A superconducting coplanar waveguide ring resonator as quantum bus for circuit quantum electrodynamics

We introduce a superconducting coplanar waveguide, in the form of a ring resonator, as a quantum bus for circuit quantum electrodynamics (circuit QED). Due to its unique symmetry, the amplitude of the electromagnetic field inside is uniform along the resonator, which in principle leads to a location independent coupling between qubits and the resonator. For an experimental demonstration, we designed and fabricated a ring resonator capacitively coupled to two tunable Xmon superconducting qubits. We showed that nearly identical coupling was achieved between individual qubits and the resonator. In addition, the resonator was also used as a high quality quantum bus for generating Bell states between the qubits with a fidelity of F = 98.6 %, although an interference effect must be included for understanding the effective coupling between the two qubits. Our results suggest that ring resonators can be a useful addition to the toolbox of circuit QED.