Multi-pole Purcell filters for fast qubit readout

For dispersive readout in circuit QED, fast and accurate measurements require strong couplings between the readout resonators and the signal line. However, increased coupling strengths also lead to reduced qubit coherence lifetimes through Purcell decay. To suppress the Purcell decay rate, coupling the readout resonators either to individual filters or to a shared single-mode filter is a common strategy [1, 2]. While effective, both these Purcell filters introduce additional design constraints. Individual filters have large on-chip footprints and single-mode filters are restricted by their resonance wavelength. Instead, we implement a multi-pole Purcell filter by coupling multiple lumped-element resonators [3]. The filter has a wide pass band with a sharp cutoff and an on-chip footprint of 0.5 mm × 0.5 mm. This allows for multiplexed readout of several resonators, while simultaneously reducing the Purcell decay rate by up to two orders of magnitude. We integrate the filter in a readout circuit and demonstrate multiplexed readout of three qubits.

1. Spring, P. A. et al. PRX Quantum 6, 020345 (June 2025).

2. Chen, L. et al. npj Quantum Information 9, 26 (Mar. 2023).

3. Park, S. H. et al. Applied Physics Letters 124, 044003 (Jan. 2024)