Quantum entanglement might cut a photon's effective wavelength in half.

Yoked Photons Break a Light Barrier

By exploiting entanglement, the quintessential "spooky" phenomenon in quantum mechanics, physicists have come up with a method for drawing tiny features on a microchip that would be impossible according to the classical theory of light. If it proves practical--always a big "if" where quantum effects are concerned--the technique could enable chip designers to circumvent a physical barrier that has limited etching resolution.

As microcircuitry grows ever finer, chipmakers run smack into the so-called Rayleigh limit. This principle dictates that the smallest feature a light beam can write on a chip is half the wavelength of the light. So manufacturers in search of finer circuits must use ever shorter wavelengths. Short wavelengths, however, are both hard to control and tough on chips.

To smash that barrier, Jonathan Dowling and colleagues at the Jet Propulsion Laboratory in Pasadena, California, imagine "entangling" two photons. When shot at a beam splitter from opposite directions, the photons will always wind up moving in lockstep. Thus yoked, the photons will remain inseparable until they strike a target--in this case, the chip-in-progress. The entangled photons could be made out of red light, say, which has relatively long wavelengths and is easy to work with. But when the two photons hit the target together, their combined energy might equal that of a single ultraviolet photon--a particle with a shorter wavelength. "It acts like UV for all intents and purposes," Dowling says. That should make it possible to etch transistors twice as fine as the Rayleigh limit allows, he adds. The team describes their idea in the 25 September issue of Physical Review Letters.

"Dowling had a very brilliant idea to use this for lithography," says Yan-hua Shih, a physicist at the University of Maryland, Baltimore County, who is trying to put the scheme into effect. Other scientists, however, think it will take more than tinkering to rout Rayleigh. Paul Kwiat, a physicist at Los Alamos National Laboratory in New Mexico, suspects that the difficulty of creating bright beams of entangled light will limit the usefulness of the technique. "But it's good to have people think about these things," he says.