operators in the unitary group $ U (2^ N) $. We run gradient-based optimization in the Lie
algebra $\mathfrak u (2^ N) $ and use the exponential map to parametrize unitary matrices.
We argue that $ U (2^ N) $ is not only more general than the search space induced by an
ansatz, but in ways easier to work with on classical computers. The resulting approach is
quick, ansatz-free and provides an upper bound on performance over all ans\" atze on $ N …