Optically detected magnetic resonance

In physics, optically detected magnetic resonance (ODMR) is a technique for detecting quantum objects that are both paramagnetic and optically active. In the case of photoluminescent point defects (color centers) in crystals, the “ODMR signal” usually means a decrease in the defect’s fluorescence intensity under continuous illumination due to a simultaneously applied AC magnetic field. The AC magnetic field induces Rabi oscillations of the fluorescing electrons, which as a result rapidly transition between an optically active state and an optically inactive state, decreasing the overall fluorescence signal. By varying the frequency of the AC magnetic field, (often referred to as the RF field due to the typical frequency used), the resonance frequency of a particular transition can be measured since the resonant RF field induces a marked decrease in fluorescence intensity. There may be many such transitions, and their characteristics as observed with ODMR (principally frequency and linewidth) depend sensitively on the conditions of the measurement, motivating the use of ODMR as a technique for quantum sensing.

Like electron paramagnetic resonance (EPR), ODMR makes use of the Zeeman effect in unpaired electrons. The negatively charged nitrogen vacancy centre (NV⁻) has been the target of considerable interest with regards to performing experiments using ODMR.

ODMR of NV⁻s in diamond has applications in magnetometry and sensing, biomedical imaging, quantum information and the exploration of fundamental physics.