Prof Thomas Udem Max-Planck Institute of Quantum Optics
Abstract: Precision Hydrogen Spectroscopy and towards XUV Spectroscopy of He+
The sharpest transition in atomic hydrogen occurs between the metastable 2S state and the 1S ground state with a natural line width of only 1.3 Hz. Its transition frequency has been measured with almost 15 digits [1]. Combining the 1S-2S transition frequency with the precisely measured 2S-6P transition frequency [2] and the measured Lamb shift in muonic hydrogen [3], we have recently been able to verify the predictions of QED at the sub-part-per-trillion level. However, further progress is prevented by the lack of laser cooling of hydrogen/antihydrogen that is not already trapped.
We are currently developing methods to load atomic hydrogen into an optical dipole trap without using laser cooling [4]. This approach could improve spectroscopic data and eventually result in an optical lattice clock with a calculable clock transition. This could lead to a redefinition of the second in terms of a fundamental constant. Using defined values of constants (such as the speed of light) rather than physical objects has several advantages. The SI second is the last SI unit that is still based on an object (the Cs atom).
In a second approach, we are developing a narrow-band XUV laser system operating at 60.8 nm, which would enable excitation of the 1S–2S transition in hydrogen-like helium ions [5]. He⁺ can be easily loaded into an ion trap and cooled sympathetically. This essentially eliminates all the leading systematic shifts experienced in the atomic beam experiments. He⁺ is more sensitive to higher-order QED terms, as these scale with large powers of the nuclear charge. Furthermore, the nuclear charge radius is much better understood, and the Lamb shift in muonic He⁺ has already been measured. Finally, a miniaturised He⁺ spectrometer could potentially serve as an optical clock of the type mentioned above.