Quantum entanglement is the phenomenon where the quantum state of each
particle in a group cannot be described independently of the state of the
others, even when the particles are separated by a large distance. The topic
of quantum entanglement is at the heart of the disparity between classical
physics and quantum physics: entanglement is a primary feature of quantum
mechanics not present in classical mechanics.
Measurements of physical properties such as position, momentum, spin, and
polarization performed on entangled particles can, in some cases, be found to
be perfectly correlated.