Researchers at the University of Waterloo's Institute for Quantum Computing (IQC) have developed a method to efficiently produce nearly perfect entangled photon pairs from quantum dot sources, paving the way for secure quantum communications. “The combination of a high degree of entanglement and high efficiency is needed for exciting applications such as quantum key distribution or quantum repeaters, which are envisioned to extend the distance of secure quantum communication to a global scale or link remote quantum computers,” said Michael Reimer, professor at IQC and Waterloo's Department of Electrical and Computer Engineering. “Previous experiments only measured either near-perfect entanglement or high efficiency, but we're the first to achieve both requirements with a quantum dot.” The entangled photon source, an indium-based quantum dot embedded in a semiconductor nanowire (left), and a visualization of how the entangled photons are efficiently extracted from the nanowire (right). Courtesy of the University of Waterloo. By embedding semiconductor quantum dots into a nanowire, the researchers created a source that creates near-perfect entangled photons 65 times more efficiently than previous work. This new source, developed in collaboration with the National Research Council of Canada in Ottawa, can be excited with lasers to generate entangled pairs on command. The researchers then used high-resolution single photon detectors provided by Single Quantum in the Netherlands to boost the degree of entanglement. “Historically, quantum dot systems were plagued with a problem called fine structure splitting, which causes an entangled state to oscillate over time,” said Matteo Pennacchietti, a PhD student at IQC and Waterloo's Department of Electrical and Computer Engineering. As a result, slow detection systems could not measure the entanglement. The team overcame this by combining their quantum dots with a fast and precise detection system. “We can basically take a timestamp of what the entangled state looks like at each point during the oscillations, and that's where we have the perfect entanglement,” Pennacchietti said. To showcase future communications applications, Reimer and Pennacchietti worked with IQC professors Norbert Lütkenhaus and Thomas Jennewein and their teams. The group was able to simulate quantum key distribution (QKD), by using the generated oscillating two-photon Bell state to establish a secure key for QKD. The team believes that the developed quantum dot source holds significant promise in the future of secure quantum communications. The researchers expect their findings to lay the foundation for creating large arrays of deterministically positioned quantum dot sources for QKD networks. The research was published in Communications Physics (www.doi.org/10.1038/s42005-024-01547-3).