Ziyang Chen

Local local oscillator (LLO) continuous-variable quantum key distribution (CV-QKD) offers enhanced security and simplified implementation compared to transmitting local oscillator (TLO) schemes, but generally requires high-power pilot tones for carrier phase recovery. Among various LLO schemes, the sequential LLO scheme features low hardware complexity, yet suffers from limited quantum signal repetition frequency due to its alternating pilot-signal structure, which reduces the secret key rate. To address this, we propose an optimized scheme that increases the proportion of quantum signals and applies exponentially weighted phase prediction. A particle filter (PF)-based algorithm is further introduced to compensate for reduced pilot tone ratio. Experimental results over 30 km fiber demonstrate that the optimized scheme suppresses excess noise below 0.008 SNU, stabilizes transmittance around 0.25, and improves the secret key rate by over 147%, even when accounting for algorithmic complexity.

Continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) can address vulnerabilities on the detection side of a QKD system. The core of this protocol involves continuous-variable Bell measurements performed by an untrusted third party. However, in high-speed systems, spectrum broadening causes Bell measurements to deviate from the ideal single-mode scenario, resulting in mode mismatches, reduced performance, and compromised security. Here, we introduce temporal modes (TMs) to analyze the security and performance of CV-MDI QKD under continuous-mode scenarios. The mismatch between Bob’s transmitting mode and Bell-measurement mode has a more significant effect on system performance compared to that on Alice’s side. When the Bell receiver is close to Bob and the mismatch is set to just 5%, the transmission distance drastically decreases from 87.96 km to 18.50 km. In comparison, the same mismatch for Alice reduces the distance to 86.83 km. This greater degradation on Bob’s side can be attributed to the asymmetry in the data modification step. These results indicate that, in scenarios involving continuous-mode interference, such as large-scale MDI network setups, careful consideration of each user’s TM characteristics is crucial. Rigorous precalibration of these modes is essential to ensure the system’s reliability and efficiency.

Quantum key distribution (QKD) can provide unconditionally secure keys at the physical layer for communication system. In practical environments, communication usually occurs in multi-user and multi-scenario, and point-to-point QKD can no longer meet the modern complex network communication needs. The downstream access network downstream, as an essential component of modern networks, requires QKD technology to ensure its security. Here, we complete a four-user high-speed QKD downstream access network experiment. The repetition frequency of the system is 100 MHz, considering block size of $10^8$, four users achieved secret key rates of 430 kbps, 450 kbps, 150 kbps, and 130 kbps at channel attenuation of 4.4 dB, 4.2 dB, 5.6 dB, and 5.8 dB, respectively. Our experimental results demonstrate the feasibility of multi-user downstream CV-QKD access networks, further advancing the practical application of quantum networks in real-world environments.

Quantum networks provide opportunities and challenges across a range of intellectual and technical frontiers, including quantum computation, communication and others. Unlike traditional communication networks, quantum networks utilize quantum bits rather than classical bits to store and transmit information. As an important part of the networks, the access network can connect multiple end users to the backbone network and provide the so-called last-mile service. In our work, the first four-end-users quantum downstream access network in continuous variable quantum key distribution with a local local oscillator has been experimentally demonstrated. Our results show that each user can get a low level of excess noise and can achieve secret key rate of 546 kbps, 535 kbps, 522.5 kbps and 512.5 kbps under transmission distance of 10 km, respectively with the finite-size block of 1×10⁸. More importantly, the successful demonstration of our quantum downstream access network also paves the way for secure broadband metropolitan and quantum networks.

Continuous-variable quantum key distribution (CV-QKD) provides a way for two remote participants called Alice and Bob to establish symmetric keys through an unsafe channel \cite{weedbrook2012gaussian,grosshans2003quantum}. Continuous-variable quantum key distribution (CV-QKD) based on commercial devices such as lasers and coherent detectors is moving towards practical. Experimental implementation of the CV-QKD systems using Gaussian-modulated coherent states (GMCS) has made significant progress recently \cite{zhang2019continuous}. At the mean time, the problems of performance degradation caused by imperfections of those experimental devices remain unsolved absolutely \cite{pirandola2020advances}. A non-orthogonal measurement angular error between quadrature components $X$ and $P$ from coherent detection is always ignored in the current experimental scheme. The optical phase shifter that constantly rotates the local oscillator phase is a necessity in continuous-variable quantum key distribution systems using heterodyne detection. In previous experimental implementations, the optical phase shifter is generally regarded as an ideal passive optical device that perfectly rotates the phase of the electromagnetic wave of $90^\circ$ \cite{wang2020high}. However,under the action of external force, the fibre is stretched or compressed within the elastic deformation range, and parameters such as the fibre change's geometrical size and refractive index change, thus causing the phase change of the transmitted signal in the fibre. Therefore, the phase shifter is somewhat susceptible to environmental changes and can hardly shift the phase by $90^\circ$ exactly Considering this, we propose a concrete interpretation of measurement angular error in practical systems and the corresponding entanglement-based description. Simultaneously, an estimation method of the measurement angular error and corresponding compensation scheme are demonstrated in some ways. We conclude that measurement angular error severely degrades the security, but the proposed calibration and compensation method can significantly help improve the performance of the practical CV-QKD systems. Undoubtedly, it is worth observing that our work is to strengthen practical security resulted from devices' imperfection.

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).