Development of a broadband reflective T-filter for voltage biasing high-Q superconducting microwave cavities

We present the design of a reflective stop-band filter based on quasi-lumped elements that can be utilized to introduce large dc and low-frequency voltage biases into a low-loss superconducting coplanar waveguide (CPW) cavity. Transmission measurements of the filter are seen to be in good agreement with simulations and demonstrate insertion losses greater than $20\,{\rm dB}$ in the range of ${\rm 3\,to\,10\,GHz}$. Moreover, transmission measurements of the CPW's fundamental mode demonstrate that loaded quality factors exceeding $10^5$ can be achieved with this design for dc voltages as large as ${\rm 20\,V}$ and for the cavity operated in the single-photon regime. This makes the design suitable for use in a number of applications including qubit-coupled mechanical systems and circuit QED.