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How Can a Single Photon Produce a Whole Image?

Short answer: it can't
slow light



UNIVERSITY OF ROCHESTER
Researchers announced last week that they had slowed down faint pulses of laser light and retrieved an image from that light after speeding it back up. In principle such "slow-light" technology might be used to build a kind of traffic stop for light signals, called an optical buffer, that would be cheaper, more powerful and faster than converting light beams into electrical signals. That is why the Defense Advanced Research Projects Agency (DARPA), an arm of the Department of Defense, asked a group at the University of Rochester to explore the technology, says group leader, quantum optics researcher John Howell.

So naturally Howell was bemused that some media outlets focused in on one aspect of the report: that an entire image was somehow produced from a single photon, the smallest unit of light. "A lot of people are getting excited about the single photon," he says.

A statement from the University of Rochester said that Howell's team made their image using a single pulse of light. To those familiar with the rudiments of quantum physics, the claim was, well, odd, to say the least. "The people that are more aware of this are wondering how we're measuring an image from a photon—and we're not," Howell says. So what's the story?

Consider the famous double-slit experiment, in which individual photons are beamed through a pair of adjacent gaps (slits) in a screen. As long as researchers do not try to determine which of the two slits each photon is passing through, light shined through the screen will create a so-called diffraction pattern of alternating bright and dark spots.

The double-slit experiment demonstrates that a photon can in some sense "feel" both slits, just like a wave passing through both at once. Like a wave, each individual photon propagates away from the screen spread out and carrying the whole diffraction pattern—but that does not mean each photon creates a weak image of the whole diffraction pattern when it hits the far wall. Instead, each photon lands in just one spot, and many photons together create the pattern.

The new technique worked similarly. The group prepared weak pulses of light that on average contained less than one photon. (That's possible because the pulses were each in a superposition, or mixture of quantum states; some pulses did not contain any photons and others contained one photon, Howell says.)

The researchers shined the pulses through a stencil of the initials "UR" and into a four-inch-long cavity filled with hot cesium vapor, which acted as a drag on the light, slowing it down. After emerging from this cavity, the pulses struck a four-square-millimeter region in a single pinpoint location.

Each pulse struck the region somewhere in the "UR" shape carved out of the light pulses by the stencil. But generating the whole image required up to 100 million pulses, in part because a single photon detector had to scan back and forth over the whole detection region, as described in the January 26 edition of Physical Review Letters.

One can imagine the photons as bulletlike cylinders fired at the target, Howell says, only each cylinder is two-to-three millimeters in diameter and up to a meter long. As the cylinders pass through the stencil, the parts that hit the opaque material are absorbed and no longer represent locations at which the photon can potentially be measured.

"It's just like when you put Play-Doh through one of those stencils," Howell says. Like the Play-Doh, each pulse that passes through the stencil does carry the whole "UR" shape, but, as with the two-slit diffraction pattern, one photon does not produce the whole image on being detected.

Howell says the experiment is the first demonstration that optical buffering, or delaying of light, can reliably transmit two-dimensional information—in this case an image. The kind of information sent down optical fibers is normally encoded along the length of the pulse, he says.

For now, optical buffering is a "hammer without a nail," meaning its exact applications are unclear, Howell says. But he adds, "I'm pretty excited about it. I'm looking for something to hit on really hard."

Graham P. Collins contributed to this article.

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