Doublet splitting due to intercell tunneling in three-Josephson-junction flux qubit

It has been shown that for a three-Josephson-junction flux qubit with non-negligible loop inductance, the intercell tunneling in the three-dimensional potential of the system can cause doublet splitting in the ground state and the first excited state. This paper presents the calculation results of the magnitude of doublet splitting for various values of parameters of the system. For realistic values of parameters, intercell splitting can be larger than 100 MHz, which should be observable in measurements. The observation of two branches in the excitation spectra of a four-Josephson-junction flux qubit in our recent study can be explained in terms of the presence of two transition processes among the four levels produced by intercell tunneling. These levels may be useful for novel applications such as cooling and lasing.