Kvazidelci v superprevodnih kvantnih vezjih

The density of quasiparticles (QPs) in superconducting devices is often significantly higher than predicted by thermal equilibrium, leading to quantitatively important effects such as decoherence. Previous theoretical considerations relied on first obtaining the effective description of circuits in the superconducting ground state and incorporating QPs afterwards. By projecting the microscopic Hamiltonian onto relevant subspaces of the Hilbert space, we obtain an effective description of circuits, with quasiparticles explicitly taken into account throughout the derivation. We also consider the role of QPs in a specific circuit element, the quantum dot Josephson junction. We present a variational Ansatz and solve the variational equations. The solutions are in good agreement with independent results obtained by the numerical renormalization group (NRG) approach.