Quantum mechanics, as formulated by Schrodinger is a very

established method for describing the motion of a single particle.

When we consider two particles (that is a pair) then quantum

states of each individual particle may not be specified. Instead

of that a quantum state of the system consisting of the pair

as a whole has to be specified.

Quantum entanglement is an area of vigorous research activity.

Experimentally, the idea of entanglement has been experimentally

verified for systems of photons, electrons and so on. The application

of the idea of entanglement is seen in areas of communication

and quantum computation.

Interestingly, Einstein, in his last years was very much interested

in this subject, he wrote several papers on it. A famous work is

on the EPR paradox (1935).