Abstract
Magnetic flux tunability is an essential feature in most approaches to quantum computing based on superconducting qubits. Independent control of the fluxes in multiple loops is hampered by crosstalk. Calibrating flux crosstalk becomes a challenging task when the circuit elements interact strongly. We present a novel approach to flux crosstalk calibration, which is circuit model independent and relies on an iterative process to gradually improve calibration accuracy. This method allows us to reduce errors due to the inductive coupling between loops. The calibration procedure is automated and implemented on devices consisting of tunable flux qubits and couplers with up to 27 control loops. We devise a method to characterize the calibration error, which is used to show that the errors of the measured crosstalk coefficients are all below 0.17%.
- Received 5 June 2021
- Revised 10 September 2021
- Accepted 14 September 2021
- Corrected 10 November 2021
DOI:https://doi.org/10.1103/PRXQuantum.2.040313
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)
Corrections
10 November 2021
Correction: The byline footnote for the seventh author was erroneously assigned to the last author during the proof production cycle and has been fixed. A corresponding reordering of footnotes has also been made.
Popular Summary
Superconducting circuits are one of the most promising platforms for building large-scale quantum computers. A key advantage of these systems is the ability to perform quantum control based on electrical signals. In particular, controlling the external fluxes in superconducting loops is a key element of functionality of these systems. Along with these benefits, flux biasing also comes with challenges to accurately calibrate the crosstalk between the bias lines. Crosstalk calibration is made difficult by the fact that a direct measurement of bias flux is hampered by magnetic fields generated by other circuit elements, which are impossible to predict without accurate models of a circuit, which themselves depend on flux crosstalk.
We address these challenges by developing a method that relies on the fundamental symmetries of superconducting circuit properties on external fluxes, which is independent of the underlying circuit model. An iterative calibration procedure is developed based on periodicity, with each iteration improving the calibration accuracy. We present a detailed description of the measurement procedure and of the data analysis tools used to automate the procedure. Our new crosstalk calibration protocol is applied to superconducting circuits based on flux qubits, with up to 27 flux-bias lines, which are among the largest in the community. Since the procedure relies only on the periodic response of superconducting circuits, it is widely applicable to other circuits with flux-bias controls.
One area in which this work is particularly relevant is the operation of quantum annealers with individual control of qubits and couplers. This control capability enables quantum annealing with custom schedules and high coherence, which has the potential to improve the performance of quantum annealing on some computational problems.
