Beyond radar

Quantum illumination may one day let us get lots of data via hyper-long-distance systems

By John Dyer
Globe Correspondent / March 14, 2011

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Imagine two photons, basic units of light that are infinitesimally small. Scientists shoot one photon into the sky. The other they keep in the lab. The photon in the sky strikes an airliner or a satellite. Almost immediately, the scientists pick up the flying object’s location. How do they know their sliver of light has hit a target miles away? They can tell by comparing one photon to the other.

It’s an exotic concept called quantum illumination, an outgrowth of quantum physics now under study by scientists at Raytheon BBN Technologies in Cambridge. Funded by a $2.1 million grant from the Department of Defense, the company’s researchers believe their work could lead to incredibly powerful radar systems, as well as advancements in telescopes and other technologies that use light.

Quantum illumination has not been tested at distances beyond 90 miles on the earth’s surface, so the notion of tracking a satellite in space is still theoretical. But the work is nonetheless on the verge of yielding long-awaited applications for groundbreaking concepts developed a century ago by giants like Albert Einstein, scientists said.

“We’ve learned about quantum physics in the lab. Now we ask ourselves, ‘Can we translate the most crucial ideas into the practical domain?’ ’’ said a Boston University physics professor, Alexander Sergienko. He is not involved in Raytheon’s project but has worked on similar Defense Department-funded research. “We call it quantum-inspired technologies. We’re becoming quantum engineers.’’

Traditional electronic devices use around 100 photons to transmit a single bit of information — either a one or a zero, the binary code that is the mathematical language of computers, said a Raytheon BBN senior scientist, Jonathan Habif. By exploiting quantum physics, however, scientists believe they can stream 10 bits of information, or 10 zeros or ones, on a single photon — a leap in data transmission.

“We’re very interested in how we can perform imaging or how we can perform communications with less light,’’ Habif said. “How can we very, very efficiently use the light that we generate and send out into the world to extract information?’’

Quantum illumination starts with scientists passing a laser through filters that thin the beam into photons, the essence of light, Habif said. The filters create identical photons that are linked to each other. “The laser beam is the father and generates one pair of twins,’’ he said. “Those twins are entangled pairs of photons.’’

Scientists then split the entangled photons and release one at a target. When the released photon hits something, it bounces. In the process, the impact alters the released photon. Scientists then detect and compare the released photon’s reflection to its unaltered twin in the lab. The difference between the two generates an enormous amount of data about whatever the photon struck, scientists said.

The process is similar to radar, where electromagnetic waves rebound off objects to create an image of something so far away the naked eye can’t see it. But because of quantum physics, the smaller photon delivers far more data far more quickly and efficiently, scientists said.

Classical physics describes the interaction of mass and energy in the visible world — for example, Isaac Newton’s law of gravity — but quantum physics describes mass and energy on a subatomic scale. It’s the science meant to understand the behavior of quanta, the tiny building blocks of atoms. Photons are quanta of light.

Quantum physics led to the paradoxical discovery that quanta behave as both particles, which have a well-defined location in space, and waves, which are spread out. That means quanta are, in a sense, two things at once, a duality that opens the door for scientists to use quantum engineering to develop new technologies, like quantum radars.

A quantum radar could conceivably carry more data than, say, a traditional radar, because of the different qualities photons express as both particles and waves.

“Quantum information has much richer, much larger parameters,’’ said Saikat Guha, another Raytheon BBN scientist.

Quantum illumination might be used to create hyper-long-distance communications systems, because the process of matching one photon with another allows interference from outside light sources like the sun to be filtered out, Habif said. The same principal means a quantum radar might use very little light and could avoid detection as it scans the skies, he said.

The effect is so fast, it encouraged some to believe science was standing on the frontier of science fiction. Quantum illumination sends so much information so quickly and clearly, Sergienko said, it seems almost instantaneous, leading some enthusiastic early boosters to suggest it was a path to teleportation devices. Unfortunately, he said, that wasn’t the case.

“You cannot instantly change anything at a distance,’’ Sergienko said. “People want to use teleportation like in ‘Star Trek.’ This is not like that. It’s teleportation of information.’’