Regulation of non-Hermitian couplings in a spontaneous six-wave mixing process

Non-Hermitian quantum systems — characterized by parity–time symmetry and the emergence of exceptional points — have attracted increasing interest, particularly in light–matter interaction platforms where dissipation and radiative losses play a fundamental role. In this work, we investigate the modulation of coupling states between atomic energy levels during the generation of backward W-state triphotons via a spontaneous six-wave mixing (SSWM) process. By employing tunable external control beams, we realize dynamic modulation from the weak to the strong coupling regime, accompanied by a qualitative transition in triphoton coincidence statistics from anti-bunching to damped Rabi oscillations. Remarkably, the appearance of single exponential decays in the triphoton coincidence count traces marks the onset of an exceptional point in the non-Hermitian system. Furthermore, triphoton correlations in the τ21 and τ31 directions can be independently modulated by selectively tuning of the coupling state via ED1 and ED2, respectively. These results provide a versatile and coherent platform for the one-step generation of backward entangled triphoton states and offer a promising approach to engineering high-dimensional entanglement, with potential applications in quantum communication and quantum information processing.