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High density optical storage on a single pulse

22 Jan 2007

New technique developed at the University of Rochester stores and retrieves an entire digital image from a single photon pulse.

Researchers at the University of Rochester, NY, US, say they have made an optics breakthrough that allows them to encode an entire image's worth of data into a single pulse of photons, slow the image down for storage, and then retrieve the image intact. While the initial test image comprises only a few hundred pixels, a tremendous amount of information can be stored with the new technique.

The image - "UR" for the University of Rochester - was made using a single pulse of light and the team can fit as many as 100 of these pulses at once into a tiny, 100 mm cell. The researchers say that squeezing that much information into so small a space and retrieving it intact opens the door to optical buffering - storing information as light.

"It almost sounds impossible, but instead of storing just ones and zeros, we're storing an entire image," said John Howell, associate professor of physics and leader of the team that created the device, which is revealed in Physical Review Letters (98 043902). "It's analogous to the difference between snapping a picture with a single pixel and doing it with a camera - this is like a 6-megapixel camera."

"You can have a tremendous amount of information in a pulse of light, but normally if you try to buffer it, you can lose much of that information," said Ryan Camacho, Howell's student and lead author on the PRL article. "We're showing it's possible to pull out an enormous amount of information with an extremely high signal-to-noise ratio even at low light levels."

Optical buffering is currently a particularly hot field of research because engineers are trying to speed up computer processing and network speeds using light, but their systemsare slowed down when they have to convert light signals to electronic signals to store information.

Howell's group employed a new approach that preserves all the properties of the pulse. The buffered pulse is essentially a perfect original; there is almost no distortion, no additional diffraction, and the phase and amplitude of the original signal are all preserved. Howell is also working to demonstrate that quantum entanglement remains unscathed.

To produce the UR image, Howell shone a beam of light through a stencil with the U and R etched out. Anyone who has made shadow puppets knows how this works, but Howell turned down the light so much that a single photon was all that passed through the stencil.

Quantum mechanics dictates some strange things at that scale, so that bit of light could be thought of as both a particle and a wave. As a wave, it passed through all parts of the stencil at once, carrying the "shadow" of the UR logo with it. The pulse of light then entered a 100 mm cell of cesium gas at a warm 100°C, where it was slowed and compressed, allowing many pulses to fit inside the small tube at the same time.

"The parallel amount of information Howell has sent all at once in an image is enormous in comparison to what anyone else has done before," said Alan Willner, professor of electrical engineering at the University of Southern California and president of the IEEE Lasers and Optical Society. "To do that and be able to maintain the integrity of the signal is a wonderful achievement."

Howell has so far been able to delay light pulses by 100 ns and compress them to 1% of their original length. He is now working toward delaying dozens of pulses for as long as "several milliseconds", and as many as 10,000 pulses for up to 1 ns.

"Now I want to see if we can delay something almost permanently, even at the single photon level," said Howell. "If we can do that, we're looking at storing incredible amounts of information in just a few photons."

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