A 3-setting Bell-type inequality enforced by the indeterminacy relation of complementary local observables is proposed as an experimental test of 2-qubit entanglement. The proposed inequality has the advantage of being a sufficient and necessary criterion of separability. Therefore any entangled 2-qubit state cannot escape the detection by this kind of test. It turns out that the orientation of the local testing observables plays a crucial role in our perfect detection of entanglement.

• Received 8 January 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.217903