Solution
= Solution
The <Helstrom-Holevo bound> for equal priors is
$$
P_{\max}=\frac12\left(1+\sqrt{1-|\langle\alpha_0|\alpha_1\rangle|^2}\right).
$$
Perfect cloning twice changes overlap $c$ to $c^2$, so discrimination would succeed with $\frac12(1+\sqrt{1-c^4})$, strictly exceeding $\frac12(1+\sqrt{1-c^2})$ for $0<c<1$. For $|0\rangle,|+\rangle$, $c=1/\sqrt2$, producing $\frac12(1+\sqrt3/2)>\frac12(1+1/\sqrt2)$, a contradiction.