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Unbreakable code – the keys to your secrets in quantum states of light

Particles such as photons – the smallest unit of light – have unusual qualities that make them perfect for carrying secret keys. Image: Shutterstock/ leungchopan
Particles such as photons – the smallest unit of light – have unusual qualities that make them perfect for carrying secret keys. Image: Shutterstock/ leungchopan

Tiny units of light known as photons have unusual qualities which are now being used to generate secure keys to encode communications and make them impossible to crack.

Uncrackable communications would have a major impact on e-business, banks and the defence industry, protecting them and their customers from damaging hacks.

In the world of quantum mechanics, particles such as photons – which are the smallest unit of light – have unusual qualities that make them perfect for carrying secret keys.

One feature is that, at the quantum scale, observing a particle such as a photon can modify the information it carries.

‘There is just no easy way to send a single photon from, say, Rome to London.’

Professor Nicolas Gisin, of the Group of Applied Physics at the University of Geneva

Another feature, known as entanglement, is that two different photons can be linked so that an action carried out on one photon also has an effect on the other. This effect can continue even when the photons are separated over hundreds of kilometres.

Both these features mean that it is easy to see if a photon sent to a receiver has been intercepted by a third party.

Spotting eavesdroppers

Dr Romain Alléaume is coordinator of the four-year EU-funded Q-CERT quantum cryptography project, which has developed a way to use photons to encode information so it is impossible to crack.

Because the method uses the principles of quantum mechanics to distribute secret keys over optical networks, it is known as quantum key distribution.

‘The security of quantum key distribution is based on the fact that you are sending very precisely controlled quantum states of light and that you have good detectors that detect those states in a controlled way,’ said Dr Alléaume, an associate professor at Telecom ParisTech in France.

The Q-CERT project is working to promote the adoption of quantum key distribution by improving the hardware used, and finding ways to reduce the numbers of errors generated by current systems.

‘Provided you do not detect too much noise, you can guarantee that there has been no eavesdropping,' said Dr Alléaume.

Range problem

Systems using these quantum-generated keys are already in daily use by banks, where secure communication is crucial. But difficulties in the range of quantum encryption techniques, and the difficulty of scaling them up, are hurdles in the way of wider adoption of the technology.

One of the biggest problems with systems that use entanglement is ensuring that transmissions remain clear over long fibre optic cables, because the entangled state can’t be maintained for long for individual photons.

‘Even with the best quality fibre optics, there is a limit of about 300 to 400 kilometres,’ said Professor Nicolas Gisin, of the Group of Applied Physics at the University of Geneva, and coordinator of the EU-funded QuReP project. ‘There is just no easy way to send a single photon from, say, Rome to London (while maintaining its entangled state).’

So the three-and-a-half-year QuReP project has been developing specialised repeater stations, where an entangled photon pair could be received securely and its information be imparted to a new pair of entangled photons.

‘It looks like magic, but it is now a well-known and well-understood physical process,’ Prof. Gisin said.

The next steps for the technology are to develop systems that can be secure over a network, rather than just from one point to another, and in scaling up to continental distances.

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