  Nitrogen-vacancy (NV) centers in diamond work as a quantum electrometer.
Using an ensemble state of NV centers, we propose vector electrometry and
demonstrate measurements in a diamond electronic device. A transverse electric
field applied to the N-V axis under a high voltage was measured while applying
a transverse magnetic field. The response of the energy level shift against the
electric field was significantly enhanced compared with that against an axial
magnetic field. Repeating the measurement of the transverse electric field for
multiple N-V axes, we obtained the components of the electric field generated
in the device.
