Observation of Tunable Four-Photon Quantum Interference in a Dissipatively Coupled Anti-PT System

We report the first experimental observation of tunable four-photon quantum interference in a non-Hermitian anti-Parity-Time (anti-PT) symmetric system. While PT symmetry has been extensively explored in the semiclassical limit and two-photon regime, the effects of non-Hermiticity on multipartite states and higher-order correlations remain largely uncharted. Using a thin-film lithium niobate (TFLN) photonic chip, we engineer dissipative coupling between two spontaneous parametric down-conversion (SPDC) generation pathways via a shared loss reservoir. We demonstrate that this architecture supports a protected Dark state exhibiting remarkable robustness against defects, thereby enabling dissipative purification into a state residing in a decoherence-free subspace. Experimentally, we measure tunable four-photon correlations with high visibility. Crucially, our results reveal a distinct interference pattern that cannot be explained by the post-selected non-Hermitian effective Hamiltonian approach. By comparing our data with a Lindblad master equation model, we identify that quantum jumps—representing continuous environmental measurement—play a decisive role in shaping higher-order quantum correlations. This work not only establishes anti-PT photonics as a promising platform for resilient quantum state engineering but also provides fundamental insights into the irreversible nature of quantum measurement in open quantum systems.