by Jenny Hogan

A fundamental law of classical physics has been broken by two teams of physicists who have linked particles of light together in a way that enhances its normal properties.

Their method for 'entangling' photons could one day allow information to be more densely crammed and read from CDs and other memory devices.

A physical principle called the diffraction limit says that light cannot be used to see or inscribe features that are smaller than half its wavelength. This limits the density of data on a CD, for example, and the size of the circuits that can be carved into microchips.

But now researchers have got round this limit by "entangling" the photons that they use. This process leaves the particles of light sharing a single quantum state, which makes them behave like a single photon with a shorter wavelength and higher energy.

Morgan Mitchell and colleagues at University of Toronto, Canada, entangled three photons, which then behaved like one photon with a wavelength one third as long. And Philip Walther's team, at the University of Vienna, Austria, managed to entangle four photons, which increased four-fold the resolution of the measurements they could make.

Other researchers have done similar things with two photons, but the multi-photon measurement has taken years to master. "These entangled states don't appear in nature. You have to work pretty hard to make them," explains Jeff Lundeen from the Toronto team.

Pits and grooves

Lundeen's team fired a pair of entangled photons and a lone photon from a laser simultaneously at opposite sides of a partially reflecting glass plate. The three photons became entangled only if they all ended up on the same path after encountering this obstacle. Walther and colleagues compiled their four-photon state from two pairs of entangled photons.

Either technique could ultimately be used to create bigger groups of entangled photons that could make even more precise measurements and write smaller features.

"Looking some years ahead, it doesn't seem unrealistic to expect commercial applications of entangled photons," writes Dirk Bouwmeester, a physicist at the University of California, Santa Barbara, in the issue of Nature where both teams report their results.

For example, says Lundeen, entangling the photons from the red lasers in CD players would instantly increase the amount of data that could be stored on the discs. If the photons were put into groups of three, the pits and grooves that encode the data could be made three times shorter and narrower, increasing the CD's capacity by a factor of nine.

Journal reference: Nature (vol 429, p 158/161)