Identifying the quantum correlations in light-harvesting complexes

TL;DR

This study investigates quantum correlations in light-harvesting complexes, demonstrating that quantum discord and entanglement measures are significant during excitation transfer, with simplified analytic forms and numerical simulations revealing their roles at different temperatures.

Contribution

It provides an analytic expression for the relative entropy of entanglement in specific quantum states and compares quantum discord and entanglement in biological systems during excitation transfer.

Findings

Quantum discord and entanglement are equal in simulations within the single-excitation subspace.

Quantum correlations are significant during the first picosecond of excitation transfer.

The relative entropy of entanglement is lower than quantum discord when not restricted to single-excitation states.

Abstract

One of the major efforts in the quantum biological program is to subject biological systems to standard tests or measures of quantumness. These tests and measures should elucidate if non-trivial quantum effects may be present in biological systems. Two such measures of quantum correlations are the quantum discord and the relative entropy of entanglement. Here, we show that the relative entropy of entanglement admits a simple analytic form when dynamics and accessible degrees of freedom are restricted to a zero- and single-excitation subspace. We also simulate and calculate the amount of quantum discord that is present in the Fenna-Matthews-Olson protein complex during the transfer of an excitation from a chlorosome antenna to a reaction center. We find that the single-excitation quantum discord and relative entropy of entanglement are equal for all of our numerical simulations, but a proof of their general equality for this setting evades us for now. Also, some of our simulations demonstrate that the relative entropy of entanglement without the single-excitation restriction is much lower than the quantum discord. The first picosecond of dynamics is the relevant timescale for the transfer of the excitation, according to some sources in the literature. Our simulation results indicate that quantum correlations contribute a significant fraction of the total correlation during this first picosecond in many cases, at both cryogenic and physiological temperature.