# Slow down, Light!



## RAXL

Scientists have slowed down and even stopped the fastest substance in the universe: light. As this ScienCentral News video reports, the research may lead to faster, more powerful computers.

Quantum Computers

Light sets the universe's speed limit. Nothing else moves faster. In just one second, an ordinary ray of light travels a distance equal to seven trips around the earth.

But now, physicist Matt Sellars has found a way to hit the brakes on light, slowing a speeding laser pulse and capturing it inside a crystal. 
Sellars and his research team at the Laser Physics Center at the Australian National University in Canberra, Australia, managed to slow the laser light down from 670 million miles an hour to a mere 670 miles an hour — about the speed of a bullet being fired from a gun — before stopping it altogether.

"It's quite a bit faster than you can drive a car, but… we now have the ability to hold it for seconds at this stage and possibly longer," Sellars explains

That's longer than ever before, and long enough to be able to use light for quantum computing. 

Using pulses of light to transfer and store digital information as quantum bits, instead of bits of electric current, would make today's supercomputers seem like yesterday's antiques. 

"Its processing power exponentially grows at a much faster rate than a classical computer with bits. You don't have to get that many quantum bits in your computer before you have a very powerful device," says Sellars.


Laser beams.

Quantum computers will one day exploit quantum mechanics to perform incredibly complex mathematical operations at high speed. And if the researchers can find a way to store the light pulses for a very long time, they will also have memory that operates on a quantum scale.

The team made a quantum bit by shining two laser beams at a silicate crystal containing atoms of a rare element called praseodymium that can absorb these light beams. Previous attempts to freeze light in the laboratory have used the atoms in a vapor, not a solid.

"The advantage of a crystal is that the atoms in a crystal are fixed, they're not moving. In a vapor, the atoms are randomly moving around," he explains. "What we're trying to do in these experiments is to store information in the material. That information is lost as the atoms move."

Laser light pulses fired at the crystal are normally absorbed by these atoms, and so don't pass through the crystal. But when a secondary laser was directed at the crystal, it became transparent, allowing light from the first laser to pass through it. When the second beam was turned off, the atoms then trapped the light from the first laser beam. The secondary laser was directed onto the crystal once again to release the pulse.

"What we're doing is storing the quantum information of the light on the atoms. These atomic systems can only store that information for so long," Sellars says. "The atoms which are storing the information interact with their environment, and the information gets slowly dissipated. If we leave it too long, and we try to get the pulse out, the pulse that we get out has a lot of noise and basically gets very weak because we've lost the information that's required to reconstitute the pulse."


Light beams pass through silicate crystal.

The researchers can transfer information onto light beams using the "nuclear spin" of the photons in the laser beam. The spin is basically the orientation of each the particles, and can be oriented either up or down — or both. Because of the laws of quantum mechanics, particles like photons can sometimes be oriented both up and down at the same time, until they are observed or measured. This arrangement is known as quantum superposition, and can be exploited to create a unit of information known as a "qubit" (or quantum bit), which can hold more information than the traditional digital bit, which can only be a zero or a one.

"That's the peculiar effect of quantum mechanics that we're trying to exploit to make more powerful computers," he says.

The amazing processing power of a quantum system is a direct result of the superposition state. Sellars is already studying how to store multiple pulses of light at the same time — the next step toward making a quantum computer — but he says that building a working quantum computer could take ten years or more. Which means the more work he does on slowing down light, the faster we'll get .


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