High resolution spectroscopy of the 1S-2S transition in atomic hydrogen

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 10^12, 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 10^11 by comparison with a reference frequency synthesized from an 88 THz CH4-stabilized He-Ne laser standard. From the result, f_1S-2S = 2446061413.182(45) MHz, we deduce a new value for the Rydberg constant, R_inf = 109737.3156841(42) cm^-1. We have furthermore measured the 1S Lamb shift in hydrogen within 1 part in 10^5 by directly comparing the 1S-2S transition frequency with the 2S-4S/4D transition frequency. The experimental value of 8172.84(9) MHz agrees with the QED calculation of 8172.94(6) MHz. Finally, we have found an isotope shift of delta_f_1S-2S = 670994.337(22) MHz for the 1S-2S transition between hydrogen and deuterium. The experiment employs a novel electro-optic modulator at 84 GHz.