Pure dephasing is the dominant leak mechanism in photonic cat qubits because its phase errors disrupt the parity protection, rendering the qubit vulnerable to energy relaxation. In this paper, we reveal that this dephasing mechanism conceals an interesting physical phenomenon: it induces asymmetric leakage from the cat-state subspace, where even- and odd-parity cat states decay at different rates. This leak asymmetry enables the dynamics of the system to be described by a non-Hermitian Hamiltonian, thereby transforming the cat qubit into a platform with controllable gain and loss for probing non-Hermitian physics. Importantly, we demonstrate that the cat-state manifold provides unique hardware-level protection for non-Hermitian simulation, including the exponential suppression of frequency detuning errors and the capability to distinguish stochastic single-photon loss via real-time parity monitoring. These features enable our protocol to determine the location of exceptional points with significantly higher precision than traditional systems, such as coupled linear resonators, which are susceptible to undesired environmental noise. Within this platform, we demonstrate the possibility to control the parity-time symmetry phase transition in a single cat qubit by adjusting its amplitude. Moreover, we couple two cat qubits to realize an entanglement phase transition induced by the exceptional point. Our work constructs a controllable non-Hermitian system simulator, overturning the conventional paradigm that treats dephasing as harmful noise.

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• PT-symmetric quantum mechanics
• Quantum optics
• Quantum simulation