Peking University research team has built the first large-scale quantum communication chip network

Peking University School of Physics Official WeChat announced on February 12th that the team led by Professor Jianwei Wang and Professor Qihuang Gong from the Institute of Modern Optics, along with Researcher Lin Chang from the School of Electronics, published a groundbreaking research paper titled “Large-Scale Quantum Communication Network Based on Integrated Photonic Quantum Chips” in the top-tier international academic journal Nature.

The research team successfully developed fully integrated high-performance quantum key distribution (QKD) chips and optical microcavity frequency comb light source chips. Building on this, they constructed the world’s first large-scale quantum key distribution network based on integrated photonic quantum chips—the “Weiming Quantum Network.” This network supports 20 chip users communicating in parallel, with pairwise communication distances reaching 370 kilometers and breaking the no-repeater limit. The network’s capability (client count × communication distance) reaches 3700 kilometers, achieving internationally leading levels in both chip user scale and network capacity.

The study further validated the advantages of material systems based on indium phosphide and silicon nitride in photonic quantum chip manufacturing, characterized by high yield, high performance, and strong scalability at wafer level, laying a technological foundation for low-cost, large-scale production. This breakthrough provides a solid chip-level solution for future practical quantum secure communication networks that cover longer distances, accommodate more users, and support larger scales.

Quantum key distribution (QKD) is based on quantum mechanics principles and can theoretically achieve unconditional secure communication. China has made a series of major achievements in quantum satellite key distribution and integrated space-ground quantum networks, leading globally. Among these, twin-field QKD (TF-QKD) combines measurement-device independence with ultra-long-distance transmission advantages. Chinese scientists have achieved point-to-point key distribution over thousands of kilometers in fiber. This protocol is naturally suitable for star network architectures, allowing expensive superconducting single-photon detectors to be centralized at the node, significantly reducing user-end costs, and is regarded as an important approach for scaling quantum communication networks. However, implementing TF-QKD heavily relies on stable single-photon interference between remote independent lasers, requiring high-precision phase locking and noise suppression, which is challenging with current experiments mostly based on bulk or discrete fiber devices, and mostly limited to two-user point-to-point systems.

Quantum key distribution chips (QKD chips) are a key pathway toward miniaturizing quantum communication systems, making devices practical, and scaling networks. Since Japan’s NTT first proposed the concept of integrated QKD chips in 2004, the past two decades have seen continuous improvements in their functions and performance. The team at Peking University has long been engaged in this field, achieving several internationally leading results, including quantum entanglement distribution and quantum teleportation between two chips [Nature Physics 16, 148 (2020)], high-dimensional entanglement quantum networks across multiple chips [Science 381, 221 (2023)], and vortex beam entanglement chips for space quantum communication [Nature Photonics 19, 471 (2025)]. Since 2019, the laboratory has continued research on QKD chips and quantum networks, accumulating over six years of technical expertise, culminating in a significant breakthrough in multi-user, long-distance, large-scale quantum communication network systems based on photonic quantum chips.

Figure 1 shows the large-scale quantum key distribution network “Weiming” based on photonic quantum chips: a, the architecture of the twin-field QKD chip network; b, physical photos of 20 QKD chips and microcomb light source chips.

In typical TF-QKD applications, frequency and phase references are distributed among users to establish coherence between remote independent lasers, with secure keys generated via single-photon interference at untrusted nodes. The team used wavelength-division multiplexing (WDM) technology to build a large-scale quantum communication network, enabling users to send quantum signals in parallel. Signals are transmitted over long-distance fiber to a central server node, where demultiplexing, interference, and single-photon detection occur to extract secure keys. Traditional multi-wavelength, multi-user systems based on discrete optical components are highly complex, but integrated photonics offers a feasible path to miniaturization and high stability. In this work, the team used high-Q silicon nitride microcavity frequency combs as seed light sources at the central server node, generating ultra-low noise, Hz-level linewidth, coherent dark pulse frequency combs in the communication band via self-injection locking, without complex electronic control systems or desktop lasers. The comb lines are distributed via downstream fiber to user nodes for demultiplexing. Each user employs an indium phosphide quantum chip integrating all key modules—laser, modulator, attenuator, key encoding and decoding devices—achieving wafer-level manufacturing, high yield, low cost, and high performance. The seed light from the comb is injected into local lasers at user nodes, significantly suppressing phase noise. Subsequently, the system encodes quantum states using weak coherent states, which are sent via uplink fiber to the server, where single-photon interference and measurement are performed. The complete network architecture is shown in Figure 1.

Figure 2 presents key performance characterizations of the integrated photonic quantum chips. a, the silicon nitride microcavity frequency comb seed laser at the server. b, the dark pulse comb spectrum. c, the phase noise power spectral density of comb lines. d, the fully integrated indium phosphide QKD sender chip. e, the wavelength tuning range of on-chip lasers at the user end. f, the half-wave voltage and modulation depth of on-chip modulators.

Figure 2 demonstrates the critical performance of the microcavity comb and QKD sender chips. As shown in 2a–c, self-injection locking not only generates the dark pulse frequency comb but also significantly suppresses phase noise. The comb operates at 1550 nm communication band, with a free spectral range of 30 GHz; after locking, the comb line phase noise spectral density baseline is about 13 Hz^2/Hz, corresponding to a short-term linewidth of approximately 40 Hz, exhibiting excellent coherence and stable operation over 12 hours. Figure 2d shows the structure and physical photo of the indium phosphide QKD sender chip at the user end. The on-chip distributed Bragg reflector (DBR) laser tuning range is shown in 2e; under injection locking, its frequency and phase closely replicate the seed light, with linewidths comparable to the seed. Testing 120 phase modulators (forming 60 intensity modulators) across 20 user chips shows an average half-wave voltage of about 5.8 V and an interference extinction ratio exceeding 33 dB, with 117 devices functioning normally and a yield of 97.5%. Notably, the study also demonstrated that microcavity comb chips and QKD sender chips exhibit high uniformity and yield at wafer level, indicating the potential for low-cost, scalable manufacturing—crucial for constructing large-scale quantum communication networks.

Figure 3 shows experimental results of multi-user TF-QKD chip networks. a–c depict phase fluctuations in long fiber channels. d–e show the bit error rate (BER). f–g display the final key generation performance of 20 QKD chips.

The team further built a multi-chip cooperative quantum network, enabling multiple users to run send–not-send TF-QKD protocols in parallel. By introducing a dual-wavelength phase-tracking scheme, despite rapid phase fluctuations in the reference light (Figure 3a) and quantum light (Figure 3b) over long fiber links, the co-propagation of both in the same fiber and their high coherence (originating from the same frequency comb) result in only slow relative phase drift (Figure 3c). This allows effective phase compensation of quantum signals via reference light monitoring. The team evaluated linear crosstalk and nonlinear Raman noise introduced by multi-wavelength co-propagation, optimizing filtering to reduce noise close to detector dark counts. Ultimately, the system achieved low BER operation over uplink links of 204 km and 370 km (Figures 3d,e), surpassing the no-repeater PLOB bound at 370 km, with a maximum relative increase of 251.4% over the theoretical limit (Figures 3f,g). Additionally, under longer downlink conditions (equivalent to a closed-loop fiber Mach–Zehnder interferometer of 490 km total length), the system maintained stable phase tracking and secure key generation, demonstrating the feasibility of the scheme in practical quantum networks.

(Article source: Cailian News)