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Improved Measurement of the Hydrogen<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>1</mml:mn><mml:mi>S</mml:mi><mml:mo>โ</mml:mo><mml:mn>2</mml:mn><mml:mi>S</mml:mi></mml:math>Transition Frequency
We have measured the 1S - 2S transition frequency in atomic hydrogen via two photon spectroscopy on a 5.8 K atomic beam. We obtain $f_{1S-2S} = 2 466 061 413 187 035 (10)$ Hz for the hyperfine centroid. This is a fractional frequency uncertainty of $4.2\times 10^{-15}$ improving the previous โฆ