Test of many-electron QED effects in the hyperfine splitting of heavy   high-Z ions
  A rigorous evaluation of the two-photon exchange corrections to the hyperfine structure in lithiumlike heavy ions is presented. As a result, the theoretical accuracy of the specific difference between the hyperfine splitting values of H- and Li-like Bi ions is significantly improved. This opens a possibility for the stringent test of the many-electron QED effects on a few percent level in the strongest electromagnetic field presently available in experiments. 
A. V. Volotka, D. A. Glazov, O. V. Andreev, V. M. Shabaev, I. I.   Tupitsyn, G. Plunien
physics.atom-ph quant-ph