Hybrid mechanics of coupled qubits

We find that a technique analogous to hybrid mechanics previously developed for Gaussian coherent states and molecular vibrations, can be extended to SU(2) coherent states describing coupled qubits. This formulation reveals a sparse structure of the propagator, leading to a computationally efficient scheme while maintaining numerical accuracy. As a result, the method enables simulations of larger qubit systems on classical computers. In particular, we demonstrate accurate real-time propagation for systems of up to 21 qubits, which would be computationally demanding using straightforward quantum-dynamics approaches.