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.
Now, the “uncrackable codes” made by exploiting the branch of physics called quantum mechanics have been sent down kilometres of standard fibre. This “quantum key distribution” has until now needed a dedicated fibre separate from that used to carry data.
A new technique reported in Physical Review X shows 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.
“Certainly in a corporate environment it’s already affordable, and as time goes on I’m sure we’ll see the technology get cheaper” - Andrew Shields, Toshiba Cambridge Research Centre
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.
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.
“Trying to use such low-level signals over ‘lit fibre’ has been rather like trying to see the stars whilst staring at the Sun,” said computer security expert Alan Woodward from the University of Surrey. ”What these researchers have developed is to use a technique that rapidly switches between the various light sources using the fibre such that one source isn’t swamping the other,” he told BBC News.