Visit the campus of the University of Maryland, Baltimore County on any cloudless afternoon, and you're likely to happen on an intriguing sight: a slender fellow bent over a contraption that looks like a cross between an 1890s camera and a bulky steamer trunk.
That would be Sanjit Karmakar, a post-doctoral physics fellow who's using his "magic box" to take pictures by following the sun across the sky. One day, the pictures will be of objects thousands of miles away.
In a deeper sense, he is trying to help answer a question that still engages scientists: What is the nature of light?
"Many people still don't understand its true properties," said Karmakar, who earned his Ph.D. in applied physics in May.
Karmakar created a buzz in physics circles last year when he used his apparatus — better known as a quantum camera — to become the first person ever to produce a "ghost image" using the sun as a light source.
Ghost imaging is a form of picture-taking in which a camera need not "see" an object in order to capture its image. A device like Karmakar's observes the light that falls on an object, then uses the mathematics of quantum physics to compute an image.
Insiders say they'll soon be able to produce pictures of objects across continents, even on other planets, and with none of the distortion often seen in traditional photography.
"We're sure that [sunlight] ghost imaging will do this and more within a few years," says Ron Meyers, a ground-breaking physicist at the U.S. Army Research Laboratory in Adelphi. "It already has many applications, in everything from the military to astronomy and medicine."
UMBC's quantum optics department has earned an international reputation over the years, thanks largely to the work of Karmakar's mentor, Beijing-born physics professor Yanhua Shih, who did some of the field's earliest research. But you'd never know his stature from the stream of passers-by who stop and ask Sanjit what he's doing.
He's happy to explain and does so repeatedly. He uses the $50,000 black box as a teaching tool.
It sits on a tripod, lightproof everywhere but on one end, where a small opening admits beams of sunlight. Inside are two discrete compartments, each of which contains a photon detector (a device that takes measurements of light).
The first detector (Detector A) registers all the light that enters its chamber. The second, Detector B, sits a few inches from the object Karmakar is "photographing" — today, a slotted card — and registers only the photons that bounce off the object. The detectors "click" in unison hundreds of times, establishing a record of paired light intensities as the day wears on.
Only quantum physics makes sense of the data.
As it happens, whenever two photons from the same source hit their targets at the same time they bear a relationship governed by a mathematical algorithm: Multiply Detector A's results by Detector B's results, and you get a series of numbers which, when fed into a computer, generate an image.
The image starts out ghostlike. As data are added, it grows in detail. Eventually it takes full shape on Karmakar's computer screen — a picture of the card made entirely from photons that never fell on it in the first place.
"With some elegant implementations, the ghost image [is] indistinguishable from a photograph," said Meyers, who has been involved in the field since 2006, when he produced the first ghost image of a remote object at the Army Research Lab.
Scientists from Europe to Asia have long been able to produce ghost images using artificial light as a source, but until last summer, no one could pull it off using sunlight. That was when Karmakar, a native of India, decided to add a filter that limits bandwidth and accentuates the interference that occurs when photons collide.
His breakthrough means scientists might soon be able to photograph any object on which sunlight falls – in other words, any object in the world. Think tanks in the Iraqi desert or ships in the Strait of Hormuz.
Many physicists still resist the idea that light is even a quantum phenomenon — in other words, that photons act as though they were both particles and waves, and that they affect one another at a distance. But Shih says his protege couldn't have pulled off his experiment any other way.
"Those guys just don't accept this approach," he said, citing some of the field's better-known names. "They don't like [quantum theory] because we don't really know how some of it works. But it does work. One day everyone will accept this [model]."
Karmakar has made a splash at several professional conferences, including this year's SPIE convention, which was held at the Baltimore Convention Center in April. (SPIE, the international society for optics and photonics, is dedicated to advancing light-based technologies.)
"His talk was the most important one," said Juan F. Ospina, a professor of mathematical physics at EAFIT University in Medellin, Colombia. "Applying quantum theory to engineering problems is still seen as controversial, but experiments like Dr. Sanjit's are changing minds."
Count Meyers as a believer. Karmakar already works for him at the Army Research Lab as part of an agreement between the Army and UMBC, and hopes to continue his research with the Army after he becomes a citizen.
"I can't tell you everything Sanjit [is] working on, of course," Meyers said. "But he has done good research in an area that is very interesting."
What is Ghost Imaging?
GHOST IMAGING: A form of picture-taking in which the camera need not directly "see" its object – and which uses quantum physics to compute a final image.
ADVANTAGES: With sunlight ghost imaging, scientists can theoretically take pictures from thousands of miles, through smoke, clouds and heat, and without using a lens.
IMAGE QUALITY: For a digital camera to attain the same resolution a sunlight ghost-imaging camera gets from 10 km away, it would need a lens 92 meters wide.