Fig. 2: Experimental demonstrations on fixed NDs.

a Sketch of the experimental setup. The NDs are dispersed and fixed on a coverslip, which is placed in a confocal microscope. Yellow wire indicates the coplaner waveguide to radiate microwave. b Rabi oscillation. The top is time-domain data, while the bottom is a FFT spectrum. The highest peak indicates Ω = 65 MHz. The input microwave power is 0.6 W. c Zero-field EPR spectra of P1 centers with different relative driving index κ. The three vertical dash lines marked 1, 2, and 3 indicate the theoretical peak positions of ¹⁴N P1 centers. Inset indicates a two-peak Lorentz fit on a representative spectrum to extract the peak contrasts and positions. d Dependence of the peak position on κ. The points are fitted results, where error bars are fitting errors. The horizontal lines indicate the mean values of 129.9 MHz (peak 2) and 148.9 MHz (peak 3). e Dependence of the signal contrast on κ. The points are fitted results, where error bars are fitting errors. The lines are fits according to Eq. (6).