Scientists at Toshiba's European research headquarters have developed a novel single-photon detector based on quantum dots that could dramatically improve the performance of quantum cryptographic communications.

"One of the reasons we're very excited about this work is that it could increase the bit rate in a quantum cryptographic system," said Dr Andrew Shields, leader of the quantum information group at Toshiba Research Europe in Cambridge.

However, what makes it particularly suitable for quantum cryptography is its inherently low noise. Existing systems, including Toshiba's own, use avalanche photodiodes, which limit the maximum bit rate as a result of afterpulses, which are manifested as spurious detection events.

Using the new detector might mean that the limit to higher bit rates becomes the speed of the detector. Shields said the 150ns resolution achieved so far is due to the capacitance of the packaging, and could theoretically be reduced to less than a nanosecond.

"Gigahertz clock rates would mean we could increase the bit rates by two orders of magnitude," said Shields, "and that would be a fantastic advance."

The current device is built in a GaAs process. Shields said the next step is to make a similar structure in indium phosphide, operating in the infra red. The work appeared in the journal Physical Review Letters.

www.toshiba-europe.com/research/crl