Hochauflösende Spektroskopie und Stabilitätsanalyse eines Magnesium-Frequenzstandards

authored by
Klaus Hendrik Zipfel
supervised by
Ernst Maria Rasel
Abstract

Optical frequency standards, reaching instabilities and uncertainties in the low 10^-18 regime, already surpassed their microwave counterparts by many orders of magnitude. Besides the precise measurement of time, such accurate clocks might answer fundamental physical questions or can even be used in relativistic geodesy. In the case of optical lattice clocks, a laser-cooled atomic ensemble of neutral atoms is probed while being trapped in an optical lattice. In order to not disturb the atomic transition frequency, the lattice has to be operated at the magic wavelength. Here, the polarizability of both clock states is identical in first order, such that the differential energy shift induced by the AC-Stark-effect vanishes. In the scope of this thesis, an optical lattice clock based on bosonic magnesium-24 has been realized for the first time. Compared to state-of-the-art lattice based frequency standards with strontium or ytterbium, magnesium offers an almost one order of magnitude lower sensitivity to black body radiation. This systematic shift, induced by the temperature of the environment, has the biggest contribution to the error budget for strontium and ytterbium at room temperature and limited their uncertainty for a long time. One key aspect of this thesis describes the investigation and reduction of broadening mechanisms during spectroscopy. At the beginning of this thesis, the resolved linewidth was broadened to several kHz, caused by tunneling in the lattice. An improved lattice laser system allowed reaching lattice depths of up to 42 ER. In this regime, tunneling induced broadening contributes only via 24 Hz, which made other broadening effects to become dominant such as the spectroscopy fields or the state preparation within the lattice. Therefore investigations of the clock laser, the involved magnetic field and the lattice state occupation has been performed and subsequently homogenized. With these improvements a linewidth of only 51(3) Hz has been observed, which gives rise to the best line quality factor demonstrated for magnesium of Q = 1.3×10^13. The second part of this thesis covers the lock of the clock laser to the atomic transition and the investigation of the overall stability. By utilizing a self-comparison, an instability of 5.1 +2.9/-1.1×10^-16 has been demonstrated. The integration behavior of 1.1×10-14(t/s)^(-1/2) was completely limited by detection noise, which was significantly bigger than the Dicklimit of 1×10-15(t/s)^(-1/2). The real instability of the magnesium frequency standard cannot be deduced via a self-comparison due to the high common mode suppression. Therefore an instability analysis against independent optical frequency standards at the Physikalisch-Technische Bundesanstalt in Brunswick has been performed. For this a 73 km long fiber link has been used which allows the Leibniz Universität Hannover the exceptional access to a growing European frequency network. By comparing against a strontium lattice clock, an instability limit of 2×10^-15 for the magnesium frequency standard has been observed. Intensity fluctuations of the clock laser and thus varying AC-Stark shifts have been identified to cause this limitation. By measuring these time dependent shifts in situ, the total instability in a comparison against an ytterbium ion clock could be reduced to 7.7 +5.0/-1.3×10^-16 via a post correction. This represents the lowest instability of a magnesium frequency standard in an independent frequency comparison so far.

Organisation(s)
QUEST-Leibniz Research School
Faculty of Mathematics and Physics
Type
Doctoral thesis
No. of pages
136
Publication date
2019
Publication status
Published
Electronic version(s)
https://doi.org/10.15488/4548 (Access: Open)
 

Details in the research portal "Research@Leibniz University"