Fig. 3: Experimental data from correlation spectroscopy measurement with an ensemble of NV centres.

(a) Signal of the correlation spectroscopy measurement vs. the time difference between the two DD sequences (black dots) and fits of the exponential (blue line) and power-law models (orange line) with their goodness of fit and the corresponding envelopes shown as dashed lines. The power-law model shows a better fit to the data, especially at long times. In both fits we fixed the initial amplitude of the fitted signal to an estimate from an independent power spectrum measurement. (b) Fast Fourier transform (FFT) of the experimental data together with the FFT from the fitted data to the exponential and the power-law decay models, shown here for illustrative purposes. Similarly to the single NV centre experiments, the power-law model provides a better fit of the sharp-peaked spectral line, thus allowing for better frequency estimation.