people already send light beams over ocean-length optical fibers (that's how we're talking). this is achieved by the use of erbium-doped fiber amplifiers, which preserve wavelength and phase information, but even without amplification you can reach 500 km: https://phys.org/news/2023-06-scientists-km-quantum-key.html

and of course we're only talking about these rube goldberg setups because we're perversely trying to build a telescope with a giant rock in the middle of it. if we build the telescope in solar orbit, we can just shine a light through space for the phase reference