Multiple imperfect photon sources can be combined on a single silicon chip to produce a much higher-quality source, say researchers in Australia. Such “photons on demand” will help create extremely compact optical chips. Researchers at the Australian Research Council Centre of Excellence for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS) at the University of Sydney say that developing a chip that can deliver one photon at a time at very high rates would provide scalability for diverse quantum technologies that could enhance computing and communication infrastructure. “This result has applications in the development of complex quantum technologies, including completely secure communications, quantum measurement, the simulation of biological and chemical systems and of course quantum computing,” said team leader Dr. Alex Clark. While the creation of a single photon in an optical circuit has been possible for some years, previous demonstrations were difficult to implement and scale up, or have been excessively noisy. This has limited the single-photon technology to being either very slow or having a high probability of error. A single laser is coupled to a monolithic silicon chip to pump an ultracompact array of silicon PhCWs. Photon pairs are generated in the PhCW region, wavelength-separated by integrated arrayed-waveguide gratings (AWGs) and the heralding photons detected using single-photon detectors. The remaining photons go through a delay line, while a fast electronic logic gate sets the state of the PLZT switch. The selected heralded photon is then routed to the common fiber output. The orange box region represents the experimental setup for results presented in this work. Inset: our first implementation used a device with two separate but monolithically fabricated PhCWs, designated A1 and B1. Courtesy of Nature Communications. “It is easy for us to generate photons at high rates, but it’s much harder to ensure they come out one by one because photons are gregarious by nature and love to bunch together,” said lead author Matthew Collins, a Ph.D. student at CUDOS. “For that reason, the quantum science community has been waiting over a decade for a compact optical chip that delivers exactly one photon at a time at very high rates.” A pulse of laser light approaches the silicon PhCW shown at a speed of c/n0, where c is the speed of light and n0 is the refractive index of silicon. When the pulse enters the PhCW, it is spatially compressed and travels at a slower speed through the medium c/ng, where ng is the group index in the PhCW. From this, a slow-down factor of S=ng/n0 is defined. Two photons of the pulse are annihilated to generate signal and idler photons of higher and lower energy, shown as blue and red circles, with the process depicted in b. The process must obey energy and momentum conservation, depicted at right. The remaining pulse and photons then exit the PhCW into the fast-light regime. Courtesy of Nature Communications. “We’ve shown how multiple imperfect sources of photons on a single chip can be combined to produce a much higher quality source,” Clark said. They generated the photons using a pulsed laser. “A key breakthrough for this research was the CUDOS development of photonic chips that slow light,” said professor Ben Eggleton, CUDOS director and co-author of the research. “This makes single-photon generation more likely, reducing energy demands and allowing extremely compact devices with lengths no longer than 200 microns.” “The smaller these systems are, the more we can fit onto a chip, and the more we can fit onto a chip the more likely we are to guarantee a single photon when we want it,” said co-author Michael Steel, associate professor at Macquarie University and CUDOS’ science leader for Quantum Integrated Photonics. The CUDOS research team at the University of Sydney: (Left to right) Dr. Alex Clark, Michael Steel, professor Benjamin Eggleton, Jiakun He, Shayan Shahnia, Dr. Chunle Xiong, Trung Vo and Matthew Collins. Courtesy of the University of Sydney. The work is part of a wider collaboration involving Australian and international universities, including Macquarie, the universities of St. Andrews and York, as well as the Australian Defence Science and Technology Organisation. The next step is to integrate all the components onto a single chip so that an on-demand “push button” single-photon source can be deployed in future photonic quantum technologies. The work appears in Nature Communications (doi:10.1038/ncomms3582). For more information, visit: http://sydney.edu.au