Quantum entanglement is a physical phenomenon that occurs when a group of particles are generated, interacted, or exist in ways such that the quantum state of each particle cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical 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 be found to be perfectly correlated. For example, if a pair of entangled particles is generated such that their total spin is known to be zero, and one particle is found to have clockwise spin on a first axis, then the spin of the other particle, measured on the same axis, will be found to be counterclockwise. However, this behavior gives rise to paradoxical effects: any measurement of a property of a particle results in an irreversible collapse of that particle's wave function and an alteration of the original quantum state.

This leads to correlations between measurements on the entangled system. Entanglement is often considered a "spooky action at a distance" because it appears to involve instantaneous communication between particles, regardless of distance. This challenges the principle of locality in physics, which states that objects are only directly influenced by their immediate surroundings.