(covers information from several alternate timelines)
This page contains information regarding new Star Trek material, and thus may contain spoilers.
A quantum signature was the unique signature with which all matter in a universe resonated. It could not be changed by any known process, and was considered to be the basic foundation of existence. However, the quantum signature did differ between different quantum universes, such as the mirror universe. (TNG: "Parallels"; DIS: "Despite Yourself")
During the first Federation-Klingon War, USS Discovery found itself transported to the mirror universe due to an incident with its spore drive. The first data suggesting that it was no longer in its native universe came when it detected the quantum signature of other ships were inconsistent with its own. (DIS: "Despite Yourself")
In 2370, Lieutenant Worf was thrown into a state of quantum flux as a result of an encounter with a quantum fissure, causing him to shift between different quantum realities. Detecting this flux in Worf's cellular RNA, which extended to the subatomic level, Lieutenant Commander Data was able to determine that Worf's quantum signature was not consistent with the resonance of matter in the rest of the universe, and that he had originated from a different quantum universe. (TNG: "Parallels")