  The quantum nature of bulk ensemble NMR quantum computing --the center of
recent heated debate, is addressed. Concepts of the mixed state and
entanglement are examined, and the data in a 2 qubit liquid NMR quantum
computation are analyzed. It is pointed out that the key problem in the current
debate is the understanding of entanglement in a mixed state system. The
following points are concluded in this Letter: 1)Density matrix describes the
"state" of an average particle in an ensemble. It can not describe the state of
an individual particle in an ensemble in detail; 2) Entanglement is a property
of the wave function of a quantum particle(such as an molecule in a liquid NMR
sample). Separability of the density matrix can not be used to measure the
entanglement of mixed ensemble; 3)The evolution of states in bulk-ensemble NMR
quantum computation is quantum mechanical; 4) The coefficient before the
effective pure state density matrix, $\epsilon$, is an measure of the
simultaneity of the molecules in an ensemble. It reflects the intensity of the
NMR signal and has no significance in quantifying the entanglement in the bulk
ensemble NMR system. We conclude that the liquid NMR quantum computation is
genuine, not just classical simulations.
