This last of a series of three measurements improves the comparison of antiproton ͑p͒ and proton ͑ p͒ by almost a factor of 10 6 over earlier exotic atom measurements, and is the most precise CPT test with baryons by a similar large factor. Measuring the cyclotron frequencies of a simultaneously trapped p and H 2 ion establishes that the ratio of q͞m forp and p is 20.999 999 999 91 6 0.000 000 000 09, more than 10 times the accuracy over our previous measurement. This 9 3 10 211 comparison makes the first use of simultaneously trapped particles for sub-ppb spectroscopy. [S0031-9007(99)08869-9]

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