Spin-selective reactions of radical pairs act as quantum measurements

TL;DR

This paper introduces a quantum measurement perspective to model spin-selective radical pair reactions, predicting faster decay of superpositions than traditional phenomenological models, though differences may be experimentally subtle.

Contribution

It provides an alternative derivation of rate expressions using quantum measurement theory, offering a potentially more accurate description of radical pair spin dynamics.

Findings

Quantum measurement approach predicts faster decay of superpositions.

Differences between models are subtle and hard to detect experimentally.

New model aligns with existing phenomenological results but offers deeper theoretical insight.

Abstract

Since the 1970s, spin-selective reactions of radical pairs have been modelled theoretically by adding phenomenological rate equations to the quantum mechanical equation of motion of the radical pair spin density matrix. Here, using a quantum measurement approach, we derive an alternative set of rate expressions which predict a faster decay of coherent superpositions of the singlet and triplet radical pair states. The difference between the two results, however, is not dramatic and would probably be difficult to distinguish experimentally from decoherence arising from other sources.