The ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry-protected topological (SPT) phase hosts a robust boundary qubit at zero temperature. At finite energy density, the SPT phase is destroyed and bulk observables equilibrate in finite time. Nevertheless, we predict parametric regimes in which the boundary qubit survives to arbitrarily high temperature, with an exponentially longer coherence time than that of the thermal bulk degrees of freedom. In a dual picture, the persistence of the qubit stems from the inability of the bulk to absorb the virtual ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ domain walls emitted by the edge during the relaxation process. We confirm the long coherence times via exact diagonalization and connect it to the presence of a pair of conjugate almost strong zero modes. Our results provide a route to experimentally construct long-lived coherent boundary qubits at infinite temperature in disorder-free systems. To this end, we propose and analyze an implementation using a Rydberg optical-tweezer array and demonstrate that the difference between edge- and bulk-spin autocorrelators can be distinguished on timescales significantly shorter than the typical coherence time.

• Received 8 February 2020
• Accepted 9 October 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.200506