Nano antennas could pave way for quantum computing networks

A team of scientists has developed a way to control the direction of light on the nanoscale, by developing miniaturized television aerials made from gold nanorods, which can pave the way for quantum computing networks in the future.

At the moment, quantum physicists use cumbersome apparatus to try to keep track of photons, for instance, building large vacuum cavities with mirrored walls to guide light.

"It's funny that to control the small quantum world, you need huge pieces of equipment," said Holger Hofmann, at the Department of Quantum Matter at Hiroshima University in Japan.

Now, according to a report in Nature News, Hofmann and his colleagues have developed a way to control the direction of light on the nanoscale.

Their technique is based on the workings of the 'Yagi-Uda' antenna commonly used to transmit and detect radio waves and often seen on rooftops as television aerials.

Hofmann stumbled on the idea by accident, while teaching his electromagnetism class how antennas work.

"The textbook didn't explain it well, and while trying to come up with my own picture, I realized that the same technique could work on the nanoscale," he said.

A standard Yagi-Uda antenna is made up of a set of parallel metal rods that gradually decrease in length.

An electrical signal is fed into the second longest rod, setting it vibrating and producing a driving electromagnetic wave that spreads out in all directions.

This stimulates the neighbouring rods to oscillate and emit secondary waves.

Both the length and spacing of adjacent rods are carefully set at fractions of the wavelength of the driving wave, so that the secondary waves interfere with the driving wave, amplifying it along the forward direction and reducing it along the sideways and backward directions.

Hofmann and his colleagues realized that gold nanorods should produce the same effect on the nanoscale - but here, the length-to-width ratio of the rods, rather than their length and spacing, is important.

The team etched their mini gold antenna into a glass substrate and drove it directly with red laser light.

The tricky part was to ensure that just one nanorod was driven by the incoming light, just as only one metal rod is in the Yugi-Uda antenna.

To ensure this, the team tilted the chosen nanorod by 45 degrees relative to its neighbours and stimulated it using laser light that was polarized at the same angle.

They then monitored the direction of light transmitted out of the glass substrate.

The result was actually better than their theory predicted, according to Hoffman, with roughly two-thirds of the input light being directed largely forwards.