Linewidth-tunable superconducting microwave circuits based on niobium nano-constrictions

Superconducting circuits have proven invaluable tools for advancing quantum technologies in recent years. Of particular importance are circuits with Josephson junctions (JJs) and superconducting quantum interferometers (SQUIDs), which have enabled the implementation of nonlinear and highly frequency-tunable low-loss microwave modes. Reports of superconducting circuits, in which their decay rate instead of their resonance frequency is tunable, are scarce though, despite their potential importance for, e.g., in-situ impedance matching, dissipative interaction engineering, or circuit reset protocols. Here, we present a concept and a preliminary experimental implementation of a linewidth-tunable microwave circuit, which has been realized by integrating two SQUIDs into the device instead of a single one. Our design allows for precise control over the external decay rate without altering the resonance frequency or for an arbitrary ratio of dispersive-to-dissipative tunability, respectively, which is a significant challenge in standard circuit QED systems. In further contrast to usual Al SIS JJ circuits, our device is based on niobium and neon-ion-beam patterned nano-constrictions, accompanied by a small circuit nonlinearity and potentially high magnetic-field tolerance.