A fictitious superweapon that the Doctor pictured himself firing as Emergency Command Hologram, taken for real by alien spies watching his daydreams in Tinker, Tenor, Doctor, Spy.
The photonic cannon does not exist. In the Voyager episode Tinker, Tenor, Doctor, Spy, the Doctor alters his programme so that he can daydream, and in his favourite fantasy he takes command of the ship as the Emergency Command Hologram and destroys a Borg vessel with the weapon.
A crew of the Hierarchy, hidden in a nearby nebula, has tapped into his perceptions and cannot tell his fantasies from fact. Their observer reports that Voyager is formidably armed, then realises his mistake and, fearing for his job, secretly contacts the Doctor to ask for help.
When the Hierarchy ships attack, Janeway puts the Doctor in the captain's chair, and he bluffs the raiders into retreat by ordering the photonic cannon brought to bear.