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npj Quantum Information

Cambridge quantum network

J. F. Dynes, A. Wonfor, W. W. -S. Tam, A. W. Sharpe, Ririka Takahashi, Marco Lucamarini, Andrew Plews, Zhiliang L. Yuan, A. R. Dixon, J. Cho, Yoshimichi Tanizawa, J.-P. Elbers, H. Greißer, Ian H. White, Richard V. Penty, Andrew J. Shields

Published Nov 21, 2019• 272 citations
Publisher

Abstract

Abstract Future-proofing current fibre networks with quantum key distribution (QKD) is an attractive approach to combat the ever growing breaches of data theft. To succeed, this approach must offer broadband transport of quantum keys, efficient quantum key delivery and seamless user interaction, all within the existing fibre network. However, quantum networks to date either require dark fibres and/or offer bit rates inadequate for serving a large number of users. Here we report a city wide high-speed metropolitan QKD network—the Cambridge quantum network—operating on fibres already populated with high-bandwidth data traffic. We implement a robust key delivery layer to demonstrate essential network operation, as well as enabling encryption of 100 Gigabit per second (Gbps) simultaneous data traffic with rapidly refreshed quantum keys. Network resilience against link disruption is supported by high-QKD link rates and network link redundancy. We reveal that such a metropolitan network can support tens of thousands of users with key rates in excess of 1 kilobit per second (kbps) per user. Our result hence demonstrates a clear path for implementing quantum security in metropolitan fibre networks.

DOI
10.1038/s41534-019-0221-4

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Abstract Future-proofing current fibre networks with quantum key distribution (QKD) is an attractive approach to combat the ever growing breaches of data theft. To succeed, this approach must offer broadband transport of quantum keys, efficient quantum key delivery and seamless user interaction, all within the existing fibre network. However, quantum networks to date either require dark fibres and/or offer bit rates inadequate for serving a large number of users. Here we report a city wide high-speed metropolitan QKD network—the Cambridge quantum network—operating on fibres already populated with high-bandwidth data traffic. We implement a robust key delivery layer to demonstrate essential network operation, as well as enabling encryption of 100 Gigabit per second (Gbps) simultaneous data traffic with rapidly refreshed quantum keys. Network resilience against link disruption is supported by high-QKD link rates and network link redundancy. We reveal that such a metropolitan network can support tens of thousands of users with key rates in excess of 1 kilobit per second (kbps) per user. Our result hence demonstrates a clear path for implementing quantum security in metropolitan fibre networks.
Quantum key distribution (QKD) allows authenticated communicating entities to generate and distribute random keys with security guaranteed by the principles of quantum mechanics. Usually, the deployment of optical fibre-based QKD links requires a dark fibre for the quantum channel of the QKD link. However, the availability of dark fibre is scarce for field deployments and involves higher capital expenses. In this paper, we propose a QKD solution where the quantum channel and classical channel of the QKD link are multiplexed on the same fibre carrying a data traffic of 100 Gbps using wavelength division multiplexing (WDM). The solution will facilitate rapid upgradation of existing communication networks with quantum security, especially in metropolitan networks.
Abstract Quantum communication has seen rapid progress towards practical large-scale networks, with quantum key distribution (QKD) spearheading this development. While fibre-based systems have been shown to be well suited for metropolitan scales, suitable fibre infrastructure may not always be in place. Here, we make the case for an entanglement-based free-space quantum network as a practical and efficient alternative for metropolitan applications. We developed a deployable free-space QKD system and demonstrated its use in realistic scenarios. For a representative 1.7-km free-space link, we showcase its ad hoc deployability and achieve secure key rates of up to 5.7 kbps, with 2.5 kbps in direct noon sunlight. By extrapolating experimental data, we show that kbps key rates are achievable even for 10-km distances and multi-user scenarios. We anticipate that our work will establish free-space networks as a viable solution for metropolitan applications and an indispensable complementary building block in the future global quantum internet.
We describe recent progress in quantum secured optical networks in the UK. The Cambridge Quantum Network has been operating for several years with 3 nodes separated by between 5-10 km of installed fibre. All links are secured by QKD systems operating with secure key rates in excess of 1 Mb/s, the highest recorded long term key rates in a deployed network. The network operates in the presence of 100Gb/s classical traffic with no significant reduction of secure key generation rate. In addition, the Bristol Quantum Network has four nodes 1-3km apart connected in a mesh protected by two pairs of QKD systems. The network is designed to be very dynamic, switching both QKD and WDM classical traffic to enable rapid reconfiguration and is used as a testbed for QKD protected dynamic applications. The two metropolitan networks are being connected by a 410 km QKD link, with 4 spans, the longest of which operates over 129km of fibre with an attenuation of 28dB achieving secure key rates of 2.7kb/s, the longest and highest loss QKD field trial to date. A 120km extension of the UK quantum network from Cambridge to BT Labs, Adastral Park operates with fully commercially available components and is an important testbed comprising 3 intermediate nodes and operates with 5 x 100Gb/s classical channels. This helps determine how to proceed with a large-scale commercial deployment of QKD.
Quantum Key Distribution (QKD) is attracting much interest for the distribution of cryptographic keys using single photon signals. Currently QKD is often used to provide secure distribution of cryptographic keys for the encryption of data transmitted using conventional classical communication systems. This paper reports major field trials carried out over several months on the Cambridge UK Quantum Network showing the operation of QKD systems alongside high-speed classical transmission systems encrypted QKD derived with AES keys. Quantum Key transmission at record secure key rates of 3.3Mbps, 3.2Mbps and 2.5Mbps has been achieved over 5km, 9.5km and 10.5km long links respectively with corresponding average Quantum bit error rates (QBER) of 2.9%, 2.4% and 3%. Using a 33km link attached to the network with a loss of 7.5 dB, a secure key rate of 1.4 Mbps is achieved with an average QBER of 3.4%. Under loop back conditions this link provides a 66 km transmission path with a 16dB fibre loss, enabling a field trial using the QKD signals multiplexed with two wavelengths each transmitting 100Gb/s classical data to be carried out. This achieves an average secure key rate of 80.2 kbps and a mean QBER of 6.6%, in line with theoretical predictions. During the trial duration, the statistics of the QBER were found to be Gaussian distributed with a standard deviation of 0.5. The results of the field trial suggest that the system works stably and has considerable potential for applications in metropolitan networks. Further measurements will be reported at the conference.
Quantum key distribution (QKD) provides an attractive means for securing communications in optical fibre networks. However, deployment of the technology has been hampered by the frequent need for dedicated dark fibres to segregate the very weak quantum signals from conventional traffic. Up until now the coexistence of QKD with data has been limited to bandwidths that are orders of magnitude below those commonly employed in fibre optic communication networks. Using an optimised wavelength divisional multiplexing scheme, we transport QKD and the prevalent 100 Gb/s data format in the forward direction over the same fibre for the first time. We show a full quantum encryption system operating with a bandwidth of 200 Gb/s over a 100 km fibre. Exploring the ultimate limits of the technology by experimental measurements of the Raman noise, we demonstrate it is feasible to combine QKD with 10 Tb/s of data over a 50 km link. These results suggest it will be possible to integrate QKD and other quantum photonic technologies into high bandwidth data communication infrastructures, thereby allowing their widespread deployment.
Quantum key distribution (QKD) offers unconditional information security by allowing two distant users to establish a common encryption key resilient to hacking. Resultingly, QKD networks interconnecting critical infrastructure and enabling the secure exchange of classified information, can provide a solution to the increasing number of successful cyberattacks. To efficiently deploy quantum networks, the technology must be integrated over existing communication infrastructure, such as optical fibre links. Yet, QKD poses stringent requirements on the conditions of the network over which it is deployed. This work demonstrates the first quantum communication network in Cyprus via the deployment of a multi-node quantum network, exploiting existing commercial underground optical fibre. The network employs bidirectional occupation of fibres and wavelength multiplexing in a ring architecture to achieve, with minimal use of dark fibres, high-rate QKD. Results obtained reveal consistent key generation rates across all nodes, confirming reliable operation in a real-world environment. This deployment highlights the feasibility of leveraging existing telecom infrastructure for quantum-secured communication, marking a significant step toward scalable and cost-effective quantum networks suited for critical applications.