A collaborative research team representing Linköping University, KTH Royal Institute of Technology, Stockholm University, and Chalmers University of Technology has reported the physical deployment of a long-haul, trusted-node Quantum Key Distribution (QKD) link integrated into a dynamically reconfigurable telecommunications network. Detailed in an open-access preprint on arXiv, the field trial established secure quantum channels over a total distance of 303 km in southeastern Sweden. The network topology connects a university lab in Linköping with a national quantum hub in Stockholm through an intermediate trusted node, combining standard long-haul single-mode fiber (SMF) with a multi-core fiber (MCF) access segment to emulate heterogeneous enterprise infrastructures.
Hardware Customization and Space-Division Multiplexed Routing
The experimental architecture bridges a 270 km deployed dark fiber link rented from GlobalConnect with a 33 km spooled seven-core multi-core fiber access link. To circumvent the high transmission losses across the two main spans—measured at 23 dB for the 110 km Linköping-to-Nyköping sub-link and 36 dB for the 160 km Nyköping-to-Stockholm segment—the researchers modified commercial ThinkQuantum (QuKy EDU Pro) systems. The receivers were retrofitted to interface with external Superconducting Nanowire Single-Photon Detectors (SNSPDs), replacing the standard internal gated-mode Indium Gallium Arsenide (InGaAs) avalanche photodiodes. The SNSPDs provided detection efficiencies up to 93% and ultra-low dark count rates down to ≤ 1 count per second, yielding an immediate baseline upgrade in the Secret Key Rate (SKR) from 0.16±0.02 kbit/s up to 4.75±0.71 kbit/s over the initial 110 km section.
Within the space-division-multiplexed Linköping access link, quantum channels were actively routed through the fiber using a multi-port Polatis optical fiber switch. The system maintained positive key rates while dynamically switching the QKD channel between two designated low-loss cores, forcing the automated polarization controllers to autonomously re-align the polarization states mid-session within tens of seconds. This quantum traffic coexisted alongside a active classical 10 Gbps Ethernet data channel operating at 1546.12 nm with a 0 dBm launch power, supplemented by continuous broadband optical noise injected via a 1550 nm light-emitting diode (LED) to simulate cross-talk contamination from parallel telecommunication services.
Key Management Buffering and Cryptographic Payload Optimization
Operating continuously over 92 hours, the physical layer fed raw key blocks into integrated Key Management Systems (KMSs) configured to automatically execute a trusted-node key relay protocol. Because the Linköping-Nyköping span maintained a higher average key generation throughput than the high-loss Nyköping-Stockholm span, local KMS storage buffers absorbed the rate discrepancies. This buffering capability prevented key starvation on the end-to-end virtual link during localized hardware pauses, such as the periodic 24-hour helium condensation cycle native to the sorption-cooled SNSPDs at the Linköping node.
To validate the real-world utility of the fluctuating key rates, the generated keys were applied to One-Time Pad (OTP) information-theoretically secure image transmission over bounded 100-second windows. The researchers compared the performance of classical wavelet-based JPEG 2000 compression against the deep-learning-based JPEG AI codec across a 2,100-image subset of the NUS-WIDE database. The testing demonstrated that under highly constrained secret-key budgets, the neural-network-driven JPEG AI codec minimized the required bit allocation per payload, preserving higher perceptual similarity metrics (LPIPS) and Peak Signal-to-Noise Ratios (PSNR) than traditional transform-based approaches when operating under severe network noise injection up to 3.4 µW.
The complete technical manuscript describing the hardware configurations, fiber cross-talk modeling, and image encryption parameters can be accessed via the open-access arXiv repository here.
June 8, 2026
