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The term antiprotonic helium refers to a helium atom in which one of the electrons is replaced by an antiproton. This is achieved in an experiment by passing an accelerator-produced beam of antiprotons through a helium medium. Antiprotonic helium was first discovered in 1991, although its existence had been predicted theoretically by Kondo as early as 1964. This exotic atom is interesting in that it has an unusual energy level diagram. The antiprotonic helium exhibits fine and hyperfine level splittings, due to the interaction between the magnetic moments of the antiproton and the electron. In 1998, D Bakalov (Bulgaria) and V I Korobov (JINR, Dubna) showed theoretically that each of the two energy levels of the hyperfine doublet must in turn split into two close sublevels. This results from the interaction of the antiproton and electron spin magnetic moments with the antiproton orbital magnetic moment. This splitting has now been discovered for the first time by a group of European and Japanese researchers headed by E Widmann. To obtain atoms of antiprotonic helium, a CERN accelerator was used. The presence of the two sublevels was registered by quantum transitions between them. Using a laser, excess population of the upper sublevel was achieved, and then quantum transitions to the lower sublevel occurred under the action of microwave radiation. The frequency of the microwave radiation was chosen such as to initiate allowed transitions between the sublevels involved. The resulting population of the levels was controlled by a second laser which caused transitions from the metastable levels to short-living ones, thus leading to a rapid annihilation of the antiproton and producing a burst. By varying the frequency of the laser pulses it was possible to determine the shape of the transition lines and to measure the energies of the states involved. The experiment has confirmed with a high degree of accuracy the predictions made by D Bakalov and V I Korobov. Source: http://prl.aps.org; Phys.Rev.Lett. 89 243402 (2002)