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

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 123 Sb 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 73 Ge 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.