News in Science

Proving the need for quantum computers

Quantum proof Scientists have reached a milestone in the search for proof that quantum computers are really necessary.

Dr Matthew Broome, of the University of Queensland, and colleagues, report their findings today in the journal Science.

"Quantum computers generate so much excitement because they allow one to solve problems that you couldn't do efficiently using a conventional computer," says Broome.

Quantum computers rely on the laws of quantum mechanics such as "superposition" and "entanglement".

"We've built small scale quantum computers but none of them to date have been able to outperform the standard classical computer," says Broome.

Since quantum computers are difficult to build, scientists want to be sure they are worth the effort by finding a problem that can be "mathematically proven" to be unsolvable by classical computers.

One candidate for this is the "boson sampling problem" which involves sending a number of single photons through a complex photonic network, and working out which outputs the photons exit.

Scientists calculate that if 20 to 30 photons were put into the system, a classical supercomputer would have difficulty solving the problem.

"It would be able to solve it but it would take a very long time and meantime a quantum computer would have done it," says Broome.

If experimental proof verified this then the boson sampling problem could indeed be used to prove the need for a quantum computer.

But since producing multiple photons at the same time is not as easy as it sounds, Broome and colleagues wanted to test the boson sampling problem at a small scale.

"You're not going to go ahead and do the full-scale 20-photon experiment first up," says Broome. "The first thing you're going to do is to make sure this thing works which is what we did in our experiment."

Broome and colleagues built a simple quantum computer, called a boson sampling computer, especially designed to solve the boson sampling problem.

He and colleagues sent three photons through six fibre-optic inputs in the computer - a system which was simple enough to be modelled and solved on a classical computer.

Both classical computer and boson sampling computer came up with the same results, which means the stage is now set to try and solve the problem at a larger scale, says Broome.

But he says right now, scientist are being held back by the number of single photons they can produce at the same time. Broome and colleagues hope to be able to simultaneously produce eight photons soon.

The boson sampling computer built by Broome was among the first in the world - another one has been built by other researchers who published in the same issue of Science.

While boson sampling computers are only simple quantum computers, scientists like Broome hope they will also be capable of solving other problems that are beyond the realm of even the best world's supercomputer.