Fragile aspects of topological transition in lossy and parity-time symmetric quantum walks.

Quantum walks often provide telling insights about the structure of the system on which they are performed. In [Formula: see text]-symmetric and lossy dimer lattices, the topological properties of the band structure manifest themselves in the quantization of the mean displacement of such a walker. We investigate the fragile aspects of a topological transition in these two dimer models. We find that the transition is sensitive to the initial state of the walker on the Bloch sphere, and the resultant mean displacement has a robust topological component and a quasiclassical component. In [Formula: see text] symmetric dimer lattices, we also show that the transition is smeared by nonlinear effects that become important in the [Formula: see text]-symmetry broken region. By carrying out consistency checks via analytical calculations, tight-binding results, and beam-propagation-method simulations, we show that our predictions are easily testable in today's experimental systems.