The hydrogen atom is a touchstone for the foundations, evolution, and frontiers of quantum theory. Key spectral lines of this atom have been determined to remarkable precision. Our research focuses on the study of antihydrogen, the antimatter counterpart of hydrogen. We test fundamental symmetries of nature (such as simultaneous charge conjugation, parity inversion, and time reversal or CPT symmetry) through precision comparisons of these atomic systems. Recent 1S – 2S spectroscopy measurements on trapped antihydrogen have reached relative precisions of parts-per-trillion. However, the ground-state hyperfine splitting, which is sensitive to the internal structure of the antiproton, has only been measured to 400 parts-per-million. Here, we report a 4 part-per-million (ppm) measurement of the antihydrogen ground-state hyperfine splitting energy 𝑎 , advancing the state-of-the-art by two orders of magnitude. From microwave spectroscopy experiments with roughly 24,000 anti-atoms, we determine 𝑎 /ℎ = 1,420,404.8 ± 1.1(stat. ) ± 5.6 (sys. ) kHz in a 1 T magnetic field, consistent with expectations for hydrogen. At this level our measurement is sensitive to the internal structure of the antiproton, which contributes at about 40 ppm, and is approaching the limit of existing theoretical analyses. The gains we report are the product of significant advances in magnetic trap field control, stabilization, and characterization; anti-atom spin-state manipulation; and improved antihydrogen accumulation rate.

R. Akbari, L. O. de Araujo Azevedo, C. J. Baker, W. Bertsche, N. M. Bhatt, G. Bonomi, A. Capra, I. Carli6, C. L. Cesar2, M. Charlton, A. Cridland Mathad, A. Del Vencio, D. Duque Quiceno, S. Eriksson, A. Evans, J. Fajans, T. Friesen, M. C. Fujiwara, L. M. Golino, M. B. Gomes Gonçalves, J. S. Hangst, M. E. Hayden, P. Heidari, D. Hodgkinson, C.A. Isaac, S. A. Jones, S. Jonsell, N. Madsen, V. R. Marshall, J. T. K. McKenna, T. Momose, J. Nauta, A. N. Oliveira, A. Powell, C. Ø. Rasmussen, T. Robertson-Brown, F. Robicheaux, R. L. Sacramento, E. Sarid, J. Schoonwater, D. M. Silveira, J. Singh, G. Smith, C. So, S. Stracka, J. Suh, A. G. Swadling, T. D. Tharp, K. A. Thompson, R. I. Thompson, E. Thorpe-Woods, A. J. Uribe Jimenez, M. Urioni, D. P. van de Werf, S. G. Wilson, P. Woosaree & J. S. Wurtele