  We will construct a simple four qubits Hamiltonian of the Rs. molischianum
purple bacteria light harvesting complex II (LH-II) ring which yields energy
levels that carry the ring's oscillator strength. In an excitonic
representation, these levels are associated with the second and the third
lowest electronic excitations of the ring. We will assume that qubits form a
closed loop lattice and the interaction between qubits is only due to the
exchange effect. As we will show, eigenstates are constructed in a such way
that as we subsequently divide qubits of the Rs. molischianum LH-II ring into
the subsystem A consisting of only one qubit and the subsystem B consisting of
the remaining qubits, respective entropies of entanglement increase until the
value of 1 for the maximally entangled state (bipartite system) is reached.
Since the Hamiltonian in essence introduces a two-level approximation for the
LH-II ring, we will go one step further and assume that interactions between
qubits closed loop lattices and an electromagnetic field are described by the
Jaynes-Cummings Hamiltonian. This assumption is interesting by itself, since it
leads to behavior where the qubits lattice and field oscillate back and fourth
exchanging a quantum of energy, at the Rabbi frequency. This opens a
challenging opportunity to experimentally study the Rs. molischianum LH-II ring
in the regime of cavity QED.
