Optimized protocol for twin-field quantum key distribution

Abstract

Twin-field quantum key distribution (TF-QKD) and its variant protocols are highly attractive due to the advantage of overcoming the rate-loss limit for secret key rates of point-to-point QKD protocols. For variations of TF-QKD, the key point to ensure security is switching randomly between a code mode and a test mode. Among all TF-QKD protocols, their code modes are very different, e.g. modulating continuous phases, modulating only two opposite phases, and sending or not sending signal pulses. Here we show that, by discretizing the number of global phases in the code mode, we can give a unified view on the first two types of TF-QKD protocols, and demonstrate that increasing the number of discrete phases extends the achievable distance, and as a trade-off, lowers the secret key rate at short distances due to the phase post-selection.

Twin-Field Quantum Key Distribution with Fully Discrete Phase Randomization

“…Essentially, this is the approach of Ref. [23]. However, note that if Alice and Bob use continuous phase-randomization, the probability that they select exactly the same phase θ is zero, and the resulting protocol is not implementable in practice.…”

“…Here, we use the same test-mode phase-postselection idea as in Ref. [23], but we employ discrete phase randomization, which results in a protocol that is actually implementable. In this case, Eq.…”

“…We note that the concept of phase postselection has appeared in other TF-QKD variants [14,15,23], although in combination with continuous-phase-randomized signals. Refs.…”

“…Ref. [23] assumes that signals with an identical phase are postselected. While certainly interesting from a theoretical point of view, this protocol is not implementable in practice, since Alice and Bob will never choose exactly the same phase when using continuous phase randomization.…”

Phase-Matching Quantum Key Distribution with Discrete Phase Randomization

“…An earlier security analysis of discrete phase randomization appears in the decoy state Bennet-Brassard-1984 (BB84) in Reference [ 33 ], which points out, when the number of discrete phase values is larger, that the performance of discrete phase randomization is close to that of continuous phase randomization, and the number is said to be ten [ 33 ]. Similar security analysis methods are used for several other protocols, the measurement-device-independent (MDI) QKD in Reference [ 34 ], the NPP-TF-QKD in References [ 35 , 36 ], the SNS-TF-QKD in Reference [ 37 ], the PM-QKD in Reference [ 38 ]. Therein, Reference [ 38 ] uses a different security poof method with Reference [ 8 ], and there is no in-depth formula derivation in the decoy state PM-QKD with discrete phase randomization.…”