Erwin Schrödinger introduced quantum entanglement in a 1935 paper delivered to the Cambridge Philosophical Society in which he argued that the state of a system of two particles that have interacted generally cannot be written as a product of individual states of each particle.
|Particle A interacting with B〉 ≠ |A〉|B〉
Such a state would be an entanglement of individual states in which one cannot say with any certainty which particle is in which state. Disentanglement occurs when a measurement is made. This is what gave rise to Schrödinger’s famous (or infamous) cat illustration, which will be useful in understanding the role of measurement and the following consequent for a quantum version of many worlds.
The non-interactive state of two particles cannot be expressed as a certain conjunction of the two states. An example of an entangled state is