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.
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.
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.
Abstract Integrating quantum key distribution (QKD) into optical networks is a crucial step toward the adoption of quantum technologies in existing telecommunication fiber infrastructures. However, state-of-the-art solutions face significant challenges, including sensitivity to classical noise, particularly spontaneous Raman scattering, limited transmission distances, and varying network conditions. In this work, we present a novel discrete-variable QKD system running a time-bin BB84 protocol operating in the O-band (1295.56 nm), where the impact of dominant noise sources is effectively reduced. Combined with passive narrow-spectral filtering at the receiver, our system demonstrates robustness across diverse dense wavelength-division multiplexing scenarios, making it practical for real-world deployments. We validate its performance through extensive testing, showing stable key generation coexisting with different classical traffic conditions up to 17 dBm of total launch power. These results represent a significant advancement toward the integration of QKD into existing fiber networks, paving the way for secure quantum communication on a large scale.
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.
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.
We report an implementation of decoy-state quantum key distribution (QKD) over 200 km optical fiber cable through photon polarization encoding. This is achieved by constructing the whole QKD system operating at 320 MHz repetition rate, and developing high-speed transmitter and receiver modules. A novel and economic way of synchronization method is designed and incorporated into the system, which allows to work at a low frequency of 40kHz and removes the use of highly precise clock. A final key rate of 15 Hz is distributed within the experimental time of 3089 seconds, by using super-conducting single photon detectors. This is longest decoy-state QKD yet demonstrated up to date. It helps to make a significant step towards practical secure communication in long-distance scope.
Quantum key distribution (QKD) is approaching industrial maturity, but its integration with high-power classical signals in shared optical fiber is hindered by noise from nonlinear effects like spontaneous Raman scattering and four-wave mixing. Hollow-core fiber (HCF) offers a promising solution by inherently suppressing these nonlinearities. However, a comprehensive noise analysis for pure HCF or standard single-mode-fiber–HCF hybrid links, validated experimentally under realistic high-power conditions, remains unavailable. This work bridges that gap through a thorough theoretical and experimental investigation of an HCF-featured coexistence system. We demonstrate a silicon-photonics-based QKD system operating with an optical transport network (OTN). The system achieves multiplexed launch powers of 20 dBm for counter-propagation and 14 dBm for co-propagation, meeting the highest requirements of modern OTNs. Over a 10 km HCF link, it simultaneously supports 12×583 Gbps classical channels across the C-band and achieves secure key rates of 5.7 kbps (counter-propagation) and 2.5 kbps (co-propagation) over a 1550 nm quantum channel. Our results provide critical benchmarks and a practical framework for deploying HCF-based quantum-classical networks, paving the way for future inherently secure telecommunications.