Temperature Dependence of Transmon Relaxation Due to Non-Equilibrium Quasiparticles: Experiment and Analysis

Understanding and decreasing the energy relaxation rate of transmon qubits is important for their use in quantum computing. We measured the relaxation time T₁ of multiple Al/AlOx/Al transmons. The devices were fabricated using different processes, mounted in different 3D aluminum cavities, and measured in two different dilution refrigerators. In some devices we found that T₁ increased by as much as a factor of two (up to 90 µs) when increasing the temperature of the device from 15 mK to 100 mK. We will discuss our results and compare them to a model based on the behavior of non-equilibrium quasiparticles residing in regions of the transmon with slightly different superconducting energy gaps. Fits to the model allow us to extract key parameters such as the values of the superconducting energy gap and density of non-equilibrium quasiparticles in each region.