Reflection-less filter for superconducting quantum circuits

TL;DR

This paper presents a superconducting reflection-less microwave filter designed to improve quantum circuit performance by providing broadband reflection suppression, low insertion loss, and thermal photon suppression, with detailed design, fabrication, and cryogenic testing.

Contribution

It introduces a novel superconducting reflection-less filter with broad bandwidth, low loss, and thermal photon suppression, suitable for integration with quantum circuits.

Findings

Achieves below 1 dB insertion loss over 80% bandwidth at 8 GHz

Provides over 10 dB return loss from DC to above 14.5 GHz

Fabricated using Al on Si with NiCr resistors for integration

Abstract

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 using a superconducting version of a special class of microwave filters known as reflection-less filters. These filters can simultaneously permit low pass band loss to preserve quantum efficiency and broad band reflection-less characteristics in the stop and pass bands. The filter also suppresses thermal photons emitted in its pass band from the termination resistors by the nature of the dual network topology. This work will review the application, theory, design, and modeling of a superconducting reflection-less filter, followed by fabrication details and the presentation of cryogenic performance measurements of a monolithic device. The filter was fabricated using Al on Si, incorporating NiCr resistors, which allows for simple integration with other superconducting quantum devices. The filter with an area of 0.6 $\mathrm{\mathbf{mm^2}}$ achieves insertion loss below 1 dB, including its connectorized package over a 80\% fractional bandwidth centered at 8 GHz, and 10 dB packaged return loss from DC to above 14.5 GHz.