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
because a singlet is a zero spin particle and there are two possible outcomes for a particle’s spin—up or down. The first particle could be up, , or down, . When a measurement is made one of the possible states collapses and produces what is observed. After a measuring apparatus is introduced an unknown state,
the whole state becomes
Remember, the state above is just a function state in conjunction with a measuring apparatus. Such a state then evolves according to the Schrödinger equation
into a non-collapsed state
When the observer makes an observation the observer becomes entangled as well.
Schrödinger found this to be so incredibly “sinister” because when states are added to previous states for measurement, larger states of affairs, etc. anything and everything becomes entangled. Thus, it seems to be the case that the entire universe becomes one large state of entanglement.
For the following example, consider the observer happy if a spin up is perceived. Let denote the observer’s state prior to measurement,
after perceiving spin up, and
after perceiving spin down. Using the Hamiltonian
which totals the energies involved in a given situation, the measurement is to be described by the unitary Schrödinger time evolution operator
U will then clearly meet
Thus, in a non-collapsed state of entanglement an observer of the superposition
according to Everett, would appear as
and not as
It should be clear by that the conjunction of outcomes becomes actualized rather than one of the possible outcomes.
 Erwin Schrödinger, “Discussion of Probability Relations Between Separated Systems,” Mathematical Proceedings of the Cambridge Philosophical Society 31 no. 4 (Oct. 1935).
 Jonathan Allday, Quantum Reality: Theory and Philosophy (Boca Raton, FL: Taylor & Francis, 2009), 374-75.