Photodetectors typically require high gain to respond to the energy of a single photon, but gain mechanisms take time to reset between photon events and thus limit the detector bandwidth to approximately 10 MHz. High gain also makes single-photon detectors susceptible to noise, because even an extremely small initial noise is amplified many times. A new superconducting bridge photodetector, however, has demonstrated a bandwidth greater than 10 GHz and a noise floor of less than 0.001 count per second.When electron pairs in a superconductor absorb photons, they split and create areas of resistance. By monitoring the changes in voltage in a superconducting strip of niobium nitride, researchers have developed an ultrafast single-photon detector with negligible dark noise. Courtesy of the University of Rochester.Researchers at the University of Rochester and at Moscow State Pedagogical University constructed the detector by depositing a 1-µm-long, 200-nm-wide, 5-nm-thick stripe of niobium nitride on a sapphire substrate. Roman Sobolewski of the Rochester group said that potential applications for the device already include sensing ultraweak electroluminescence and integration into high-data-rate free-space optical communications systems.At 4.2 K, NbN becomes a superconductor. Its electrons form Cooper pairs, and its resistance drops to zero. The electron pairs can be split when they absorb energy two to three orders of magnitude less than in a semiconductor photodetector. And when an electron pair absorbs the energy from a photon, not only do the initial two electrons heat up, but they also transfer energy to other electrons through electron-electron or electron-phonon interactions.This increase in electron energy that surrounds the point of photon absorption creates a region of normal resistance. Current flowing through the strip can travel the preferred zero-resistance path only by passing to the side of this local hot spot. If the strip is biased with the proper current flow, however, the current in the regions neighboring the hot spot will exceed the critical current and form a nonsuperconducting barrier across the entire strip. This produces a measurable drop in voltage that the researchers can interpret as a detection event.The area created by photon absorption is cooled by electron-phonon interactions and returns to the superconducting state in approximately 30 ps, enabling high-bandwidth operation. Because the device does not require intrinsically high gain, its noise is extremely low; in a 1000-s measurement sequence, the blocked detector yielded zero counts. In addition, the new detector responds to wavelengths from 400 nm to 3 µm and displays a quantum efficiency of 20 percent at 810 nm.Further avenuesSobolewski said that the researchers intend to improve the device's efficiency by constructing a detector with a meander-strip geometry. They also are designing SuperBob, a 10-GHz single-photon-detection and -counting system for quantum cryptography, and hope to build devices using MgB2, which should display even faster response times.Even then, the need for speed will not be satisfied. "Ultimately, we hope to fabricate structures out of high-temperature superconductors," said Sobolewski, "which we know from our time-resolved experiments are characterized by single-picosecond, or even subpicosecond response times."The researchers reported the device in the Aug. 6 issue of Applied Physics Letters.