In quantum physics, what name is given to the rule that two particles can behave as one linked system?

In quantum physics, quantum entanglement is the rule that two particles can behave as one linked system.

An entangled state cannot generally be written as separate states for its individual particles. Measurements on the particles can show correlations stronger than classical theories with local hidden properties allow. These correlations persist even when the particles are separated by large distances.

Albert Einstein, Boris Podolsky, and Nathan Rosen highlighted the conceptual problem in their 1935 EPR paper. John Bell later showed how experiments could test local hidden-variable theories. Experiments based on Bell inequalities have repeatedly observed quantum correlations, including increasingly rigorous tests in the twenty-first century.

Entanglement does not allow usable information to travel faster than light. Each individual measurement has a random result, and the correlation becomes evident only when the results are compared through ordinary communication. Entanglement is also different from interference: interference concerns amplitudes combining, while entanglement concerns a joint state that cannot be separated into independent parts.