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A frictionless liquid that flows forever: that's superfluidity. Helium-3 only achieves it at thousandths of a degree above absolute zero. The theory of this state was developed by Lev Landau and Richard Feynman. Here, quantum order is protected by an energy gap — an invisible armor, much like a taut string: as long as the pluck is gentle, the tone stays pure. In a new experiment, helium was confined in channels hundreds of nanometers wide and made to vibrate like a tiny string. At low amplitude, superfluidity held, but a stronger 'pluck' sent the frequency sliding downward. The string went out of tune: the flow breached the defense, suppressed the gap, and the system's entropy, a measure of disorder, grew.

By measuring the frequency drop, scientists calculated the critical speed at which the protection vanishes. The results matched the theory. This approach paves the way for ultra-sensitive sensors and quantum devices based on superfluid liquids.