There has been some debate as to whether this technology was synonymous with photon torpedoes of the twenty-third and twenty-fourth centuries or an early forerunner. Regardless, the information here applies only to weapons of the twenty-second century.
Photonic torpedoes were torpedo weapons carried aboard Starfleet vessels during the twenty-second century. These torpedoes were the most powerful weapons in the standard vessel's armament and were the successor to earlier spatial torpedoes.
Photonic torpedoes were equipped with a variable yield antimatter warhead. They could be launched while a ship was traveling at warp speed as well as when it was traveling at sublight speeds. They had over fifty times greater range than the previous spatial torpedoes. The variable yield was described by Enterprise NX-01 armory officer Lieutenant Malcolm Reed as being able to "knock the com array off a shuttlepod without scratching the hull or put a three-kilometer crater into an asteroid." Antimatter to the warheads on NX-class starships was provided by the ship's antimatter reserves.
Starfleet began to utilize photonic torpedo technology in 2153 when it began installation of the torpedoes aboard Enterprise NX-01 following its return to Earth. The use of antimatter warheads aboard Earth starships came as quite a surprise to attacking Klingons. In 2153, a bird-of-prey sustained enough damage that they were forced to discontinue their pursuit of Enterprise. This form of torpedo, however, still could not effectivel penetrate the deflector shielding of a typical Klingon bird-of-prey when most shield power was transferred to the section under attack.
Klingon torpedo technology of the time appeared to be much more advanced than both Earth and Vulcan technology as some of their ships were already equipped with photon torpedoes.