Aarhus University Seal / Aarhus Universitets segl

Quantum cryptography on the standard fibre network



One of the most secure encryption methods is quantum cryptography, where the key is built one photon at a time. However, this is both difficult and expensive. Researchers are working to make it easier and cheaper – by transmitting quantum encrypted data via standard fibre networks.

What could be worse than having an unauthorised person finding out how to decrypt your encrypted communication? That you do not discover this and continue to send confidential information that is no longer confidential, in fact.

The German military learned this the hard way during World War II when they used the Enigma encryption machine without realising that the British forces had cracked the code and were thereby peeking over their shoulders.

“The advantage of quantum cryptography is actually that outsiders are unable to intercept the key to the encrypted data without the legitimate transmitters and receivers of the information noticing it,” says Professor Martin Kristensen.

Quantum cryptography is mainly about sending to a receiver a stream of individual photons, each of which is randomly encoded via a fibre optic cable. The photons can be encoded by either polarising them in different ways or giving them different phases.

When the transmitter has sent a sufficient number of photons that can be used as an encryption key, he or she tells the receiver how to set up their apparatus in order to ‘read’ each photon correctly.

Technology can detect eavesdropping
Should an unauthorised person try to intercept the code by measuring the phase of each photon passing through the fibre optic cable, it will be discovered immediately. It is actually a fundamental condition of quantum mechanics that it is impossible to measure photons without disturbing them.

And it is worth mentioning that any attempt to eavesdrop will be discovered while the key is being distributed – i.e. before the encrypted information begins to flow through the cable.

“By combining quantum mechanics with the theory of relativity, you can work out where the eavesdropper is located. Based on the number of bit errors, the pattern of the errors and the absolute time of arrival, you can locate the eavesdropper with an accuracy of one centimetre on a 50-kilometre cable,” says Professor Kristensen.

And it is impossible for any eavesdroppers to hide when listening in on long cables. Quantum cryptography actually works on relatively short distances because the photons are lost in transit in the fibre optic cable. It is impossible to insert amplifiers along the way because exactly the same thing happens as when you are eavesdropping – the signal is distorted.

Global race for encryption distances
Until now, quantum encryption has not succeeded in working at distances of more than 120 kilometres, and only on special high-quality cables that are directly laid between the users.

This has sparked a global race to make quantum encryption more widely accessible – both by extending the ‘shelf life’ of the photons and by using standard fibre networks.

A fully developed system is nearly ready in Professor Kristensen’s laboratory today, and it can be used in standard fibre optic cables, based on phase encoding. The advantage of this system is that it can very quickly generate secure encryption keys over a distance of more than 100 kilometres and store them on a standard computer chip.

Not ready for home computers
Even though Professor Kristensen’s system works on standard fibre networks, it is too early to expect to use it for home online banking services just yet.

To start with, the end user must have a chip installed in the box where the fibre optic cable enters. Secondly, the Internet provider must allow quantum communication and reserve channels for it in the fibre optic cables, so that standard data traffic does not act as a jammer.

PHOTO TOP: A quantum cryptography system is nearly ready in Professor Kristensen’s laboratory. It can generate keys that are close to indestructible. Aarhus University has thus joined the global competition to develop new technology for cyber security. (Photo: Lars Kruse)