Securing Critical Infrastructure using Trackside Fibre

  • Toshiba QKD-MU system deployed over a 46 km fibre pair, running 5m from busy railway
  • Robust quantum key generation enabled through advanced active stabilization technology
  • Performance monitoring dashboard built by users using SNMPv3

A trackside fibre communication link in the Czech Republic has recently been upgraded to a quantum-secure connection using Toshiba’s multiplexed QKD technology. Despite heavy railway traffic in close proximity, the built-in stabilisation technologies in Toshiba’s QKD systems ensured robust, long-term performance, suitable for securing critical transport infrastructure communications.

Toshiba have developed a number of quantum-secure networks across Europe, working with various partners as a member of the European OpenQKD project [1]. This has focused on the integration of QKD to particularly challenging real-world fibre networks to show the seamless integration of QKD with practical Use Cases. In a recent collaboration with the Czech Technical University in Prague, CyberSecurity Hub z.ú., and the Czech National Railway Administration (Správa železnic), we showcased the potential of trackside fibre for quantum-safe communication networks.

Trackside fibre refers to the fibre optic cables that run alongside railways lines. This includes vast networks across Europe (in the UK, for example, trackside fibre spans over 17,000 km[2]), where the trackside fibre is used for both critical transport infrastructure-related and private sector communications.

While optical fibre technology may initially appear unrelated to railways, the two types of infrastructure share many similarities – they both aim to connect high-density population areas, ideally in a linear route that requires minimal construction work to surrounding areas. Such fibre infrastructure could thus be very valuable for use in quantum communications networks. Additionally, with the with the rapid advancement of quantum computing meaning that current cryptographic methods may soon be obsolete, there is a pressing need ensure that national communications infrastructure can be made quantum-safe.

Prior to this use case, it was an open question, however, whether the harsh vibrations and noise induced by railways affects QKD signals on co-located fibre. The use of single-photon and phase-based encoding typically makes quantum systems much more sensitive to the environment than in classical communications.

Our results demonstrate that robust QKD on trackside fibre is both possible and a promising opportunity.

Deployment Details

The Czech National Railway Administration provided a 46-km length of trackside fibre, running within five metres of the main railway line from Prague Smichov Station to Beroun. The fibre experienced significant vibrations and mechanical noise due to high railway traffic, with a passenger or freight train typically passing every five minutes during the day.

Toshiba contributed a QKD-MU system, which operates with a 1310 nm quantum channel and enables high-density multiplexing of user traffic over the same fibre pair. The system was deployed and provisioned by network engineers from the Czech Technical University in Prague, taking advantage of the simple out-the-box installation routine for the system. They first set up the system in their laboratory to obtain baseline performance data, then moved it to the railway sites.

The engineers from the Czech Technical University users were also able to develop a real-time dashboard for monitoring the QKD deployment using SNMP, (as the Toshiba equipment supports SNMP v2 and v3.). The retrieved secure bit rate and quantum bit error rate data was plotted in an interactive web-based user interface, enabling quick inspection of link performance over time. Throughout the entire railway deployment, lasting more than two weeks, QKD performance was excellent. The system generated quantum keys at a secure bit rate of 112 kbps over a distance of 46 km (quantum channel loss > 15 dB), with a quantum bit error rate (QBER) of 3.90% ± 0.41%. Technically, the standard deviation of the QBER on trackside fibre of 0.41% was marginally higher than the 0.17% value which was measured in the laboratory, likely related to railway-induced noise. This trackside QKD performance is still excellent, however, and the key generation rate of 112 kbps over 46 km is market leading. Such QKD keys can then be used for regular rekeying of encryptors for bringing quantum-safety to railway communications.

Map of railways in the Czech Republic, with QKD nodes shown

Stabilisation Features of Toshiba QKD

The sensitive nature of phase-encoded light and quantum signals is well-known, so how do Toshiba QKD devices ensure stable operation even in challenging environments? Scientifically, mechanical vibrations and thermal fluctuations result in perturbative strains on the fibre, affecting various properties of propagating light such as polarization, phase and timing delays. This can increase error rates in QKD, where high-performance systems like Toshiba QKD-MU rely on precision phase-encoding of light with picosecond timing accuracy and our proprietary high-speed detectors which require a fixed polarization state. 

Toshiba QKD systems solve this problem using active stabilization technology, ensuring long-term optimal operation without any user intervention. When the systems first start-up, they perform a complex ‘alignment’ algorithm to establish the baseline of the communication channel between the two nodes and they self-calibrate themselves. Then, during normal operation, dedicated stabilization pulses are embedded in the encoding pattern, generated by one node and measured by the other. These references enable continual real-time feedback algorithms to automatically correct the polarisation, timing and phase of light, minimising the error rate.

Active stabilisation technology in Toshiba QKD systems.

This use case has demonstrated that QKD can be successfully deployed along trackside fibre, with high performance and robust long-term operation, despite railway-related disturbances. The high-rate-low-noise performance was enabled by active stabilization technology within Toshiba’s QKD system. This is a promising indicator of how QKD can bring quantum-security to critical infrastructure communications, while also highlighting the opportunities for using trackside fibre in future quantum communication networks.



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