This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Quantum key distribution allows two users to establish secret keys whose security is grounded in the laws of quantum mechanics. Extending this capability to multiple users is an essential step toward quantum networks that support secure communication among many participants.
Quantum cryptographic conferencing addresses this need by enabling multiple users to share the same secure key, which they can then use to protect group communications. In an article published in Physical Review Letters, a team led by professor Xiao-Song Ma at Nanjing University reports the experimental realization of asynchronous measurement-device-independent quantum cryptographic conferencing, or AMDI QCC. The demonstration addresses two major challenges in developing practical quantum networks: maintaining useful key-generation rates as networks grow and reducing the complexity of controlling optical phases.
As illustrated above, three users independently send optical pulses to a shared GHZ measurement station, where a fiber-based multipath interferometer enables interference between signals from different users and single-photon detection. Based on the detection results publicly announced by the measurement node and their own encoding information, the users obtain a shared secure conference key through processes such as single-photon detection event pairing, key mapping and classical post-processing. The measurement-device-independent design protects against attacks targeting the detection equipment, removing the need to trust the shared measurement station.
Conventional measurement-device-independent quantum conferencing protocols rely on multiphoton coincidence events to generate secure keys. These events become increasingly rare as transmission losses increase and more users join the network, sharply limiting communication performance and scalability. The asynchronous protocol, theoretically proposed by professor Zeng-Bing Chen and Hua-Lei Yin's group and collaborators, overcomes this bottleneck by assembling key-generation events from single-photon detections recorded at different times.
Rather than requiring the relevant detections to occur simultaneously, it pairs suitable events during data processing, making more efficient use of the detected signals. This change brings a fundamental improvement in how the key rate scales with transmission loss. In an ideal N-user network, the rate changes from a dependence proportional to ηN in conventional MDI conferencing to proportional to η, where η denotes single-user channel transmittance.
The loss-scaling exponent therefore no longer grows with the number of users. Although practical rates still depend on system parameters and finite data, this improvement offers a promising route to higher-rate, larger-scale quantum conferencing. The research team generated secure keys under a maximum total system loss of approximately 59.6 dB, compared with approximately 21.5 dB achieved in the same group's earlier polarization-encoded MDI quantum conferencing experiment.
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