# Quantum nondemolition measurement of an electron spin qubit

## TL;DR

This paper demonstrates an all-electrical quantum nondemolition measurement of a single electron spin in a quantum dot, enabling repeated, high-fidelity measurements with minimal disturbance, crucial for quantum information processing.

## Contribution

It introduces a novel all-electrical QND measurement protocol for electron spins using an exchange-coupled ancilla qubit, achieving rapid, repetitive, and minimally invasive measurements.

## Key findings

- Readout fidelity increases monotonically over 100 measurements
- Measurement rate is two orders of magnitude faster than spin relaxation
- Allows observation of quantum jumps in an isolated spin system

## Abstract

Measurement of quantum systems inevitably involves disturbance in various forms. Within the limits imposed by quantum mechanics, however, one can design an "ideal" projective measurement that does not introduce a back action on the measured observable, known as a quantum nondemolition (QND) measurement. Here we demonstrate an all-electrical QND measurement of a single electron spin in a gate-defined quantum dot via an exchange-coupled ancilla qubit. The ancilla qubit, encoded in the singlet-triplet two-electron subspace, is entangled with the single spin and subsequently read out in a single shot projective measurement at a rate two orders of magnitude faster than the spin relaxation. The QND nature of the measurement protocol is evidenced by observing a monotonic increase of the readout fidelity over one hundred repetitive measurements against arbitrary input states. We extract information from the measurement record using the method of optimal inference, which is tolerant to the presence of the relaxation and dephasing. The QND measurement allows us to observe spontaneous spin flips (quantum jumps) in an isolated system with small disturbance. Combined with the high-fidelity control of spin qubits, these results pave the way for various measurement-based quantum state manipulations including quantum error correction protocols.