On-chip cryogenic low-pass filters based on finite ground-plane coplanar waveguides for quantum measurements.

Quantum technology exploits fragile quantum electronic phenomena whose energy scales demand ultra-low electron temperature operation. The lack of electron-phonon coupling at cryogenic temperatures makes cooling the electrons down to a few tens of millikelvin a non-trivial task, requiring extensive efforts on thermalization and filtering high-frequency noise. Existing techniques employ bulky and heavy cryogenic metal-powder filters, which prove ineffective at sub-GHz frequency regimes and unsuitable for high-density quantum circuits such as spin qubits. In this work, we realize ultra-compact and lightweight on-chip cryogenic filters based on the attenuation characteristics of finite ground-plane coplanar waveguides. These filters are made of aluminum on sapphire substrates using standard microfabrication techniques. The attenuation characteristics are measured down to a temperature of 500 mK in a dilution refrigerator in a wide frequency range of a few hundred kHz to 8.5 GHz. We find their performance is superior by many orders compared to the existing filtering schemes, especially in the sub-GHz regime, negating the use of any lumped-element low-pass filters. The compact and scalable nature makes these filters a suitable choice for high-density quantum circuits such as quantum processors based on quantum dot spin qubits.