Diamond based quantum sensing is a fast-emerging field with both scientific and technological importance. The nitrogen-vacancy (NV) center, a crystal defect in diamond, is a unique model system for microwave sensing application due to its excellent photo-stability, long spin coherence time in ambient conditions. In this work, we systematically optimized the measurement parameters for microwave sensing. The system noise is analyzed, and 1/f noise is suppressed by introducing a differential algorithm. The gain of avalanche photodiode and the gating window of the pulsed fluorescence is optimized to further suppress the noise floor. The decoherence of spin is characterized by varying the duration of the laser and microwave. The minimal detectable power on a standard microstrip is characterized with sampling time down to 1 ms, showing flat frequency dependence. The results have important implications toward fast measurement of broadband microwave power, especially in the field of IC testing and radar signal processing under intense electromagnetic interference.

With the increasing integration and complexity of chips, the problem of wafer‐level electromagnetic compatibility (EMC) is becoming more and more prominent, and the spatial resolution and operating frequency of existing EMC test techniques can no longer meet the demand for wafer‐level EMC testing. In this work, a surface H‐field imaging system based on a micron‐sized diamond crystal containing nitrogen‐vacancy centers is proposed, which is micrometer resolved, quantum calibrated, frequency tunable, and nonintrusive to the H‐field of the device to be tested. The surface H‐field of a limiter chip at different input power is scanned for imaging, revealing how the energy is dissipated across the chip when exceeding limiting power. The result is essential for understanding the workings of the limiter chip and for optimizing the chip design.

Diamond based quantum sensing for on‐chip electromagnetic field imaging is an emerging field. While smaller diamond probes offer higher spatial resolution, their lower fluorescence intensity poses a challenge for the traditional Avalanche Photo Diode. In this work, we propose a tapered fiber diamond probe containing Nitrogen‐vacancy center and utilize a single photon counter module as the detector. A time domain differential pulse sequence is introduced to suppress noise caused by laser intensity fluctuation. By analyzing the relationship between the signal‐to‐noise ratio (SNR) and parameters of differential pulse sequence, we derived the formula of SNR and the basic cycle (M $M$), and obtained the optimal SNR in the same measurement time through trade‐off between two different kinds of noise. The technique was applied to a waveguide microstrip filter chip, accurately imaging the current density distribution. Our results have potential applications in on‐chip testing and failure analysis.