For the first time, a heralded single photon has been generated from a silicon chip. An international consortium of researchers from the University of California, San Diego, the National Institute of Standards and Technology (NIST) and the Politecnico di Milano of Milan, Italy, has overcome an important barrier to generate single photons using a tiny, chip-scale device constructed from silicon. Their discovery could lead to applications in cryptography, radiometry, imaging and telemetry, and to 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. Researchers have demonstrated that quantum light sources can be fabricated using silicon, the most widely used material underpinning modern electronics. Shown is a silicon photonic chip containing several dozen devices designed and fabricated by graduate students at UC San Diego. (Image: UC San Diego) 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 × 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. “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.” 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. Graduate student Junrong Ong is a finalist in the Maiman student paper competition. (Image: John Hanacek/Calit2, UC San Diego) “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. Dr. Shayan Mookherjea, associate professor of electrical and computer engineering, in the Micro-/Nanophotonics Lab at UC San Diego. A comprehensive test suite, including optical, electrical and radio-frequency characterization equipment, is available for developing chip-scale devices for both classical and quantum device applications. (Image: UC San Diego) “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 Politecnico di Milano. 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.” Results of the research will be presented May 10 at the Conference on Lasers and Electro-Optics (CLEO) in San Jose. For more information, visit: www.ucsd.edu