Two-Qubit Circuit Depth and the Monodromy Polytope

For a native gate set which includes all single-qubit gates, we apply results from symplectic geometry to analyze the spaces of two-qubit programs accessible within a fixed number of gates. These techniques yield an explicit description of this subspace as a convex polytope, presented by a family of linear inequalities themselves accessible via a finite calculation. We completely describe this family of inequalities in a variety of familiar example cases, and as a consequence we highlight a certain member of the XY-family for which this subspace is particularly large, i.e., for which many two-qubit programs admit expression as low-depth circuits.