  With microwave irradiation, the switching current of a Josephson junction
coupled to a microscopic two-level system jumps randomly between two discrete
states. We modeled the switching process of the coupled system with quantum
jump approach that was generally used in quantum optics. The parameters that
affect the character of the quantum jumps between macroscopic quantum states
are discussed. The results obtained from our theoretical analysis agree well
with those of the experiments and provide a clear physical picture for the
macroscopic quantum jumps in Josephson junctions coupled with two-level
systems. In addition, quantum jumps may serve as a useful tool to investigate
the microscopic two-level structures in solid-state systems.
