  Quantum transduction between microwave and optical photons plays a key role
in quantum communication among remote qubits. Although the quantum transduction
schemes generating communication photons have been successfully demonstrated by
using optomechanical interfaces, the low conversion efficiency remains an
obstacle to the implementation of a quantum network consisting of multiple
qubits. Here, we present an efficient quantum transduction scheme using a
one-dimensional diamond optomechanical crystal cavity tuned at a color-center
emission without optomechanical coupling. The optomechanical crystal cavity
incorporates a thin aluminum nitride (AlN) pad piezoelectric coupler near the
concentrator cavity region, while retaining ultrasmall mechanical and optical
mode volumes of about 1.5 \times 10^{-4} and ~0.2({\lambda}/n)^3, respectively.
The energy level of a coherent color-center electron is manipulated by a strong
mechanical-mode-color-center electron-coupling rate up to 16.4MHz. In our
system, we theoretically predict that the population-conversion efficiency from
a single microwave photon into an optical photon can reach 15% combined with
current technologies. The coherent conversion efficiency is over 10% with a
reasonably pure decay time of T^2_{*} > 10 ns. Our results imply that an atomic
color center strongly coupled to the optomechanical crystal cavity will offer a
highly efficient quantum transduction platform.
