Relationship between two physically separated particles under special circumstances.
Two photons may be "quantum entangled" if produced by the same particle interaction and emerging in opposite directions. The two photons remain quantum entangled with each other even when separated by very large distances (even when light-years apart). In such a circumstance, the two quantum entangled photons, if each forced to make a decision to choose among two equally probable pathways, will make the identical decision and will do so at the same instant in time. Since there is no possible communication link between two quantum entangled photons, classical physics would predict that their decisions would be independent. But two quantum entangled photons make the same decision and do so at the same instant in time.
During the early Information Age, experiments demonstrated that even if there were an unknown communication path between them, there is not enough time for a message to travel from one photon to the other at the speed of light.
Text by Ray Kurzweil
Initially published on 19 December 2001.