A Superconducting RF Low-Pass Filter Based on Ti/TiN Artificial Transmission Line for Detector and Qubit Readout.

Cryogenic detector and qubit readout systems often include radio-frequency low-pass filters (LPFs) at the input/output paths to protect the sensitive devices from stray radiation traveling down the coaxial lines. For this application, a popular solution is to use custom-made copper powder filters, which are LPFs with injection of copper powder-loaded epoxy. They are very effective in attenuating high frequency (THz and far infrared) radiation but often introduce a few dBs of undesired insertion loss at the readout band below 10 GHz. Here, we describe a LPF design concept based on low- T c ( ≈ 700 mK) Ti/TiN superconducting artificial (lumped-element-loaded) transmission lines. We carefully design the unit inductance L u and unit capacitance C u to make the cut-off frequency f c = 1 / (π L u C u ) ≈ 30 GHz and the characteristic impedance Z 0 = L u / C u ≈ 50 Ohm. Above the gap frequency ( ≈ 50 GHz), the transmission line behaves as a lossy line made of normal metal which naturally attenuates the high-frequency radiations. In addition, the cavity resonances and spurious pass-bands at high frequencies can be effectively suppressed by adding a layer of microwave absorber below the substrate and by introducing an aperiodic loading of unit structures. A detailed design and electromagnetic simulation using realistic material parameters are presented. Our LPF design offers the advantages of close-to-zero insertion loss below 10 GHz, precise and sharp frequency roll-off, and linear phase response. The LPF can be placed in a separate package with compact size and easily connected to other superconducting circuits. [ABSTRACT FROM AUTHOR]