A quantum calculation cracking one of the most common
forms of data encryption has been demonstrated on a silicon
chip.
The study demonstrates that complex quantum circuits can be
built relatively easily out of silicon and silica – a significant
milestone on the road to full-blown quantum computing.
Fifteen years ago, Peter Shor, a computer scientist at the
Massachusetts Institute of Technology, predicted that quantum
computers could beat even the most powerful supercomputers and
crack the widely used RSA encryption algorithm.
Code cracking
RSA relies on a mathematical asymmetry: it's easy to calculate
the result of multiplying together two large prime numbers, but
hard to work backwards from the result and find the two factors.
RSA encryption uses the product of two large primes to make a
public "key", safe in the knowledge that only those authorised to
know the factors used to make it can decode the message.
There is a way to crack the code, however: by carrying out a
relatively basic calculation again and again a computer can search
for patterns and break the code. But in reality the pattern will be
so large that a conventional computer would take an impossibly long
time to find it.
Shor predicted that a quantum computer could do it much faster,
though. Thanks to quantum entanglement, all the numbers spewed out
by a quantum computer are interconnected, so just a few of them
hint at the code-breaking pattern. Shor wrote an algorithm for
future quantum computers that would allow them to take a shortcut
to decoding confidential material.
Now a proof-of-principle version of Shor's idea has been
demonstrated on a silicon chip for the first time.
Quantum chip
The 26-millimetre-long chip was designed and built using
standard fabrication processes by Jeremy O'Brien, Jonathan Matthews
and Alberto Politi at the University of Bristol, UK. It can run
Shor's algorithm in cut-down form – confirming that 3 and 5
multiply to form 15.
Unlike the silicon chips inside conventional computers, the
Bristol team's chip uses light rather than electricity.
Light-transmitting silica on a silicon wafer guides photons with
entangled quantum properties around, an approach first demonstrated
by the same team last year.
Glad grads
"We've made rapid progress, which is testimony to the strength
of this approach," says O'Brien.
Dan Browne, a quantum physicist at University College London who
was not involved in the study, agrees. He worked on one of the
first quantum circuits to run Shor's algorithm in 2007: a table-top
setup that sent photons travelling through the air rather tiny
guides on a chip.
"A free-space optics experiment looks a mess, with many mirrors
and lenses for an experiment with say, 4 or 5 photons," Browne
says. "Imagine being the poor grad student who has to align all
those mirrors."
Light classical
The new chip saves time, says O'Brien. "It's almost as simple as
stamping the design out onto the chip and it's there and working,"
he says.
Andrew White, a quantum physicist at the University of
Queensland in Brisbane, Australia, is impressed with the progress
towards shrinking quantum-circuit size.
"It is very important to shrink the circuitry, and the Bristol
group has shown the quantum community that this can be done using
well-established techniques from classical photonics," he says.
But White points out that the technology used to generate
individual photons to feed into the chip, and to detect them as
they emerge, is not efficient, fast or compact enough yet. Although
the new chip is only 26 mm long, it has to be surrounded by a whole
table top of that equipment.
Journal reference: Science, DOI: 10.1126/science.1173731