Abstract
Quantum error correction is of crucial importance for fault-tolerant quantum computers. As an essential step toward the implementation of quantum error-correcting codes, quantum nondemolition measurements are needed to efficiently detect the state of a logical qubit without destroying it. Here we implement quantum nondemolition measurements in a Si/SiGe two-qubit system, with one qubit serving as the logical qubit and the other serving as the ancilla. Making use of a two-qubit controlled-rotation gate, the state of the logical qubit is mapped onto the ancilla, followed by a destructive readout of the ancilla. Repeating this procedure enhances the logical readout fidelity from to after 15 ancilla readouts. In addition, we compare the conventional thresholding method with an improved signal processing method called soft decoding that makes use of analog information in the readout signal to better estimate the state of the logical qubit. We demonstrate that soft decoding leads to a significant reduction in the required number of repetitions when the readout errors become limited by Gaussian noise, for instance, in the case of readouts with a low signal-to-noise ratio. These results pave the way for the implementation of quantum error correction with spin qubits in silicon.
- Received 29 November 2019
- Accepted 3 March 2020
DOI:https://doi.org/10.1103/PhysRevX.10.021006
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)
Popular Summary
For many quantum information processing tasks, including quantum error correction, it is important to measure a qubit state accurately while perturbing the qubit as little as allowed by quantum mechanics. This can be achieved by repeatedly mapping information from a data qubit to an ancilla qubit, each time followed by destructive measurement of the ancilla. The state of the data qubit is then inferred from the sequence of ancilla measurements. Here, we implement an ancilla-based repetitive readout of an electron-spin qubit and analyze its performance.
In ancilla-based information storage, there are two approaches to retrieving the data. In hard decoding, ancilla measurements are converted one by one to bit values, followed by a majority vote to determine the most likely state of the data qubit. This strategy may discard useful information. In soft decoding, the noisy signals from many ancilla measurements can be processed simultaneously. This strategy ideally extracts all available information in the measurements to optimally infer the state of the data qubit.
We encode quantum information in a single electron spin stored in one of two coupled quantum dots in silicon. A second electron spin, located in the other dot, acts as an ancilla qubit. We then measure and reinitialize the ancilla. After 15 repetitions, the measurement fidelity increases from 75.5% to 94.5%. Moreover, when Gaussian white noise dominates the overall measurement error, soft decoding is more efficient than hard decoding, thereby easing detector efficiency requirements for operation at a higher temperature and gate-based readout.
This work paves the way toward quantum error correction in silicon.