Spectroscopy on a Single Nonlinear Mode Recognizes Quantum States

The ability to efficiently characterize optical quantum states is essential for the development of quantum technologies. While traditional approaches to perform quantum state tomography are often experimentally demanding, neuromorphic architectures provide an attractive alternative. In this work, we demonstrate that a quantum nonlinear driven-dissipative mode is sufficient to act as a quantum reservoir. We show that training with the optical emission spectrum alone is sufficient to characterize the quadrature angle and squeezing strength of continuous wave squeezed states. We demonstrate how the method works for a finite-bandwidth squeezed source (e.g., a degenerate optical parametric oscillator) processed by a nonlinear polariton microcavity, highlighting its transferability. These results suggest a scalable and experimentally frugal path to characterizing nonclassical states with nonlinear optical systems.