  Efficient teleportation is a crucial step for quantum computation and quantum
networking. In the case of qubits, four different entangled Bell states have to
be distinguished. We have realized a probabilistic, but in principle
deterministic, Bell-state analyzer for two photonic quantum bits by the use of
a nondestructive controlled-NOT gate based on entirely linear optical elements.
This gate was capable of distinguishing between all of the Bell states with
higher than 75% fidelity without any noise substraction due to utilizing
quantum interference effects.
