  The ability to perform fast, high-fidelity readout of quantum bits (qubits)
is essential to the goal of building a quantum computer. However, coupling a
fast measurement channel to a superconducting qubit typically also speeds up
its relaxation via spontaneous emission. Here we use impedance engineering to
design a filter by which photons may easily leave the resonator at the cavity
frequency but not at the qubit frequency. We implement this broadband filter in
both an on-chip and off-chip configuration.
