A subspace differential pulse was a method utilized by the crew of the USS Enterprise to scan a quantum fissure in 2370.
Using the pulse, they scanned over ten million different quantum states within the fissure, in an attempt to locate the quantum state which shared a specific quantum signature with Lieutenant Worf, who had been shifting between quantum realities due to an encounter with the fissure. Later, after coming under attack from a Bajoran vessel, the subspace pulse experienced an energy surge, destabilizing the fissure. This had the effect of breaking down the barriers between quantum universes, causing approximately 285,000 different quantum versions of the Enterprise to emerge from their respective realities. (TNG: "Parallels")