An ultrafast, super-efficient single-photon detector has led to direct entanglement of three photons. Researchers at the University of Waterloo, with help from a team at the National Institute of Standards and Technology (NIST), made the discovery using a superconducting nanowire detector developed at NIST. Critical for quantum information systems, entanglement has long been impossible for more than two photons without also destroying their fragile quantum states. Superconducting nanowire single-photon detectors assisted in the successful three-photon entanglement. Images courtesy of NIST. “The NIST detectors enabled us to take data almost 100 times faster,” said Krister Shalm, a postdoctoral researcher at Waterloo and also a NIST physicist. The detector incorporates tungsten silicide, which greatly boost efficiency. In the study, the researchers used a blue photon that was polarized both vertically and horizontally. The photon was then sent through a crystal that converted it into two entangled red daughter photons, each with half the original energy. The system ensured that this pair had identical polarization. Next, one daughter photon was sent through another crystal to generate two near-infrared granddaughter photons, which entangled with the remaining daughter photon. NIST’s single-photon detectors were able to detect very low light levels. This resulted in three entangled photons with the same polarization, either horizontal or vertical, potentially representing 0 and 1 in a quantum computer or communications system. The granddaughter photons possessed a common wavelength, as well, allowing them to be transmitted through fiber for practical applications. The photon triplet finding is quite rare, the researchers said, because the initial stages of this cascaded down-conversion process are not typically successful. The researchers also measured one of each of a succession of triplets to show that they could herald the presence of remaining entangled pairs. Such an on-demand system could prove useful in quantum repeaters, extending the range of quantum communications systems and sharing of secret data encryption keys. Improvements in conversion efficiency could make possible the addition of more stages to the down-conversion process, potentially generating four or more entangled photons. The work was funded by the Ontario Ministry of Research and Innovation Early Researcher Award, Quantum Works, the Natural Sciences and Engineering Research Council of Canada, Ontario Centers of Excellence, Industry Canada, the Canadian Institute for Advanced Research, Canada Research Chairs and the Canadian Foundation for Innovation. The research was published in Nature Photonics (doi:10.1038/nphoton.2014.218). For more information, visit www.uwaterloo.ca.