Quantum spin liquids:
Materials with interacting quantum spins that nevertheless do not order magnetically down to the lowest temperatures are candidates for a materials class called quantum spin liquids (QSLs). QSLs are characterized by long-range quantum entanglement and are tricky to study theoretically; an even more difficult task is to experimentally prove that a material is a QSL. [Science]
Whether they even exist is something of an open question. Noted in “The 50-year quest to create a quantum spin liquid may finally be over,” Karmela Padavic-Callaghan, NewScientist (9 May 2026; paywall):
This isn’t any ordinary rock. Anarakite – later renamed herbertsmithite – could be a rare type of matter known as a quantum spin liquid (QSL). Whether these occur naturally is hotly debated, but if the physicists who think they can are right, nature could be creating highly entangled states. Physicists know how to create entanglement, too, but only in limited ways, such as entangling particles of light or ultracold atoms. Entangling particles within a chunk of stuff has so far eluded them.
