Charge and Flux Noise from Nonequilibrium Quasiparticle Energy Distributions in Superconducting Wires

The quasiparticle density observed in low-temperature superconducting circuits is several orders of magnitude higher than the value expected at thermal equilibrium. The tunneling of this excess of quasiparticles across Josephson junctions is recognized as one of the main loss and decoherence mechanisms in superconducting qubits. Here we propose an additional loss mechanism arising from nonequilibrium quasiparticle densities: Ohmic loss due to quasiparticles residing in superconducting wires away from the junctions. Our theory leverages the recent experimental demonstration that the excess quasiparticles are in quasiequilibrium [T. Connolly et al., https://doi.org/10.1103/PhysRevLett.132.217001Phys. Rev. Lett. 132, 217001 (2024)] and uses a generalized fluctuation-dissipation theorem to predict the amount of charge and flux noise generated by them. We show that the resulting charge noise can be larger than dielectric loss due to amorphous two-level systems at frequencies in the MHz range, and find a logarithmic-in-frequency ``nearly white" contribution to flux noise that is comparable to the flux noise observed in experiments. This shows that wire-resident quasiparticles are a universal source of loss and decoherence even when the quasiparticles are far away from Josephson junctions.