Quantum maths on a silicon chip

A quantum calculation cracking one of the most common forms of data encryption has been demonstrated on a silicon chip.



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


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.


Journal reference: Science, DOI: 10.1126/science.1173731

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This was first published in September 2009


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