Yichen Zhang

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.

We report a unidimensional two-way continuous-variable quantum key distribution protocol, which shows the potential of secure communication with simple modulation method in noisy situations.

We propose a high efficiency reconciliation method for continuous-variable quantum key distribution over free-space channel based on rate compatible codes, which achieves stable reconciliation efficiency of more than 95% under the fluctuation of the SNR (as low as -16 dB).

We report the polarization-multiplexed CV-QKD with four-state modulation capable of full use of two orthogonal polarization channels and experimentally investigate it. We design a polarization and phase compensation scheme by introducing a pair of rather than one single regularly spaced reference data timemultiplexed with weaker signal data. The polarization mixing and relative phase can be estimated simultaneously by using the transmitted reference data and the corresponding detection data to calculate a overall rotation matrix, and compensated by rotating Bob's received data. The results show that combined with our efficient polarization and phase compensation scheme this low-complexity scheme can further improve the secret key rate and prompt CV-QKD to be network-compatible and on-chip integrated.

Quantum key distribution (QKD) is designed to establish symmetric keys among two legitimate parties. Continuous variable (CV) QKD that uses the coherent states and homodyne detection can only apply the cost-effective telecommunication components[1]. The field test of CV-QKD has reached over 50 km[2], and under the laboratory conditions, experimental demonstration of over 200km has been reported [3], thus, has revealed great potentials in practical implementations. The access network that allows multitude end-users to connect to the nodal network is a necessary in the modern network infrastructure since it is suitable for general home-to-home scenarios. Quantum access network was first proposed [4] and demonstrated in field tests [5] for discrete variable QKD. Here, we report an upstream access network based on CV-QKD. In our experimental demonstrations, two transmitters Alice are deployed as optical network units that simultaneously send signals to the network, the receiver Bob is acted as the optical line terminal. The optical distribution network is located between the optical network units and the optical line terminal to couple the signals. The signals generated from each optical network unit are required to pass through a variable delay line to calibrate the arriving time at the optical distribution network before being transmitted. The signals are then simultaneously sent to the optical distribution network through fibers of 5.3 km and 12.3 km respectively. When the signals approach to the optical distribution network, dynamic polarization control modules are firstly applied in each path to pre-compensate the polarization. The signals are then coupled through a beamsplitter and forwarded to the optical line terminal. With a system repetition frequency of 2.5 MHz, we obtain the averaged secret key rates of 55 kbps and 22 kbps for Alice No. 1 and Alice No. 2 respectively. The total secret key rates has reached 77 kbps which suggests a higher network capacity. The excess noise is relatively stable, yet there are small fluctuations in the secret key rates. To one step further reduce the loss, the wavelength division multiplexing devices can be used at the optical distribution network. This is especially beneficial when the quantum signal has to co-propagate with classical data channels in the same fiber [6,7]. The upstream access network implementations can be easily extended to a higher repetition frequency system or to support more users. The demonstration experiments provide the possibility of building practical large-scale CV-QKD networks. This work is supported by the Key Program of National Natural Science Foundation of China under Grants No. 61531003, and the Fund of CETC under Grant No. 6141B08231115. References 1. C.Weedbrook, S. Pirandola, R. Garc´ıa-Patr´on, N. J. Cerf, T. C. Ralph, J. H. Shapiro and S. Lloyd, Gaussian quantum information, Rev. Mod. Phys. 84, 621 (2012). 2. Y. Zhang, Z. Li, Z. Chen, C. Weedbrook, Y. Zhao, X. Wang, Y. Huang, C. Xu, X. Zhang, Z. Wang, M. Li, X. Zhang, Z. Zheng, B. Chu, X. Gao, N. Meng,W. Cai, X.Wang, G.Wang, S. Yu and H. Guo, Continuous- variable QKD over 50 km commercial fiber, Quantum Sci. Technol. 4, 035006 (2019). 3. Y. Zhang, Z. Chen, S. Pirandola, X. Wang, C. Zhou, B. Chu, Y. Zhao, B. Xu, S. Yu and H. Guo, Long- distance continuous-variable quantum key distribution over 202.81 km fiber, arXiv:2001.02555 (2020). (Accepted by Phys. Rev. Lett.) 4. 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). 5. B. K. Park, M. K. Woo, Y. S. Kim, Y. W. Cho, S. Moon and S. W. Han, User-independent optical path length compensation scheme with sub-nanosecond timing resolution for a 1* N quantum key distribution network system, Photon. Res. 8, 296 (2020). 6. T. A. Eriksson, T. Hirano, B. J. Puttnam, G. Rademacher, R. S. Lu´ıs, M. Fujiwara, R. Namiki, Y. Awaji, M. Takeoka, N. Wada and M. Sasaki, Wavelength division multiplexing of continuous variable quantum key distribution and 18.3 Tbit/s data channels, Commun. Phys. 2, 9 (2019). 7. B. Chu, Y. Zhang, Y. Zhao, Y. Xu, X. Chen, X. Wang and S. Yu, Crosstalk-induced impact of coexisting DWDM network on continuous-variable QKD, 16th International Conference on the Design of Reliable Communication Networks DRCN, Milano, Italy, pp. 1-5 (2020).