  This paper concerns the existence of multiple rotating quasi-periodic
solutions for second order Hamiltonian systems with sub-quadratic potential.
Such solutions have the form $x(t+T)=Qx(t)$ for some orthogonal matrix $Q$. To
deal with such quasi-periodic solutions, we introduce the $\mathcal{Q}(s)$
index which is a development of the well known $S^1$ index. Applying the
$\mathcal{Q}(s)$ index, we give an estimate of the number for rotating
quasi-periodic orbits with a fixed period.
