Building Europe’s largest and most complex quantum network with scalable QKD technology and software-defined networking

• Multiple Toshiba QKD systems have been deployed as part of the Madrid “MadQCI” network, delivering the highest key rate of any system on Europe’s largest and most complex quantum network
• The metro-scale network enables quantum-safe communications between nine production sites in real, active telecom infrastructure
• Use of a modern software-defined networking (SDN) paradigm makes the network vendor-agnostic, flexible and scalable

The sheer power of quantum computing opens up a world of possibilities, especially when compared to the abilities of classical computing, but it comes with great risks. Current methods used for securing communications stand to be rendered completely obsolete once quantum computers become more widely available in the coming years. As a result, there is now a clear need to start developing communications networks that can withstand this before the tipping point comes.

Savvy organisations are already doing so. In cooperation with BT, Toshiba launched the Quantum Secure Metro Network (QSMN) in London in 2022, using Quantum Key Distribution technology (QKD) to protect communications between sites in Canary Wharf and datacentres in Slough. HSBC went on to test commercial use cases in 2023.

QKD is resistant to quantum attacks because it relies on the physical properties of light. It involves encoding individual photons with quantum information that then forms the basis of an encryption key. No amount of computing power can crack this key, as merely observing a single photon changes its state, therefore alerting the system and causing it to discard the key that’s being transferred.

However, the same physical properties of light that make QKD so secure also present challenges when trying to scale the technology. The complexity of real-life networks make maintaining effective key rates without disrupting existing traffic challenging.

As a leader in QKD technology, Toshiba joined the MadQCI project to explore how QKD networks can be scaled using real telecom infrastructure. Led by the Universidad Politécnica de Madrid as part of the EU-funded OpenQKD project, MadQCI brought Toshiba together with several other partners in tests to deploy QKD devices to commercial multi-tenant telecom infrastructure (see Figure 1), and embrace modern management solutions to explore real use cases in an extended network.

In particular, the project tested two key elements. Firstly, whether QKD technology can operate effectively with management solutions orchestrating traffic across busy, commercial networks, such as Software-Defined Networking (SDN). And secondly, whether multiple QKD services could co-exist on the same network without disrupting existing traffic, while at the same time maintaining effective key rates for practical use cases based on potential commercial applications. Validating both of these elements is vital if QKD is to be scaled to widespread, commercial use on today’s networks.

Combining QKD and SDN

Software-defined networking (SDN) is an emerging next-generation technique based on centralised, programmable network management. It has been growing in popularity because the ability to increasingly control networks through software rather than hardware streamlines operations and provides additional capabilities to enhance network operations.

Previous tests have validated that QKD and SDN can be used together. However, the MadQCI work demonstrated this concept at scale on a real-life, commercial network. It also embraced the recently published ETSI QKD 015 standard with a network architecture comprising an SDN “agent” in each node and a centralised SDN “controller” (see Figure 2). QKD-related operational information is extracted from QKD hardware at each node using software-based solutions – for example, Toshiba’s devices support industry-standard SNMP protocol as well as offering a CLI and GUI for management and monitoring.

The project found that Toshiba’s commercial QKD systems are fully compatible with SDN in large-scale networks and could be relatively easily deployed while meeting current telecommunications data centre best practices. This opens up the avenue for QKD to be deployed in more networks using SDN.

Addressing the scalability challenge

Another aim of the project was to test the deployment and integration of QKD systems and key-consuming applications from multiple vendors into active, commercial telecom networks to assess the complexity and impact on performance.

The MadQCI network leveraged existing fibres from two network providers (Telefonica and RediMadrid) carrying live traffic. It was essential to install QKD without degrading service for the existing users, which can prove challenging because quantum signals need to co-propagate on the same fibre as classical traffic, resulting in “noise” that affects transmissions. Toshiba provided four QKD-MU systems, which are ideal for integrating into lit metro networks  as they use an “O-band” quantum channel and include precision filtering to reduce noise.

Other vendors, including ID Quantique, Huawei and the Austrian Institute of Technology (AIT), also provided QKD systems that were tested on the network. Despite using different QKD protocols and different local management interfaces, the SDN approach in MadQCI enabled simple orchestration of the whole network, demonstrating how modern networking techniques can help to develop truly commercial networks without vendor lock-in .

In the published paper, the network operators also compared the recorded performance data from various deployed devices, with Toshiba’s QKD systems shown to perform very reliably, delivering a secure bit rate performance up to an order of magnitude better than other vendors’ QKD systems.

Reliable high-performance QKD technology is essential when building commercial quantum networks so that users have flexibility and are future proofed. Networks must enable diverse use cases and be set-up to serve multiple users as technology further develops. The MadQCI project concluded by demonstrating various use cases which highlight the benefits of QKD, , including:

• Protection of critical infrastructure by securing industrial data traffic (SCADA)
• Securing 5G communications with low latency
• QKD as a cloud service with up to 1000 requests per second
• Ordered proof of transit protocol for network security and attestation

MadQCI represents a milestone in quantum-safe networking as a flexible, heterogenous, vendor-agnostic production network with a simple, unified management approach. This validates the potential of QKD for bringing quantum-secure communications to diverse, real use cases and offers a blueprint and vision for scaling quantum networks.

Please see the npj Quantum Information journal paper for full details on the project.