Energy Decay in Josephson Qubits from Non-equilibrium Quasiparticles

Understand

We calculate the energy decay rate of Josephson qubits and superconducting resonators from non-equilibrium quasiparticles.

• The decay rates from experiments are shown to be consistent with predictions based on a prior measurement of the quasiparticle density n_qp = 10/um^3, which suggests that non-equilibrium quasiparticles are an important decoherence source for Josephson qubits.
• Calculations of the energy-decay and diffusion of quasiparticles also indicate that prior engineered gap and trap structures, which reduce the density of quasiparticles, should be redesigned to improve their efficacy.
• This model also explains a striking feature in Josephson qubits and resonators - a small reduction in decay rate with increasing temperature.