Speck trios make secret codes

Researchers from the Canadian National Research Council have devised a way to use quantum dots -- tiny bits of semiconductor -- to print invisible secret codes onto surfaces like documents.

The quantum dots the researchers used in their prototype measured between 3 and 6 nanometers in diameter. A nanometer is one millionth of a millimeter.

The method could eventually be used to authenticate valuable documents like passports and certificates, according to the researchers.

Quantum dots can be made to emit one wavelength of light when hit with a second wavelength of light. The researchers method uses three quantum dots that emit three different colors of light. The intensity levels of the three lightwave peaks represent a three-digit code.

The code can be kept secret because the intensity levels change depending on the color of the light source. For example, three single-color quantum dots can emit fluorescence corresponding to the code of 2-7-3 when hit with 470-nanometer light waves, but the code changes to 3-5-3 when hit with 450-nanometer light waves and 6-9-5 when hit with 360-nanometer light waves.

The correct code can be read only by a person who knows the key, which is the correct wavelength of light for each set of three quantum dots contained in the cryptograph.

The method could be used practically in two to five years, according to the researchers. The work appeared in the June 28, 2004 issue of Optics Express.