Observing the dynamics of quantum states generated inside nonlinear optical cavities.

Observing non-classical properties of light is a long-standing interest to advance a wide range of quantum applications. Optical cavities are essential to generate and manipulate non-classical light. However, detecting changes in cavity properties induced by the quantum state remains a critical challenge in the optical domain due to the weak material nonlinearity. Here, we propose a framework for observing the dynamics of quantum states generated inside nonlinear optical cavities. We leverage the symmetry-breaking process of a bistable system, which is highly sensitive to the initial state, enabling detection of quantum state displacement through an asymmetric equilibrium of a macroscopic observable. With a nonlinear response at the single photon level, our approach directly imprints the cavity field distribution onto the statistics of bistable cavity steady-states. We experimentally demonstrate our approach in a degenerate optical parametric oscillator, generating and reconstructing different quantum states. As a validation, we reconstruct the Husimi Q function of the cavity squeezed vacuum state. In addition, we observe the evolution of the quantum vacuum state inside the cavity as it undergoes phase-sensitive amplification. By enabling generation and measurement of quantum states in a single nonlinear optical cavity, our method paves a way for studying exotic dynamics of quantum optical states in nonlinear driven-dissipative systems.