Charge-coupled devices, or CCDs, have become commonplace in modern consumer electronics. They are used in digital cameras and camcorders and in document scanners. Introduced in the late 1970s, they have become the workhorse light detector for astronomers. But CCDs have a number of limitations. In particular, they do not detect the wavelength (and hence the color) of light. Digital cameras get around this by having red, blue and green filters over individual pixels or over three separate CCD arrays. Filters, however, reduce the sensitivity and are of no use for measuring wavelengths with any precision. Now a group of researchers at the Jet Propulsion Laboratory and the California Institute of Technology, led by Peter K. Day of JPL, has demonstrated a detector based on superconducting technology that can detect individual photons and identify their wavelength. Best of all, the detector seems well suited to being engineered into a large array like a CCD.
The heart of the detector is made out of a thin film of aluminum on a sapphire substrate. The aluminum is etched by standard photolithographic processes to form a meandering strip. When cooled to near absolute zero (less than one kelvin), the aluminum becomes superconducting. Like the vibrations of a tuning fork, current in the aluminum strip oscillates at a resonant frequency.
This article was originally published with the title Seeing Single Photons.