  Coupling of quantum emitters in a semiconductor relies, generally, on
short-range dipole-dipole or electronic exchange type interactions.
Consistently, energy transfer between exciton states, that is, electron-hole
pairs bound by Coulomb interaction, is limited to distances of the order of
10~nm. Here, we demonstrate polariton-mediated coupling and energy transfer
between excitonic states over a distance exceeding 2~$\mu$m. We accomplish this
by coupling quantum well-confined excitons through the delocalized mode of two
coupled optical microcavities. Use of magnetically doped quantum wells enables
us to tune the confined exciton energy by the magnetic field and in this way to
control the spatial direction of the transfer. Such controlled, long-distance
interaction between coherently coupled quantum emitters opens possibilities of
a scalable implementation of quantum networks and quantum simulators based on
solid-state, multi-cavity systems.
