Number 273 (Story #2), May 31, 1996 by Phillip F. Schewe and Ben Stein SUPERCONDUCTING TUNNEL JUNCTIONS (STJ) , under development as efficient detectors of x rays, can now also be used as single-photon detectors at visible wavelengths. In this regard they will be welcomed by astronomers who increasingly record incoming light with charge-coupled device (CCD) arrays. In contrast to the silicon-based CCDs, which are insensitive to a photon's energy (one photon engenders one electron in the detector), the niobium-based STJ's do discriminate as to energy (one photon, depending on its energy, can generate thousands of electrons). Determining a photon's energy would allow astronomers to forego filters, which lower the detector's overall efficiency. A STJ device developed by an Oxford-Cambridge-European Space Agency (Netherlands) collaboration can detect light in the wavelength range 200-500 nm with a spectral resolution of 45 nm (this should improve to 20 nm or better). The STJ can also determine the photon's time of arrival at the millisecond level, a property the would be handy for studying fast astronomical processes such as pulsars. (A. Peacock et al., Nature, 9 May 1996.)