Entanglement

From Qwiki

Entanglement is a property of quantum states of two or more systems that is akin to correlation. In classical physics two systems are correlated when finding out something about one system also tells you something about the other. For instance, in my sock drawer, the colors of socks of a pair are highly correlated. Entangled quantum systems have a similar property, but the correlation is stronger than is possible in classical physics. The way to show that mathematically is, e.g., through Bell inequalities. That is, probabilities associated with classical systems (in particular: 2 systems separated by a sufficiently large distance such that no communication between the two can take place within a certain time interval) always obey certain inequalities, but entangled quantum states can violate them.

In the field of quantum information, entanglement is used as a resource: entanglement can be used to perform certain tasks in a better way. Better may mean more efficiently (as in quantum computing), or more securely (as in quantum cryptography).

See measures of entanglement that satisfy the entanglement monotone requirements, as well as particularly interesting entangled quantum states.