24th August 2019 Breakthrough in quantum teleportation Austrian and Chinese scientists report the first teleportation of three-dimensional quantum states, or "qutrits", which are more complex than two-dimensional qubits.
Researchers from the Austrian Academy of Sciences and the University of Vienna – in collaboration with quantum physicists from the University of Science and Technology of China – have experimentally demonstrated what was previously only a theoretical possibility. Writing in Physical Review Letters, they describe the teleportation of complex, high-dimensional quantum states. In their study, the researchers teleported the quantum state of one photon (light particle) to another distant one. Previously, only two-level states ("qubits") had been transmitted by this method, i.e. information with values of "0" or "1". However, the team succeeded in teleporting a three-level state – known as a "qutrit". In quantum physics, unlike in classical computer science, "0" and "1" are not an 'either/or' – both values simultaneously, or anything in between, is also possible. The Austrian-Chinese team has now demonstrated a third possibility, "2". It has been known since the 1990s that multidimensional quantum teleportation is theoretically possible. However, it took until now to design an experimental method and to develop the necessary technology. The quantum state to be teleported is encoded in the possible paths a photon can take. These can be imagined as three optical fibres, with a single photon able to be located in all three optical fibres at the same time. Teleporting this multi-dimensional information required the so-called Bell measurement based on a multiport beam splitter, directing photons through several inputs and outputs and connecting all optical fibres together. In addition, the scientists used "auxiliary" photons in the beam splitter, which can interfere with the other photons. Through clever selection of certain interference patterns, the quantum information was transferred to another photon far from the input photon, without the two ever physically interacting. This breakthrough is an important step towards practical applications such as a future quantum internet, since high-dimensional quantum systems can transport larger amounts of information than qubits. "This could help to connect quantum computers with information capacities beyond qubits," explains Anton Zeilinger, quantum physicist at the Austrian Academy of Sciences. The experimental concept is not limited to three dimensions, but could in principle be extended to any number of dimensions – according to study co-author Manuel Erhard, from the Austrian Academy of Sciences. In future work, the quantum physicists will focus on how to extend the newly gained knowledge to enable teleportation of the entire quantum state of a single photon or atom.
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