To address the challenges of frame drift and link interruptions in underwater quantum key distribution (QKD), we design a polarization-encoded reference-frame-independent quantum key distribution (RFI-QKD) system. Temporal coding of synchronized optical pulses combined with quantum signal discrimination enables the system to re-establish timing synchronization after a link interruption and resume key generation. With an average output photon number of 0.1, we experimentally demonstrate RFI-QKD over a 15.3 dB-loss channel, achieving a final key rate of 357 bps. Furthermore, over a 17 m underwater channel with an attenuation of 2.76 dB, the system attains a final key rate of 24.14 kbps.
To address the challenges of frame drift and link interruptions in underwater quantum key distribution (QKD), we design a polarization-encoded reference-frame-independent quantum key distribution (RFI-QKD) system. Temporal coding of synchronized optical pulses combined with quantum signal discrimination enables the system to re-establish timing synchronization after a link interruption and resume key generation. With an average output photon number of 0.1, we experimentally demonstrate RFI-QKD over a 15.3 dB-loss channel, achieving a final key rate of 357 bps. Furthermore, over a 17 m underwater channel with an attenuation of 2.76 dB, the system attains a final key rate of 24.14 kbps.
During free-space quantum key distribution, the rotation and fluctuation of reference frame degrades the performance of quantum key distribution (QKD). Reference-frame-independent QKD (RFI-QKD) overcomes this issue effectively. To date, much theoretical and experimental research has been conducted on the performance of free-space RFI-QKD. However, these studies are all based on free-space air and satellite ground, and none have investigated the performance of RFI-QKD in an underwater channel. Therefore, this paper constructed a channel model that considered both scattering and optical attenuation to obtain an RFI-QKD secret key rate in an underwater channel. The simulation results confirm that even in a relatively harsh underwater scenario, RFI-QKD maintains good performance.
Abstract Rapidly and randomly drifted reference frames will shorten the transmission distance and decrease the secure key rate of realistic quantum key distribution (QKD) systems. In this article, we present a free-running reference-frame-independent (RFI) QKD scheme, where measurement events are classified into multiple slices with similar estimated classification parameter. We perform the free-running RFI QKD experiment with a fiber link of 100 km and reference frame misalignment more than 29 periods in 50.7 h. A key rate as high as 742.98 bps is achieved at the total loss of 31.5 dB benefiting from both the new protocol design and the 80 MHz repetition rate system in use. Our system runs 50.7 h freely without any reference frame alignment. In the experiment, the misalignment variation rate tolerance of the experiment is 0.262 rad/s, and could be optimized to 1.309 rad/s. Therefore, our free-running RFI scheme can be efficiently adapted into the satellite-to-ground and drone-based mobile communication scenarios.
Free-space quantum key distribution (QKD) based on mobile platforms, such as satellites, drones, and vehicles, is considered a promising way to overcome the rate-distance limit without a quantum repeater. Real-time reference frame calibration is required in most recent implemented polarization encoded QKD systems due to the relative motion between sender and receiver. Although active compensations can be used to calibrate the reference frame, doing so increases the complexity of the system and reduces the key rate. To overcome this problem, the reference-frame-independent (RFI) QKD was proposed in which fixed deviations of the reference frame between the two parties are tolerated automatically. In this Letter, we report the experimental implementation of a time-bin encoded RFI QKD in an urban environment through free space. The quantum bit error rate for key-distill is as low as 1% over a 2 km free-space link with a total equivalent loss of 31.5 dB. Our demonstration shows that a stable RFI QKD can be implemented in the free-space channel.
A global quantum secure network (QSN) based on quantum key distribution (QKD) has emerged as a significant trend since it provides an unconditionally secure way to defeat the threat posed by quantum computers. The nodes, based on ground or mobile platforms, are connected with two different channels, free-space and fiber. In this paper, a qubit-based synchronization reference-frame-independent (RFI) QKD is demonstrated in the field with a free-space/fiber integrated channel, in which the bright classical reference-frame calibration and clock synchronization signals are removed, and the quantum state is directly converted between two different channels without extra coding/decoding operations. It not only reduces the complexity of QKD and QSN but also enhances the practical security (e.g., removing calibration attacks and trusted repeaters). A key rate of 799.2 bps is achieved under 7 km urban free-space and 75 km fiber link. And the long time tests of a continuous 12 h without human intervention also demonstrate the stability of our system under such an integrated channel. Thus, we believe this work opens a way to implement a simple, flexible, and low-cost QSN.
Quantum key distribution (QKD) is a cryptographic technique that uses quantum mechanical principles to enable secure key distribution, offering information-theoretic security guaranteed by physical laws. Practical deployment of QKD requires robust, cost-effective systems that can operate in challenging field environments. A major challenge is achieving reliable clock synchronization without adding hardware complexity. Conventional approaches often use separate classical light signals, which increase costs and introduce noise that degrades quantum channel performance. To address this limitation, we demonstrate a QKD system incorporating a recently proposed qubit-based distributed frame synchronization method, deployed over a metropolitan fiber network in Nanning, China. Using the polarization-encoded one-decoy-state BB84 protocol and the recently proposed qubit-based distributed frame synchronization method, our system achieves synchronization directly from the quantum signal, eliminating the need for dedicated synchronization hardware. Furthermore, to counteract dynamic polarization disturbances in urban fibers, the system integrates qubit-based polarization feedback control, enabling real-time polarization compensation through an automated polarization controller using data recovered from the qubit-based synchronization signals. During 12 hours of continuous operation, the system maintained a low average quantum bit error rate of 1.12 ± 0.48%, achieving a secure key rate of 26.6 kbit/s under 18 dB channel loss. Even under a high channel loss of 40 dB, a finite-key secure rate of 115 bit/s was achieved. This study represents a successful long-term validation of a frame-synchronization-based QKD scheme in a real urban environment, demonstrating exceptional stability and high-loss tolerance, and offering an alternative for building practical, scalable, and cost-efficient quantum-secure communication networks.
Reference-frame-independent measurement-device-independent quantum key distribution (RFI-MDI-QKD) eliminates detector side-channel attacks and avoids reference-frame calibration. While its feasibility has been widely demonstrated, existing implementations typically assume fixed or slowly drifting reference-frame misalignment, conditions rarely satisfied outside the laboratory. In realistic environments, rapid and free-running reference-frame variations can severely degrade both the key rate and transmission distance of conventional RFI-MDI-QKD. Here we propose a free-running RFI-MDI-QKD protocol that maintains high-rate key generation under rapid reference-frame variations. By introducing a classification-distillation method that reclassifies total detection events, secure keys can be extracted without modifying the experimental setup. Our protocol achieves a key rate more than nine times higher than the best previous RFI-MDI-QKD scheme and tolerates channel losses exceeding 24 dB, where earlier approaches fail. These results enable practical quantum key distribution on mobile platforms, including satellite-to-ground links and airborne nodes.