Round-robin differential phase-time-shifting protocol for quantum key distribution: theory and experiment

TL;DR

This paper introduces a new quantum key distribution protocol combining time and phase encoding, providing enhanced security and higher key rates over long distances without increased complexity.

Contribution

The paper proposes the RRDPTS protocol that enlarges the Hilbert space using time and phase degrees-of-freedom, with security proofs and experimental demonstration over 80 km fiber.

Findings

Higher secret key rate compared to RRDPS at high error rates

Security proven against collective attacks

Successful experimental implementation over 80 km fiber

Abstract

Quantum key distribution (QKD) allows the establishment of common cryptographic keys among distant parties. Many of the QKD protocols that were introduced in the past involve the challenge of monitoring the signal disturbance over the communication line, in order to evaluate the information leakage to a potential eavesdropper. Recently, a QKD protocol that circumvents the need for monitoring signal disturbance, has been proposed and demonstrated in initial experiments. Here, we propose a new version of this so-called round-robin differential phase-shifting (RRDPS) protocol, in which both time and phase degrees-of-freedom are utilized to enlarge the Hilbert space dimensionality, without increasing experimental complexity or relaxing security assumptions. We derive the security proofs of the round-robin differential phase-time-shifting (RRDPTS) protocol in the collective attack scenario and benchmark the new protocol against RRDPS for different experimental parameters. Furthermore, a proof-of-concept experiment of the RRDPTS protocol, using weak coherent pulses and decoy-state method, is demonstrated over 80 km of fiber link. Our results show that the RRDPTS protocol can achieve higher secret key rate in comparison with the RRDPS, in the condition of high quantum bit error rate.