Microwave nonlinearity and sensing with Rydberg atoms

Rydberg electromagnetically induced transparency (EIT) has emerged as a powerful platform for quantum microwave field sensing, offering broadband operation and intrinsic SI-traceability. In most microwave sensing protocols, the system response is assumed to remain in a linear regime with respect to the applied microwave field. However, as the microwave Rabi frequency increases, nonlinear effects become increasingly significant and can fundamentally alter the spectral response.

In this talk, I will present our experimental investigation of nonlinearity in a cold-atom Rydberg EIT system. We employ microwave coupling between Rydberg states and extract field amplitudes through multi-parameter spectral fitting. Since microwave nonlinear effects in such systems remain largely unexplored, it is not immediately clear whether the fitted parameters accurately reflect the true physical fields. To address this, we perform a series of controlled validation experiments to directly test the reliability of the fitting procedure.