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The Zeeman splitting of a localized single spin can be used to construct a highly sensitive magnetometer offering almost atomic spatial resolution. While sub-μT sensitivity can be obtained in principle using pulsed techniques and long measurement times, a fast and easy method without laborious data postprocessing is desirable for a scanning-probe approach with high spatial resolution. In order to measure the resonance frequency in real time, we applied a field-frequency lock to the optically detected magnetic resonance signal of a single electron spin in a nanodiamond. We achieved a sampling rate of up to 100 readings per sec with a sensitivity of 6 μT/sqrtHz. Images of the field distribution around a magnetic wire were acquired with ∼30 μT resolution and 4096 submicron sized pixels in 10 min. The response of several spins was used to reconstruct the field orientation.
Schoenfeld et al. (2011) studied this question.