  Simon in his FOCS'94 paper was the first to show an exponential gap between
classical and quantum computation. The problem he dealt with is now part of a
well-studied class of problems, the hidden subgroup problems. We study Simon's
problem from the point of view of quantum query complexity and give here a
first nontrivial lower bound on the query complexity of a hidden subgroup
problem, namely Simon's problem. Our bound is optimal up to a constant factor.
