Quantum sensor

A sensor apparatus for determining and/or monitoring a process variable of a medium capable of filling a containment to a plurality of different fill levels, the sensor apparatus comprising:

• a crystal body including at least one defect;
• a magnetic field system configured to generate a magnetic field in a region of the crystal body, and in a region of the medium capable of filling the containment to the plurality of different fill levels, within the containment, wherein the crystal body and the magnetic field system are respectively configured as to be arranged at a wall of the containment from the outside;
• a detection unit configured to detect a magnetic field-dependent fluorescence signal emitted from the crystal body in a manner dependent upon the magnetic field, wherein the detection unit comprises an excitation unit configured to optical excite the at least one defect of the crystal and comprises a detector configured to detect the fluorescence signal;
• and
• an evaluation unit configured to determine at least one piece of information about the process variable of the medium capable of filling the containment to the plurality of different fill levels, based on the fluorescence signal;
• and
• wherein the process variable includes a fill level of the medium, and the magnetic field in the region of the medium is dependent on a change to the fill level.