The photonic cannon destroys a Borg sphere.
The photonic cannon is an imaginary weapon of mass destruction, created by the USS Voyager's EMH, after he added daydreaming subroutines to his program. This weapon was supposedly impervious to all conventional sensors, and had enough power to destroy a Borg sphere with a single volley of two lightning-like energy beams.
The Doctor "used" the photonic cannon to prevent an attack on Voyager by a Hierarchy vessel, with the help of Phlox who convinced his superior officer about the weapon's awesome power. (VOY: "Tinker, Tenor, Doctor, Spy")