Quantum non-demolition measurement of an electron spin qubit through its low-energy many-body spin environment

TL;DR

This paper demonstrates a high-fidelity, quantum non-demolition measurement of an electron spin qubit via its many-body spin environment, supporting quantum Darwinism and enabling repeated, heralded readouts without wavefunction collapse.

Contribution

It introduces a method for measuring a spin qubit through off-resonant coupling with nuclear spins, achieving near-perfect fidelity and observing quantum jumps linked to phonon bath fluctuations.

Findings

Single-shot readout fidelity of approximately 99.85%

Observation of quantum jumps due to phonon bath fluctuations

Repeated measurements enable heralded initialization and dynamics probing

Abstract

The measurement problem dates back to the dawn of quantum mechanics. Here, we measure a quantum dot electron spin qubit through off-resonant coupling with thousands of redundant nuclear spin ancillae. We show that the link from quantum to classical can be made without any "wavefunction collapse", in agreement with the Quantum Darwinism concept. Large ancilla redundancy allows for single-shot readout with high fidelity $\approx99.85\%$. Repeated measurements enable heralded initialization of the qubit and probing of the equilibrium electron spin dynamics. Quantum jumps are observed and attributed to burst-like fluctuations in a thermally populated phonon bath.