Reflectionless filter for superconducting quantum circuits

Protecting superconducting quantum circuits from non-ideal return loss, including out-of-band circulator behavior, and enhancing the performance of broadband quantum-limited amplifiers can be accomplished simultaneously with low passband loss using a special class of microwave filters known as reflectionless filters. This work will review the theory, design, and modeling of a superconducting reflectionless filter, followed by the presentation of cryogenic measurements of the device. The filter has been fabricated monolithically using Al on Si, incorporating NiCr resistors, which allows for simple integration with other superconducting quantum devices. The filter achieves both low insertion loss over a 50\% fractional bandwidth and return loss superior to the out-of-band performance of commercial circulators, from DC to above 10 GHz. The thermal behavior in both the passband and stopband will be characterized. Finally, examples of how the filters can be applied in quantum systems will be presented.