Fragility of time-reversal symmetry protected topological phases

Time-reversal symmetry (TRS) underpins many interesting phenomena in quantum mechanics, including certain topological phases such as topological insulators and the Haldane phase. However, even if present at the microscopic level, TRS is effectively broken in the dynamics of open systems. In this talk, I will argue that phenomena protected by TRS are fundamentally unstable against coupling to their surroundings. System-environment interactions lead to the breaking of TRS in the sense that the system propagates irreversibly, and this same mechanism gives rise to processes that would be forbidden by TRS in an isolated system, thus compromising TRS-protected phenomena. Specifically, I will demonstrate that topological bound states at the edges of 1D topological systems protected by TRS are inevitably subject to decoherence. Analogously, in 2D systems this same mechanism compromises the quantized conductance of helical edge modes. Our results elucidate potential challenges in utilizing topological systems for quantum technologies, and may account for resistance measurements seen in experiments on quantum spin Hall systems.

MM & Nigel Cooper, Nature Physics (2020)
MM & Nigel Cooper, arXiv:2009.14650