  We theoretically investigate the macroscopic quantum dynamics of a pi
junction with a superconductor (S) and a multiferroic material or a
ferromagnetic insulator (FI). By deriving the effective action from a
microscopic Hamiltonian, a pi-junction qubit (a S-FI-S superconducting quantum
interference device ring) is proposed. In this qubit, a quantum two-level
system is spontaneously generated and the effect of the quasiparticle
dissipation is found to be very weak. These features make it possible to
realize a quiet qubit with high coherency. We also investigate macroscopic
quantum tunneling (MQT) in current-biased S-FI-S pi junctions and show that the
influence of the quasiparticle dissipation on MQT is negligibly small.

Mesoscale and Nanoscale Physics;Superconductivity;Quantum Physics