For the first time, a heralded single photon has been generated from a silicon chip. The discovery – made by a consortium of researchers from the University of California, San Diego, the National Institute of Standards and Technology (NIST) and the Polytechnic Institute of Milan in Italy – overcomes an important barrier to generating single photons using a tiny, chip-scale device constructed from silicon. It could lead to applications in cryptography, radiometry, imaging and telemetry, and could pave the way for new devices for quantum communication, ultralow-power computing and other technologies, now that all three basic components of a quantum transceiver – sources, controllable circuits and detectors – have been demonstrated using silicon photonics. Heralded photons are the second in a pair of spontaneously generated photons: When the first hits a detector and provides timing information, it “heralds” the companion photon, which is then in a quantum mechanical single-photon state. The researchers fabricated the 0.5 x 0.5-mm device using CMOS-compatible processes on 200-mm silicon-on-insulator wafers at an external collaborative research foundry. The device operates at room temperature and generates quantum light in the near-1550-nm wavelength range. “This is in the infrared range, and it is technologically important because those wavelengths are used in today’s optical fiber networks,” said Shayan Mookherjea, an associate professor of electrical and computer engineering at UC San Diego’s Jacobs School of Engineering. “Chip-scale single-photon sources could be used in quantum devices, networks and systems to bring about enormous improvements over their classical counterparts, in terms of speed or security or computational complexity.” Dr. Shayan Mookherjea, associate professor in the Micro-/Nanophotonics Lab at UC San Diego. Courtesy of UC San Diego. In a recent demonstration, silicon waveguide circuits consisting of a network of controllable couplers and interferometers showed quantum interference and entanglement manipulation using off-chip light sources, and on-chip single-photon counters were formed using a superconducting layer deposited as a cladding of a silicon nanophotonic waveguide. “Silicon is not an efficient light emitter, so creating a single-photon source using silicon was challenging,” said Junrong Ong, a graduate student at UC San Diego. “Our demonstration of an on-chip, single-photon source is a first step towards achieving on a single silicon chip all the three main components needed for fully integrated quantum photonics.” “While a variety of single-photon sources have been developed, they often involve nonstandard fabrication processes or require cryogenic cooling,” said Kartik Srinivasan of NIST. “The devices studied by our team, in contrast, operate at room temperature and are built using mature fabrication techniques already applied in the manufacturing of computer chips.” To generate single photons, the scientists split pump photons into pairs at different wavelengths resulting from the optical nonlinearity present in the device. They next demonstrated the process of heralded single-photon generation using a novel silicon nanophotonic waveguide consisting of a linear array of coupled microresonators. “Our novel device not only provides plug-and-play resonant enhancement of desired processes, but it also suppresses undesired processes by filtering out nonresonant pump noise effects,” Mookherjea said. The devices used in the project were measured using telecommunications-band single-photon counters developed by professor Alberto Tosi and collaborators at Polytechnic Institute. The scientists performed a photon correlation measurement in which the heralded light was split into two separate paths and detected using single-photon counters. They confirmed that, when working with single photons, it should not be possible to see heralded photons on both detectors simultaneously, known as “anti-bunching.” The research was presented at CLEO:2012, the Conference on Lasers and Electro-Optics, held in May in San Jose.