Abstract We report the implementation and field testing of an atmospheric quantum key distribution (QKD) system obtained by adapting the commercially available fiber-based ViPNet-QTS platform. The phase-time state-preparation module, detector electronics, and post-processing procedures were retained, whereas the fiber channel was replaced by external optical devices forming a 400 m urban free-space link. The 1550 nm quantum-channel signal was coupled into a multimode fiber (MMF) at the receiver and analyzed by a free-space multimode delay interferometer, allowing the quantum-cryptographic equipment to remain inside trusted zones. Synchronization and bidirectional pointing were implemented using spectrally separated 850 and 780 nm auxiliary channels. We characterized the channel loss, background-count probability, atmospheric turbulence, and performance of the fast and low-bandwidth tracking loops. Although feedback based on fast steering mirrors strongly reduced the fluctuations of the tracking-beam position at the CMOS sensor, it increased the mean power coupled into the MMF by only 4.07% under the tested conditions. Long-term alignment was instead maintained by low-bandwidth feedback to the motorized mounts, demonstrating that continuous high-bandwidth fine tracking was not required for the investigated 400 m link. During an autonomous four-day QKD experiment, the system operated fully unattended, with no manual intervention in alignment, system control, or post-processing. Secret keys were generated over most of the measurement interval under changing day–night and weather conditions. Temporary interruptions in key generation coincided with strong daylight background, whereas the measured turbulence variations had no similarly direct effect on system operation. These results demonstrate a practical approach to extending existing fiber-based QKD platforms to atmospheric links using an MMF-coupled receiver and a comparatively simple tracking architecture.
Abstract We report the implementation and field testing of an atmospheric quantum key distribution (QKD) system obtained by adapting the commercially available fiber-based ViPNet-QTS platform. The phase-time state-preparation module, detector electronics, and post-processing procedures were retained, whereas the fiber channel was replaced by external optical devices forming a 400 m urban free-space link. The 1550 nm quantum-channel signal was coupled into a multimode fiber (MMF) at the receiver and analyzed by a free-space multimode delay interferometer, allowing the quantum-cryptographic equipment to remain inside trusted zones. Synchronization and bidirectional pointing were implemented using spectrally separated 850 and 780 nm auxiliary channels. We characterized the channel loss, background-count probability, atmospheric turbulence, and performance of the fast and low-bandwidth tracking loops. Although feedback based on fast steering mirrors strongly reduced the fluctuations of the tracking-beam position at the CMOS sensor, it increased the mean power coupled into the MMF by only 4.07% under the tested conditions. Long-term alignment was instead maintained by low-bandwidth feedback to the motorized mounts, demonstrating that continuous high-bandwidth fine tracking was not required for the investigated 400 m link. During an autonomous four-day QKD experiment, the system operated fully unattended, with no manual intervention in alignment, system control, or post-processing. Secret keys were generated over most of the measurement interval under changing day–night and weather conditions. Temporary interruptions in key generation coincided with strong daylight background, whereas the measured turbulence variations had no similarly direct effect on system operation. These results demonstrate a practical approach to extending existing fiber-based QKD platforms to atmospheric links using an MMF-coupled receiver and a comparatively simple tracking architecture.
Abstract We report on the adaptation of a commercially available fiber-based quantum key distribution (QKD) system with phase-time encoding to free-space operation. The proposed approach minimizes architectural modifications by replacing the single-mode receiving interferometer with a free-space multi-mode delay interferometer, enabling efficient detection of spatially distorted quantum states typical for atmospheric channels. Several interferometer configurations are experimentally investigated, and a multi-mode Michelson interferometer with polarization routing is identified as the optimal solution in terms of interference visibility, loss, and alignment robustness. The impact of chromatic and modal dispersion on phase-time interference is analyzed and shown to be manageable under realistic system parameters. A laboratory QKD experiment demonstrates stable operation with a quantum bit error rate of about 4.5% and a secret key rate of hundreds of bits per second, confirming the feasibility of extending fiber-based QKD platforms to free-space links.
Abstract We demonstrate a free-space quantum key distribution (QKD) system based on phase-time encoding by reconfiguring a commercially available fiber-based QKD platform. The main goal of this work is to develop a free-space QKD system with minimal modifications, primarily through the implementation of a free-space multi-mode (MM) delay interferometer at the receiver. The use of a MM interferometer is crucial for enabling phase encoding over atmospheric channels, where direct coupling into single-mode fibers suffers from prohibitive losses due to atmospheric turbulence and beam distortions. We review existing methods for detecting spatially distorted quantum states, propose technological improvements, and experimentally evaluate several interferometer designs. A free-space MM Michelson interferometer with polarization routing is identified as the optimal solution, offering high interference visibility, low loss, and ease of integration into the system. A laboratory QKD experiment conducted with the modified system achieved a quantum bit error rate of 4.5% and a stable secret key rate of hundreds bit s−1, demonstrating the feasibility of secure key distribution over free-space links using adapted fiber-based technology. Detailed theoretical and experimental studies of chromatic and modal dispersion effects confirm that their impact can be effectively managed. This approach provides a practical and scalable route for extending quantum-secure communications into scenarios where optical fiber deployment is impractical or impossible.
Abstract We demonstrate the experimental feasibility of a commercially available fiber-based phase-encoding quantum key distribution system ViPNet QTS, operating over a communication link incorporating a multi-mode fiber (MMF) segment of up to 20 km. Since ViPNet QTS is designed for deployment in single-mode fiber (SMF) communication links, all experiments were conducted in an SMF-MMF-SMF configuration. We report systematic measurements of transmission losses, as well as secret key generation performance for different MMF link lengths. As a result, stable secret key generation was achieved at a rate of 1.75 kbit min −1 over MMF links of up to 20 km.
Quantum key distribution (QKD) enables information-theoretically secure communication by exploiting the quantum properties of single photons. Real-world deployment of QKD, however, requires reliable operation under uncontrolled environmental conditions where background illumination, atmospheric loss, coupling fluctuations, and polarization drift can significantly degrade performance. In this work, we experimentally demonstrate entanglement-based free-space QKD over an atmospheric channel with a passive, low-bandwidth polarization-correction strategy that avoids high-bandwidth real-time polarization tracking during the QKD acquisition. Using a 50-m free-space optical link implementing the BBM92 protocol, we compensate polarization scrambling arising from fiber birefringence and reference-frame misalignment by optimizing Bob’s measurement bases through quantum state tomography. QKD performance is evaluated across three 24-h acquisition campaigns spanning daylight and nighttime conditions, where high background noise, atmospheric fluctuations, and polarization drift pose significant challenges for secure key generation. By combining spectral, temporal, and spatial filtering with optimized coincidence-window selection, threshold-compatible average QKD performance is maintained across these acquisition campaigns. The optimized daylight key rate and QBER are ( 6.39 ± 0.81 k H z , 8.05 ± 1.48 % ) , while nighttime operation yields ( 6.98 ± 0.81 k H z , 8.25 ± 0.85 % ) . Under representative operating conditions with 8.3 % QBER, we obtain a final secure key rate of 351 H z after reconciliation and privacy amplification. To assess loss tolerance, we further emulate attenuation equivalent to a 1 k m free-space link using controlled attenuation, demonstrating approximately 6 k H z raw key rate under 30 % additional link loss. This attenuation study reproduces the additional loss budget only and does not constitute a physical kilometer-scale free-space demonstration. These results demonstrate that tomography-assisted measurement-basis correction can enable stable free-space entanglement-based QKD without high-bandwidth polarization tracking during key acquisition, providing a basis for future studies of more dynamic terrestrial and satellite-relevant quantum communication links.
We investigate the feasibility of satellite-to-ground quantum key distribution (QKD) at multi-GHz clock rates, where secure key generation time is constrained, due to low-Earth orbit (LEO) satellite overpasses ( ≈ 300 s). Higher repetition rates offer a direct route to increased secure key rate (SKR), though system performance is fundamentally limited by detector timing jitter, optical coupling losses, and atmospheric turbulence, depending on receiver architecture. We combine finite-key modeling, detailed detector timing characterization, adaptive optics (AO) modeling, and real-time free-space QKD experiments at 1 GHz to evaluate practical receiver configurations based on multi-mode fiber-coupled avalanche photodiodes (MMF-APDs) and single-mode fiber-coupled superconducting nanowire single-photon detectors (SMF-SNSPDs), at projected rates >1 GHz. The results indicate clear operating regimes across a range of atmospheric conditions: MMF-APDs maximize secure key rates below ∼ 1 GHz, mainly due to their low coupling loss. However, low-jitter SMF-SNSPDs are required to obtain positive key rates at higher clock rates—with modeling and measured timing responses of our hardware indicating an estimated optimal operating point near ∼ 7.2 GHz—though AO correction is required for effective single-mode coupling under turbulence. The experimental and projected results provide quantitative performance bounds and realistic design targets for future high-rate satellite QKD systems, while highlighting the technological requirements needed to achieve end-to-end operation beyond the GHz regime.
Intermodal quantum key distribution at telecom wavelengths provides a hybrid interface between fiber connections and free-space links, both essential for the realization of scalable and interoperable quantum networks. Although demonstrated over short-range free-space links, long-distance implementations of intermodal quantum key distribution remain challenging, due to turbulence-induced wavefront aberrations which limit efficient single-mode fiber coupling at the optical receiver. Here, we demonstrate a real-time intermodal quantum key distribution field trial over an 18 km free-space link, connecting a remote terminal to an urban optical ground station equipped with a 40 cm-class telescope. An adaptive optics system, implementing direct wavefront sensing and high-order aberration correction, enables efficient single-mode fiber coupling and allows secure key generation of 200 bit/s using a compact state analyzer equipped with room-temperature detectors. We further validate through experimental data a turbulence-based model for predicting fiber coupling efficiency, providing practical design guidelines for future intermodal quantum networks.