Instantons and Rarefaction Pulses as Pathways to Global Phase Coherence in Gain-Based Optical Networks

Abstract

We investigate how to reliably remove unwanted global phase windings in gain-based optical oscillator networks, thereby ensuring convergence to the true synchronized configuration corresponding to the XY Hamiltonian's global minimum. Focusing on one-dimensional rings and two-dimensional toroidal lattices, we show that two key strategies greatly enhance the probability of reaching the defect-free state. First, operating at a low effective injection rate just above threshold, exploits the amplitude degree of freedom, allowing the system to form transient zero-amplitude holes: instantons in one dimension, or vortex-antivortex rarefaction pulses in two-dimensional space, that enable phase slips. Second, preparing the initial condition with amplitude or phase inhomogeneities can directly seed these amplitude collapses and prevent the system from getting trapped in higher-energy states with nonzero winding. Using the Stuart--Landau and Ginzburg--Landau equations as models for fast-reservoir lasers, we derive analytic and numerical evidence that even relatively minor amplitude dips can trigger global unwinding in 1D. We further demonstrate that the pump's slow annealing favours these amplitude-driven events, leading to improved success in finding the globally coherent ground state. These findings highlight the critical role of amplitude freedom in analogue solvers for XY optimization problems, showing how local amplitude suppression provides a direct route to ejecting topological defects.