DOUBLE-LOOP JOSEPHSON-JUNCTION FLUX QUBIT WITH CONTROLLABLE ENERGY GAP

摘要

A double-loop four-Josephson-junction (4-JJ) flux qubit has an advantage over a three-Josephson-junction flux qubit in that the energy gap DELTA at the degeneracy point can be controlled in situ by varying the magnetic flux in one of the two loops. We report the results of microwave spectroscopy and ground-state measurements of a 4-JJ qubit. The value of DELTA is estimated from the resonances due to one-photon and two-photon processes. The appearance of a qubit step on a two-dimensional flux map is in agreement with the theoretical prediction. The dependence of the maximum slope of the qubit step on the control flux indicates the tunability of the barrier height.