Abstract
Residual noise photons in a readout resonator become a major source of dephasing for a superconducting qubit when the resonator is optimized for a fast, high-fidelity dispersive readout. Here, we propose and demonstrate a nonlinear Purcell filter that suppresses such an undesirable dephasing process without sacrificing the readout performance. When a readout pulse is applied, the filter automatically reduces the effective linewidth of the readout resonator, increasing the sensitivity of the qubit to the input field. The noise tolerance of the device we have fabricated is shown to be enhanced by a factor of 3 relative to a device with a linear filter. The measurement rate is enhanced by another factor of 3 by utilizing the bifurcation of the nonlinear filter. A readout fidelity of 99.4% and a quantum nondemolition fidelity of 99.2% are achieved using a 40-ns readout pulse. The nonlinear Purcell filter will be an effective tool for realizing a fast, high-fidelity readout without compromising the coherence time of the qubit.
- Received 8 September 2023
- Revised 25 September 2023
- Accepted 1 December 2023
DOI:https://doi.org/10.1103/PRXQuantum.5.010307
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Near-perfect qubit readout is essential for performing error correction on a quantum computer. Researchers working on superconducting qubits have been improving the speed and accuracy of the readout by making the qubit interact more and more strongly with a microwave pulse. However, this has also made the qubit more and more sensitive to noise coming from the same waveguide as the pulse, limiting the coherence time of the qubit. We propose and demonstrate a nonlinear filter that enhances the noise tolerance of the qubit without sacrificing the readout performance.
The trick is to make the qubit sensitive to incoming microwaves only during a readout. Our nonlinear filter achieves this by automatically deactivating with the application of a readout pulse. We also utilize the nonlinearity of the filter to revisit the technique of bifurcation readout, which was last used more than a decade ago to amplify the readout signal.
Our work creates a new paradigm by being the first to address the problem of the noise in the readout waveguide through device design instead of by reducing the noise. Further research in this direction will be an essential part of the collective effort toward the goal of realizing a fault-tolerant superconducting quantum computer.
