Broadband Bandpass Purcell Filter for Circuit Quantum Electrodynamics

In circuit quantum electrodynamics (QED), qubits are typically measured using dispersively-coupled readout resonators. Coupling between each readout resonator and its electrical environment however reduces the qubit lifetime via the Purcell effect. Inserting a Purcell filter counters this effect while maintaining high readout fidelity, but reduces measurement bandwidth and thus limits multiplexing readout capacity. In this letter, we develop and implement a multi-stage bandpass Purcell filter that yields better qubit protection while simultaneously increasing measurement bandwidth and multiplexed capacity. We report on the experimental performance of our transmission-line--based implementation of this approach, a flexible design that can easily be integrated with current scaled-up, long coherence time superconducting quantum processors.