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Quantum ‘Kiss’ Changes Color of Space

CAMBRIDGE, England, DONOSTIA-SAN SEBASTIÁN, Spain, and PARIS, Nov. 8, 2012 — Just as a little boy blushes when a girl ever so slightly brushes his cheek with her lips, so, too, does the minute gap between nanosize gold spheres flush red as electrons jump from one side to the other.

Using optical methods, scientists from Cambridge University, and the universities of the Basque Country and Paris have shown that spaces smaller than a nanometer between metals undergo color changes, thanks to the movement of electrons in the surrounding area — meaning that researchers can see quantum mechanics in action in air at room temperature.

Because electrons in a metal move easily, shining light onto a small crack pushes electric charges onto and off each crack face in turn at optical frequencies. The oscillating charge across the gap produces a plasmonic color for the ghostly region in between — but only when the gap is small enough.

The image shows, in an artistic manner, the change in color when a quantum tunnel effect is produced in a subnanometric gap. Courtesy of Cambridge University.

Illuminating the gap between electrons accumulating on the gold surfaces with white light caused a tunneling effect, enabling the electrons to jump from one to the other without the spheres coming into contact with one another. This reduced the accumulated charge on the surface of each sphere, changing the color of the gap. The scientists observed a color shift from red to blue as the gap shrank from 1 nm to just below 0.35 nm.

“It’s as if you can kiss without quite touching lips,” said project leader Jeremy J. Baumberg, a professor at Cambridge University’s Cavendish Laboratory. This quantum “kiss” is like the tension building between a flirtatious couple staring into each other’s eyes. As their faces get closer, the tension mounts, and only a kiss discharges the energy, he said.

“Lining up the two nanoballs of gold is like closing your eyes and touching together two needles strapped to the end of your fingers,” said Matthew M. Hawkeye, a research fellow at Cambridge. “It has taken years of practice to get good at it.”

For the scientists to be able to predict the color changes confirmed in the study, Dr. Javier Aizpurua of the Center for Materials Physics and the Donostia International Physics Center in Donostia-San Sebastián fused classical and quantum views of the world together.

“Modeling of so many electrons oscillating within the gold particles in response to a beam of light could not be described with existing theories,” Aizpurua said.

The results of the study have established a fundamental quantum limit for how tightly light can be trapped and could provide new ways to measure the world down to the scale of single atoms and molecules, paving the way for smaller optoelectronic devices.

The work appeared in Nature (doi: 10.1038/nature11653).

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