In StarCraft II, the Protoss unit that can create defensive Photon Cannons is the Probe.
The Probe is the Protoss worker unit. It gathers minerals and vespene gas, constructs Protoss structures, and can place a Photon Cannon as part of a defensive setup. Protoss structures are warped in after construction begins, allowing a Probe to start a building and return to resource gathering rather than remaining beside it.
Photon Cannons provide both ground and air defense and can detect cloaked or burrowed enemies. The Probe itself is not a combat unit, although it can attack in an emergency. It should not be confused with the Sentry, a combat support unit that uses force fields and guardian shields.