**Bell States **(named after John S. Bell)** **or** EPR pairs** (after Einsetin, Podolski and Rosen Paradox) are the maximally entangled quantum states of a two qubit system (i.e. a quantum mechanical system composed of two interacting two-level subsystems).

In the *computational basis* the four maximally entangled Bell states are expressed as

,

.

To understand the correlations present in these states consider the Bell state . If a measurement is performed on the first qubit the result may be 0 or 1, after which the two qubit state collapses to or respectively. Then, a measurement on the second qubit *must* result in 0 or 1, yielding the same result as the measurement on the first one. That is, the outcomes of the measurements are *correlated*.

John Bell proved that the correlations present in EPR pairs* cannot be accounted for by any classical theory*. See this article for more.

Go to the Dictionary of Quantum Information and Quantum Computation.