Accurate Hydrogen Spectroscopy
     For many years, ATRAP collaborators from the Max Planck Institute for Quantum Optics (MPQ) in Garching have been leaders in extremely accurate laser spectroscopy of atomic hydrogen.  The most accurate measurements come from measuring the frequency of light that induces transitions between the 1s ground state and the long lived 2s excited state.  The transition takes place when the atom absorbs two photons of light, each of which is ultraviolet (too blue to see with the human eye) with a wavelength of 243 nm (0.000 000 243 meters).
     The accuracy with which hydrogen transition frequencies have been measured has improved dramatically over the years, as illustrated in the figure below. 
We expect the structure of antihydrogen and hydrogen to be similar enough that we can use the very sensitive experimental tools developed to investigate hydrogen to similarly learn about antihydrogen.  This will allow us to compare the structure of antihydrogen and hydrogen at a very high accuracy. 		      The most accurate measurement of the 1s to 2s transition frequency to date is shown below.

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If we could get a similar signal from antihydrogen as well (this will be difficult since we are likely to have orders of magnitude less antihydrogen atoms) then we could compare measured antihydrogen and hydrogen frequencies to approximately 0.1 kHz out of 2.5 x 1015 Hz.  We would be sensitive to differences in the structure of antihydrogen and hydrogen which are only 0.4 parts in a trillion.

     To illustrate the incredible accuracy of this measurement, suppose that you adjust your monitor so that the graph above is 8 centimeters wide.  To extend this graph to include all of the frequencies that could have been measured, all the way down to a frequency of 0 Hz, would require a graph so wide that it could be wrapped around the earth's equator more than 200 times.
For the most up-to-date information about the progress of hydrogen spectroscopy check the hydrogen spectroscopy web site.