  Efficient switching and routing of photons of different wavelengths is a
requirement for realizing a quantum internet. Multimode optomechanical systems
can solve this technological challenge and enable studies of fundamental
science involving widely separated wavelengths that are inaccessible to
single-mode optomechanical systems. To this end, we demonstrate interference
between two optomechanically induced transparency processes in a diamond
on-chip cavity. This system allows us to directly observe the dynamics of an
optomechanical dark mode that interferes photons at different wavelengths via
their mutual coupling to a common mechanical resonance. This dark mode does not
transfer energy to the dissipative mechanical reservoir and is predicted to
enable quantum information processing applications that are insensitive to
mechanical decoherence. Control of the dark mode is also utilized to
demonstrate all-optical, two-colour switching and interference with light
separated by over 5 THz in frequency.
