Co-designed reflective and leaky-waveguide low-pass filter for superconducting circuits

Abstract: A stepped-impedance low-pass filter with integrated hollow waveguide absorbers is presented. The filter combines low insertion loss in the passband with strong attenuation at high frequencies, making it well suited for superconducting quantum computing applications, where qubits are sensitive to both near-band and far out-of-band radiation. The structure is implemented in a rectangular coaxial geometry, with inductive sections coupled to circular hollow waveguides oriented orthogonally to the transmission axis. Above their cutoff frequency, these waveguides efficiently couple to radiation inside the stepped-impedance filter, absorbing energy that would otherwise cause Cooper pair breaking in conventional superconductors. Optimal dimensions were obtained using a differential evolution algorithm applied to interpolated electromagnetic simulation data. A prototype was fabricated and characterized using a calibrated vector network analyzer up to 67 GHz. Measurements confirm a 3 dB cutoff frequency at 13.5 GHz, insertion loss below 0.45 dB for frequencies under 8 GHz, and more than 52.7 dB rejection above 17.3 GHz. The design offers a compact, low-loss solution for near-band filtering and suppression of quasiparticle-generating radiation in cryogenic quantum systems.