25 Oct 2025 – Quantum cryptography took a big step forward with a new distance record for secure communication. A European research team demonstrated quantum key distribution (QKD) over 120 km of optical fiber while sharing that fiber with normal data traffic. This is the longest distance to date for “co-propagating” QKD – where encrypted quantum signals travel alongside standard internet data in the same cable.
In the experiment, reported in Physical Review Letters on October 23, the group used a continuous-variable QKD (CV-QKD) system with highly optimized hardware. They sent faint entangled light pulses (coherent states) through 120 km of ultra-low-loss fiber, which was simultaneously carrying ordinary high-speed data. Thanks to clever engineering – including a “local oscillator” design that naturally filtered out noise, and adjustments to suppress phase noise – the quantum keys could be exchanged successfully at record distance. In fact, they generated secure keys over 100 km of fiber even under strict, realistic conditions (finite-size key analysis) and with the fiber fully loaded with classical network traffic.
Crucially, the presence of the regular data signals did not significantly increase noise or prevent the quantum exchange. Past QKD demonstrations usually required dedicated dark fibers or showed much shorter range when multiplexed with classical signals (tens of kilometers at most). Breaking the 100+ km barrier in a real network environment is a major proof-of-concept that quantum cryptography can integrate into existing telecom infrastructure.
The researchers achieved all this without needing special wavelength channels or extra filtering tricks – the system’s design inherently handled the cross-talk. As they noted, their entangled-light scheme is essentially “plug-and-play” with standard fiber networks. The secure key rates were low but usable, and importantly, the quantum signal survived 20 dB of loss (equivalent to 120 km) which is well beyond typical previous limits.
This result brings quantum-secure communication closer to practical deployment. It shows that one can send quantum encryption keys over long distances on existing network backbones, protecting data from future quantum decryption threats (the classic “harvest now, decrypt later” problem). With international efforts underway to build quantum-safe networks, demonstrations like this one build confidence that QKD can scale up. Next steps will include improving key rates and extending to even longer fibers or satellite links. But for now, 120 km with co-propagating traffic is a landmark – it suggests that telecom companies could potentially upgrade to quantum-safe links without overhauling their whole infrastructure. As the research team put it, quantum keys can be distributed “with minimal disruption” to normal communications, a big win for the practicality of quantum cryptography.
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