  Nitrogen-vacancy (NV) centers are defects in diamonds, which, due to their
electronic structure, have been extensively studied as magnetic field sensors.
Such field detection applications usually employ the NV centers to detect field
components aligned with the direction of the internally-defined spin axis of
the NV center. In this work we detect magnetic fields which are slightly
misaligned with the NV center axis. In particular, we demonstrate that the NV
center can measure the square of the angle between the magnetic field and the
NV center axis with high sensitivity which diverges as the external field
approaches a value pre-defined by NV center's internal parameters, in agreement
with predictions. These results show that NV centers could be used as sensitive
transducers for making quantum nondemolition (QND) measurements on systems such
as nanomechanical oscillators.
