Strong zero modes in a class of generalized Ising spin ladders with plaquette interactions

We study a class of spin-1/2 quantum ladder models with generalized plaquette interactions in the presence of a transverse field. We show that in certain parameter regimes, these models have strong zero modes responsible for the long relaxation times of edge spins. By exploiting an infinite set of symmetries in these systems, we show how their Hamiltonians can be represented, in each symmetry sector, by a transverse field Ising chain. Due to the presence of an extensive number of conserved quantities, even if the original system has no disorder, most of these symmetry sectors feature a quasirandom transverse field profile. This representation of the ladder system in terms of a disordered Ising chain allows us to explain the features of the edge autocorrelation function of the original system. Furthermore, we find what appears to be an interesting mechanism for slow decorrelation: Even in parameter regimes where the full ladder model does not possess an obvious strong zero mode, some of the initial information stored in the edge spins can be preserved for long times as a consequence of the existence of strong zero modes within individual symmetry sectors.