Fig. 4: WFMI of an AC gradient magnetic field.

a Pulse sequences of WFMI, which consists of three parts: quantum sensing, coherence decay, and Fourier imaging. An AC current with a period of 5 μs is sent to the gradient microcoils in the y direction, which generates a magnetic field with a uniform gradient in the imaging area. The Hahn echo sequence with free precession time τ₁ = 5.6 μs is applied in the sensing period. b–d Widefield real-space imaging of the alternating gradient magnetic field when Fourier imaging is not introduced in a. The different magnetic gradients are listed in these figures. e–g The WFMI of the alternating gradient magnetic field. h–j The profiles in y direction taken by averaging all the pixels in each corresponding row of x direction. The blue lines are connections of the data points, and the red curves are fitting by smoothing for h, multiple Gaussian functions for (i), and fast Fourier transform with zero padding for j.