  We derive a unified quantum theory of coherent and incoherent energy transfer
between two atoms (donor and acceptor) valid in arbitrary Markovian
nanophotonic environments. Our theory predicts a fundamental bound $\eta_{max}
= \frac{\gamma_a}{\gamma_d + \gamma_a}$ for energy transfer efficiency arising
from the spontaneous emission rates $\gamma_{d}$ and $\gamma_a$ of the donor
and acceptor. We propose the control of the acceptor spontaneous emission rate
as a new design principle for enhancing energy transfer efficiency. We predict
an experiment using mirrors to enhance the efficiency bound by exploiting the
dipole orientations of the donor and acceptor. Of fundamental interest, we show
that while quantum coherence implies the ultimate efficiency bound has been
reached, reaching the ultimate efficiency does not require quantum coherence.
Our work paves the way towards nanophotonic analogues of efficiency enhancing
environments known in quantum biological systems.
