Measurement of critical photon number of the 2nd excited state of a Transmon qubit

Abstract

The cavity response was measured while varying the state of the transmon qubit, and a nonlinear signal was observed at higher powers. This signal exhibited a rapid phase change when the amplitude was low, deviating from the typical resonance behavior. This phenomenon is attributed to the photon blockade effect arising from cavity photon bistability, commonly referred to as bifurcation. When the previous model to describe the branch was applied, there was a discrepancy with the experimental results. However, by employing measurement-induced state transitions and semi-classical photon dynamics considering the effective resonance, it was able to apply these to the experimental results and provide an explanation for the aspects that previous models explain insufficiently.