@article {Ahmadi:2020aa,
	title = {Investigation of the fine structure of antihydrogen},
	journal = {Nature},
	volume = {578},
	number = {7795},
	year = {2020},
	pages = {375{\textendash}380},
	abstract = {<p>At the historic Shelter Island Conference on the Foundations of Quantum Mechanics in 1947, Willis Lamb reported an unexpected feature in the fine structure of atomic hydrogen: a separation of the 2S1/2 and 2P1/2 states1. The observation of this separation, now known as the Lamb shift, marked an important event in the evolution of modern physics, inspiring others to develop the theory of quantum electrodynamics2\&ndash;5. Quantum electrodynamics also describes antimatter, but it has only recently become possible to synthesize and trap atomic antimatter to probe its structure. Mirroring the historical development of quantum atomic physics in the twentieth century, modern measurements on anti-atoms represent a unique approach for testing quantum electrodynamics and the foundational symmetries of the standard model. Here we report measurements of the fine structure in the n = 2 states of antihydrogen, the antimatter counterpart of the hydrogen atom. Using optical excitation of the 1S\&ndash;2P Lyman-αtransitions in antihydrogen6, we determine their frequencies in a magnetic field of 1 tesla to a precision of 16 parts per billion. Assuming the standard Zeeman and hyperfine interactions, we infer the zero-field fine-structure splitting (2P1/2\&ndash;2P3/2) in antihydrogen. The resulting value is consistent with the predictions of quantum electrodynamics to a precision of 2 per cent. Using our previously measured value of the 1S\&ndash;2S transition frequency6,7, we find that the classic Lamb shift in antihydrogen (2S1/2\&ndash;2P1/2 splitting at zero field) is consistent with theory at a level of 11 per cent. Our observations represent an important step towards precision measurements of the fine structure and the Lamb shift in the antihydrogen spectrum as tests of the charge\&ndash;parity\&ndash;time symmetry8 and towards the determination of other fundamental quantities, such as the antiproton charge radius9,10, in this antimatter system.</p>},
	isbn = {1476-4687},
	doi = {10.1038/s41586-020-2006-5},
	url = {https://doi.org/10.1038/s41586-020-2006-5},
	author = {Ahmadi, M. and Alves, B. X. R. and Baker, C. J. and Bertsche, W. and Capra, A. and Carruth, C. and Cesar, C. L. and Charlton, M. and Cohen, S. and Collister, R. and Eriksson, S. and Evans, A. and Evetts, N. and Fajans, J. and Friesen, T. and Fujiwara, M. C. and Gill, D. R. and Granum, P. and Hangst, J. S. and Hardy, W. N. and Hayden, M. E. and Hunter, E. D. and Isaac, C. A. and Johnson, M. A. and Jones, J. M. and Jones, S. A. and Jonsell, S. and Khramov, A. and Knapp, P. and Kurchaninov, L. and Madsen, N. and Maxwell, D. and McKenna, J. T. K. and Menary, S. and Michan, J. M. and Momose, T. and Munich, J. J. and Olchanski, K. and Olin, A. and Pusa, P. and Rasmussen, C. {\O}. and Robicheaux, F. and Sacramento, R. L. and Sameed, M. and Sarid, E. and Silveira, D. M. and So, C. and Starko, D. M. and Stutter, G. and Tharp, T. D. and Thompson, R. I. and van der Werf, D. P. and Wurtele, J. S. and The ALPHA Collaboration}
}
