Bingjie Xu

A coherent-state point-to-multipoint protocol is proposed to simultaneously support multiple independent quantum key distribution links between a single transmitter and massive receivers. Every prepared coherent state is measured by all receivers to generate raw keys, then processed with a secure and high-efficient key distillation method to remove the correlations between different links. The simulation results show that it can achieve remarkably high key rates even with a hundred of access points. Further, a proof-of-principle experiment with one network node and four end users has been demonstrated, where the average secret key rate of 4.1 Mbps between the transmitter and each one receiver is achieved, resulting in two orders-of-magnitude higher than previous networks. This scheme is a promising step towards a high-rate multi-user solution in a scalable quantum secure network.

We report a 16QAM-modulated continuous-variable quantum key distribution system employing semidefinite programming to guarantee composable security, achieving a record-breaking secret key rate of 18.93 Mb/s over a 25 km fiber channel. Our system offers a performance advantage of more than one order of magnitude compared to previous continuous-variable quantum key distribution systems, while maintaining low complexity and being cost-effective.

We develop a chip-based continuous-variable quantum key distribution system using an integrated optical switch for real-time shot noise calibration. Experimental results demonstrate a secure key rate of 12.30 Mbps over 25.3 km, establishing foundational capabilities for practical applications.

Quantum key distribution (QKD) which enables the secure distribution of symmetric keys between two legitimate parties is of great importance in future network security [1, 2]. Access network that connects multiple end-users with one network backbone can be combined with QKD to build security for end-users in a scalable and cost-effective way. Access network can have upstream stream transmission direction and downstream transmission direction. For upstream transmission, signals are transmitted from the end-users optical network units (ONUs), combined at the optical distribution network (ODN), and then forwarded to the optical line terminal (OLT) through single fiber. For downstream transmission direction, signals are sent from the OLT and separated at the ODN, then distributed to ONUs in the network. Though previous QKD access network demonstrations are all based on upstream transmission direction [3], the downstream access network on the other hand may offer extra advantages, since no time multiplexing technique is applied, the crosstalk is minimized, also, only passive beam- splitter is sufficient to distribute the signals, and no active controls or calibrations are required at the intermediate optical distribution network node, signals are simply broadcasted to the ONUs [4]. However, it is not straight- forward to integrate QKD into the downstream access network, for discrete-variable QKD, the quantum signals cannot be deterministically distributed to the ONUs. More importantly, since every ONU gets a copy of the transmitted quantum signals, it is crucial that the final secret key is private against other ONUs in the downstream access network. Here, we prove that QKD downstream access network can be realized by using continuous-variable (CV) QKD [5], the corresponding implementation can deterministically perform QKD [6] with the activated ONU, the network still only applies passive beamsplitter to distribute quantum signals. The secrecy against other parties in the network is achieved by considering a reinforced Eve during the security analysis. The security analysis can be conducted with only the optical line terminal and the activated ONU, and no other parties assistances are required. Our work provides the security analysis framework for realizing QKD in the downstream access network which will boost the diversity for constructing practical QKD networks. This work was supported by the Key Program of National Natural Science Foundation of China under Grant No. 61531003, National Natural Science Foundation of China under Grant No. 62001041, China Postdoctoral Science Foundation under Grant No. 2020TQ0016, Sichuan Science and Technology Program under Grant No. 2020YFG0289 and the Fund of State Key Laboratory of Information Photonics and Optical Communications. [1] V. Scarani, H. Bechmann-Pasquinucci, N. J. Cerf, M. Dusek, N. Lütkenhaus, and M. Peev, The security of practical quantum key distribution, Rev. Mod. Phys. 81, 1301 (2009). [2] F. Xu, X. Ma, Q. Zhang, H.-K. Lo, and J.-W. Pan, Secure quantum key distribution with realistic devices, Rev. Mod. Phys. 92, 025002 (2020). [3] B. Fr¨ohlich, J. F. Dynes, M. Lucamarini, A. W. Sharpe, Z. Yuan and A. J. Shields, A quantum access network, Nature 501, 69-72 (2013). [4] ITU. G.984.1: Gigabit-capable passive optical networks (gpon): General characteristics. ITU-T (2008). [5] S. Pirandola, et al., Advances in quantum cryptography, Adv. in Opt. and Photon. 12, 1012 (2020). [6] Y. Zhang, et al., Continuous-variable QKD over 50km commercial fiber, Quantum Sci. Technol. 4, 035006 (2019).