English: The Bell state is a quantum state |\Psi> = a|00> + b|01> + c|10> + d|11>, where a = d = 1/sqrt(2) and b = c = 0. Therefore it can be fully described by the plane spanned by the basis vectors |00> and |11>, as in the picture.

The unit sphere (in C^4) intersects the plane in a circle. The state |\Psi> lies on the unit spheres surface.

a = d = 1/sqrt(2), meaning that |a|^2 = |b|^2 = 1/2. There is equal probability of measuring this state to |00> or |11>, and zero probability of measuring it to |01> or |10>.