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Stopped light means go for quantum computers (eventually)

‘Stationary light’ could lead to quantum logic gates – building blocks for quantum computers. Cathal O'Connell reports.

A cold cloud of atoms (red smear in the centre) holds light in place.
Ben Buchler / ANU

Australian physicists have brought quantum computing a step closer by bringing light to a standstill. This kind of system, reported in Nature Physics, could be used to store light in a quantum memory or build optical gates – two vital components in the futuristic goal of assembling a light-based quantum computer.

The researchers from the Australian National University in Canberra liken the experiment to a scene from the 2015 film Star Wars: The Force Awakens when the character Kylo Ren used the Force to stop a laser blast mid-air.

“Of course, we’re not using the Force, we’re using a light-matter interaction,” says study co-author Geoff Campbell, adding that the movie scene does give an intuitive idea about what the experiment was about.

The work follows 20 years of research into slowing or stopping the fastest phenomenon in the universe. Light barrels along through a vacuum at three hundred billion metres per second.

In 1999 physicists managed to slow it to 17 metres per second in a cloud of cold gas.

And by 2013, scientists at the University of Darmstadt in Germany stopped it entirely, for a full minute, inside an opaque crystal.

But what physicists call ‘stopped light’ is not quite what you might imagine from that Star Wars scene.

When physicists stop light, it’s actually only the light’s information that’s held in place – imprinted on surrounding atoms as light is absorbed. They can then retrieve this information by setting it in motion again as another light wave, for instance.

This storage and retrieval of light information could be vital for building light-based quantum computers.

The new experiment is a new variation on the stopped light technique, called ‘stationary light’. To pull it off, the Australian team shone infrared lasers into an ultra-cold cloud of rubidium atoms which excited atoms in two locations.

The two excited groups of atoms then exchanged photons in a self-sustaining interaction – a bit like two groups of excited supporters exchanging chants at a football game.

This optical chanting is called stationary light, because it preserves the information of the original light sent into the cloud – although only for a fraction of a second.

While light has ground to a halt before, the Australian team managed to create a self-correcting arrangement – something which has never been done but makes preparation a lot easier. They were also able to image the cloud of atoms side-on and show the light exchange in action.

The physicists see the experiment as an important step towards building a quantum logic gate, a critical element of optical quantum computers.

Although some quantum logic gates have been built, they have been probabilistic, meaning they only work some of the time and can't be scaled up. Building more reliable quantum gates hinges on finding a way to get two particles of light to interact.

“The problem is photons tend not to talk to one another,” says Ben Buchler, who led the research.

Using this technique, holding the light in place could give it more of a chance to interact, he adds: “That’s the building block for a quantum gate which is essential to a quantum computer."

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Cathal O'Connell is a science writer based in Melbourne.
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