50 Years of Physical Review A: The Legacy of Three Classics – Physics

Decades after Einstein described quantum entanglement as spooky action at a distance, it was still seen as a quirkyif fascinatingfeature of quantum theory. But that changed in the early 1990s when physicists predicted that entanglement would allow the state of one particle to be transferredor, teleportedto another. Suddenly entanglement was not just something weird, but something useful, says Guifre Vidal, a graduate student at the time.

To know how useful a quantum state was, you had to know how much entanglement it contained, and this was exactly what Vidal and other theorists wanted to quantify. In a 2002 paper, Vidal, by then a postdoc at the University of Innsbruck, Austria, and his coauthor Reinhard Werner of the Technical University of Braunschweig, Germany, focused on a measure of entanglement that would apply to mixed states. Compared with the pure states that had been considered before, mixed states are more representative of those that can be created in experiments.

The measure, known as logarithmic negativity, was attractive to experimentalists because it allowed them to compare entanglement in different setups, says Vidal. Negativity is still used today to assess, for example, new ways of creating entangled photons for quantum information technologies. But Vidal, who now works at X (formerly, Google X), values the entanglement measures for a different reason. In his view, it led researchers to powerful mathematical tools for describing quantum systems, called tensor networks. This formalism has influenced the development of quantum gravity, quantum field theory, quantum simulation, and artificial intelligence.

Jan Sperling, a theorist who recently began leading a group at Paderborn University in Germany, says Vidal and Werners paper influenced the way physicists characterize a quantum systems unique characteristicsin essence, its quantumness. It also laid the groundwork for so-called quantum resource theories. Much like thermodynamics sets limits on the efficiency of an engine, a resource theory defines the capabilities of a quantum system.

Sperling says he was drawn to the fundamental and application-oriented sides of entanglement, which he sees as increasingly converging. It is interesting to observe how basic concepts of quantum physics have inspired and revolutionized quantum information technologies, potentially benefiting society as a whole.

G. Vidal and R. F. Werner, Computable measure of entanglement, Phys. Rev. A 65, 032314 (2002).

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50 Years of Physical Review A: The Legacy of Three Classics - Physics