• Open Access

Characterization and Tomography of a Hidden Qubit

M. Pechal, G. Salis, M. Ganzhorn, D. J. Egger, M. Werninghaus, and S. Filipp
Phys. Rev. X 11, 041032 – Published 15 November 2021

Abstract

In circuit-based quantum computing the available gate set typically consists of single-qubit gates acting on each individual qubit and at least one entangling gate between pairs of qubits. In certain physical architectures, however, some qubits may be “hidden” and lacking direct addressability through dedicated control and readout lines, for instance, because of limited on-chip routing capabilities, or because the number of control lines becomes a limiting factor for many-qubit systems. In this case, no single-qubit operations can be applied to the hidden qubits and their state cannot be measured directly. Instead, they may be controlled and read out only via single-qubit operations on connected “control” qubits and a suitable set of two-qubit gates. We first discuss the impact of such restricted control capabilities on the performance of specific qubit coupling networks. We then experimentally demonstrate full control and measurement capabilities in a superconducting two-qubit device with local single-qubit control and iswap and controlled-phase two-qubit interactions enabled by a tunable coupler. We further introduce an iterative tune-up process required to completely characterize the gate set used for quantum process tomography and evaluate the resulting gate fidelities.

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  • Received 21 December 2020
  • Revised 1 July 2021
  • Accepted 26 August 2021

DOI:https://doi.org/10.1103/PhysRevX.11.041032

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)

Quantum Information, Science & Technology

Authors & Affiliations

M. Pechal1,*, G. Salis1, M. Ganzhorn1, D. J. Egger1, M. Werninghaus1, and S. Filipp2,3,4

  • 1IBM Quantum, IBM Research–Zurich, Säumerstrasse 4, 8803 Rüschlikon
  • 2Physik-Department, Technische Universität München, 85748 Garching, Germany
  • 3Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
  • 4Munich Center for Quantum Science and Technology (MSQCT), Schellingstraße 4, 80799 München, Germany

  • *Present address: Department of Physics, ETH Zurich, CH-8093 Zurich, Switzerland.

Popular Summary

In state-of-the-art quantum computing systems, each quantum bit can be individually controlled and measured. Large systems of qubits thus require a large amount of control electronics and signal lines, which are costly resources. One way to increase the number of qubits without also multiplying auxiliary resources is simply to add “hidden” qubits that are not directly controlled or read out. These are operated indirectly, using interactions with their neighboring qubits and do not require a separate control line. In our experiment, we demonstrate full control over a superconducting two-qubit system even though only one of the qubits is directly addressable.

The addition of hidden qubits does come at a cost: increased complexity of their manipulation, which can lead to lower fidelity. By estimating the quantum volume—a quantity describing the computational power of quantum devices—we find that adding hidden qubits to a system with a fixed number of control lines would enable higher quantum volumes once error rates are reduced by only 1 order of magnitude from their current values.

With expected future reductions in error rates, hidden qubits could soon become practical and turn into a useful quantum engineering tool.

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Vol. 11, Iss. 4 — October - December 2021

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