Speaker
Description
Due to its simple structure, the Hydrogen atom is a powerful platform for precision tests of fundamental physics, more explicitly quantum electrodynamics (QED). The energy levels in atomic Hydrogen can be calculated up to a high degree of precision and can be written as:
$E_{n,l,j}=hc\mathbf{R_y}\left(-\frac{1}{n^2}+f_{n,l,j}(\alpha,\frac{m_e}{m_p},...)+\delta_{l,0}\frac{C_{NS}}{n^3}{\mathbf{r_p}}^2\right)$,
where $f_{n,l,j}$ is the QED series expansion in the fine structure constant $\alpha$, containing various corrections to the leading Bohr-level term. The last term describes the contribution due to the finite size effect, i.e. the fact that the proton in the atom core is not a point-like particle but has a charge distribution with the RMS charge radius $r_p$ to which the S-states ($l = 0$) are sensitive to, due to their finite spatial overlap of the wave function with the atom core. As other required parameters, such as $\alpha$ or the electron to proton mass ratio $m_e/m_p$ can be determined very accurately by other experiments in atom interferometers and Penning traps, $R_y$ and $r_p$ remain to be ascertained by spectroscopy [1]. Thus, two transition measurements in hydrogen are required to fix $R_y$ and $r_p$ and more to check for consistency. Contributing to that quest, the 1S-3S experiment at MPQ in Garching delivered its first result in 2020 with a fractional uncertainty of 10$^{-13}$ [2]. The measurement result differs by 2.1 standard deviations from the value obtained by colleagues at the Laboratoire Kastler Brossel in Paris [3]. Since then, several ideas for improvements of the experimental setup towards a lower uncertainty measurement have been devised and implemented. The main modification concerns the spectroscopy laser, a mode-locked titanium-sapphire laser $820 \;\text{nm}$ with a pulse duration $\tau \approx 1.2 \;\text{ps}$ and a repetition rate of $f_{rep}\approx160 \;\text{MHz}$, that has been constructed from scratch to fit the requirements of the experiment.
In this poster, an overview of the 1S-3S experiment at MPQ is given, together with an outlook on the improvements expected from the new laser system, as well as other modifications that are currently being planned.
References:
[1] E. Tiesinga, P. J. Mohr, D. B. Newell, and B. N. Taylor: CODATA recommended values of the fundamental physical constants:
2018, Journal of Physical and Chemical Reference Data, 50(3): 033105, sep 2021
[2] A. Grinin, A. Matveev, D. C. Yost, L. Maisenbacher, V. Wirthl, R. Pohl, T. W. Hänsch, and T. Udem: Two-photon frequency
comb spectroscopy of atomic hydrogen, Science, 370(6520): 1061–1066, nov 2020
[3] H. Fleurbaey, S. Galtier, S. Thomas, M. Bonnaud, L. Julien, F. Biraben, F. Nez, M. Abgrall, and J. Guéna: New measurement
of the 1S-3S transition frequency of hydrogen: Contribution to the proton charge radius puzzle, Physical Review Letters,
120(18): 183001, may 2018