Experimental verification of superconducting metamaterial quarter-wavelength transmission line resonators with non-uniform mode spacing.

Superconducting transmission line resonators (TLRs) have been widely applied to circuit quantum electrodynamical systems for superconducting quantum computation and single-photon detections. Recently, such a right-handed TLR has been generalized to electromagnetic metamaterials (MTMs), and its unconventional mode structure has been experimentally characterized (Wang et al. in Phys Rev Appl 11:054062, 2019). Here, we demonstrate another kind of MTM TLR, i.e., the superconducting quarter-wavelength composite right/left-handed (CRLH) TLRs composed of lumped elements. We design the devices and analyze their transport properties by developing a real-space approach, wherein the physical parameters at the device boundaries are utilized conveniently. Superconducting MTM TLRs with typical three unit cells were experimentally prepared by etching a superconducting aluminum film on a SiO[Formula: see text] substrate, and their microwave transport properties were measured at low temperatures of 50 mK. The results show that the modes spacing in such a quarter-wavelength CRLH-TLR is non-uniform, due to the nonlinear dispersion relation. This implies that it could be utilized to encode the superconducting qubits, analogously to the usual ones encoded by using the Josephson devices.