We develop and implement automated methods for optimizing quantum circuits of
the size and type expected in quantum computations that outperform classical
computers. We show how to handle continuous gate parameters and report a
collection of fast algorithms capable of optimizing large-scale quantum
circuits. For the suite of benchmarks considered, we obtain substantial
reductions in gate counts. In particular, we provide better optimization in
significantly less time than previous approaches, while making minimal
structural changes so as to preserve the basic layout of the underlying quantum
algorithms. Our results help bridge the gap between the computations that can
be run on existing hardware and those that are expected to outperform classical
computers.Comment: 21 page