  We report X-ray fluorescence (XRF) imaging of nanoscale inclusions of
impurities for quantum technology. A very bright diffraction-limited focus of
the X-ray beam produces very high sensitivity and resolution. We investigated
gallium (Ga) dopants in silicon (Si) produced by a focused ion beam (FIB).
These dopants might provide 3/2-spin qubits or p-type electrical contacts and
quantum dots. We find that the ion beam spot is somewhat larger than expected,
and the technique provides a useful calibration for the resolution of FIBs.
Enticingly, we demonstrate that with a single shot detection of 1 second
integration time, the sensitivity of the XRF would be sufficient to find
amongst background a single isolated inclusion of unknown location comprising
only 3000 Ga impurities (a mass of just 350 zg) without any need for
specialized nm-thickness lamellae, and down from >105 atoms in previous reports
of similar work. With increased integration we were able to detect 650
impurities. The results show that planned facility upgrades might achieve
single atom sensitivity with a generally applicable, non-destructive technique
in the near future.
