No unitary can copy arbitrary unknown quantum state: U|ψ⟩|0⟩ = |ψ⟩|ψ⟩ fails by linearity. Foundation of quantum cryptography.
No unitary can copy arbitrary unknown quantum state: U|ψ⟩|0⟩ = |ψ⟩|ψ⟩ fails by linearity. Foundation of quantum cryptography.