Studying the fine-structure constant by spectroscopy of dipole transitions in a Paul trap
Dr. Guido Saathoff (Max Planck Institut fuer Quantenoptik)

Frequency measurements on single trapped ions are usually performed in the strong-binding limit. In this resolved-sideband regime, the interaction of a laser with an electromagnetic transition of the ion does not affect its motional state. However there is strong interest in spectroscopy of MHz-broad dipole transitions of ions that can only be weakly bound. In this regime of unresolved sidebands, however, a spectroscopy laser exerts a detuning-dependent force on the ion and thus strongly distorts the line shape.

We have recently demonstrated a method [1], based on sympathetic cooling of an ion chain and spatially resolved fluorescence detection, by which such line shape distortions are strongly suppressed. This scheme was employed [2] to measure the absolute frequencies of both fine structure components of the 3s-3p transition in 24Mg+ and 26Mg+, which are useful for comparison with quasar absorption spectra to search for possible space-time variations of the fine-structure constant.

Recent progress in the measurement and theoretical calculation of the fine structure in the metastable triplet P state of helium promises to derive a competitive absolute value of the fine structure constant [3]. However it is at present limited by the insufficient knowledge of uncalculated higher-order QED terms. Proper estimations of the size of these terms could come from a fine structure measurement in a He-like ion [3]. I will present prospects and status of such an experiment on metastable Li+.

[1] M. Herrmann et al., Phys. Rev. Lett. 102, 013006 (2009).
[2] V. Batteiger et al., Phys. Rev A 80, 022503 (2009).
[3] C. Pachucki, V.A. Yerokhin, Phys. Rev. Lett. 104, 070403 (2010).