Improving Superconducting Qubit Lifetimes with Broadband Filters

In circuit quantum electrodynamics, the state of the qubit is read out via a resonator at a different frequency than that of the qubit. Spontaneous qubit decay via the resonator may be suppressed by engineering an impedance mismatch at the qubit frequency, while still allowing a large coupling between the resonator and external environment necessary for fast, high fidelity readout. We present a stepped-impedance filter with a large stop-band in the qubit frequency range and demonstrate its effect on qubit lifetime. This filter is also effective when used in an off-chip geometry.