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Maximizing the Nondemolition Nature of a Quantum Measurement Via an Adaptive Readout Protocol

Arjen Vaartjes1, Rocky Yue Su1, Laura A. O’Neill1, Paul Steinacker1,2, Gauri Goenka1, Mark R. van Blankenstein1, Xi Yu1, Benjamin Wilhelm1, Alexander M. Jakob3 et al.

Fay E. Hudson1,2, Kohei M. Itoh4, Chih Hwan Yang1,2, Andrew S. Dzurak1,2, David N. Jamieson3, Martin Nurizzo1, Danielle Holmes1, Arne Laucht1,2, and Andrea Morello1,*

  • 1School of Electrical Engineering and Telecommunications, UNSW Sydney, Sydney, NSW 2052, Australia
  • 2Diraq Pty. Ltd., Sydney, NSW, Australia
  • 3School of Physics, University of Melbourne, Melbourne, VIC 3010, Australia
  • 4School of Fundamental Science and Technology, Keio University, Kohoku-ku, Yokohama, Japan

  • *Contact author: a.morello@unsw.edu.au
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PRX Quantum 7, 020330 – Published 15 May, 2026

DOI: https://doi.org/10.1103/jtn1-wzyl

Abstract

Quantum error correction benefits from high-fidelity, noninvasive measurements for fault-tolerant quantum computing. Deviations from ideal quantum nondemolition (QND) measurements can disturb the encoded information. To address this challenge, we develop a readout protocol for a D-dimensional system that, after a single positive outcome, switches to probing only the D−1 remaining subspace. This adaptive switching strategy minimizes measurement-induced errors by relying on negative-result measurement results that do not perturb the Hamiltonian. We apply the protocol on an eight-dimensional 123Sb nuclear qudit in silicon and achieve an increase in the readout fidelity from (98.93±0.07) % to (99.61±0.04) %, while reducing threefold the overall readout time. To highlight the broader relevance of measurement-induced errors, we study a ten-dimensional 73Ge nuclear spin read out through Pauli spin blockade, revealing nuclear spin flips arising from hyperfine and quadrupole interactions. We discuss the applicability of our method to other quantum hardware platforms such as color centers in diamond, spins in lithographic quantum dots, clusters of donors in silicon, and dual-species neutral atom arrays. These results unveil the effect of nonideal QND readout across diverse platforms and introduce an efficient readout protocol that can be implemented with minimal field-programmable gate array (FPGA) logic on existing hardware.

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Popular Summary

Quantum computers promise to solve problems far beyond the reach of classical computers, but today’s devices are extremely sensitive to errors. One major source of errors is the act of measurement. The best we can do is to use quantum nondemolition (QND) measurements, which repeatedly extract information while introducing only the minimum disturbance dictated by quantum mechanics. Deviations from QND introduce further errors, which we wish to avoid.

Quantum information is often encoded in spins. A spin behaves like a tiny magnet that aligns itself with the axis of an external magnetic field. Measuring a spin projects it onto this measurement axis, forcing it into one of the few discrete outcomes. To avoid disturbing the spin, we often measure it indirectly through a secondary spin, or ancilla, which acts like a messenger carrying the measurement information. But like in a game of Chinese whispers, each time the message is passed, there is a chance it gets distorted. Repeated measurements via the ancilla introduce a small tilt in the measurement axis, causing the spin to change its orientation. We address this challenge with an adaptive measurement protocol that minimizes the amount of error-producing “whispers.” Once the system is detected in a state, instead of repeating the full measurement, the protocol checks only the remaining dark states, which produce no signal and leave the system undisturbed.

We demonstrate the protocol on an eight-level nuclear spin measured via an electron ancilla and reduce measurement errors below the estimated threshold for fault-tolerance. This approach applies to many quantum platforms and directly supports building scalable, fault-tolerant quantum computers.

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PRX Quantum 7, 020330– Published 15 May, 2026
  • Received 23 November 2025
  • Revised 9 February 2026
  • Accepted 2 April 2026
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