Memories made of light

Physicists find more efficient way to store quantum information

Sun-drenched summer vacations may yield pleasant memories, but physicists in Australia have harnessed the power of light to remember something else: quantum information.

LIGHT MEMORY A laser penetrates a crystal containing the rare earth element praseodymium, which retains a memory of quantum information contained within the light. Matthew Sellars/Australian National University

Researchers have coaxed laboratory crystals to capture and release information carried within a light pulse at the highest efficiency yet. The work, reported in the June 24 issue of Nature, could one day lead to new types of secure communications that take advantage of the weird properties of the quantum world.

“It’s quite an important step towards our dream of extending the distances over which we can do quantum communication,” says Wolfgang Tittel, a physicist at the University of Calgary in Canada who was not involved in the work.

From a physicist’s perspective, light is the ideal carrier of information, because it is literally lightning-fast. Until now, researchers have tried to fashion “quantum memories” for light primarily by sending lasers into a vapor made of atoms. The atoms preserve information in the light that can then be read out again, like playing back the data on a DVD. But quantum memories based on atomic vapors are inefficient. The best such system reported to date has an efficiency of 17 percent, meaning that of 100 light particles put into the system, only 17 make it out. Physicists don’t need 100 percent efficiency, as it’s acceptable to lose some information in the transfer, but a system does need at least 50 percent recall to be useful in quantum applications.

The new work instead uses a solid crystal, in which the atoms are rigidly packed together instead of bouncing around diffusely as they would in a vapor. That control allows researchers to achieve a memory efficiency of 69 percent, reports a team led by Morgan Hedges of the Australian National University in Canberra.

That high efficiency is an impressive breakthrough, says Thierry Chanelière, a physicist at the National Center for Scientific Research in Orsay, France.Most researchers working on quantum memories study atomic vapors as opposed to solid-state crystals, but Chanelière says that could soon change if the crystals continue to show such promise.

As the light pulse enters the crystal, it begins to slow down, its front reaching one end of the crystal and stopping as the rest of the light squeezes itself in to fill the whole crystal. The crystal is mostly transparent but can absorb one particular color very strongly. The researchers switch on an electric field gradient, which changes the strongest absorption color in different parts of the crystal, so that one end of it absorbs strongly at the blue end of the spectrum and the other end toward the red. Quantum information from the light is stored in the oscillations of the crystal’s atoms; by reversing the electric field, the scientists get the atoms to re-emit light containing the same information as the original pulse.

The crystal that Hedges and his colleagues created is made of praseodymium, a rare-earth element, along with the elements yttrium, silicon and oxygen. The scientists are now expanding beyond praseodymium to study quantum memories using other rare earth elements, such as europium. Their next goal: to coax the crystal to retain quantum memory for longer than a few microseconds. “The idea is to combine the high efficiencies with very long storage times,” says Hedges.

If the crystals can be made to hold quantum information for practical lengths of time , they could form the basis for a a quantum repeater, a device that would allow  a stream of light particles to be sent a distance of many miles in a quantum communication system. Such a system would be secure, because the information would vanish as it was read out from the crystal. “No one can come and look at what’s left in the memory to try to work out what had been stored,” says team member and lab head Matthew Sellars.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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