters. Here, we present a variant of
MA-MDI-QKD structure that relies on only a single physical QM: a
nitrogen-vacancy center embedded into a cavity where its electronic spin
interacts with photons and its nuclear spin is used for storage. This enables
us to propose a simple but efficient MA-MDI-QKD scheme resilient to memory
errors and capable of beating, in terms of rate and reach, existing QKD
demonstrations. We also show how we can extend this setup to a quantum repeater
system, reaching, thus, larger distances.
