Quantum Trickery Could Lead to Stealth Radar

Stealthy radar systems and the ability to transmit large amounts of data over long distances are a step closer thanks to a technique that could improve the efficiency of modern optics by a factor of 1000.

Traditional methods of transmitting data, such as fibre optics or laser-based radar, require roughly 100 photons to transmit a single bit of data. Now a team led by Saikat Guha at Raytheon BBN Technologies in Cambridge, Massachusetts, say they can transmit 10 bits on a single photon - a 1000-fold improvement.

To do this, the researchers exploit a phenomenon known as entanglement, in which the quantum state of one photon is linked to that of another, regardless of how far apart they are. Once created, each entangled pair of photons is separated by passing a laser beam made up of them through a filter made from a non-linear crystal. This splits the beam in two so that each exiting beam contains one photon of an entangled pair.

The team are now developing transmitters that will release one of the beams at a target, while keeping the other near the transmitter. When a photon hits the target, it bounces back towards the transmitter in a process that alters the photon. When it returns to the detector, the altered photon is no longer strictly entangled with its pair, but Guha says his team has shown "there is still some remnant memory that remains between the two photons".

The researchers hope to discern information about the target such as its distance, size and velocity based on changes to the properties of the recaptured photon - such as polarisation, frequency and momentum - compared to its former partner.

The team has shown in laboratory experiments that this method can be used to glean more data from each photon than conventional methods. This means significantly less light is needed to get the same imaging quality as conventional laser radar.

"If you do it with less [light] energy, the object you are interrogating will never know you are probing it," says Alexander Sergienko of Boston University, who was not involved in the research. "That is why DARPA is interested." Raytheon received $2.1 million in August 2010 from the US Defense Advanced Research Projects Agency (DARPA) to develop the technology.

The technology could also be used to transmit data at greater rates than conventional light or radio transmissions. But developing transmitters and receivers for both applications will be difficult, Sergienko says. "The science is known but in practice it's very difficult to do," he says.

Quantum Trickery Could Lead to Stealth Radar New Scientist reports, explores and interprets the results of human endeavour set in the context of society and culture, providing comprehensive coverage of science and technology news.

Image Credit: Casper Benson/FStop/Corbis