Hybrid rings for circuit quantum electrodynamics

During the last years, superconducting circuit quantum electrodynamics (circuit QED) has attracted increasing interest. Experiments in this field require detection schemes for microwave signals on the single photon level. In particular, devices acting as microwave beam splitters are necessary. Using niobium thin films on silicon and sapphire substrates, we fabricated superconducting 180 microstrip hybrid ring couplers, acting as beam splitters with center frequencies of about 6 GHz. For the magnitude of the coupling and isolation we find -3.5±0.5 dB and at least -15 dB, respectively, in a bandwidth of 2 GHz. We also investigate the effect of reflections at the contact between the superconducting hybrid ring and the normal conducting wiring using low temperature laser scanning microscopy. Our measurements indicates that our hybrid rings are well suited for on-chip applications in circuit quantum electrodynamics experiments.