The present invention relates to a
magnetometer (100) using optically detected magnetic
resonance (ODMR), where a
solid state material (10), such as
diamond, with an ensemble of paramagnetic defects, such as
nitrogen vacancies centers NV, is applied. An
optical cavity (20) is optically excited by an
irradiation laser (25) arranged therefore. A
coupling structure (30) causes a
microwave excitation (Ω) of the paramagnetic defects, and a permanent
magnetic field (40, B_C) causes a Zeeman splitting of the energy levels in the paramagnetic defects. A probing volume (PV) in the
solid state material is thereby defined by the spatially overlapping volume of the optical excitation by the
irradiation laser (25), the
coupling structure (30) also exciting the defects, and the constant
magnetic field. The
magnetometer then measures an unknown
magnetic field by detecting emission (27), e.g.
fluorescence, from the defects in the probing volume (PV) from the double excitation of the defects by the
irradiation laser, and the
coupling structure exciting these defects.