Superconductor-ferromagnet-superconductor junctions in flux and phase qubits

Abstract

π-junction qubit based on superconducting quantum interference device ring has been already proposed. In this qubit, a quantum two-level system is spontaneously generated without any bias flux applied. Flux qubit with a single π-junction concerns the large inductance L of the loop, which makes the qubit vulnerable to decoherence caused by magnetic fluctuations in the environment. To overcome this difficulty one can use three-junction π-qubit, comprising three π-junctions in a loop with negligible small inductance or unharmonic two-junction interferometer exploiting non-sinusoidal current-phase relations of the junctions. In the paper we theoretically investigate the macroscopic quantum dynamics of proposed qubits. The measurement process in two types of qubits has been analyzed in the frame of Lindblad equation formalism. Different mechanisms giving a non-sinusoidal CPR in SFS structures are analyzed as well. It is shown, that the largest second harmonic can be obtained for the SFS junction in the "clean" limit with a thin ferromagnet layer at low temperature.