  We study coherent energy transfer of a single excitation and quantum
entanglement in a dimer, which consists of a donor and an acceptor modeled by
two two-level systems. Between the donor and the acceptor, there exists a
dipole-dipole interaction, which provides the physical mechanism for coherent
energy transfer and entanglement generation. The donor and the acceptor couple
to two independent heat baths with diagonal couplings that do not dissipate the
energy of the non-coupling dimer. Special attention is paid to the effect on
single-excitation energy transfer and entanglement generation of the energy
detuning between the donor and the acceptor and the temperatures of the two
heat baths. It is found that, the probability for single-excitation energy
transfer largely depends on the energy detuning in the low temperature limit.
Concretely, the positive and negative energy detunings can increase and
decrease the probability at the steady state, respectively. In the high
temperature limit, however, the effect of the energy detuning on the
probability is neglectably small. We also find that the probability is
neglectably dependent on the bath temperature difference of the two heat baths.
In addition, it is found that quantum entanglement can be generated in the
process of coherent energy transfer. As the bath temperature increases, the
generated steady state entanglement decreases. For a given bath temperature,
the steady-state entanglement decreases with the increasing of the absolute
value of the energy detuning.
