Quantum-Nondemolition Determination Of An Unknown Werner State

A nondestructive approach to determine an unknown two-qubit Werner state of an ensemble is proposed. The approach is feasible for all four types of Werner states which contain different kinds of Bell states. Our scheme can detect the magnitude of the real parameter of an initial Werner state. An easy two-step method is also presented to determine which type of Bell state is included in the Werner state. The initial state can be preserved during the measurements. Our nondemolition determinations are demonstrated experimentally in a nuclear-magnetic-resonance quantum-information processor.