Quantum leap

ANU researchers have set a new world record for stopping light, meaning the next generation of computers could soon see the light of day.

At the heart of the experiment is a small clear, colourless crystal. This glass-like cube sits in a cryogenic vessel standing on a large optical table, surrounded by electronic boxes.

Crisscrossing the table are fine pencils of orange laser light, steered onto the crystal by a forest of optical components.

With this finely calibrated set-up, ANU researchers have set a new world record for stopping light in its tracks, holding it for 1,000 times longer than previous efforts. They are now one step closer to developing the new generation of computers, which will make use of the more bizarre aspects of quantum mechanics.

“Stopping light is not just a neat trick, it is the basis of a quantum memory — a device capable of storing and recalling the quantum states of light. This is one of many quantum computing components under development in our lab,” explains Dr Matthew Sellars. Along with Professor Neil Manson, Dr Sellars leads the quantum computing program in the Laser Physics Centre, part of the Research School for Physical Sciences and Engineering.

“Unlike the semiconductor computers currently in use, our quantum RAM will store data on nuclear spin states and will be written to and read out using pulses of light.”

Creating a functional quantum computer has become the Golden Fleece for teams of physicists around the world, spurred on in part by defence and other government agencies that are concerned about the potential ramifications for data security. The intrinsically parallel nature of quantum computations means it is potentially much more powerful than classical computing. This massively increased processing ability could easily handle the immensely large mathematical problems used in breaking encryptions.

“Initially the defence agencies are interested in finding out whether it’s going to be possible to build a quantum computer. If it is proved to be possible, they’re going to have to take a long hard look at how they can keep their information secure,” Dr Sellars says.

But there is more to building a quantum computer than buying a kit from the local Tandy store. Quantum information is very fragile — the slightest interaction with the outside world can corrupt the quantum states. Until recently, one of the major stumbling blocks has been how to store the information for more than a few milliseconds.

“We’ve developed new techniques, using the nuclear spins of the ions in a solid, to store quantum information for periods longer than a minute. But we needed a way of moving the data about.

"Light is very good at transmitting information. The only trouble is it’s very, very fast. You need to hold it for a while, or otherwise it just zips off. That is where the stopped light comes in. This is a means of mapping the quantum state of the light onto the nuclear spins in the crystal, where we can store it. By reversing the process we can recreate the original light pulse, in principle, down to the last photon.

“We use an yttrium silicate crystal doped with a rare-earth element, praseodymium. It is on the praseodymium nuclear spins that we store the light pulse. When we shine a laser pulse at this crystal, it’s normally absorbed. The light doesn’t get through the crystal. Then we add a second laser beam that turns on the coupling between the nuclear spins and the light. This coupling makes the crystal transparent. So when we now fire the first laser beam at it, it gets through, but the odd thing about it is that it takes a very long time to do so.”

Using this method, the researchers have slowed the speed of light down from 300,000 kilometres per second to a few hundred metres per second.

“To store the light in there, we turn the second laser beam off. The signal from the first laser beam is trapped inside the crystal. To get the signal out again, we turn the coupling beam on again. We can now store light for seconds, and potentially quite a bit longer.”

The team are now trying to refine the process so they can store a single photon, which will require a larger crystal, ten times the size of the one currently in use.

“If we can store a single photon we will have demonstrated the world’s first quantum memory,” Dr Sellars says. If this happens, a tiny crystal could precipitate a giant technological leap.

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