Martin Nurizzo

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.

Pauli-spin-blockade (PSB) measurements have so far achieved the highest fidelity of spin readout in semiconductor quantum dots, overcoming the 99% threshold. Moreover, in contrast to energy-selective readout, PSB is less error prone to thermal energy, an important feature for large-scale architectures that could be operated at temperatures above a few hundreds of millikelvins. In this work, we use rf reflectometry on a single-lead quantum dot to perform charge sensing and to probe the spin state of a double quantum dot at 0.5 K. At this relatively elevated temperature, we characterize both singlet-triplet and parity readout, which are complementary measurements to perform a complete readout of a two-spin system. We demonstrate high-fidelity spin readout with an average fidelity above 99.9 % for a readout time of 20 µs and 99 % for 4 µs. Finally, we succeed in initializing a singlet state in a single dot with a fidelity higher than 99 % and separate the two electrons while retaining the same spin state with a 95.6 % fidelity.

Recent demonstrations using electron spins stored in quantum dot array as qubits are promising for developing a scalable quantum computing platform. An ongoing effort is, therefore, aiming at the precise control of the quantum dot parameters in larger and larger arrays which represents a complex challenge. Partitioning of the system with the help of the inter-dot tunnel barriers can lead to a simplification for tuning and offers a protection against unwanted charge displacement. In a triple quantum dot system, we demonstrate a nanosecond control of the inter-dot tunnel rate permitting to reach the two extreme regimes, large GHz tunnel coupling, and sub-Hz isolation between adjacent dots. We use this development to isolate a subpart of the array in a metastable configuration while performing charge displacement and readout in the rest of the system. The degree of control over tunnel coupling achieved in a unit cell should motivate future protocol development for tuning, manipulation, and readout including this capability.