It was 1987, a year after high-temperature superconductivity was discovered in the cuprates. Over a lunch table in a small Chinese restaurant in downtown Tokyo, a group of physicists were excitedly discussing Philip Anderson’s new paper,1 which proposed that the insulating phase of the cuprates is a quantum spin liquid (QSL). Similar conversations probably took place among physicists around the world.

One of us (Imai), then a young graduate student, wondered aloud what the big deal was. An unamused old-timer snapped, “You can surely try to read the new paper!” Still, he went on to explain that in 1973 Anderson had examined the possibility of a peculiar destruction of magnetism exhibited by spins arranged in a triangular lattice.2 The new paper extended his original work to the square-lattice geometry found in the newly discovered cuprate superconductors.

The QSL, in theory, represents a new state of matter. Unlike conventional...

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