Researchers at the University of Toronto are working to address the challenges of transmitting quantum information securely over great distances using optical fiber communication. They have developed a prototype for a key element for all-photonic quantum repeaters, called all-photonic time-reversed adaptive (TRA) Bell measurement, that could eliminate or reduce many of the shortcomings of standard quantum repeaters. With collaborators at Osaka University, Toyama University, and NTT Corporation, University of Toronto professor Hoi-Kwong Lo and his team have demonstrated proof-of-concept of their work. Professor Hoi-Kwong Lo (ECE) and his collaborators have performed a proof-of-principle experiment on a key aspect of all-photonic quantum repeaters. Courtesy of Jessica MacInnis. One of the most promising technologies for transmitting quantum data is quantum key distribution (QKD), a technique that exploits the fact that any third party eavesdropping on a quantum system would leave behind a clearly detectable trace. Until now, this type of quantum security has been demonstrated in small-scale systems only. The team’s TRA measurement, based only on optical devices, without any quantum memories or any quantum error correction, passively but selectively performs the Bell measurement on single photons that have successfully survived their lossy travel over optical channels. The team implemented an experiment based on a scheme it obtained by invoking the concept of time-reversal in all-photonic quantum repeaters and then combining a local delayed preparation of a multipartite Greenberger-Horne-Zeilinger (GHZ) state with a special feature of a type-II fusion gate. The experiment showed that only the survived single-photon state was teleported without disturbance from the other lost photons. The researchers said that in principle, once the GHZ state was treated in a lossless manner, their scheme could double the achievable distance of QKD. “We have developed all-photonic repeaters that allow time-reversed adaptive Bell measurement,” Lo said. “Because these repeaters are all-optical, they offer advantages that traditional, quantum memory-based-matter repeaters do not. For example, this method could work at room temperature.” Because light signals lose potency as they travel long distances through fiber optic cables, repeaters are used to amplify the signals and help transmit the information along the line. Existing repeaters for quantum information require storage of the quantum state at the repeater sites, making the repeaters error-prone, difficult to build, and very expensive because they often operate at cryogenic temperatures. “An all-optical network is a promising form of infrastructure for fast and energy-efficient communication that is required for a future quantum internet,” Lo said. “Our work helps pave the way toward this future.” The research was published in Nature Communications (https://doi.org/10.1038/s41467-018-08099-5).