ABSTRACT
Entanglement in quantum theory is a consequence of the superposition principle for probability amplitudes. The term relates to a pair of quantum systems whose state cannot be expressed as the product of the single state of each system. The simplest example is the singlet, or zero angular momentum, state of a pair of spin-half particles. This has the form ð1= ffiffiffi2p Þðjþi1ji2 ji1jþi2Þ where jþii is the spin up, and jii the spin down, state for particle i.
