An Etymological Dictionary of Astronomy and Astrophysics
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The act of entangling. The state of being entangled.

Verbal noun of → entangle; → -ment.

A quantum → phenomenon that occurs when two or more particles (→ photons or → atomic particles) that have a common origin remain linked together when they travel apart. A measurement of one of the particles determines not only its → quantum state but the quantum state of the other particle as well. A change in one is instantly reflected in the other. To use a familiar example, it is as if you have a pair of dice entangled in such a way that when you throw them the sum of the two is 7. Any time you cast them, if the first die shows 2, 5, 3, etc. the other will show 5, 2, 4, etc., respectively. Quantum entanglement is rooted in the → superposition principle. But, in contrast to → quantum coherence, the states in a superposition are the shared states of two entangled particles rather than those of the two split waves of a single particle. There are several ways for entangling atomic particles. Photons can be entangled using → cascade transitions, as was done by Alain Aspect and colleagues in the early 1980s (→ Aspect experiment). Calcium atoms are put into a highly-excited energy level where the electron is forbidden to return to the → ground state by emitting a single photon. As a result, the atoms → decay by emitting two photons which are entangled. Like quantum coherence, quantum entanglement plays an essential role in quantum technologies, such as quantum teleportation, quantum cryptography, and super dense coding. See also → EPR paradox.

→ quantum; → entanglement.