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We report comparison of the interfacial sharpness characterization of germanium (Ge) isotopic multilayers between laser-assisted atom probe tomography (APT) and secondary ion mass spectrometry (SIMS). An alternating stack of 8-nm-thick naturally available Ge layers and 8-nm-thick isotopically enriched 70Ge layers was prepared on a Ge(100) substrate by molecular beam epitaxy. The APT mass spectra consist of clearly resolved peaks of five stable Ge isotopes (70Ge, 72Ge, 73Ge, 74Ge, and 76Ge). The degree of intermixing at the interfaces between adjacent layers was determined by APT to be around 0.8 ± 0.1 nm which was much sharper than that obtained by SIMS.