Three transitions satisfy the uncertainty constraint described above, including hydrogen (1s -2s), indium II (6s -6p), and cesium I (5s -5p) (Fig. ). These three values have relative uncertainties of 0.012, 0.17, and 6.7 ppt, respectively. Apparent diffusivity is then calculated based on a linear regression of (𝐸 * -𝐸) vs. (12𝑚 e 𝜈 em ), where 𝐸 * is the energy level of the excited state, 𝐸 is the energy level of the ground state, and 𝑚 e is the rest mass of the electron. The resulting experimental value for 𝜄 0 is 6.061 579 2527(16) × 10 -5 m 2 /s with a relative uncertainty of 260 parts per trillion.