T1 and thermally-excited quasiparticles effects on multi-mode transmons

In this study, we investigate the impact of thermally-excited quasiparticles on the performance degradation of multi-mode transmon qubits. Thermalization within the dilution refrigerator is a concern when utilizing non-superconducting cables for DC current distribution, which have a finite resistance at cryogenic temperature. Previous studies have shown that thermally excited states below a certain effect temperature unique to the device deviate from Maxwell-Boltzmann predictions, with a residual population of 0.1%. This residual quasiparticle population directly impacts the T1 time of qubits. We extend this analysis to multi-mode transmons, specifically modes A, B, and C, with frequencies of 4.09 GHz, 4.39 GHz, and 5.02 GHz, respectively. We predict the T1 degradation due to thermal quasiparticles and compare it with the empirical results. Our results show that the T1 times fall exponentially with increasing temperature, and we propose a methodology to mitigate the thermal effects. This includes optimizing the material properties and fridge setup to reduce thermal influx and maintain qubit performance within an optimal range. These findings are critical for the continued development of high-coherence multi-mode transmon qubits for scalable quantum computing.