In quantum information theory, which theorem says an unknown quantum state cannot be copied perfectly?

The no-cloning theorem says that an unknown quantum state cannot be copied perfectly.

Wootters and Zurek, and independently Dieks, published the result in 1982. It follows from the linearity of quantum mechanics: a device that copied two different states would produce contradictory results when given a superposition of them.

The theorem does not prohibit copying known classical information or preparing many systems in a known state. It also does not prevent quantum teleportation, because teleportation transfers a state while destroying the original and requires classical communication.

A related result, the no-broadcasting theorem, concerns mixed quantum states. Together, these limits help explain why quantum information cannot be treated exactly like ordinary digital data.