Fig. 3: Single ion detection.

a Inhomogeneous distribution of Er ions, measured by the fluorescence intensity in the waveguide. Gray curve is a Gaussian fit with FWHM of 160 GHz. Orange and blue lines indicate the frequency that the cavity is tuned to for fluorescence spectrum measurement shown in panels b and c. b Fluorescence spectrum inside the resonance when it is tuned to 1533.274 nm. A continuous emission spectrum shows that an ensemble of ions are probed. c Fluorescence spectrum when the resonance is tuned further to the tail of inhomogeneous broadening, at 1534.064 nm. Discrete peaks suggest single ion emission. The star mark indicates the point for g⁽²⁾ measurement. d Second-order autocorrelation function g⁽²⁾ for the single ion emission. The x axis is the number of offset between excitation pulses, y axis is normalized coincidence. Calculated value of g⁽²⁾(0) = 0.38 ± 0.08 proves that most of the collected photons come from a single ion. The data is symmetric around zero since a single detector is used. The error bars show the standard deviation of the measurement results.