Memory-assisted quantum key distribution resilient against multiple-excitation effects

Abstract

Memory-assisted measurement-device-independent quantum key distribution (MA-MDI-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-MDI-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 memoryless QKD schemes, the quantum memories used in the MA-MDI-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-MDI-QKD both relying on single-photon sources 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 single-photon sources. By finding the root cause of the problem, we then propose another setup, which can outperform single memory-less setups even if we allow for some imperfections in our single-photon sources. 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.

Advances in quantum cryptography

“…In fact, even the simplest setup, when there is only one repeater node in the middle, can offer certain advantages for QKD applications. These setups, known as memory-assisted QKD [662,663], can soon offer better rate-versus-distance scaling than most conventional QKD systems in operation by using existing quantum memory technologies [664][665][666][667][668][669]. Over long distances, however, probabilistic repeater would suffer from a low rate, or require a large number of memories to perform well [670][671][672].…”

Finite-key analysis for memory-assisted decoy-state quantum key distribution

“…In this scheme, we are also delaying the writing of the second photon of the EPP until we learn about the success of teleportation. While there is a chance that the transfer of this photonic state to the QM may fail, this delayed writing process has the advantage that the QM initialisation is not necessary in each round [11], but only when a writing procedure has been attempted. This helps with maximising the repetition rate of the protocol especially when the initialisation phase is time consuming.…”

“…Several analyses of MA-QKD have already been carried out, under varying assumptions and for different implementations of QMs. However, most of them [7,11,12] assume single-photon sources, which are difficult to attain in practice. In many QKD experiments, attenuated laser sources are used, instead.…”

“…Previous work [6] on MA-QKD has only considered the asymptotic limit in which the users exchange an infinite number of signals, and under simplified assumptions on the loading of QMs with attenuated laser sources. In our finite-key analysis, we compare MA-QKD performance with that of a no-memory MDI-QKD system, by using parameters from state-of-the-art experiments on quantum memories [11]. We find that MA-QKD is inherently more resistant to finite-key effects, and it experiences a lower reduction in secret key rate than MDI-QKD.…”

Memory-assisted quantum key distribution with a single nitrogen-vacancy center

“…The results in [9], while promising, may suffer from the limited coherence time of the electron spin of the NV center [13,14], which makes the key rate of this system to drop to zero at a distance around 500 km [9]. Once other non-idealities, such as the additional background noise from frequency converters [15], are accounted for, the window over which the NV-based system outperforms the no-memory one would even become narrower. It is therefore important to come up with a system that has a wide window of opportunity, so that in practice part of it can be exploited.…”