New mirror made for quantum research could catch counterfeit cash

Monday, April 13, 2020

En français.

Researchers at the University of Waterloo’s Institute for Quantum Computing (IQC) have developed a new kind of mirror that could be used to protect against counterfeit banknotes.

Photons—quantum particles of light—can be left or right-handed, like humans. In some applications, researchers need to be able to sort left from right-handed photons, for example to study the arrangement of molecules in a drug.

In a recent study led by Behrooz Semnani, a second-year postdoctoral fellow at IQC and the University of Waterloo's electrical and computer engineering department, researchers created a 2D structure that, just like a mirror, reflects photons with one handedness but, unlike an ordinary mirror, lets photons with the other handedness through. The structure is a photonic crystal mirror: a thin membrane with a repeating chiral pattern of holes and looks like a very fine strainer. The term chiral means a pattern possessing handedness, and not matching its mirror image, like the letters S or Z. 

The development of this new mirror will open up photon sorting in a wide range of industries, including the detection of counterfeit currency. Because the structure is less than a micrometer thick, it could be printed on government documents—such as banknotes—as a hidden security feature against counterfeiting.

“The structure is invisible to the naked eye,” Semnani said. “Its unique properties would allow the structure to be detected only upon inspection with light of the correct wavelength and polarization handedness. The structure can be mass produced, which is important for practical applications like detecting counterfeit currency.”

The research proves that photon sorting by left or right handedness can be done with thin 2D structures. This is counter to the previous belief that thick and complicated 3D structures were needed.

The study Spin-Preserving Chiral Photonic Crystal Mirrorby Semnani, Flannery, Al Maruf, and Bajcsywas published in Light Science and Applications by Nature Publishing Group on February 20, 2020.

This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund (CFREF).