LAUSR

We present a theoretical study of the temporal and spatial coherence properties of a topological laser device built by including saturable gain on the edge sites of a Harper--Hofstadter lattice for photons. In the comoving frame of its chiral motion, the lasing edge mode behaves very similarly to a one-dimensional system, and the correlations of long-wavelength fluctuations display the peculiar Kardar-Parisi-Zhang (KPZ) scaling. At longer times, the finite size of the device starts to matter, and the functional form of the coherence decay changes from a KPZ stretched exponential to a pure exponential. Still, the nonlinear dynamics of KPZ fluctuations reflects into an enhanced linewidth as compared to a single-mode Schawlow-Townes phase diffusion. While all these features are general to extended laser arrays, the role of topology in protecting the coherence from static disorder is finally highlighted. This opens exciting possibilities and intriguing open questions both for fundamental studies of non-equilibrium statistical mechanics and for concrete applications to laser devices.