  In many physical systems, there are specific electronic states called dark
state that are protected from the rapid radiative decay imposed by the system
symmetry. Although their long-lived nature indicates their potential for
quantum information and spintronic applications, their high stability comes at
the expense of optical accessibility. Breaking the symmetry by using magnetic
and electric fields has been employed to hybridize dark and bright states thus
making them optically active, but high-frequency and on-chip operation remains
to be developed. Here we demonstrate the strain-induced coherent coupling of
dark and bright exciton states in a GaAs mechanical resonator. The in-plane
uniaxial strain breaks the rotational symmetry of the crystal, allowing the
dark states to be optically accessible without any external fields. Such
dark-bright coupling is tailored by the local strain distribution, which
enables the coherent spin operation in the gigahertz regime and opens the way
to on-chip excitonic quantum memories and circuits.
