[LINK] QKD on a normal broadband service

stephen at melbpc.org.au stephen at melbpc.org.au
Thu Nov 22 00:23:11 AEDT 2012

Quantum cryptography done on standard broadband

Jason Palmer, Science and technology reporter, BBC News. 20 November 2012 
(and) http://prx.aps.org/abstract/PRX/v2/i4/e041010

The "uncrackable codes" made by exploiting the branch of physics called 
quantum mechanics have been sent down kilometres of standard broadband 

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 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 

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 

Slice of time
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.

"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.

Most QKD systems are currently restricted to tightly controlled lab 
conditions Paul Townsend of University College Cork led research 
published in the New Journal of Physics in 2011 aiming to do the same 
trick over 10km of fibre - but the new work was carried out over 90km of 
fibre at data rates hundreds of times higher.

"The work of this group, our own and others is showing how to address 
some of the critical practical problems that have to be addressed in 
order to get QKD out of the lab and into real fibre networks," he told 
BBC News. 

"This is a major advance in this respect."

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.

"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."


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