  Atomically-thin transition metal dichalcogenide crystals (TMDCs) hold great
promise for future semiconductor optoelectronics due to their unique electronic
and optical properties. In particular, electron-hole pairs (excitons) in TMDCs
are stable at room temperature and interact strongly with light. When TMDCs are
embedded in an optical microcavity, the excitons can hybridise with cavity
photons to form exciton polaritons (polaritons herein), which display both
ultrafast velocities and strong interactions. The ability to manipulate and
trap polaritons on a microchip is critical for future applications. Here, we
create a potential landscape for room-temperature polaritons in monolayer
WS$_2$, and demonstrate their free propagation and trapping. We show that the
effect of dielectric disorder, which restricts the diffusion of WS$_2$ excitons
and broadens their spectral resonance, is dramatically reduced in the strong
exciton-photon coupling regime leading to motional narrowing. This enables the
ballistic transport of WS$_2$ polaritons across tens of micrometers with an
extended range of partial first-order coherence. Moreover, the dephasing of
trapped polaritons is dramatically suppressed compared to both WS$_2$ excitons
and free polaritons. Our results demonstrate the possibility of long-range
transport and efficient trapping of TMDC polaritons in ambient conditions.
