Disentangling unitary dynamics with classically simulable quantum circuits

We explore to which extent it is possible to construct efficient classical simulations of quantum many body systems using a combination of tensor network methods and the stabilizer formalism. For this, we study both quantum circuits and Hamiltonian dynamics. We find that expectations of Pauli operators can be simulated efficiently even for deep Clifford circuits doped with T-gates or more general phase gates, provided the number of non-Clifford gates is smaller or approximately equal to the system size. This is despite the fact that the resulting states exhibit both extensive entanglement and extensive nonstabilizerness. For the Hamiltonian dynamics, we find that the classical simulation generically quickly becomes inefficient, but suggest the use of matchgate circuits alongside tensor networks for the efficient simulation of many-body quantum systems near free-fermion integrability.