Emergence of a measurement basis in atom-photon scattering

TL;DR

This paper demonstrates how a measurement basis naturally emerges in the evolution of a single trapped atomic ion's spin due to spontaneous photon scattering, aligning with decoherence theory and enabling high-fidelity measurement.

Contribution

It provides experimental visualization of the measurement basis emergence in atom-photon scattering, confirming decoherence theory predictions and illustrating the classical correlation of pointer states.

Findings

Measurement basis aligns with photon propagation direction.

Photon scattering minimally increases entropy of pointer states.

Photon polarization measurement accurately determines the spin state.

Abstract

The process of quantum measurement has been a long standing source of debate. A measurement is postulated to collapse a wavefunction onto one of the states of a predetermined set - the measurement basis. This basis origin is not specified within quantum mechanics. According to the theory of decohernce, a measurement basis is singled out by the nature of coupling of a quantum system to its environment. Here we show how a measurement basis emerges in the evolution of the electronic spin of a single trapped atomic ion due to spontaneous photon scattering. Using quantum process tomography we visualize the projection of all spin directions, onto this basis, as a photon is scattered. These basis spin states are found to be aligned with the scattered photon propagation direction. In accordance with decohernce theory, they are subjected to a minimal increase in entropy due to the photon scattering, while, orthogonal states become fully mixed and their entropy is maximally increased. Moreover, we show that detection of the scattered photon polarization measures the spin state of the ion, in the emerging basis, with high fidelity. Lastly, we show that while photon scattering entangles all superpositions of pointer states with the scattered photon polarization, the measurement-basis states themselves remain classically correlated with it. Our findings show that photon scattering by atomic spin superpositions fulfils all the requirements from a quantum measurement process.