Readout of Superconducting Qubits with Purcell Filters: Experiment and Theory

The performance of a wide range of quantum computing algorithms and protocols depends critically on the fidelity and speed of the employed qubit readout. In this work, we demonstrate a new technique to improve qubit readout beyond the current state-of-the-art in a quantum processor. By dynamically tuning the qubit-resonator detuning during the readout process, we nearly quadruple the signal-to-noise ratio of the detection. We use a Purcell-filter in conjunction with a standard readout resonator to maximize detection speed and fidelity while protecting the qubits from decay. We interpret the effect of the reduction of qubit-resonator detuning as an enhancement of the hybridization between the readout resonator and its filter, effectively doubling its linewidth and thus reducing its response time. In addition, we analyze and characterize the interplay between the Signal-to-Noise Ratio (SNR) of the readout process and its dependence on the couplings and frequencies of the transmon–readout-resonator–Purcell-filter system. Our experimental results show excellent agreement with our theoretical model. We anticipate that our findings will significantly enhance the performance of new and existing algorithms and protocols that critically rely on high-fidelity, fast measurements.