Britton Plourde, University of Syracuse
Low-loss resonators formed from lumped circuit elements or distributed transmission lines can be coupled to superconducting qubits. This is the basis for the numerous investigations of circuit-QED over the past decade. In this case, one is primarily interested in the coupling between the qubit and one or a few modes. If the resonator is instead formed from a metamaterial transmission line, it is possible to generate a structure with a quite different response, capable of exhibiting a dense spectrum of modes with which a qubit can then interact. We have fabricated a variety of superconducting thin-film metamaterial resonators and characterized their transmission spectra at low temperatures. In parallel, we have performed various numerical simulations of these structures to compare with our measured spectra and to investigate the standing-wave patterns of the various modes. By adding a coplanar waveguide section onto the end of a metamaterial resonator, we are able to produce a region where a flux-tunable asymmetric transmon qubit can couple to multiple modes. We have characterized this coupling by measuring splittings in microwave transmission measurements of the various modes as we tune the qubit transition energy with a magnetic flux. The dispersive couplings between the qubit and the different closely spaced modes lead to complex transmission spectra for one of the modes when a second microwave tone is driven near a neighboring mode.