  We propose a scheme for identifying an unknown Bell diagonal state. In our
scheme the measurements are performed on the probe qubits instead of the Bell
diagonal state. The distinguished advantage is that the quantum state of the
evolved Bell diagonal state ensemble plus probe states will still collapse on
the original Bell diagonal state ensemble after the measurement on probe
states, i.e. our identification is quantum-state nondestructive. It is also
shown finally how to realize our scheme in the framework of cavity
electrodynamics.
