The detection of photons emitted from single quantum objects is highly desirable for the diagnosis of nanoscale devices using microscopes. An extremely tiny probe (̃0.1 nm) with high current recently became available for aberration-corrected scanning transmission electron microscopy, and it is possible for individual atoms in nanoscale devices to be excited using such a highly focused probe. Here, we demonstrate the successful detection of characteristic X-ray signals from single erbium atoms using energy-dispersive X-ray spectroscopy. The intensities of the erbium L and M lines from a single erbium atom were extremely weak in comparison to the N edge of electron energy-loss spectroscopy, demonstrating the intrinsic difficulty in sensing single atoms using X-ray spectroscopy. Nevertheless, this work will certainly help in the advance towards obtaining X-ray spectra from single atoms and to evaluate the fluorescence yield on a single-atom basis.
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