Quantensysteme

Controllable coupling between the odd and even fundamental modes of an asymmetric half-wavelength superconducting coplanar waveguide resonator is demonstrated. The resonant frequency of the even mode Xe could be tuned by an external magnetic field, while the resonant frequency of the odd mode Xo is field independent. To realize the tunability of Xe, the central conductor of the NbN-based resonator was galvanically coupled to an array of Al-based rf-SQUIDs (radio frequency superconducting quantum interferometer device). These rf-SQUIDs are placed in only one resonator gap, ensuring its strong asymmetry. By adjusting the appropriate external magnetic flux U, equal frequencies of both modes were obtained. At this resonant point XoðUÞ ¼ XeðUÞ an avoided level crossing of the eigenfrequencies was observed, demonstrating the coupling between the odd and even fundamental modes.