Coherent exciton dynamics in supramolecular light-harvesting nanotubes revealed by ultrafast quantum process tomography

TL;DR

This paper introduces a novel quantum process tomography method to analyze exciton dynamics in light-harvesting nanotubes, revealing energy transfer pathways and coherence properties at room temperature.

Contribution

It presents the first experimental quantum process tomography in a complex, warm system, enhancing understanding of exciton coherence and transfer in supramolecular structures.

Findings

Unidirectional energy transfer from outer to inner wall excitons

Absence of nonsecular processes in exciton dynamics

150 femtoseconds of coherence between wall states

Abstract

Long-lived exciton coherences have been recently observed in photosynthetic complexes via ultrafast spectroscopy, opening exciting possibilities for the study and design of coherent exciton transport. Yet, ambiguity in the spectroscopic signals has led to arguments for interpreting them in terms of the exciton dynamics, demanding more stringent tests. We propose a novel strategy, Quantum Process Tomography (QPT) for ultrafast spectroscopy, to reconstruct the evolving quantum state of excitons in double-walled supramolecular light-harvesting nanotubes at room temperature. The protocol calls for eight transient grating experiments with varied pulse spectra. Our analysis reveals unidirectional energy transfer from the outer to the inner wall excitons, absence of nonsecular processes, and an unexpected coherence between those two states lasting about 150 femtoseconds, indicating weak electronic coupling between the walls. Our work constitutes the first experimental QPT in a 'warm' and complex system, and provides an elegant scheme to maximize information from ultrafast spectroscopy experiments.