We report the observation of the 1S-2S transition in magnetically trapped atoms of antihydrogen in the ALPHA-2 apparatus at CERN. We determine that the frequency of the transition, driven by two photons from a frequency stabilised laser at 243 nm, is consistent with that expected for hydrogen in the same environment. This represents the first laser excitation of an internal quantum state of an atom of antimatter, and the most precise measurement performed on an anti-atom. Our result is consistent with CPT invariance at a relative precision of ~ 2x10-10.

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