  We show that the universal cover of a compact complex surface $X$ is the
bidisk $\HH \times \HH$, or $X$ is biholomorphic to $\PP^1 \times \PP^1$, if
and only if $K_X^2 > 0$ and there exists an invertible sheaf $\eta$ such that
$\eta^2\cong \hol_X$ and $H^0(X, S^2\Omega^1_X (-K_X) \otimes \eta) \neq 0$.
The two cases are distinguished by the second plurigenus, $P_2(X)\geq 2$ in the
former case, $P_2(X)= 0$ in the latter. We also discuss related questions.
