Repetitive quantum non-demolition measurement and soft decoding of a silicon spin qubit

TL;DR

This paper demonstrates quantum non-demolition measurements and soft decoding techniques in silicon spin qubits, significantly improving logical qubit readout fidelity and efficiency, advancing quantum error correction efforts.

Contribution

It introduces a QND measurement scheme using a two-qubit controlled-rotation gate in silicon spin qubits and compares soft decoding with thresholding for enhanced readout accuracy.

Findings

Logical qubit readout fidelity improved from 75.5% to 94.5% with repeated measurements.

Soft decoding reduces the number of repetitions needed under Gaussian noise conditions.

Results support the feasibility of quantum error correction with silicon spin qubits.

Abstract

Quantum error correction is of crucial importance for fault-tolerant quantum computers. As an essential step towards the implementation of quantum error-correcting codes, quantum non-demolition (QND) measurements are needed to efficiently detect the state of a logical qubit without destroying it. Here we implement QND 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 $75.5\pm 0.3\%$ to $94.5 \pm 0.2\%$ 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.