Beating direct transmission bounds for quantum key distribution with a multiple quantum memory station

Abstract

Overcoming repeaterless bounds for the secret key rate capacity of quantum key distribution protocols is still a challenge with current technology. D. Luong et al. [Applied Physics B 122, 96 (2016)] proposed a protocol to beat a repeaterless bound using one pair of quantum memories. However, the required experimental parameters for the memories are quite demanding. We extend the protocol with multiple pairs of memories, operated in a parallel manner to relax these conditions. We quantify the amount of relaxation in terms of the most crucial memory parameters, given the number of applied memory pairs. In the case of high-loss channels we found that adding only a few pair of memories can make the crossover possible.

Simulating quantum repeater strategies for multiple satellites

“…To this end, we have developed a large scale numerical Monte Carlo simulation for quantum repeaters that can faithfully represent loss and imperfect quantum memories, as well as other sources of noise such as dark counts. While there are existing approaches dealing with the computation of key rates for different repeater setups [4,25,26,32], the generalization of these methods to longer distances and other error models is by no means straightforward, e.g. an analytical approach also involves an intricate analysis of entanglement swapping strategies [33].…”

“…The main difference from protocols in Refs. [25,26] is that the satellites with entangled pair sources do not have the information about whether a qubit has been loaded into memory successfully. Therefore, it makes sense for the source not to wait until confirmation from the satellite with the memory but to instead continuously send out entangled pairs.…”

“…However, untrusted node operation for beyond-line-of-sight distances towards truly global scales requires the implementation of a QR protocol enabled by on-board quantum memories (QMs) [21,22]. Furthermore, QMs do not only help increase the overall network range but also offer a solution to low detection rates in entanglement-based schemes [23][24][25][26][27][28] and thus facilitate memory-assisted QKD (MA-QKD) protocols which can be thought of as a single-node QR link. By synchronizing otherwise probabilistic detections, a single repeater station with a QM would change the scaling of the key rate from η ch to √ η ch where η ch is the channel transmission.…”

Proposal for space-borne quantum memories for global quantum networking

“…no QM (grey dashed) 42 ; (ii) uplink configuration with protocol presented in ref. 31,32 (blue) and finally (iii) downlink scenario with a single memory pair (m = 1, red) and 100 memory pairs (m = 100, green) 43 . Parameters used in simulations to generate Figs.…”

“…This also requires long-lived QMs with storage times in the order of seconds to achieve similar performance to the previous method. An extension of this protocol in which the central pair of QMs are replaced with m pairs of QMs 43 reduces the required storage time.…”