Recursive Cartan decompositions for unitary synthesis

Abstract: Recursive Cartan decompositions (CDs) provide a way to exactly factorize quantum circuits into smaller components, making them a central tool for unitary synthesis. Here we present a detailed overview of recursive CDs, elucidating their mathematical structure, demonstrating their algorithmic utility, and implementing them numerically at large scales. We adapt, extend, and unify existing mathematical frameworks for recursive CDs, allowing us to gain new insights and streamline the construction of new circuit decompositions. Based on this, we show that several leading synthesis techniques from the literature-the Quantum Shannon, Block-ZXZ, and Khaneja-Glaser decompositions-implement the same recursive CD. We also present new recursive CDs based on the orthogonal and symplectic groups, and derive parameter-optimal decompositions. Furthermore, we aggregate numerical tools for CDs from the literature, put them into a common context, and complete them to allow for numerical implementations of all possible classical CDs in canonical form. As an application, we efficiently compile fast-forwardable Hamiltonian time evolution to fixed-depth circuits, compiling the transverse-field XY model on $10^3$ qubits into $2\times10^6$ gates in 22 seconds on a laptop.