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 20dB in the range of 3to10GHz. Moreover, transmission measurements of the CPW’s fundamental mode demonstrate that loaded quality factors exceeding 105 can be achieved with this design for dc voltages as large as 20V 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.