Two‐photon spectroscopy of the hydrogen and deuterium 1S–2S transition in a cold atomic beam has reached a resolution of 7.3 parts in 1012, yielding order of magnitude advances in several precision measurements. We have determined the absolute frequency of the 1S–2S transition of hydrogen within 1.8 parts in 1011 by comparison with a reference frequency synthesized from an 88 THz CH4‐stabilized He–Ne laser standard. From the result, ν1S–2S=2 466 061 413.182(45) MHz, we deduce a new value for the Rydberg constant, R=109 737.315 684 1(42) cm1. We have furthermore measured the 1S Lamb shift in hydrogen within 1 part in 105 by directly comparing the 1S–2S transition frequency with the 2S–4S/4D transition frequency. The experimental value of 8 172.84(9) MHz agrees with the QED calculation of 8 172.94(6) MHz. Finally, we have found an isotope shift of δν1S–2S=670 994.337(22) MHz for the 1S–2S transition between hydrogen and deuterium. This experiment employs a novel electro‐optic modulator at 84 GHz.

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