Dispersive readout: Universal theory beyond the rotating-wave approximation

We present a unified picture of dispersive readout of quantum systems in and out of equilibrium. A cornerstone of the approach is the backaction of the measured system to the cavity obtained with non-equilibrium linear-response theory. It provides the dispersive shift of the cavity frequency in terms of a system susceptibility. It turns out that already effortless computations of the susceptibility allow one to generalize former results beyond a rotating-wave approximation. Examples are the readout of detuned qubits and thermally excited multi-level systems. For ac-driven quantum systems, we identify the relevant Fourier component of the susceptibility and introduce a computational scheme based on Floquet theory. The usefulness is demonstrated for two-tone spectroscopy and interference effects in driven two-level systems. This reveals that dispersive readout does not necessarily measure excitation probabilities.