Increasing the secret key rates and point-to-multipoint extension for experimental coherent-one-way quantum key distribution protocol

TL;DR

The paper addresses the challenge of boosting secret key rates and enabling multi‑user deployment of coherent‑one‑way QKD in practical fiber networks. It demonstrates that dual SPDs on the data line can significantly raise SKR while keeping QBER within acceptable bounds, and it extends the protocol to a point‑to‑multipoint topology with OTP post‑processing for three users. Security analysis under a conservative collective beam‑splitting attack shows how the dual‑receiver configuration affects Eve’s information and guides parameter choices such as the mean photon number $\mu$, with $\mu=0.2$ often outperforming $\mu=0.5$ for longer links. The results indicate scalable, multi‑user time‑bin QKD is feasible and underscore the need to account for stronger attack models in real‑world deployments, while the methods remain applicable to other time‑bin implementations.

Abstract

Using quantum key distribution (QKD) protocols, a secret key is created between two distant users (transmitter and receiver) at a particular key rate. Quantum technology can facilitate secure communication for cryptographic applications, combining QKD with one-time-pad (OTP) encryption. In order to ensure the continuous operation of QKD in real-world networks, efforts have been concentrated on optimizing the use of components and effective QKD protocols to improve secret key rates and increase the transmission between multiple users. Generally, in experimental implementations, the secret key rates are limited by single-photon detectors, which are used at the receivers of QKD and create a bottleneck due to their limited detection rates (detectors with low detection efficiency and high detector dead-time). We experimentally show that secret key rates can be increased by combining the time-bin information of two such detectors on the data line of the receiver for the coherent-one-way (COW) QKD protocol with a minimal increase in quantum bit error rate (QBER, the proportion of erroneous bits). Further, we implement a point-to-multipoint COW QKD protocol, introducing an additional receiver module. The three users (one transmitter and two receivers) share the secret key in post-processing, relying on OTP encryption. Typically, the dual-receiver extension can improve the combined secret key rates of the system; however, one has to optimise the experimental parameters to achieve this within security margins. These methods are general and can be applied to any implementation of the COW protocol.

Figures (10)

• Figure 1: Experimental architecture of COW QKD.
• Figure 2: COW QKD: Alice module.
• Figure 3: COW QKD: Bob module.
• Figure 4: Comparing the key rates with single detector and dual detectors on the data line for distance $L=80$ km between Alice and Bob.
• Figure 5: Comparing the key rates with single detector and dual detectors on the data line for distance $L=100$ km between Alice and Bob.