Novel mm-Wave and InfraRed Filter Design for Reducing Quasiparticles in Superconducting Qubits

Nonequilibrium quasiparticle excitations can be a limiting source of error in superconducting qubits, causing relaxation, spurious excitation, and dephasing. One major source of quasiparticles is blackbody radiation from higher-temperature stages of a cryostat traveling down microwave cables. To mitigate this source of decoherence, high frequency signals (100-300 GHz) must be attenuated. Typically filters made with Eccosorb can accomplish significant attenuation, but a long filter length is required, which causes undesirable levels of attenuation in the microwave regime. We present work on designing and testing novel filters with greater high-frequency attenuation and less microwave attenuation. We show a novel meandering coplanar waveguide filter that can achieve greater than 40dB of loss above 100GHz and less than 1dB loss at 5GHz using a Corning glass dielectric, which has been shown to have attenuation which sharply increases at higher frequencies. We have used finite-element simulations to test various iterations of the design at microwave (1-10 GHz) and mm-wave (100-300 GHz) frequencies. Our simulations show loss more than 200x higher at 110 GHz than at 5 GHz. Further design optimizations could push this ratio even higher by increasing scattering of the mm-wave radiation. We discuss these improvements and experimental tests of filter efficacy.