View larger image

The precise measurement of physical quantities based on the principles of quantum mechanics is called quantum sensing. A remarkable example is magnetic field measurements using nitrogen-vacancy (NV) centers in diamonds as quantum sensors. NV centers are a type of lattice defect in diamond crystals, in which a nitrogen atom and an atomic vacancy replace two adjacent carbon atoms. Among the many lattice defects, NV centers have unique properties that make them suitable for quantum sensing and have been applied to physical property measurements. NV centers are renowned for their high sensitivity. However, to take full advantage of their features, the accuracy (precision) and sensitivity of the measured physical quantities must be improved.

Optically detected magnetic resonance (ODMR) of NV centers measures the red photoluminescence (PL) intensity as a function of microwave frequency when green light is injected. The magnetic field can be determined from the magnetic resonance frequency of the ODMR spectrum. An unexpected phenomenon was recently reported: the ODMR spectra of NV centers in nanodiamonds (NDs) change with the optical power of the excitation light [M. Fujiwara et al., Phys. Rev. Res. 2, 043415 (2020)]. The spectral change is not drastic; hence, the phenomenon has been overlooked thus far. However, it can be problematic because it reduces the accuracy of low magnetic field measurements.

We systematically investigated the optical power dependence of ODMR spectra using NV ensembles in NDs and a bulk diamond single crystal. The experimental ODMR spectrum at zero magnetic field showed a slight splitting of the resonance frequency (Δ) owing to crystal distortion, nuclear spins near the NV centers, and so on. The Zeeman effect increased the frequency splitting in a finite magnetic field, making precise magnetic field measurements possible. We observed that Δ depended on the optical power in both NDs and bulk samples: it decayed exponentially with increasing optical power and saturated at a certain point. The decay amplitude depended on the samples, whereas the optical power at which the decay was saturated was almost independent of the samples.

The mechanism causing this unexpected phenomenon is most likely related to crystal distortion or a change in the charge state near the NV centers induced by photoexcitation. The present findings indicate that accuracy degradation can be suppressed by injecting an excitation light with an optical power above a certain value. Our achievement shows that there are unexplained phenomena even in the NV center, which has been studied for many years, and, at the same time, provides valuable guidelines for accurate magnetic field measurements using diamond quantum sensors.

(Written by Kensuke Kobayashi and Kento Sasaki on behalf of all the authors)