# Round-robin-differential-phase-shift quantum key distribution with and without monitoring signal disturbance

## TL;DR

This paper advances the security understanding of RRDPS quantum key distribution by developing a new theory applicable with and without signal disturbance monitoring, leading to improved practicality and a successful 140 km experiment.

## Contribution

It introduces a novel security proof for RRDPS QKD that accounts for phase randomization and eavesdropper's ancilla, enhancing security analysis and practical implementation.

## Key findings

- Developed a tight security bound applicable with and without disturbance monitoring.
- Achieved a proof-of-principle experiment over 140 km fiber, the longest for RRDPS to date.
- Demonstrated improved practicality and performance of RRDPS QKD systems.

## Abstract

Round-robin-differential-phase (RRDPS) quantum key distribution (QKD) protocol has attracted intensive studies due to its distinct security characteristic, e.g., information leakage in RRDPS can be bounded without learning error rate of key bits. Nevertheless, its implementation is still far from practical due to the complication of its measurement device. Moreover, on the theoretical side, its security is still not clear in view of optimal attack. Here, by observing a potential phase randomization of the encoding states and its connection with eavesdropper's ancilla, we develop a theory to bound information leakage quite tightly and differently. Our theory is applicable for both with and without monitoring signal disturbance scenarios, which is significant for the understanding of RRDPS. Based on our novel security proof, the practicality and performance of RRDPS can be both improved dramatically. Furthermore, we realize a proof-of-principle experiment up to 140 km fiber distance which is the longest achievable distance of RRDPS system until now, while the original security proof predicts no secret key can be generated in our experiment. Our results pave an avenue towards practical RRDPS.