  We show that it is possible to transfer two-bit information via encoding a
single qubit in a conventional nuclear magnetic resonance (NMR) experiment with
two very weakly polarized nuclear spins. Nevertheless, the experiment can not
be regarded as a demonstration of superdense coding by means of NMR because it
is based on the large number of molecules being involved in the ensemble state
rather than the entanglement of the NMR states. Following the discussions, an
entanglement witness, particularly applicable for NMR, is introduced based on
separate and simultaneous measurement of the individual nuclear spin
magnetizations.
