Placing Light on Hold

Gotcha! A newly designed trap freezes a beam of light for 1 second.

Researchers have frozen a pulse of light in place for a full second, a thousand times longer than the previous record. The dramatic lengthening of the pulse's pause hints at a practical approach to short-term memory devices for optical and quantum computers.

Stopping a light pulse requires a special trap of very cold and very still atoms--so still that each one is in the same quantum state. Normally this quiet clump of atoms would be opaque, but a carefully calibrated laser can cut a swath through the opacity, so that when a pulse of light arrives from another direction, the trap is transparent. But cut the laser, and poof! The trap becomes opaque again, and the light pulse stops dead inside the trap. Turn the laser back on, and the light pulse continues on its way.

The trap's secret is that it doesn't literally trap the light. Instead, it sets up a quantum conflict that traps the pulse's information. The laser makes the trapped atoms want to do one thing, the pulse another: The conflicted atoms tangle into a mix of two quantum states. When the laser turns off, the atoms absorb the light pulse. But the pulse isn't lost; the atoms are still tangled between quantum states and are imprinted with the pulse's information. As long as they don't move or change, the atoms contain everything there is to know about the pulse.

Previous light traps broke down after a millisecond or so because the atoms shifted around too much. Now, physicist Jevon Longdell and colleagues at Australian National University in Canberra describe a superior light trap made from a silicate crystal doped with praeseodymium, a rare earth element. Because the crystal is a solid, and praeseodymium can be very stable magnetically, it retains light pulse imprints much longer than did previous traps that used gases or less stable crystals, the team reports 5 August in Physical Review Letters.

"A second is just wonderful--I think it's very exciting," says Lene Hau, a physicist at Harvard University in Cambridge, Massachusetts. However, she cautions that the Australian team's crystal did not compact the light enough to contain an entire pulse. Longdell and colleagues believe that the problem can be solved and that a crystalline light trap similar to theirs could be a practical material for optical memory buffers, which could be used to temporarily store or reroute light signals for telecommunications networks and quantum computers.

Related sites
The report
NASA story on stopped light
Lene Hau's Web site

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