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.
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.
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.
This paper evaluates the feasibility of off-the-shelf quantum key distribution (QKD) systems over multimode fiber (MMF) links. QKD systems with two types of encoding, polarization-based and phase/time-bin-based, are tested using OM1, OM3, and OM4 fibers, with single mode fiber (SMF) as a reference. The connection between single mode QKD components and MMF forms an interferometric single modemulti mode-single mode (SMS) structure that exhibits intensity fluctuations. These, together with modal dispersion enabled by mode coupling, lead to decoherence in phase-encoded systems and significant depolarization in polarization-encoded systems. To minimize mode excitation, the fibers are operated in an underfilled launch regime, enabling near-single mode transmission over short spans.
Multimode fiber (MMF) telecommunication networks are ubiquitous in metropolitan-scale settings. Since many existing links are now ‘dark’ in favour of newer single-mode fiber deployment, they present an attractive opportunity for repurposing for emerging quantum technologies, including quantum key distribution (QKD). However, the topic of QKD in MMF is rather under-explored, with the only prior work in this field achieving a very short range of 550 m. In particular, well-known signal distortion effects in MMF can be algorithmically corrected in classical communication schemes, however it is not yet clear how to deal with them in QKD protocols. In this work, we critically evaluate the prospects of MMF for high-speed metro-scale QKD, including experiment consideration of design factors affecting the performance and the introduction of a new mode-matching approach with photonic lanterns. We also report a proof-of-principle realisation of a decoy-state BB84 QKD protocol with phase encoding over 17 km of MMF fibre, as well as a key generation rate over 1 Mbit/s at a distance of 1 km. This work confirms the suitability of current QKD technology for use in existing MMF links, unlocking new opportunities for quantum applications using legacy fibre and inspiring future research directions of quantum information processing in MMF more generally.
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 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.
Quantum key distribution (QKD) is increasingly considered for deployment in realistic communication networks, where long distances, heterogeneous fiber infrastructure, and coexistence with classical traffic present substantial challenges. Here, we demonstrate trusted-node QKD between Linköping University and the Stockholm hub of the Swedish national quantum communication infrastructure over 270 km of deployed single-mode fiber, extended by a 33 km multi-core fiber (MCF) segment emulating a metropolitan access link, for a total distance of 303 km. The two sub-links use commercial QKD systems whose receivers are interfaced with external superconducting nanowire single-photon detectors, enabling operation at losses beyond those supported by standard internal gated-mode detectors. We operate the link while actively switching the QKD channel between two MCF cores, with co-propagating Ethernet traffic and injected broadband optical noise in the other cores. The results demonstrate the integration of commercial QKD into demanding, dynamically reconfigurable fiber infrastructure relevant to future hybrid quantum-classical networks. Finally, using the generated secret keys, we illustrate how limited and time-varying QKD throughput affects one-time-pad-protected image transmission: image fidelity depends strongly on the available QKD-generated key budget and the choice of compression algorithm, highlighting application-level challenges for QKD-based encryption in realistic scenarios.