  We report here on a major improvement of the control and characterization
capabilities of 14N nuclear spin of single NV centers in diamond, as well as on
a new method that we have devised for characterizing quantum states, i.e.
quantum state tomography using Rabi experiments. Depending on whether we use
amplitude information or phase information from Rabi experiments, we define two
sub-methods namely Rabi amplitude quantum state tomography (RAQST) and Rabi
phase quantum state tomography (RPQST). The advantage of Rabi-based tomography
methods is that they lift the requirement of unitary operations used in other
methods in general and standard methods in particular. On one hand, this does
not increase the complexity of the tomography experiments in large registers,
and on the other hand, it decreases the error induced by MW irradiation. We
used RAQST and RPQST to investigate the quality of various two-qubit pure
states in our setup. As expected, test quantum states show very high fidelity
with the theoretical counterpart.
