We report the first long-term field test of a polarization-encoding QKD system in C-band coexistence with classical signals over the same standard optical fiber. The system combines a self-stable polarization transmitter with a polarization control unit that stabilizes the link based on quantum metrics. This enables robust QKD operation over a metropolitan fiber link for several days, without requiring knowledge or control of the classical channels. The system can produce positive secret key rates of 422 kbps while coexisting with 0.7 mW classical power along a 50-km deployed link, achieving more than 98% uptime in the quantum channel.
We report the first long-term field test of a polarization-encoding QKD system in C-band coexistence with classical signals over the same standard optical fiber. The system combines a self-stable polarization transmitter with a polarization control unit that stabilizes the link based on quantum metrics. This enables robust QKD operation over a metropolitan fiber link for several days, without requiring knowledge or control of the classical channels. The system can produce positive secret key rates of 422 kbps while coexisting with 0.7 mW classical power along a 50-km deployed link, achieving more than 98% uptime in the quantum channel.
This study introduces an advanced discrete-variable (DV) polarization-state quantum key distribution (QKD) transmitter tailored for the BB84 protocol with decoy states at 1550 nm. Demonstrating an intrinsic quantum bit error rate (QBER) of 0.15 % at a 615 MHz repetition rate and 0.34 % across a 50-km standard single-mode fiber link, our implementation marks a significant stride towards practical quantum-secured communication. The system's reliance on commercial off-the-shelf (COTS) components and low-voltage operation, coupled with a self-stable design, ensures resilience against environmental perturbations. This research pioneers the evaluation of a polarization-state QKD transmitter's performance in scenarios that involve copropagation with high classical powers within the telecom C Band on shared optical links, observing the impact of spontaneous Raman scattering on the quantum states.
The co-existence of quantum key distribution (QKD) signals with classical optical signals in a single fiber enables substantial fiber resource savings and reduces the deployment costs of QKD networks. However, conventional approaches to mitigate the impact of strong classical signals on QKD performance typically involve reducing classical optical power, which inherently conflicts with standard optical communication network specifications and impedes the widespread adoption of QKD networks. In this work, we explore the feasibility of integrating QKD networks with high-capacity classical optical networks without significant modifications to existing infrastructure and validate wavelength allocation strategies for their co-existence in field-deployed shared fibers. Notably, we demonstrate co- propagation of QKD with high-power classical signals (21.75 dBm total launch power) over 81 km of commercial backbone fiber, covering 90% of China Telecom's inter-site links. Leveraging a 400 Gbps optical transport network (OTN) system, we first, to the best of our knowledge, achieve a classical optical capacity of 15.7 Tbps. Additionally, we introduce single-fiber optical supervisory channel (OSC) configurations in quantum-classical co-existence experiments for the first time, further validating the feasibility of integrating QKD networks with classical networks requiring high-precision clock synchronization. The system achieves secure key rates (SKRs) of 2.2 kbps and 1.35 kbps under two-fiber and single-fiber OSC configurations, respectively. This study marks a significant step towards the practical deployment and maturity of QKD networks.
We present polarization entanglement distribution over an operator-owned, metropolitan-scale fiber network, employing standard wavelength-division multiplexing components to achieve coexistence of O-band photons and C-band classical light. First, we demonstrate dynamic path switching by alternating between 15-km and 30-km deployed fiber loops, enabled by automated polarization compensation achieving sub-second recovery and time-averaged CHSH S -values exceeding 2.65 over 21 hours. We then assess the impact of co-propagating 100-Gbit/s C-band classical signals by varying launch powers up to 6 dBm. Finally, by combining path switching and classical co-propagation, we achieve stable entanglement distribution over 30 km of deployed fiber, validating the feasibility of hybrid quantum–classical transmission and paving the way for entanglement-based quantum networking on existing metropolitan infrastructure.
A successful commercial deployment of quantum key distribution (QKD) technologies requires integrating QKD links into existing fibers and sharing the same fiber networks with classical data traffic. To mitigate the spontaneous Raman scattering (SpRS) noise from classical data channels, several quantum/classical coexistence strategies have been developed. O-band solutions place the QKD channel in the O-band for lower SpRS noise but with the penalty of higher fiber loss and can rarely reach beyond 80 km of fiber; another method is C-band coexistence with attenuated classical channels, which sacrifices the performance of classical channels for lower SpRS noise. In this work, a time-interleaving technique is demonstrated to enable the co- propagation of quantum and classical channels in the C-band without sacrificing either performance. By embedding QKD pulses in the gaps between classical data frames, the quantum channel is isolated from SpRS noise in both wavelength and time domains. C-band co- propagation of a polarization-encoding decoy-state BB84 QKD channel with a 100 Gb/s QPSK channel is experimentally demonstrated with quantum bit error rate (QBER) of 1.12%, 2.04%, and 3.81% and secure key rates (SKR) of 39.5 kb/s, 6.35 kb/s, and 128 b/s over 20, 50, and 100 km fibers, respectively. These results were achieved with the presence of classical launch power up to 10 dBm, which is at least one order of magnitude higher than reported works. We also demonstrated the co- propagation of a QKD channel with eight classical channels with total launch power up to 18-dBm (9-dBm per channel), which is the highest power of classical channels reported in C-band coexistence works.
Stability and robustness are important criteria to evaluate the performance of a quantum-key-distribution (QKD) system in real-life applications. However, the inherent birefringence effect of the fiber channel and disturbance caused by the variation of the deployment environment of the QKD system tremendously decrease its performance. To eliminate this adverse impact, we propose a polarization-insensitive phase-coding QKD system based on Sagnac--Mach-Zehnder interferometers. Verified by theoretical analysis and experimental tests, this QKD system is robust against channel polarization disturbance. The robustness and long-term stability of the QKD system is confirmed with a 10-day continuous operation over a 12.6-dB channel, which consists of a 50-km fiber spool and a polarization scrambler (2 rad/s). As a result, an average quantum bit error rate of 0.958 and a sustained secure key rate of 3.68 kbps are obtained. Moreover, the secure key rate of the QKD system for a typical channel loss of 10 dB reaches 6.89 kbps, and the achievable maximum transmission distance exceeds 125 km.
Current Free-Space Optical Quantum Key Distribution (FSO-QKD) systems are mainly focusing on the development of critical components needed towards the realization of satellite-based QKD, relying on bulky optical telescopes equipped with expensive optical systems. The cost and complexity of these systems hinder the establishment of a practical FSO-QKD infrastructure deployed as the wireless last-mile segment of Quantum Communication Infrastructure for applications in urban environments. In this work, we experimentally demonstrate successful transmission of polarization encoded qubits over 0.5 km free space metropolitan link with enhanced noise suppression for both nighttime and daylight operation. Βy deploying two low-cost, Single Mode Fiber-coupled, optical terminals along with a prototype quantum sender station operating in C-band, Quantum Bit Error Rate (QBER) values below 3.5% were achieved, with expected Secure Key Rates up to 1 kbps during nighttime. The FSO link was integrated with existing in-building fiber infrastructure, demonstrating stable operation and acceptable QBER values below 5.5%, even when coexisting with intense C-band light signals (> 10 mW) propagating alongside the photonic qubits. The field trial and its comprehensive verification paves the way for practical FSO-QKD in city-scale environments, enabling end users to share quantum keys over kilometer-scale wireless links between building rooftops in last-mile connectivity scenarios.