  We present a quantum algorithm for approximating maximum independent sets of
a graph based on quantum non-Abelian adiabatic mixing in the sub-Hilbert space
of degenerate ground states, which generates quantum annealing in a secondary
Hamiltonian. For both sparse and dense graphs, our quantum algorithm on average
can find an independent set of size very close to $\alpha(G)$, which is the
size of the maximum independent set of a given graph $G$. Numerical results
indicate that an $O(n^2)$ time complexity quantum algorithm is sufficient for
finding an independent set of size $(1-\epsilon)\alpha(G)$. The best classical
approximation algorithm can produce in polynomial time an independent set of
size about half of $\alpha(G)$.
