FEATURE NETWORK SECURITY
latter, but is less secure, he adds, although for attacks that are likely in the real world, both protocols are very safe.
Going the distance In addition to QKD protocol complexity, a significant implementation challenge comes when sending the very weak QKD signals down optical fibres, which can absorb the individual photons. ‘Te photons initially prepared and sent in the channel are attenuated during propagation and only a small proportion reaches the receiving side,’ Lucamarini said. ‘For a standard singlemode fibre in the third telecom window, the attenuation coefficient is 0.2dB/km, meaning only 50 per cent of the initial photons survive aſter 15km, only 25 per cent aſter 30km, and so on.’ Consequently, oſten only single-photon
detectors are sensitive enough for such applications, and even they have limitations, Takesue comments. ‘When the number of received photons is overwhelmed by the detector noise, we cannot send the key anymore,’ he said. ‘Tus the transmission fibre loss limits key distribution distance, typically to 100km.’ Increasing the key distribution distance is important to increase the market QKD can serve’, he adds. China is attempting just this with a 2,000km-long quantum backbone between Beijing and Shanghai, due to be operational by the end of 2016. ‘On the other hand,’ adds Lucamarini, ‘I
personally believe that QKD became more appealing to investors when it was demonstrated to co-exist with 10G and 100G metropolitan networks over distances of about 100km.’ Tat’s comparable to the diameter of the world’s busiest cities, he emphasises. ‘If it is true that of the 7 billion people in the world, 4 billion live in cities, then QKD is already suitable to serve more than 50 per cent of the global population.’ Te experiment to which he refers was published in 2012, when Toshiba Research Europe demonstrated that quantum key distribution is feasible over metropolitan area networks carrying live data traffic. It used wavelength multiplexing to
Figure 2: A quantum backbone network
ID Quantique systems are currently being used to set up a quantum backbone between Martlesham (Ipswich) and Cambridge as part of a coast-to-coast network that Toshiba is also involved with
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Quantum information encoded on a single photon cannot be split … meaning stolen data completely disappears
combine QKD with classical signals on the same fibre. In 2014, Toshiba achieved stable communication of quantum keys for 34 days over standard optical fibre. Te current working solution for longer-
Toshiba’s QKD modules, one for the sender Alice, and one for the recipient, Bob
distance QKD is based on “trusted nodes”, the Toshiba scientist adds. ‘Te overall distance between two endpoints in a network is split into a number of sub-links, each connecting two nodes,’ Lucamarini says. ‘If the endpoints of each sub-link are owned by a trusted authority, the overall communication is guaranteed to be secure. Sometimes you don’t want to have “trusted authorities” around. In this case, if the network is large enough, it is possible to split the initial secret into several parts and then send each part through a different route in the network. Only the endpoints – Alice and Bob – will receive all the parts necessary to reconstruct the initial secret, whereas the intermediate nodes will have only access to a meaningless part of it. Tis way, the trust in third parties can be partially removed.’ At each trusted node, one key is received and a
22 FIBRE SYSTEMS Issue 14 • Winter 2017
new one transmitted, Huttner explains. ‘Ten you do some processing to make an overall key that works from one side to the other,’ he says. ‘It’s called a trusted node because the node knows everything. Tat could be one reason why China is keen on QKD – the government is responsible for the nodes, and has access to all the secret information.’ Te Chinese project also has short distances between nodes to increase transmission rates, notes Huttner. ‘But of course you need more links, which means you need more systems, and that is more expensive.’ Yet the 100km range of ID Quantique’s
Cerberis QKD system has been enough for commercial deployments. For example, it has enabled Geneva-based companies and financial institutions to securely transfer information to a remote disaster recovery data centre 70km away. Te Geneva government has also used Cerberis in every federal and cantonal election and local internet referendum since 2007. Tis application also exploits the company’s true random number generator, Quantis, which, like QuintessenceLabs’ qStream, exploits quantum fluctuations, but using a slightly different optical setup. Quantis produces unique personal identification numbers for each citizen, and then generates encryption keys for each internet voting session. However, ID Quantique’s collaborators at the
University of Geneva have achieved QKD over 300km experimentally. In collaboration with the UK Quantum Communications Hub, a government-funded industry-academic partnership for quantum technologies research, the company is also building a quantum backbone
UK National Quantum Technologies Programme
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