Interferometric Tuning of Resonator Linewidth

Dynamic control of resonator linewidth opens new possibilities for superconducting circuits, including the suppression of qubit dephasing and relaxation, in-situ mitigation of impedance mismatches, and optimization of qubit readout. We control the linewidth of a resonator by coupling it to a semi-infinite waveguide with a tunable boundary impedance. By tuning the boundary impedance from capacitive to inductive, we tune the self-interference of scattered resonator photons, enabling dynamic, notch-filter-like suppression of photon decay. We implement this architecture with coupled qubit-resonator systems and present linewidth on/off ratios exceeding 200, together with a tenfold extension of qubit dephasing times. We further demonstrate multiplexed readout and discuss how this approach can be scaled for large-scale readout architectures.