Understanding energy dissipation in Si substrate transmon qubit through phonon kinematics

In the context of quantum error correction, correlated errors in a qubit chip have been found to be strongly associated with the generation of electron-hole pairs and phonons through the absorption of terrestrial gamma and cosmic ray muons in the qubit substrate. During this process, a big fraction of the incident energy dissipates through the substrate as phonons and propagates to the superconductor generating broken Cooper pairs of electrons which cause qubit decoherence. In order to understand how these radiation events impact the performance of a qubit detector, we simulate these processes using the Geant-4 based particle physics simulation program. In this talk, we discuss our work in understanding phonon propagation, down conversion, scattering, and surface interaction between the Si substrate and the superconducting parts of a transmon qubit along with electron-hole pair generation and recombination in the Si. This work is not only relevant in the context of quantum computers but also in building a large array of qubit based Dark Matter detectors, which is our primary focus.