Noise-induced asymmetric coupling and relaxation in giant atoms

Abstract The non-Markovian feedback effects in giant atoms can strongly suppress spontaneous emission. This protection, however, relies critically on precise symmetries in the coupling amplitudes and phases, which are vulnerable to inevitable noise in realistic settings. In this paper, we show the mechanism of breaking the symmetric coupling by introducing noise and the resultant relaxation. Furthermore, we propose a strategy to preserve the symmetry even in the presence of noise, which enables the quantum state to be remain protected. To be specific, the symmetric condition of using non-Markovian feedback to suppress relaxation is studied through a model of a two-level giant atom stochastically coupled to a waveguide. The distinct roles played by amplitude noise and phase noise in breaking the symmetric coupling are separately analyzed. While correlated amplitude noise preserves both symmetry and population, uncorrelated amplitude noise breaks the symmetric coupling and induces dissipation. In contrast, phase noises destroys the symmetric condition even when they are fully correlated. Besides, we further show that the noise spectrum shape modulates the dissipation strength. The results provide clear guidelines for designing robust giant-atom devices.