  In superconducting qubits, suppression of spontaneous emission is essential
to achieve fast dispersive measurement and reset without sacrificing qubit
lifetime. We show that resonator-mediated decay of the qubit mode to the
feedline can be suppressed using destructive interference, where the readout
resonator is coupled to the feedline at two points. This "interferometric
Purcell filter" does not require dedicated filter components or impedance
mismatch in the feedline, making it suitable for applications such as all-pass
readout. We design and fabricate a device with the proposed scheme and
demonstrate suppression of resonator-mediated decay that exceeds 2 orders of
magnitude over a bandwidth of 400 MHz.
