|An artistic rendition of quantum entanglement.|
The premise of entanglement arose in a 1935 thought experiment of Albert Einstein, Boris Podolsky, and Nathen Rosen to demonstrate, as they thought, a logical impossibility of quantum mechanics, hence the acronym EPR effect, experiment, or paradox, as it is sometimes called. 
Experimental testing on the entanglement hypothesis was first conducted by Irish physicist John Bell working at CERN, in Geneva in the mid 1960s.
The premise of entanglement is the, what seems to be, measured phenomenon that if two systems, or particles, A and B, are permitted to interact, and then separated by some great distance in space, a change in one particle or system, is said to be instantaneously detectable in the other, to the effect that the particles seem to remain entangled or ‘aware’ of each other, so to speak, to some extent or another. 
In the 1980s, a group working in Paris induced an atom to emit two photons simultaneously in opposite directions, and found that measurement of one photon on one side of the lab affected the other photon on the opposite side of the lab, instantaneously. The argument is that because the two photons originated from the same source, they are correlated with one another, and thus remain ‘entangled’ even when far apart.
In the 1990s, researchers in Geneva sent two originally entangled photons separately down 6.2 miles of fiber optic cables, finding that the two photons acted like one particle even when more than six miles apart.
The work of English researcher Rupert Sheldrake, and his theory of the morphogenetic field, gives the most comprehensive summary of work done on the idea or concept of entanglement as applied to animals and humans, to the effect that animals can sense eminent danger, that spouses (one pair of a dihumanide molecule) can sense when they are being cheated on, even when separated on opposite sides of the globe, or that twins can sense when their counterpart has been in an accident, among other studied phenomenon. 
1. Polkinghorne, John. (2002). Quantum Theory: a Very Short Introduction (pgs. 80-81). Oxford University Press.
2. Gribben, John. (2002). Quantum Physics: a Beginner’s Guide to the Subatomic World (entanglement, pgs. 62-63). DK publishing.
3. Gribben, John. (2000). Q is for Quantum: an Encyclopedia of Particle Physics (§EPR experiment, pgs. 126-27). Touchstone.
4. (a) Sheldrake, Rupert. (2003). The Sense of Being Stared At: and Other Unexplained Powers of the Human Mind. Crown Publishers.
(b) Sheldrake, Rupert. (1999). Dogs That Know When Their Owners Are Coming Home: and Other Unexplained Powers of Animals. Three Rivers Press.
● Double-slit experiment
● Homework problems
● Aczel, Amir D. (2001). Entanglement. Plume.
● Quantum entanglement – Wikipedia.
● EPR paradox – Wikipedia.