The apparent imbalance of matter and antimatter in the Universe is one of the great remaining mysteries in physics. One possible explanation, not yet proven, is the existence of a small asymmetry between the behaviour of atoms and their anti-atoms. Hydrogen is probably the most studied atom in the periodic table. However, the optical spectrum of antihydrogen, consisting of an antiproton and a positron, has not been measured and the pressing question is will it be identical to that of hydrogen? Now, a group of researchers from 12 different countries has answered the question by optically driving the 1S–2S transition in magnetically trapped antihydrogen using the ALPHA-2 set-up at CERN. A cylindrical volume (44 mm diameter, 280 mm length) of anti-atoms was illuminated by 150 mW of ultraviolet light at a wavelength of 243 nm. The apparatus includes a Fabry–Pérot cavity to enhance the interaction time between antihydrogen atoms and ultraviolet photons. Counterpropagating photons excite the 1S–2S transition at a frequency that is approximately independent of the Doppler effect; the cavity transmission is monitored by a photodiode. The frequency of the transition observed for antihydrogen is the same, within the limits of experimental uncertainty, as that of hydrogen. These results are in agreement with the CPT (charge conjugation, parity reversal and time reversal) theorem, which requires the symmetry of the two spectra.
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Pile, D. Probing antimatter. Nature Photon 11, 141 (2017). https://doi.org/10.1038/nphoton.2017.25