Analyzing weak microwave signals in the GHz regime is a challenging task if the signal level is very low and the photon energy widely undefined. A superconducting qubit can detect signals in the low photon regime, but due to its discrete level structure, it is only sensitive to photons of certain energies. With a multilevel quantum system (qudit) in contrast, the unknown signal frequency and amplitude can be deduced from the higher level ac Stark shift. The measurement accuracy is given by the signal amplitude, its detuning from the discrete qudit energy level structure, and the anharmonicity. We demonstrate an energy sensitivity in the order of ${10}^{\ensuremath{-}3}$ with a measurement range of more than
1GHz. Here, using a transmon qubit, we experimentally observe shifts in the transition frequencies involving up to three excited levels. These shifts are in good agreement with an analytic circuit model and master equation simulations. For large detunings, we find the shifts to scale linearly with the power of the applied microwave drive. Exploiting the effect, we demonstrated a power meter which makes it possible to characterize the microwave transmission from source to sample.
The authors report a high-resolution electron spectroscopic study of three-photon resonant, five-photon ionisation of xenon. The results are analysed in terms of a dynamic resonance of three photons with an AC Stark-shifted intermediate level (6s), followed by a saturated two-photon transition to the continuum. They demonstrate a linear relationship between detuning from the zero-field resonance position and the local momentary light intensity causing the five-photon ionisation. Thus, the AC Stark shift acts as a probe which selects a particular intensity from the wide range present in the laser focus. They determined the AC Stark shift of the 6s level the distribution function of the light intensities and the intensity dependence of the ionisation probability. High-resolution electron spectra are used in arguments to support the validity of their analysis.