  Doping impurity atoms is a strategy commonly used to tune the functionality
of materials including catalysts, semiconductors, and quantum emitters. The
location of dopants and their interaction with surrounding atoms could
significantly modulate the transport, optical, or magnetic properties of
materials. However, directly imaging individual impurity atoms inside materials
remains a generally unaddressed need. Here, we demonstrate how single atoms can
be detected and located in three dimensions via multislice electron
ptychography.Interstitial atoms in a complex garnet oxide heterostructure are
resolved with a depth resolution better than 2.7 nm, together with a
deep-sub-{\AA}ngstrom lateral resolution. Single-scan atomic-layer depth
resolution should be possible using strongly divergent electron probe
illumination. Our results provide a new approach to detecting individual atomic
defects and open doors to characterize the local environments and spatial
distributions that underlie a broad range of systems such as single-atom
catalysts, nitrogen-vacancy centers, and other atomic-scale quantum sensors.
