Rapid high-fidelity readout of a strongly Purcell-filtered transmon qubit*

High-fidelity quantum-nondemolition (QND) readout of qubit states is crucial for fault-tolerant quantum computation using superconducting qubits. Such a readout is commonly realized using the dispersive interaction between a qubit and a resonator. To achieve a high readout fidelity, the resonator is often accompanied by a Purcell filter, which suppresses the Purcell effect—the energy decay of the qubit mediated by the resonator. However, the rate of this decay has been found to significantly increase while a readout pulse is being applied [1].

Here, we develop a two-stage Purcell filter consisting of an intrinsic Purcell filter [2] and a band-pass Purcell filter [3] to strongly suppress the Purcell effect during the dispersive readout of a transmon qubit. Using a 36-ns readout pulse, we achieve a readout fidelity of 99.62±0.02% and a QND fidelity of 99.63±0.02%. We also develop a method to distinguish between different sources of readout error even when the amplifier for the readout signal is saturated by a strong readout pulse. We find that the largest remaining source of error is qubit excitation induced by the readout pulse.

[1] A. Petrescu et al., Phys. Rev. B 101, 134510 (2020).

[2] Y. Sunada et al., Phys. Rev. Applied 17, 044016 (2022).

[3] E. Jeffrey et al., Phys. Rev. Lett. 112, 190504 (2014).