Experimental Demonstration of Asynchronous Measurement-Device-Independent Quantum Cryptographic Conferencing

Quantum networks enable a variety of quantum information processing tasks, where multiuser quantum communication is one of the important objectives. Quantum cryptographic conferencing (QCC) serves as an essential solution to establish secure keys to realize secure multiuser communications. However, existing QCC implementations have been fundamentally limited by the low probability of multiuser coincidence detection to measure or construct the Greenberger-Horne-Zeilinger (GHZ) entangled state. In this Letter, 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 loss of 66.3 dB, with the channel loss of 51.24 dB, achieving secure key rates of 5.4  bit/s, whereas previous QCC experiments have been limited to 100 km. The results establish a new regime of scalable, multiuser quantum communication and paving the way for metropolitan quantum networks.