Twin-field quantum key distribution (TF-QKD) has emerged as a promising solution for practical quantum communication over long-haul fiber. However, previous demonstrations on TF-QKD require the phase locking technique to coherently control the twin light fields, inevitably complicating the system with extra fiber channels and peripheral hardware. Here, we propose and demonstrate an approach to recover the single-photon interference pattern and realize TF-QKD without phase locking. Our approach separates the communication time into reference frames and quantum frames, where the reference frames serve as a flexible scheme for establishing the global phase reference. To do so, we develop a tailored algorithm based on fast Fourier transform to efficiently reconcile the phase reference via data postprocessing. We demonstrate no-phase-locking TF-QKD from short to long distances over standard optical fibers. At 50-km standard fiber, we produce a high secret key rate (SKR) of $1.27\mathrm{Mbit}/\mathrm{s}$, while at 504-km standard fiber, we obtain the repeaterlike key rate scaling with a SKR of 34 times higher than the repeaterless secret key capacity. Our work provides a scalable and practical solution to TF-QKD, thus representing an important step towards its wide applications.

Silicon photonics offers a stable, miniaturized platform for quantum key distribution (QKD). As such, the chip-based system is vital for a wider adoption of QKD in the near future. However, the practical security issues presented in this alternative platform have not been fully studied. Here, we theoretically and experimentally evaluate the security of a chip-based QKD system under a powerful source attack, namely, Trojan-horse attack. We analyze the finite-key security of two prominent QKD protocols—BB84 and measurement-device-independent QKD—with the information leakage from both the polarization modulator and the intensity modulator integrated on a silicon photonic chip. Furthermore, we prove that our chip-based measurement-device-independent QKD system is feasibly resistant to the Trojan-horse attack by adding 232-dB isolation to the chip. In this sense, our work provides a guideline for the secure implementation of a leaky transmitter chip in the QKD system.

Quantum key distribution (QKD) allows remote parties to share secret keys with proven security. To guarantee the security of practical QKD, the imperfections in realistic devices need to be characterized and considered in practical security analysis. Particularly, a standard QKD system normally uses optical isolator or optical circulator in the transmitter to prevent the injection from external light. Here we find that the optical isolators and circulators, commonly based on the Faraday effect, are subject to the perturbation from external magnetic fields. With this, we provide a comprehensive analysis of the security for practical QKD due to external magnetic effect. We experimentally demonstrate the influence of magnetic fields on both the transmittance and the isolation of standard optical isolators and circulators. We analyze the security risks caused by the potential magnetic attacks, together with the corresponding countermeasures.

Quantum key distribution (QKD) provides information-theoretic security for communication. The mode-pairing (MP) protocol emerges as a promising solution for long-distance QKD by eliminating the need for a global phase reference while maintaining the repeaterlike rate-loss scaling. Recent implementations have demonstrated its potential, but they either rely on costly ultrastable lasers or struggle with phase fluctuations from commercial lasers, particularly over long distances. As a result, surpassing the repeaterless bound with a practical system remains a challenge. In this work, we demonstrate a practical high-performance MP-QKD system using commercial lasers. To address phase fluctuations, we propose a frequency-tracking scheme based on fast Fourier transformation, enabling us to extend the pairing length to 2 × 10 5 pulses ( 160 μ s ). We propose a model to carefully analyze the phase noise and optimize the system parameters. Our system achieves an optimal secret key rate of 47.8 bit / s over 403 km of standard fiber (76.5 dB loss), exceeding the repeaterless bound by a factor of 2.92. Furthermore, we compare MP-QKD and twin-field QKD under various practical conditions and clarify the distinct application scenarios of the two protocols. These results confirm the feasibility of MP-QKD using cost-effective commercial technologies, paving the way for scalable quantum communication networks.

Real-time analysis of single-photon coincidence is critical in photonic quantum computing. The large channel number and high counting rate foreseen in such experiments pose a big challenge for the conventional time tagged method and coincidence instruments. Here we propose a real-time time-tagged coincidence method and a data filtering solution, demonstrated by a 32-channel coincidence counting unit that has been implemented successfully on a field-programmable gate array system. The unit provides high counting rates, a tunable coincidence window, and a timing resolution of 390 ps. Beyond that, it is feasible to be scaled up to 104 channels and is thus ideally suited for channel consuming applications such as boson sampling. Based on the versatility and scalability the unit has shown, we believe that it is the turn-key solution for many single-photon coincidence counting applications in photonic quantum computing.