Quantum entanglement

Quantum entangelement is a physical phenomenon that occurs when pairs (or groups) of particles are generated or interact in ways such that the quantum state of each member must subsequently be described relative to the other.

Quantum entanglement is a product of quantum superposition. However, the state of each member is indefinite in terms of physical properties such as position, momentum, spin, polarization, etc. in a manner distinct from the intrinsic uncertainty of quantum superposition. When a measurement is made on one member of an entangled pair and the outcome is thus known (e.g., clockwise spin), the other member of the pair is at any subsequent time always found (when measured) to have taken the appropriately correlated value (e.g., counterclockwise spin). There is thus a correlation between the results of measurements performed on entangled pairs, and this correlation is observed even though the entangled pair may be separated by arbitrarily large distances.

Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement: there is no lightspeed or slower influence that can pass between the entangled particles. Experiments have measured entangled particles within less than one part in 10,000 of the light travel time between them; according to the formalism of quantum theory, the effect of measurement happens instantly.

This behavior is consistent with quantum theory, and has been demonstrated experimentally with photons, electrons, molecules the size of buckyballs, and even small diamonds.