# Open System Perspective on Incoherent Excitation of Light Harvesting Systems

## TL;DR

This paper investigates how incoherent natural thermal light influences the excited states and energy transfer pathways in light-harvesting systems, revealing long-lasting coherences and differences from pulsed laser excitation.

## Contribution

It provides a detailed analysis of incoherent light's effects on open quantum systems, highlighting the generation and dissipation of coherences and their impact on energy transfer pathways.

## Key findings

- Natural thermal light induces long-lasting coherences due to super-Ohmic radiation.
- Incoherent excitation leads to different energy transfer pathways compared to pulsed laser light.
- Coherence loss occurs as the light turn-on time approaches natural timescales.

## Abstract

The nature of excited states of open quantum systems produced by incoherent natural thermal light is analyzed based on a description of the generator of the dynamics. Natural thermal light is shown to generate long-lasting coherent dynamics because of (i) the super-Ohmic character of the radiation, and (ii) the absence of pure dephasing dynamics. In the presence of an environment, the long-lasting coherences induced by suddenly turned-on incoherent light dissipate and stationary coherences are established. As a particular application, dynamics in a subunit of the PC-645 light-harvesting complex is considered where it is further shown that aspects of the energy pathways landscape depend on the nature of the exciting light and number of chromophores excited. Specifically, pulsed laser and natural broadband incoherent excitation induce significantly different energy transfer pathways. In addition, we discuss differences in perspective associated with the eigenstate vs site basis, and note an important difference in the phase of system coherences when coupled to blackbody radiation or when coupled to a phonon background. Finally, an Appendix contains an open systems example of the loss of coherence as the turn on time of the light assumes natural time scales.