# Gain and loss in open quantum systems

## TL;DR

This paper models the high-efficiency energy transfer in photosynthesis using open quantum systems with gain and loss, revealing a two-step process involving rapid fluctuations and slower internal excitations, akin to biological light harvesting.

## Contribution

It introduces a novel quantum model incorporating gain and loss to explain the efficiency of photosynthetic energy transfer, highlighting a two-step process with analytical and numerical insights.

## Key findings

- Fluctuations near singular points cause rapid, efficient cross section changes.
- The excitation of internal states occurs slower, involving internal degrees of freedom.
- The overall process is highly efficient with bi-exponential decay, similar to photosynthesis.

## Abstract

Photosynthesis is the basic process used by plants to convert light energy in reaction centers into chemical energy. The high efficiency of this process is not yet understood today. Using the formalism for the description of open quantum systems by means of a non-Hermitian Hamilton operator, we consider initially the interplay of gain (acceptor) and loss (donor). Near singular points it causes fluctuations of the cross section which appear without any excitation of internal degrees of freedom of the system. This process occurs therefore very quickly and with high efficiency. We then consider the excitation of resonance states of the system by means of these fluctuations. This second step of the whole process takes place much slower than the first one, because it involves the excitation of internal degrees of freedom of the system. The two-step process as a whole is highly efficient and the decay is bi-exponential. We provide, if possible, the results of analytical studies, otherwise characteristic numerical results. The similarities of the obtained results to light harvesting in photosynthetic organisms are discussed.