Study of hyperfine structure in simple atoms and precision tests of the   bound state QED
  We consider the most accurate tests of bound state QED, precision theory of simple atoms, related to the hyperfine splitting in light hydrogen-like atoms. We discuss the HFS interval of the 1s state in muonium and positronium and of the 2s state in hydrogen, deuterium and helium-3 ion. We summarize their QED theory and pay attention to involved effects of strong interactions. We also consider recent optical measurements of the 2s HFS interval in hydrogen and deuterium. 
S. G. Karshenboim, S. I. Eidelman, P. Fendel, V. G. Ivanov, N. N.   Kolachevsky, V. A. Shelyuto, T. W. Haensch