Making a key advance

UK researchers have built the world’s fastest quantum key distribution system, a major step in the move towards transmitting large amounts of unbreakable coded information.

The team, led by Dr. Gerald Buller of Heriot-Watt University’s school of engineering and physical sciences, has successfully demonstrated a quantum key distribution (QKD) system with a speed of 2GHz.

The system transmits encrypted information down standard telecom fibre, and the researchers ultimately hope to use it to send compressed video between two points with absolute security.

This could allow it to be used in the defence and banking industries for sending sensitive video footage and for secure video conferencing.

The researchers have also built a multi-user fibre system consisting of a transmitter and 32 receivers, one of the few in existence, and by far the largest network constructed, according to Buller.

‘We have taken QKD up to the level of clock rates of GHz, where typically people have been using kHz or MHz in the past. This is taking it up by a factor of 1,000 to make it much morepractical,’ he said.

Quantum cryptography has the potential to provide verifiably secure data transmission, as it uses streams of single photons so sensitive that unwanted observation by a third party causes errors in the code, which can then be detected by the users.

However, to date such systems have been relatively slow, meaning the amount of information transmitted is low. As the key must be as long as the amount of information to ensure absolute security, the faster the system, the greater the amount of data that can be sent.

‘Our ultimate aim is to send compressed video information in pseudo-real time (or real time, but slightly delayed to allow for decryption). That means we would have to send keys on the Mb/sec level. At the moment we are the only people even close to that.’

During the experiments at 2GHz, the researchers were able to achieve bit rates in excess of 1million b/sec, although this is only possible over distances of 10km and under, making the technology suitable for local area networks such as financial districts and defence companies’ campuses.

To reach such speeds, the researchers have improved the components used to build the system. In particular, this has meant improving single photon detectors, and the team is using silicon-based detectors operating at short wavelengths of below one micron. They have been working with a team at the Politecnico di Milano, which has developed devices capable of increased speeds, with significantly reduced ‘jitter’, a phenomenon that reduces bit rates.

Unlike other researchers – including those at the US National Institute of Standards and Technology – who are developing systems capable of transmitting encoded information through ‘free space’, the Heriot-Watt team is using standard telecom fibre.

While free space systems have the advantage of being relatively easily reconfigured, making it possible to easily switch to a different receiver for example, they can be adversely affected by environmental conditions, including the weather, humidity, and even ambient light.

However, sending short wavelength light down standard telecom fibre normally results in propagation difficulties, as the light is split into different modes, all of which have different speeds and propagation rates.

So the researchers have developed a method of splicing specialist 850nm fibre, fitted to the transmitter and receiver, with standard telecom fibre in between. By splicing the fibres correctly, it ensures only the desired mode is excited and propagates down the standard fibre, while other modes are suppressed.

<b>Broadband provides an alternative</b>

A technique for ‘hiding’ information within broadband light could provide a cheaper and faster alternative to quantum encryption, Scottish engineers claim.

Dr. Andy Harvey, senior lecturer at Heriot-Watt University’s school of engineering and physical sciences, has developed a method for optically encrypting information, and is talking to fibreoptics and secure communications companies about commercialising the technology.

The system allows information to be transmitted at very high rates, he said.

‘We don’t have the sexiness of quantum encryption, but we do have the practicality of a system we could put together tomorrow for high data rates.’

The technology also allows secure data to be sent across long distances, he added. ‘If you look at quantum cryptography, typically it is limited to tens of km, and you can’t put a quantum encrypted signal through an optical amplifier, which would enable you to transmit it across the Atlantic for example. With our system the signal can be transmitted through an optical amplifier and it remains encrypted.

‘So we have long range, we have high data rates, and we can implement it using off the shelf technology,’ he said.

The system uses a transmitter to create an optical signal, which is sent down a fibre into an encryptor where it is given a specific signature, called an optical key. Unless the person receiving the message has this key, all they will see is broadband light coming out of the fibre, said Harvey.

However, if the signal is put through a decryptor, which contains a matching optical key, the information can be retrieved.

So unlike other encryption systems, a third party could not even record the coded information and attempt to decrypt it later, he said. ‘I have to have the key even to record the data, and there is nothing to stop me encrypting that data as well of course to make it doubly secure.’