Fast, 3D-integrated qubit readout using a combined bandpass and intrinsic notch filter*

Fast and accurate qubit state readout is essential for fault-tolerant quantum computation. In dispersive readout in circuit QED, this requires increasing the coupling strength of readout resonators to the detector line to allow for rapid pointer-state separation without compromising the coherence or addressability of qubits. To this end, providing each readout mode with its own dedicated Purcell resonator has emerged as an effective strategy [1-2] – enhancing both Purcell filtering and measurement addressability in parallel.

Here, we present experimental results on a 3D-integrated superconducting circuit device with CPW readout resonators that couple to dedicated Purcell resonators through a section of their transmission line. The interplay between inductive and capacitive coupling that arises can be exploited to produce an additional notch filter, resulting in an extra layer of Purcell filtering that allows readout resonators to be very strongly coupled to the detector line. This approach demonstrates a qubit readout under 100 ns with an assignment fidelity exceeding 99.6%.

[1] J. Heinsoo et al., Rapid high-fidelity multiplexed readout of superconducting qubits. Physical Review Applied, 10, 034040 (2018)

[2] F. Swiadek et al., Enhancing dispersive readout of superconducting qubits through dynamic control of the dispersive shift: Experiment and theory. arXiv:2307.07765 (2023)