Ziyang Chen

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.

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.

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.