TECHNOVA                                                       

How Can a Single Photon Produce a Whole Image ?
by Shubham Agarwal
Birla institute of Technology and Sciences, Pilani.

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.  Prepare 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.) 

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, 

One can imagine the photons as bulletlike cylinders fired at the target, , 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 As the cylinders pass through the stencil, the parts that hit the opaque material are absorbed and no longer represent locations at which the photon can potentially be measured.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," . 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. 

To produce the UR image, Howell simply 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 with it. The pulse of light then entered a four-inch cell of cesium gas at a warm 100 degrees Celsius, where it was slowed and compressed, allowing many pulses to fit inside the small tube at the same time.

"The parallel amount of information John has sent all at once in an image is enormous in comparison to what anyone else has done before," says 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—it's a wonderful achievement." 

It has been   so far been able to delay light pulses 100 nanoseconds and compress them to 1 percent 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 a nanosecond.