We experimentally investigate a highly integrated, polarization qubit based quantum key distribution system over a real-world, 12.4 km fiber link. Our system implements quantum frames, i.e. alternating sequences of high intensity optical pulses (classical data) and faint ones (qubits or quantum data). The classical data enables compensation of time-varying birefringence in the fiber link, and will facilitate synchronization, time tagging, and allow extending point-to-point quantum key distribution systems to networks. Our system can be clocked at 980 MHz, and has demonstrated good stability over 37 hours operation. We also discuss high rate error correction using a low-density parity-check code.
We experimentally investigate a highly integrated, polarization qubit based quantum key distribution system over a real-world, 12.4 km fiber link. Our system implements quantum frames, i.e. alternating sequences of high intensity optical pulses (classical data) and faint ones (qubits or quantum data). The classical data enables compensation of time-varying birefringence in the fiber link, and will facilitate synchronization, time tagging, and allow extending point-to-point quantum key distribution systems to networks. Our system can be clocked at 980 MHz, and has demonstrated good stability over 37 hours operation. We also discuss high rate error correction using a low-density parity-check code.
We propose a fibre-based quantum key distribution system, which employs polarization qubits encoded into faint laser pulses. As a novel feature, it allows sending of classical framing information via sequences of strong laser pulses that precede the quantum data. This allows synchronization, sender and receiver identification and compensation of time-varying birefringence in the communication channel. In addition, this method also provides a platform to communicate implementation specific information such as encoding and protocol in view of future optical quantum networks. We demonstrate in a long-term (37 h) proof-of-principle study that polarization information encoded in the classical control frames can indeed be used to stabilize unwanted qubit transformation in the quantum channel. All optical elements in our setup can be operated at Gbps rates, which is a first requirement for a future system delivering secret keys at Mbps. In order to remove another bottleneck towards a high rate system, we investigate forward error correction based on low-density parity-check codes.
We propose and investigate a fibre-based quantum key distribution system, which employs polarization qubits encoded into faint laser pulses. As a novel feature, it allows sending of classical framing information via sequences of strong laser pulses that precede the quantum data. This allows synchronization, sender and receiver identification, and compensation of time-varying birefringence in the communication channel. Furthermore, this method also provides a platform to communicate implementation specific information such as encoding and protocol in view of future optical quantum networks. All optical components can be operated at Gbps rates, which is a first requirement for Mbps secret key rates.
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.
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) relies on quantum communication to allow distant parties to share a secure cryptographic key. Widespread adoption of QKD in current telecommunication networks will require the development of simple, low-cost, and stable systems. However, current QKD implementations usually include additional hardware that perform auxiliary tasks such as temporal synchronization and polarization basis tracking. Here we present a polarization-based QKD system operating at 1550 nm that performs synchronization and polarization compensation by exploiting only the hardware already needed for the quantum communication task. Polarization encoding is performed by a self-compensating Sagnac loop modulator that exhibits high temporal stability and the lowest intrinsic quantum bit error rate reported so far. The QKD system was tested over a fiber-optic link, demonstrating tolerance up to about 40 dB of channel losses. Due to its reduced hardware requirements and the quality of the source, this work represents an important step towards technologically mature QKD systems.
Quantum communication technologies capable of operating reliably across heterogeneous optical channels are essential for scalable metropolitan quantum networks. Here we demonstrate high-dimensional time-bin-encoded quantum key distribution over a hybrid metropolitan link comprising 1.7 km free-space transmission and 685 m of optical fiber. Operating at a clock rate of 500 MHz in the C-band, we implement both 2- and 4-dimensional protocols, and obtain estimated secure finite-key rates of (95 +- 28) kbit/s for 4D at (25.0 +- 2.0) dB loss and (59 +- 27) kbit/s for 2D at (23.5 +- 2.3) dB loss. Crucially, we achieve continuous operation over 48 h in a fully self-referenced architecture: initial synchronization, interferometric phase stabilization, and long-term drift compensation are performed exclusively using the detected quantum signals, without auxiliary optical reference channels. Our results thus establish a practical and versatile platform for hybrid free-space-to-fiber quantum communication and show that the encoding dimensionality can be adapted to the optimal operating regime of realistic metropolitan channels, providing a pathway toward efficient, autonomous and deployable quantum network nodes.