Yuto Yamakawa | Search arXiv

arXiv2026

Geometric estimation of NV charge-state contributions from a low-dimensional spectral representation

Nitrogen-vacancy (NV) centers in diamond exist in neutral (NV0) and negatively charged (NV-) states, and quantifying their respective photoluminescence (PL) contributions is important for charge-state-based measurements. Existing methods either require additional experimental control or may suffer from limited physical identifiability. Here, we introduce a geometric method for determining NV charge-state contributions from PL spectra acquired at different excitation intensities. Each spectrum is mapped into a low-dimensional space through broadband spectral weighting, where changes in the relative NV0 and NV- contributions trace a one-dimensional trajectory. Zero-phonon-line (ZPL) information from two spectra physically calibrates this trajectory, enabling the NV-PL contribution to be determined by geometric projection. Measurements on two bulk single-crystal diamond samples yielded NV- contributions in close agreement with those obtained using an independent dual-excitation reference-spectrum method, with root-mean-square errors of 1.11 and 0.31 percentage points. Under additive spectral noise, the proposed method exhibited more than an order of magnitude less variation than a ZPL-only method. Fisher-information analysis further showed that the three-dimensional CIE XYZ representation retained approximately 81% of the information about the NV- contribution available in the full spectrum. These results establish a physically calibrated approach to low-dimensional spectral estimation that combines reduced experimental overhead with robust parameter estimation and provides a general framework for spectral sensing governed by a small number of physical degrees of freedom.

quant-ph↗

arXiv2026

Current-dipole detection of microscale metal defects

An effective electric current dipole provides a compact description of localized current disturbances and has been employed across a broad range of physical systems and length scales. Despite its extensive use in macroscopic systems, its applicability to microscale material diagnostics remains unexplored. In this study, we demonstrate that the current-dipole representation provides an effective physical framework for the detection and characterization of microscopic defects in conductive materials. When an external current is applied to a metal, defects locally perturb the current distribution, thus generating an effective in-plane current dipole that yields a characteristic magnetic-field pattern. By measuring the magnetic-field distribution above the metal and fitting it with a dipole model, we reconstruct the position and effective strength of the dipole, thereby enabling the inference of the defect location and an effective defect-volume metric. Using wide-field magnetic imaging as a measurement platform, we observe a defect-associated magnetic signal from a 38-um-long defect on the front surface of copper samples and identify a defect on the back surface using a 0.5-mm-thick copper plate. We further examine the applicability of this approach to magnetic materials, where permeability contrasts modify the field distribution. Our results establish current-dipole imaging as a general physical framework for magnetic detection of defects and highlight its potential for nondestructive inspection across a wide range of length scales.

physics.app-ph↗