Placing a sensor close to the target at the nano-level is a central challenge in quantum sensing. We demonstrate magnetic field imaging with a boron vacancy (VB−) defects array in hexagonal boron nitride with a few 10 nm thickness. VB− sensor spots with a size of (100 nm)2 are arranged periodically with nanoscale accuracy using a helium ion microscope and attached tightly to a gold wire. The sensor array allows us to visualize the magnetic field induced by the current in the straight micro wire with a high spatial resolution. Each sensor exhibits a practical sensitivity of 73.6 μT/Hz0.5, suitable for quantum materials research. Our technique of arranging VB− quantum sensors periodically and tightly on measurement targets will maximize their potential.

A negatively charged boron vacancy (VB-) in hexagonal boron nitride (hBN) has recently started to be used as a magnetic sensor.
However, VB- has magnetic resonance spectra broadened by hyperfine interactions, which decreases the signal contrast and thus degrades the magnetic field sensitivity. 
Here we propose multi-frequency composite pulse sequences to enhance the contrast of the measurement signal in the Rabi oscillation. 
We demonstrate that the sequence using five microwave frequencies enhances the signal contrast, improving the sensitivity by a factor of 2.3 compared to the conventional sequence.