Transmon-phonon coupling of plasma oscillations and lattice vibrations

In the transmon qubit we expect from conservation of momentum and energy a coupling between the plasma oscillations and the vibrations of the underlying lattice. Specifically, the electron velocities and their kinetic energy density are boosted by the underlying lattice vibrations. We consider this effect in a representative transmon comprising two semi-circular superconducting charge islands joined by a Josephson junction. In particular, we solve the Fourier transform of a two-dimensional radial current density having inversion symmetry. The resulting spectral density is ohmic but also scales quadratically with the critical current I_c and logarithmically with the size of the transmon: J(w) ~ I_c^2 w log(kR). We make positive-definite Born-Markov approximations in a generalized Fermi’s Golden Rule and estimate the phonon-induced dephasing rate is negligible compared to current experiments.