Experimental demonstration of scalable quantum cryptographic conferencing

TL;DR

This paper demonstrates a scalable quantum cryptographic conferencing method that overcomes previous limitations by eliminating coincidence detection, enabling secure multi-user communication over long distances with high success probability.

Contribution

The authors experimentally realize a new QCC protocol that removes the need for coincidence detection, significantly increasing success probability and enabling long-distance multi-user quantum communication.

Findings

Successfully performed QCC over 331.5 km of fiber

Achieved secure key rates of 5.4 bit/s

Surpassed the multi-user repeaterless bound

Abstract

Quantum network enables a variety of quantum information processing tasks, where multi-user quantum communication is one of the important objectives. Quantum cryptographic conferencing serves as an essential solution to establish secure keys to realize secure multi-user communications. However, existing QCC implementations have been fundamentally limited by the low probability of multi-user coincidence detection to measure or construct the Greenberger-Horne-Zeilinger (GHZ) entangled state. In this work, we report the experimental realization of QCC eliminating the need for coincidence detection, where the GHZ state is constructed by correlating detection events occurring within the coherence time, thereby greatly enhancing the success probability of GHZ-state measurement. Meanwhile, to establish and maintain high-visibility GHZ measurement among three independent users, we developed a three-party phase compensation scheme combined with precise temporal and polarization alignment within a time-bin-phase encoding framework. Furthermore, we designed an efficient pairing strategy to simplify subsequent data processing and enhance processing efficiency. Based on these techniques, we successfully performed QCC experiments over total channel losses of 66.3 dB, corresponding to 331.5 km of commercial fiber (0.2 dB/km), achieving secure key rates of 5.4 bit/s, whereas previous QCC experiments have been limited to 100 km. The results surpass the multi-user repeaterless bound in quantum networks, establishing a new regime of scalable, multi-user quantum communication and paving the way for metropolitan quantum networks.