Comprising an antiproton and a positron, antihydrogen is the simplest atom of antimatter. The transition frequency of 2.466×1015 Hz from the 1S ground state to the 2S metastable state, if measured with the accuracy set by the natural line width of 1.1 Hz, offers the chance to test CPT invariance and the equivalence principle.
PACS numbers: 25.43.+t
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