Figure 2
From: Using non-Markovian measures to evaluate quantum master equations for photosynthesis
Schematic illustration of the entanglement measure.
We consider a system and a copy of it acting as a well-isolated ancilla possessing the same degrees of freedom of the system. Initially, they form a maximally-entangled state 
. Then the system starts to feel contact with its environment (denoted by the gray shadow) and evolves according to 
, whereas the ancilla is kept isolated.
