An integrated photon source with the potential to deliver large-scale quantum photonics has been developed by physicists at the University of Bristol. Integrated quantum photonics is a promising platform for developing quantum technologies due to its capacity to generate and control photons in miniaturized optical circuits. Leveraging the mature CMOS silicon industry for the fabrication of integrated devices would enable circuits with the equivalent of thousands of optical fibers and components to be integrated on a single mm-scale chip. One challenge that has limited the scaling of integrated quantum photonics has been the lack of on-chip sources able to generate high-quality single photons. “Without low-noise photon sources, errors in a quantum computation accumulate rapidly when increasing the circuit complexity, resulting in the computation being no longer reliable,” researcher Stefano Paesani said. “Moreover, optical losses in the sources limit the number of photons the quantum computer can produce and process.” The silicon photonic chip used in this study to generate and interfere high-quality photons. Courtesy of Stefano Paesani. To produce high-quality photons, the researchers developed a technique called intermodal spontaneous four-wave mixing. The photon sources were fabricated in silicon using mature processes. The researchers exploited a dual-mode pump-delayed excitation scheme to engineer the emission of spectrally pure photon pairs through intermodal spontaneous four-wave mixing in low-loss spiraled multimode waveguides. Using this technique, they created near-ideal conditions for generating single photons. Working with colleagues at the University of Trento, the Bristol team benchmarked the use of such sources for photonic quantum computing in a heralded Hong-Ou-Mandel experiment and, according to the researchers, obtained the highest quality on-chip photonic quantum interference ever observed (96% visibility). “The device demonstrated by far the best performances for any integrated photon source: spectral purity and indistinguishability of 99% and > 90% photon heralding efficiency,” Paesani said. The silicon photonic device was fabricated using CMOS-compatible processes in a commercial foundry, which means that thousands of sources could be integrated on a single device using the technique developed by the Bristol team. The research could be a major step toward building quantum circuits at scale. “We have solved a critical set of noises that had previously limited the scaling of photonic quantum information processing,” Paesani said. “For example, arrays of hundreds of these sources can be used to build near-term noisy intermediate-scale quantum (NISQ) photonic machines, where tens of photons can be processed to solve specialized tasks, such as the simulation of molecular dynamics or certain optimization problems related to graph theory.” Now that the researchers have devised a way to build near-perfect photon sources, over the next few months the scalability of the silicon platform will allow them to integrate tens to hundreds of sources on a single chip. Developing circuits at such a scale could enable NISQ photonic quantum machines to solve industry-relevant problems beyond the capability of current supercomputers. “Furthermore, with advanced optimization and miniaturization of the photon source, our technology could lead to fault-tolerant quantum operations in the integrated photonics platform, unleashing the full potential of quantum computers,” Paesani said. The research was published in Nature Communications (www.doi.org/10.1038/s41467-020-16187-8).