  In this work, we have been working on the concept of quantum entanglement. At
first, we studied the theory of entanglement in its characterization and
measurement, introducing a new scheme for detection of entanglement. The new
approach links molecular-spin entities involving nuclear spins to quantum
computing as more appropriate physical systems of interest. Then, we continued
with the realization of entanglement in experiments. NMR has been the first
choice due to its well approved advantages for quantum computing. NMR, however,
has not been an appropriate system for demonstrating entangled states. Through
a mathematical proof, NMR with low spin polarization has been invalidated for
true implementations of non-local quantum algorithms, particularly supserdense
coding. The point is that high spin polarization is inevitably required to
acquire entanglement while in the current NMR it has been a formidable task to
get highly polarized nuclear spins. In order to acquire high spin polarization,
introducing electron spins can be much effective because of its
three-order-of-magnitude larger gyromagnetic ratio compared to nuclear spins.
Electron Nuclear DOuble Resonance (ENDOR) is spin manipulation technology that
enables us to deal with both electron and nuclear spins. Thus, in this context,
it can be more appropriate device for quantum computing. We emphasize that
(pseudo)entanglement and interconversion between the entangled states have been
realized with ENDOR on extremely stable organic molecular-spin entities. The
required experimental conditions to obtain true quantum entanglement are also
discussed. The appropriate entanglement witness for the corresponding ensemble
quantum computing is introduced and examined.
