Quantum cryptography done on standard broadband fibre
London: The “uncrackable codes” made by exploiting the branch of physics called quantum mechanics, have been sent down kilometres of standard broadband fibre.
This “quantum key distribution” has until now needed a dedicated fibre separate from that used to carry data.
But a new technique reported in Physical Review X has shown how to unpick normal data streams from the much fainter, more delicate quantum signal.
It may see the current best encryption used in many businesses and even homes.
The quantum key distribution or QKD idea is based on the sharing of a key between two parties - a small string of data that can be used as the basis for encoding much larger amounts.
Tiny, faint pulses of laser light are used in a bid to make single photons - the fundamental units of light - with a given alignment, or polarisation. Two different polarisations can act like the 0s and 1s of normal digital data, forming a means to share a cryptographic key.
What makes it secure is that once single photons have been observed, they are irrevocably changed. An eavesdropper trying to intercept the key would be found out.
Sending these faint, delicate quantum keys has until now been done on dedicated, so-called “dark fibres”, with no other light signals present.
That is an inherently costly prospect for users who have to install or lease a separate fibre.
So researchers have been trying to work out how to pull off the trick using standard “lit” fibres racing with data pulses of millions of photons.
Now, Andrew Shields of Toshiba’s Cambridge Research Laboratory and his colleagues have hit on the solution: plucking the quantum key photons out of the fibre by only looking in a tiny slice of time.
Dr Shields and his team developed detectors fit to catch just one photon at a time, as well as a “gate” that opens for just a tenth of a billionth of a second - at just the time the quantum key signal photons arrive, one by one.
The team achieved megabit-per-second quantum key data rates, all the while gathering gigabit-per-second standard data.
Financial institutions are likely to be the first who are interested in the technology when it does escape the lab, senior author of the paper Dr Shields told BBC News.
“We’re not too far away from that type of application already. QKD isn’t so expensive, probably comparable to a high-grade firewall - in the range of tens of thousands of pounds. So certainly in a corporate environment it’s already affordable, and as time goes on I’m sure we’ll see the technology get cheaper and cheaper,” he said.
“This is of academic interest only,” Bruce Schneier, chief security technology officer at BT, told BBC News.