Transport phenomena at the nanoscale exhibit both quantum (coherent) and classical (noisy) behaviour. Coherent and incoherent transfer are normally viewed as limiting cases of a certain underlying dynamics. However, there exist parameter regimes where an intricate interplay between environmental noise and quantum coherence emerges, and whose net effect is an increase in the efficiency of the transport process. In this chapter we illustrate this phenomenon in the context of excitation transport across quantum networks. These are model systems for the description of energy transfer within molecular complexes and, in particular, photosynthetic pigment–protein molecules, a type of biologically relevant structures whose dynamics has been recently shown to exhibit quantum coherent features. We show that nearly perfect transport efficiency is achieved in a regime that utilizes both coherent and noisy features, and argue that Nature may have chosen this intermediate regime to operate optimally.

Introduction

The dynamical behaviour of a quantum system can be substantially affected by interaction with a fluctuating environment and one might initially be led to expect a negative effect on quantum transport involving coherent hopping of a (quasi-) particle between localized sites. In this section, however, we demonstrate that quantum transport efficiency can be enhanced by a dynamical interplay of the quantum dynamics imposed by the system Hamiltonian with the pure dephasing induced by a fluctuating environment.