Quantum quirk promises smaller circuitsBy Eric Smalley, Technology Research News
When researchers talk about using quantum entanglement to produce faster computers, they are usually referring to the long-range vision of building quantum computers. But a paper published last week proposes to use linked photons to speed computers made with today's technology.
A pair of linked, or entangled, photons passing through a narrow opening behaves like a single photon with half the wavelength of one of the original photons.
This property could be used to make chips denser, and therefore faster, by allowing photolithography equipment to make smaller features than are possible with ordinary light, according to Jon Dowling, principal research scientist and supervisor of the Jet Propulsion Laboratory's Quantum Computing Technologies Group and one of the paper's authors.
Integrated circuits are made via photolithography, which involves shining a light source through a mask onto a photosensitive material. Openings in the mask allow light to strike the photosensitive material in the pattern of the desired circuits.
One physical limit to how small circuits can get is the wavelength of light. The Rayleigh diffraction limit is a law of physics that the smallest area that light can shine on through an opening is 1/2 the wavelength of the light used.
Chip manufacturers currently use ultraviolet light at a wavelength of 248 nanometers, or millionths of a millimeter, meaning the smallest circuits possible are 124 nanometers wide, said Dowling. Circuit widths can be in the tens of nanometers as researchers have demonstrated using various experimental chemistry techniques.
"The problem with this push to shorter wavelengths is that while it's fairly easy to image and reflect and focus light in the visible range, this process of manipulating the light [for photolithography] becomes harder and harder to do as you go to shorter and shorter [wavelengths]," he said.
One possible solution is to use nonlinear crystals to entangle pairs of photons, he said. Entanglement is a quantum mechanical state in which two or more atoms or subatomic particles are linked regardless of the physical distance between them.
"Even though [the entangled pair] is made out of two red photons, it acts for all practical purposes like an ultraviolet photon," said Dowling. "The neat thing is that all our optics and lenses and mirrors only need to handle the red photons because that's what's really propagating through the system. But at the point of absorption on this lithographic substrate the resolution is as if we had used UV."
As more photons are entangled, the fraction of the wavelength of the original light grows smaller, Dowling said.
"If you put together four red photons as an entangled quantum state, its imaging properties are equivalent to x-rays, without any of the imaging problems associated with really dealing with x-rays," he said.
"This is a great idea," said Yanhua Shih, a professor of physics at University of Maryland, Baltimore County. Shih is one of the researchers who discovered that entangled photons can produce half-wavelength diffraction.
Producing and sustaining entangled particles, however, is extremely difficult. For this quantum property to be useful, researchers will have to develop better methods of producing entangled photons, Dowling said.
"We're hoping this paper will stimulate people to look more intensely at how to make higher order entangled states of three, four and five entangled photons," he said.
Another challenge is developing materials that can absorb entangled photons, Dowling said.
Shih is experimenting with two-photon absorption materials. "At this moment, we're still struggling to find this kind of new material because most of the two-photon absorption material is not sensitive enough to give us very good results," Shih said. "We know it's there and the diffraction pattern is narrower, but in order to have publishable results we have to have very good contrast and lower the noise."
Researchers should be able to demonstrate the effect in six months to one year, according to Dowling. However, it could be 20 years before sources of entangled photons are bright enough to use in mass production, he said.
Dowling's co-authors were Agedi N. Boto, Pieter Kok, Daniel S. Abrams, Samuel L. Braunstein and Colin P. Williams. Their work was published in the September 25, 2000 issue of the journal Physical Review Letters. The research was funded by NASA. Shih's research is funded by the National Security Agency, the Office of Naval Research, and the National Science Foundation.
Timeline: <1 year; 20 years
TRN Categories: Integrated Circuits
Story Type: News
Related Elements: Technical paper "Quantum Interferometric Optical Lithography: Exploiting Entanglement to Beat the Diffraction Limit" in Physical Review Letters September 25, 2000
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