Quantum Entanglement

Quantum entanglement is a physical phenomenon in which two or more particles become correlated such that the quantum state of each cannot be described independently of the others, even when separated by vast distances. Measuring one particle instantaneously constrains what can be found when measuring its partner — not because information travels between them, but because they share a single quantum state that extends across space.

The phenomenon was considered paradoxical by Einstein, Podolsky, and Rosen in 1935, who argued it implied either that Quantum Mechanics was incomplete or that faster-than-light influence existed. John Bell showed in 1964 that these two possibilities could be distinguished experimentally. The experiments, beginning with Aspect in 1982, decisively confirmed that entanglement is real and irreducible. No hidden variable theory can reproduce its statistics.

Entanglement is a resource in Quantum Computing, enabling correlations between qubits that make quantum parallelism possible. It is also the basis of Quantum Teleportation, in which quantum states can be transmitted using entanglement plus a classical channel. The philosophical implications — that distant parts of the universe can share a single indivisible state — remain contested under different interpretations of quantum mechanics.