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  • Letter
  • Open Access

Extended quantum process tomography of logical operations on an encoded bosonic qubit

Mikael Kervinen, Shahnawaz Ahmed, Marina Kudra, Axel Eriksson, Fernando Quijandría*, Anton Frisk Kockum, Per Delsing†, and Simone Gasparinetti‡

  • *Present address: Quantum Machines Unit, Okinawa Institute of Science and Technology Graduate University, Onna-son, Okinawa 904-0495, Japan.
  • †Contact author: per.delsing@chalmers.se
  • ‡Contact author: simoneg@chalmers.se
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Phys. Rev. A 110, L020401 – Published 14 August, 2024

DOI: https://doi.org/10.1103/PhysRevA.110.L020401

Abstract

We demonstrate the use of coherent-state quantum process tomography (csQPT) for a bosonic-mode superconducting circuit. We have enhanced our methodology over previous implementations of csQPT by leveraging Kraus operators and constrained gradient descent to learn the underlying process. We show the results of our method by characterizing a logical quantum gate implemented using displacement and selective number-dependent arbitrary phase operations on an encoded qubit. Our use of csQPT allows for the reconstruction of Kraus operators for the larger Hilbert space rather than being limited to the logical subspace. This approach enables us to more accurately identify and quantify the various error mechanisms that can lead to gate infidelity, including those occurring outside of the computational subspace. We showcase the potential of our approach by demonstrating the ability to quantify leakage outside of the computational subspace, a key factor for developing more robust and reliable quantum gates in high-dimensional systems.

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Phys. Rev. A 110, L020401– Published 14 August, 2024
  • Received 9 April 2024
  • Accepted 14 June 2024
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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. Funded by Bibsam.

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