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Resonant quantum transitions in trapped antihydrogen atoms


The hydrogen atom is one of the most important and influential model systems in modern physics. Attempts to understand its spectrum are inextricably linked to the early history and development of quantum mechanics. The hydrogen atom’s stature lies in its simplicity and in the accuracy with which its spectrum can be measured1 and compared to theory. Today its spectrum remains a valuable tool for determining the values of fundamental constants and for challenging the limits of modern physics, including the validity of quantum electrodynamics and—by comparison with measurements on its antimatter counterpart, antihydrogen—the validity of CPT (charge conjugation, parity and time reversal) symmetry. Here we report spectroscopy of a pure antimatter atom, demonstrating resonant quantum transitions in antihydrogen. We have manipulated the internal spin state2,3 of antihydrogen atoms so as to induce magnetic resonance transitions between hyperfine levels of the positronic ground state. We used resonant microwave radiation to flip the spin of the positron in antihydrogen atoms that were magnetically trapped4,5,6 in the ALPHA apparatus. The spin flip causes trapped anti-atoms to be ejected from the trap. We look for evidence of resonant interaction by comparing the survival rate of trapped atoms irradiated with microwaves on-resonance to that of atoms subjected to microwaves that are off-resonance. In one variant of the experiment, we detect 23 atoms that survive in 110 trapping attempts with microwaves off-resonance (0.21 per attempt), and only two atoms that survive in 103 attempts with microwaves on-resonance (0.02 per attempt). We also describe the direct detection of the annihilation of antihydrogen atoms ejected by the microwaves.

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Figure 1: Hyperfine energy levels.
Figure 2: The ALPHA device.
Figure 3: Transition lineshapes and resonance conditions.
Figure 4: Appearance mode data.


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This work was supported by CNPq, FINEP/RENAFAE (Brazil), ISF (Israel), MEXT (Japan), FNU (Denmark), VR (Sweden), NSERC, NRC/TRIUMF, AITF, FQRNT (Canada), DOE, NSF (USA), EPSRC, the Royal Society and the Leverhulme Trust (UK). We thank them for their generous support. We are grateful to the AD team (T. Eriksson, P. Belochitskii, B. Dupuy, L. Bojtar, C. Oliveira, B. Lefort and G. Tranquille) for the delivery of a high-quality antiproton beam. We thank the following individuals for help: M. Harrison, J. Escallier, A. Marone, M. Anerella, A. Ghosh, B. Parker, G. Ganetis, J. Thornhill, D.Wells, D. Seddon, F. Butin, H. Brueker, K. Dahlerup-Pedersen, J. Mourao, T. Fowler, S. Russenschuck, R. De Oliveira, N. Wauquier, J. Hansen, M. Polini, J. M. Geisser, L. Deparis, P. Frichot, J. M. Malzacker, A. Briswalter, P. Moyret, S. Mathot, G. Favre, J. P. Brachet, P. Mésenge, S. Sgobba, A. Cherif, J. Bremer, J. Casas-Cubillos, N. Vauthier, G. Perinic, O. Pirotte, A. Perin, G. Perinic, B. Vullierme, D. Delkaris, N. Veillet, K. Barth, R. Consentino, S. Guido, L. Stewart, M. Malabaila, A. Mongelluzzo, P. Chiggiato, G. Willering, E. Mahner, A. Froton, C. Lasseur, F. Hahn, E. Søndergaard, F. Mikkelsen, W. Carlisle, A. Charman, J. Keller, P. Amaudruz, D. Bishop, R. Bula, K. Langton, P. Vincent, S. Chan, D. Rowbotham, P. Bennet, B. Evans, J.-P. Martin, P. Kowalski, A. Read, T. Willis, J. Kivell, H. Thomas, W. Lai, L. Wasilenko, C. Kolbeck, H. Malik, P. Genoa, L. Posada, R. Funakoshi, M. Okeane, S. Carey and N. Evetts. We thank former collaborators M. J. Jenkins, G. B. Andresen, R. Hydomako, S. Chapman, A. Povilus, R. Hayano, L. V. Jørgensen and Y. Yamazaki. We are grateful to the CERN Summer Student Program for funding the participation of undergraduate students (C.Ø.R. and S.C.N.) in our experiment.

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Authors and Affiliations



W.B., P.D.B., J.F., M.C.F., J.S.H., N.M. and D.M.S. conceived, designed and constructed the central ALPHA apparatus and participated in all aspects of the experimental and physics programme. The microwave hardware was designed and fabricated by M.D.A., E.B., W.N.H. and M.E.H., who also participated in all aspects of the experimental programme. T.F. developed the electron cyclotron resonance diagnostic and participated actively in all aspects of the experimental programme. S.S. developed the alternative event analysis and participated actively in the experimental and analysis efforts. C.A., M.B.-R., A.C., A.G., A.J.H., J.T.K.M., E.S. and C.S. participated actively in the experimental runs, data taking, on- and offline analysis, and maintenance and modification of the apparatus. D.R.G. and A.O. contributed to all aspects of the detector systems and participated actively in the experimental shift work and analysis efforts. M.C. designed and built the positron accumulator and participated in the experimental shift work, the physics planning effort, and the strategic direction of the experiment. D.P.v.d.W. designed and built the positron accumulator, contributed to the magnetic design of the atom trap, and participated in the experimental programme. F.R., with help from P.H.D., performed the spin-flip simulations reported in this paper and supported the design and experimental programme with simulations and calculations. P.N. led the design of the ALPHA silicon detector. P.P. was responsible for implementing the silicon detector at CERN and participated in the experimental and analysis programme. A.D., C.A.I., C.Ø.R., S.C.N., A.L. and C.R.S. contributed to the experimental shift work. S.J. and J.S.W. contributed theoretical support in the form of atomic or plasma physics calculations and simulations and contributed to the experimental shift work. C.L.C., S.E., S.M. and R.I.T. participated in the experimental programme and the physics planning effort. L.K. and K.O. provided offsite support for detector electronics and database management systems, respectively, and contributed to the experimental shift work. E.B., J.S.H. and M.E.H. wrote the initial manuscript, which was edited, improved and approved by the entire collaboration.

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Correspondence to J. S. Hangst or M. E. Hayden.

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Amole, C., Ashkezari, M., Baquero-Ruiz, M. et al. Resonant quantum transitions in trapped antihydrogen atoms. Nature 483, 439–443 (2012).

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