Doubling up can single out photons. A new submillimeter-wave detector promises the sensitivity to spot single photons. In contrast to existing single-photon-detection schemes, the detector does not require a magnetic field, and it offers an expanded wavelength range. The new technique could lead to better instruments for astronomy, chemistry and biology, as well as to the development of novel photonic devices.Developed by researchers at the University of Tokyo and at Japan Science and Technology Corp. in Kawaguchi, Japan, the detector uses double quantum dots. A quantum dot, built out of semiconductor material, enables electrons to move freely parallel to the surface, but it prohibits their perpendicular motion.The researchers employed two quantum dots relatively close to each other in GaAs/AlGaAs. They electrically coupled an antenna to one, while the second formed the heart of a single-electron transistor. Each dot contained a few hundred electrons, and both were cooled to 70 mK.When a 0.6-mm photon struck the antenna, it freed an electron in the first dot, causing a change in the conductance of the single-electron transistor. This detectable change indicated a submillimeter-wave event.The researchers hope to refine the technique and to lower the detection limit so that single photons can be spotted reliably. Other work will broaden the device's reach into longer wavelengths, said Oleg Astafiev, a member of the project team.