Negative temperature: Atoms beyond absolute zero may be a new form of matter

A cloud of atoms with a temperature beyond absolute zero – which is also bizarrely hotter than any positive temperature imaginable – could be a mysterious new quantum state of matter.

Luca Donini at the University of Cambridge and his colleagues have put thousands of potassium atoms into this seemingly paradoxical situation by precisely manipulating their energy levels and quantum states. “Quantum mechanics allows you to do this, while classical thermodynamics would never allow it,” he says.

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