A theoretical study reveals how to control and drive a quantum system without causing its decoherence.

Quantumness is famously fragile. Decoherence, particle loss, and other dissipative processes typically destroy delicate quantum superpositions, causing open quantum systems to behave classically. This universal, inevitable fate suggests that, even when a system’s constituents are fully quantum, its nonequilibrium critical points could be described by classical universality classes. That is, the system could belong to a group whose behavior near a critical point is identical and scale invariant regardless of microscopic details. In a new theoretical study, Rohan Mittal and his collaborators at the University of Cologne in Germany have overturned this expectation for open systems of fermions [1]. They identified a particular symmetry, which, if present, blocks most of the noise channels that would ordinarily wash out quantum behavior at large scales.