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The observation of hyperfine structure in atomic hydrogen by Rabi and co-workers 1-3 and the measurement 4 of the zero-field groundstate splitting at the level of seven parts in 10 13 are important achievements of mid-twentieth-century physics. The work that led to these achievements also provided the first evidence for the anomalous magnetic moment of the electron 5-8 , inspired Schwinger's relativistic theory of quantum electrodynamics 9,10 and gave rise to the hydrogen maser 11 , which is a critical component of modern navigation, geo-positioning and very-long-baseline interferometry systems. Research at the Antiproton Decelerator at CERN by the ALPHA collaboration extends these enquiries into the antimatter sector. Recently, tools have been developed that enable studies of the hyperfine structure of antihydrogen 12 -the antimatter counterpart of hydrogen. The goal of such studies is to search for any differences that might exist between this archetypal pair of atoms, and thereby to test the fundamental principles on which quantum field theory is constructed. Magnetic trapping of antihydrogen atoms 13,14 provides a means of studying them by combining electromagnetic interaction with detection techniques that are unique to antimatter 12,15 . Here we report the results of a microwave spectroscopy experiment in which we probe the response of antihydrogen over a controlled range of frequencies. The data reveal clear and distinct signatures of two allowed transitions, from which we obtain a direct, magneticfield-independent measurement of the hyperfine splitting. From a set of trials involving 194 detected atoms, we determine a splitting of 1,420.4 ± 0.5 megahertz, consistent with expectations for atomic hydrogen at the level of four parts in 10 4 . This observation of the detailed behaviour of a quantum transition in an atom of antihydrogen exemplifies tests of fundamental symmetries such as charge-parity-time in antimatter, and the techniques developed here will enable more-precise such tests.In an earlier experiment 12 using the original ALPHA apparatus 16 , we demonstrated microwave-induced spin flips in trapped antihydrogen. The current work was carried out using the second-generation ALPHA-2 device (Fig.

We have developed a new method, based on the ballistic transfer of preaccumulated plasmas, to obtain large and dense positron plasmas in a cryogenic environment. The method involves transferring plasmas emanating from a region with a low magnetic field (0.14 T) and relatively high pressure (10 ÿ9 mbar) into a 15 K Penning-Malmberg trap immersed in a 3 T magnetic field with a base pressure better than 10 ÿ13 mbar. The achieved positron accumulation rate in the high field cryogenic trap is more than one and a half orders of magnitude higher than the previous most efficient UHV compatible scheme. Subsequent stacking resulted in a plasma containing more than 1:2 10 9 positrons, which is a factor 4 higher than previously reported. Using a rotating wall electric field, plasmas containing about 20 10 6 positrons were compressed to a density of 2:6 10 10 cm ÿ3 . This is a factor of 6 improvement over earlier measurements.

The detection of electrostatic nonneutral plasma modes in the ATHENA (ApparaTus for High precision Experiment on Neutral Antimatter) experiment [M. Amoretti, C. Amsler, G. Bonomi et al., Nature (London) 419, 456 (2002)] is described. A complete nondestructive diagnostic of the plasma based on a fit to the line shape of the function describing the power transmitted through the plasma around the frequency of the fundamental mode is developed and the experimental results are presented and discussed.