heats up when a photon is absorbed The subsequent rise and fall of the temperature is recorded to determine the photon energy. These detectors are employed by astronomers for rocket-based observations of photons at x-ray avelengths with an energy 100 to 1, 000 times that of visible photons. but thei sensitiv ity is not suf ficient to record visible photons, with an energy of 1.5 to 3 Over the past decade, new devices have been develo ped that are more sensitive and can record visible photons 3-9. These detectors fall into two classes. The first is the bolometer which uses the heating effect as mentioned above to determine photon energy. The thermometer inside such a device, recording the temperature change is a strip of partly superconducting metal3 5; the readout amplif ier is also superconducting. As superconductivity the flow of current without resistance -is a low-temperature property, the entire structure of the device is a cryogenic environment (which is easier to engineer than a device with only some superconducting components) The second class of detector makes use of the electron excitations created in solids by the energy of an incoming photon Numerous observations have been made with such detectors developed at the european Space agency z-9, including observations of the Crab nebula and short-period binary systems, known as polars. The readout e lectron ics of these detectors, however, operates at room temperature, with the consequence that the number of channels that can be read out is limite So far, the number of pixels in an image has been limited to 36. Clever device design mig ht mean that this number of pixels can be expanded by a factor of maybe 10 to 20, but the engineering is proving dif ficult Day et a/ i now present a new approach to the problem of photon detection. Their device contains a superconducting film held at low temperature. Inside the film resistanceless current flows in the form of electron pairs. If a photon hits the device, it can break up some of these pairs, with the result that the supercurrent becomes more 'sluggish'l0. That increase in slugg ishness can be detected in the surrounding microwave circuitry. The nature of the measurement is akin to watching an object on a spring vibrating at the natural oscillation rate: the rate of the spring s vibration reveals the mass(sluggishness) of the object The authors also show that cryogenic electronics systems already available are adequate for the readout of the signal from their device Device and readout together make the desired single-photon detector. Moreover, the method of readout used can be rather easily generalized to accommodate hundreds, and perhaps thousands of pixels. Other fields of science such as fluorescence microscopy studies of single molecules, should also benef it from this new detector technologyheats up when a photon is absorbed. The subsequent rise and fall of the temperature is recorded, to determine the photon energy. These detectors are employed by astronomers for rocket-based observations of photons at X-ray wavelengths, with an energy 100 to 1,000 times that of visible photons. But their sensitivity is not suf ficient to record visible photons, with an energy of 1.5 to 3 electronvolts. Over the past decade, new devices have been developed that are more sensitive and can record visible photons3-9. These detectors fall into two classes. The first is the bolometer, which uses the same heating ef fect as mentioned above to determine photon energy. The thermometer inside such a device, recording the temperature change, is a strip of partly superconducting metal3, 6; the readout amplifier is also superconducting. As superconductivity — the flow of current without resistance — is a low-temperature property, the entire structure of the device is a cryogenic environment (which is easier to engineer than a device with only some superconducting components). The second class of detector makes use of the electron excitations created in solids by the energy of an incoming photon5. Numerous observations have been made with such detectors developed at the European Space Agency7-9, including observations of the Crab nebula and short-period binary systems, known as polars9. The readout electronics of these detectors, however, operates at room temperature, with the consequence that the number of channels that can be read out is limited. So far, the number of pixels in an image has been limited to 36. Clever device design might mean that this number of pixels can be expanded by a factor of maybe 10 to 20, but the engineering is proving dif ficult. Day et al. 1 now present a new approach to the problem of photon detection. Their device contains a superconducting film held at low temperature. Inside the film, resistanceless current flows in the form of electron pairs. If a photon hits the device, it can break up some of these pairs, with the result that the supercurrent becomes more 'sluggish'10. That increase in sluggishness can be detected in the surrounding microwave circuitry. The nature of the measurement is akin to watching an object on a spring, vibrating at the natural oscillation rate: the rate of the spring's vibration reveals the mass ('sluggishness') of the object. The authors also show that cryogenic electronics systems already available are adequate for the readout of the signal f rom their device. Device and readout together make the desired single-photon detector. Moreover, the method of readout used can be rather easily generalized to accommodate hundreds, and perhaps thousands, of pixels. Other fields of science, such as fluorescence microscopy studies of single molecules, should also benefit f rom this new detector technology