A quantum calorimetric sensor developed at the National Institute of Standards and Technology in Boulder, Colo., displays photon-number-resolving operation at 1550 nm with an efficiency of 88.6 percent. The scientists behind the work suggest that the instrument is suitable for fundamental physics applications, such as testing a local realism interpretation of quantum mechanics, and may have potential applications in quantum cryptography and subdiffraction-limit lithography.The sensor features a 25 × 25-µm square of 20-nm-thick tungsten film that is chilled to below its superconducting transition temperature of 110 mK. Application of a bias voltage keeps the electrons in the material on the edge of the transition. When the tungsten absorber layer is exposed to 1550-nm photons, its temperature changes proportionally to the energy of the photons, producing a change in the current passing through the film that the researchers measure using a superconducting quantum interference device array.Increasing the number of absorber layers in the device and optimizing the layer thickness should improve the efficiency beyond the 88.6 percent demonstrated in tests of the sensor. In theory, the researchers note, it should be possible to increase the efficiency to well above 99 percent and to develop devices that are sensitive to any desired wavelength between the ultraviolet and the near-infrared.