Optical buffering of light

Photons can carry enormous amount of information, but one of the problems in using them to encode information is that they are difficult to store for even short periods (they are moving at the speed of light after all!). University of Rochester scientists have taken a step in solving the practical problems of using photons to store information by creating an optical buffer for photons that slows them to more usable speeds:

Researchers at the University of Rochester have demonstrated that optical pulses in an imaging system can be buffered in a slow-light medium, while preserving the information of the image.

The ability to delay an entire image and retrieve it intact opens a new avenue in optical buffering--short-term storage of information as optical images. While the initial test image consists of only a few hundred pixels, a tremendous amount of information can be buffered with the new technique.

Slow-light systems have been a topic of recent interest because of their potential application in signal-processing, including all-optical buffers. University scientists used a dilute cesium vapor inside a four-inch glass cell with temperatures at about the boiling point of water to slow the light.

Much of the research into slow-light buffers has been to delay binary signals. The Rochester group demonstrated that entire images can also be buffered.

"Instead of delaying ones and zeros, we're delaying an entire image," says John Howell, assistant professor of physics and leader of the team that created the device, described in the Jan. 22 online issue of the journal Physical Review Letters. "It's analogous to the difference between snapping a picture with a single pixel and doing it with a camera--this is the optical-buffering equivalent of a six-megapixel camera."

The researchers use very weak pulses, which are three meters long and a few millimeters in diameter. Each pulse contains less than one photon on average, which means sometimes no photons will be detected, sometimes one photon will be detected, and occasionally multiple photons will be detected. Every second, three million pulses pass through a stencil with the letters "UR" for the University of Rochester. The pulses travel through the cesium vapor where they are slowed up to 300 times slower than in air.

A reprint of the paper in Physical Review Letters is available here.

A diagram of the device is below (click to enlarge):