The photonic cannon destroys a Borg sphere.
The photonic cannon was an imaginary type of directed energy cannon, a weapon of mass destruction, created by the USS Voyager's ECH, 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 concept of a photonic cannon as a bluff to prevent an attack on Voyager by a Hierarchy starship, with the help of Phlox, who convinced his superior officer about the weapon's awesome power. (VOY: "Tinker Tenor Doctor Spy")
Species 8472 bioships, seen in "Scorpion" and "Scorpion, Part II", were armed with a similar real weapon with the same level of destructive power.