Mean-chiral displacement in coherently driven photonic lattices and its application to synthetic frequency dimensions.

Characterizing topologically nontrivial photonic lattices by measuring their topological invariants is crucial in topological photonics. In conservative one-dimensional systems, a widely used observable to extract the winding number is the mean-chiral displacement. In many realistic photonic systems, however, losses can hardly be avoided, and little is known on how one can extend the mean-chiral displacement to a driven-dissipative context. Here we theoretically propose an experimentally viable method to directly detect the topological winding number of one-dimensional chiral photonic lattices. The method we propose is a generalization of the mean-chiral displacement to a driven-dissipative context with coherent illumination. By integrating the mean-chiral displacement of the steady state over the pump light frequency, one can obtain the winding number with a correction of the order of the loss rate squared. We demonstrate that this method can be successfully applied to lattices along synthetic frequency dimensions. While various photonic lattices with topologically nontrivial band structures have been realized, how to measure topological invariants in such systems with photon losses has not been clearly understood. We find that the topological winding number can be measured even in the presence of photon loss by generalizing the concept of the mean-chiral displacement originally developed for lossless systems. [ABSTRACT FROM AUTHOR]