  Memory-assisted quantum key distribution (MA-QKD) has recently been proposed
as a technique to improve the rate-versus-distance behavior of QKD systems by
using existing, or nearly-achievable, quantum technologies. The promise is that
MA-QKD would require less demanding quantum memories than the ones needed for
probabilistic quantum repeaters. Nevertheless, early investigations suggest
that, in order to beat the conventional no-memory QKD schemes, the quantum
memories used in the MA-QKD protocols must have high bandwidth-storage products
and short interaction times. Among different types of quantum memories,
ensemble-based memories offer some of the required specifications, but they
typically suffer from multiple excitation effects. To avoid the latter issue,
in this paper, we propose two new variants of MA-QKD both relying on
single-photon sources (SPSs) for entangling purposes. One is based on known
techniques for entanglement distribution in quantum repeaters. This scheme
turns out to offer no advantage even if one uses ideal SPSs. By finding the
root cause of the problem, we then propose another setup, which can outperform
single no-QM setups even if we allow for some imperfections in our SPSs. For
such a scheme, we compare the key rate for different types of ensemble-based
memories and show that certain classes of atomic ensembles can improve the
rate-versus-distance behavior.
