appear in the even-proton number (Z) isotopes beyond the proton drip-line, in which one-proton (1p) emission is energetically prohibited but the ejection of two protons is energetically allowed due to the pairing interaction. More than 40 years after its prediction, ground-state 2p radioactivity was discovered in 2002 [2,3]. Two experiments independently observed that the ground state (g.s.) of 45 Fe decays by simultaneous emission of two protons. Later 54 Zn [4], 19 Mg [5], 48 Ni [6], and 67 Kr [7] were found to be other g.s. 2p radioactive nuclei. Among the g.s. 2p emitters hitherto observed, the halflives of 45 Fe, 48 Ni, and 54 Zn are in the range of several ms,