Governments and big businesses might soon be using a new line of defence to make their communications perfectly secure: quantum cryptography.
The Toshiba Research Lab in Cambridge has reported a secure bit rate of 1 MB/sec, which is over 100 times better than previously achieved, making it suitable for commercial application.
Any secure channel is only as secure as the keys used for decoding messages. The attraction of quantum key distribution (QKD) is that it is inherently secure, assuming that the laws of quantum theory are correct. Any eavesdropper would change the quantum state of the photons being transmitted, thereby revealing themselves. The more often you send keys, the more secure your system – for example, the widely-known encryption algorithm ‘one-time pad’ is theoretically uncrackable, but needs secret keys as long as the data itself. However the practicalities of QKD – sending single photons down a fibre optic cable and detecting them some distance away – have been slow to develop, limiting one-time pad’s usefulness up until now.
“The detector has always been the limiting factor,” says Andrew Shields, who leads the group. Every time an incident photon is detected, it triggers an avalanche of millions of electrons in the semiconductor device, which can get trapped and could cause more avalanches, leading to false counts and errors in the key. The way to avoid this in the past was to turn the detector off for about 10 microseconds after detecting the first avalanche, to give the trapped electrons time to decay away. “Our detector is quite unique in that we’ve designed a circuit that allows us to detect very small avalanches with fewer electrons, so no electrons are trapped.” Now that the detector doesn’t need to be turned off, incident photons can be detected more frequently and the detector operated at much faster rates.
His group also developed a stability system to monitor the errors caused by heating of the fibre optic cables, making adjustments accordingly. This allows the system to work continuously, transmitting keys over 50 km of fibre. The previous bit rates had allowed voice transmission, but the increased rate is enough to transmit video images. The results are reported today in Applied Physics Letters.
So now the limiting factor is cost. But as demand for quantum cryptographic systems rises, manufacturing costs will fall, and soon we’ll all want one.
Posted on behalf of Sophie Hebden.