Experimental investigation of an interferometer type charge-flux qubit

We present here recently obtained results with the theoretical and experimental investigations of charge-flux qubits. A charge-flux qubit consists of a single-Cooper-pair transistor closed by a superconducting loop. In this arrangement a qubit is effectively a tuneable two-level system. The qubit inductance was probed by a superconducting high-quality tank circuit. Under resonance irradiation, with a frequency of the order of the qubit energy level separation, change of the qubit inductance was observed. We have demonstrated that this effect is caused by the redistribution of the qubit level population. We have extracted from the measured data the energy gap of a qubit as a function of the quasicharge in the transistor island as well as the total Josephson phase difference across both junctions. The excitation of the qubit by one-, two-, and three-photon processes was detected. Quantitative agreement between theoretical predictions and experimental data was found. Relaxation as well as dephasing rates were reconstructed from the fitting procedure.