Demonstration of highly-sensitive wideband microwave sensing using ensemble nitrogen-vacancy centers

Microwave magnetometry is essential for the advancement of microwave technologies. We demonstrate a broadband microwave sensing protocol using the AC Zeeman effect with ensemble nitrogen-vacancy (NV) centers in diamond. A widefield microscope can visualize the frequency characteristics of the microwave resonator and the spatial distribution of off-resonant microwave amplitude. Furthermore, by combining this method with dynamical decoupling, we achieve the microwave amplitude sensitivity of $5.2 \, \mathrm{\mu T} / \sqrt{\mathrm{Hz}}$, which is 7.7 times better than $40.2 \, \mathrm{\mu T} / \sqrt{\mathrm{Hz}}$ obtained using the protocol in previous research over a sensing volume of $2.77 \, \mathrm{\mu m} \times 2.77 \, \mathrm{\mu m} \times 30 \, \mathrm{nm}$. Our achievement is a concrete step in adapting ensemble NV centers for wideband and widefield microwave imaging.