Quantum Cryptography Can Mix With Data Traffic, Say Cambridge Boffins

A group of researchers claims to have reached a key milestone in quantum cryptography, overcoming major barriers by showing how it can work over standard fibre networks sharing networks with conventional data traffic.

Quantum cryptography encodes each bit of a digital key upon a single photon – a particle of light – but the weakness of these signals has been a limitation, and has meant that quantum crypto systems would need to have their own dedicated fibre links, with no other traffic on them – so called “dark fibre”. The Cambridge Research Laboratory of Toshiba Research Europe has shown how quantum cryptography can share standard fibre connections – with the weak quantum signal picked out from the data traffic using very clever detection methods.

Quantum cryptography is a physics-led approach which has major benefits over mathematical cryptography, through the phenomenon of “wave function collapse”. That key part of quantum theory, in layman’s terms, means that just observing an experiment can affect what is happening. The “wave function” describes the probability of finding a photon in a given state – but all states exist at the same time until the wave function is observed and collapses to a single polarised state.

How does quantum cryptography work?

In the security world, that means that if a hacker attempts to intercept data over a network protected by quantum-cryptography, that observation collapses the wave function and the photon loses any information – i.e. its part of the cryptographic key – it was carrying. This means a hacker’s presence can be detected.

To add the cryptographic key to photons in the first place, each kind of polarisation represents a bit of data – either the 1 or 0 of a binary code, depending if it is vertically or horizontally polarised. A polarised photon, has its waves lined up along one direction – and can only be detected by a filter with that same polarisation.

There have been a number of barriers preventing widespread adoption of quantum cryptography, however. Firstly, it required “dark fibre” – bespoke optical fibre separate from the fibre used in normal broadband networks.

But the physical length of a network has been a massive issue too, as light signals degrade over long distances, as there is more chance they will interact with other photons. Furthermore, ordinary data signals are far more intense than single photon signals and swamp the quantum crypto signal. In normal signals, one bit of data is carried by over 1 million photons.

Some have doubted quantum cryptography will ever appear in the enterprise, whilst others believe current cryptographic methods are doing an adequate job.

But the Toshiba research group in Cambridge has demonstrated quantum cryptography over standard fibre connections, and has found a way of extracting the weak signals used in quantum cryptography from ordinary telecom fibres transmitting data traffic, so they no longer need their own separate fibres.

They did this by using a detector that is sensitive only for 100 millionths of a micro-second, and which accepts photons at their expected arrival time. That means the detector is insensitive to scattered light caused by data signals, which overwhelm photon signals, and can still assess the polarisation of the light particle.

“The requirement of separate fibres has greatly restricted the applications of quantum cryptography in the past, as unused fibres are not always available for sending the single photons, and even when they are, can be prohibitively expensive. Now we have shown that the single photon and data signals can be sent using different wavelengths on the same fibre,” explained

Toshiba said it had implemented quantum cryptography over standard fibre, whilst transmitting data at 1Gbps in both directions. The team showed off a secure key rate of over 500kbps for 50km of fibre, about 50,000 times higher than the previous best value for this fibre length.

A quantum of solace?

There remains scepticism around the widespread adoption of quantum cryptography. Dr Michael Scott, chief cryptographic officer at CertiVox, said quantum cryptography was always potentially a big deal for military and “high value” communications, but was prohibitively expensive. The cost may continue to be a problem for all stakeholders, even in light of the Fujtsu team’s findings.

“Realistically, even now, the detectors used would still be very expensive indeed, which would be a significant barrier to mainstream adoption,” Scott told TechWeekEurope.

“Very serious questions have been raised as to whether or not the theoretical is actually achievable in practice by any vendor. The sheer cost of the detectors is also something that vendors would need to take into account.”

Other issues remain pertinent. In 2010, researchers showed how they could remotely control photon detectors, potentially eavesdropping on conversations.

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