Readout-induced degradation of transmon lifetimes: interplay of TLSs and qubit number splitting

Fast, quantum non-demolition (QND) measurements of qubit states are essential for many quantum computing tasks, in particular quantum error correction. In superconducting qubits, increasing the speed of dispersive readout by increasing drive power often leads to a marked decrease in qubit lifetimes. In this work, we show how this degradation can result from the interplay between detuned TLSs and a strongly drive-renormalized qubit spectrum. For modest to strong readout, the qubit spectrum becomes strongly non-Lorentzian, exhibiting the so-called "number splitting" effect. We show that this effect dramatically modifies spurious interactions with TLSs. Importantly, we demonstrate that both the qubit spectrum shape and linewidth broadening depend on the choice of readout frequency for a given readout rate, supported by our measurement data and numerical simulations.