By integrating single-photon detectors with nanophotonic chips, an international team of scientists has developed a way to reliably detect single photons for optical data transmission and quantum computations. Without reliable detection of single photons, it is impossible to make real use of the latest optical advances. Although numerous single-photon detector models have been developed over the past few years, thus far, none have provided satisfactory performance. Now, physicists at the Karlsruhe Institute of Technology, in cooperation with colleagues at Yale, Boston and Moscow State Pedagogical universities, have developed a single-photon detector that combines near-unity detection efficiency with high timing resolution and a very low error rate. The prototype, tested in the telecommunications wavelength range, achieves a previously unattained detection efficiency of 91 percent. The single-photon detector developed at Karlsruhe Institute of Technology is characterized by five convincing factors: 91% detection efficiency, direct integration onto the chip, counting rates on a gigahertz scale, high timing resolution and negligible dark count rates. Courtesy of KIT/CFN. The detector, developed within the Integrated Quantum Photonics project at the German Research Association’s Center of Functional Nanostructures, was realized by fabricating superconducting nanowires directly on top of a nanophotonic waveguide. This geometry is comparable to a tube that conducts light, around which a wire in a superconducting state is wound and, as such, has no electric resistivity. The nanometer-sized niobium nitride wire absorbs photons that propagate along the waveguide. When a photon is absorbed, superconductivity is lost, which is detected as an electrical signal. The longer the tube, the higher the detection probability. The direct installation of the detector onto the chip makes it possible to replicate it at random. Single-photon detectors built thus far have been stand-alone units, connected to chips with optical fibers. Arrangements of this type suffer from photons being lost in the fiber connection or being absorbed in other ways. Such loss channels are eliminated when the detector is fully embedded in a silicon photonic circuit. In addition to high detection efficiency, this design enables a remarkably low dark count rate. Dark counts arise when a photon is detected erroneously — for instance, because of a spontaneous emission, an alpha particle or a spurious field. The new design also provides ultrashort timing jitter of 18 ps. Several hundred of these detectors can be integrated on a single chip — the basic precondition for future use in optical quantum computers. The detector demonstrated in this study was designed to work at wavelengths in the telecom bandwidth, but the same architecture can be used for other wavelengths, such as those in the visible spectrum. This would allow the principle to be employed in analyses of all structures that emit little light — i.e., photons — such as single molecules or bacteria. The results were published in Nature Communications (doi: 10.1038/ncomms2307). For more information, visit: www.kit.edu