Testing Quantum Electrodynamics with Lithium Ions

• Applicant: Dr. Akira Ozawa
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 535639992

Quantum electrodynamics (QED) is considered the best tested theory in all of physics and has served as the blueprint for all other quantum field theories. Testing QED means comparing its predictions with experimental results. To do this, one looks for systems that can be both calculated and measured with the highest accuracy. Atomic hydrogen is one of the systems that allows the comparison of theory and measurement with very high precision. Spectroscopy on muonic hydrogen yielded a value for the proton charge radius that is 20 times more accurate than the CODATA world average, but more than 5 standard deviations away from it. This is known as the proton charge radius puzzle, and was initially seen as a hint of new physics beyond the Standard Model. This striking discrepancy demonstrates the importance of testing QED with systems other than just ordinary atomic hydrogen. QED calculations for three-body systems have become much more accurate in recent years. The neutral helium atom is an obvious system in this category and has been investigated thoroughly. Here we propose to extend the helium work to helium-like lithium ions to enable new types of precision QED tests. Higher order QED corrections, which scale with large powers of the nuclear charge Z, are more pronounced in Li+ than in neutral He. QED tests with neutral He have been experimentally limited by several systematic effects arising from the motion of the atoms. In this proposal we plan to perform spectroscopy on trapped Li+ ions. The motion of the trapped ions can be suppressed by applying direct laser cooling or sympathetic cooling. Systematic effects due to ion motion can be significantly reduced in a state-of-the-art ion trap. With the reduction of the measurement uncertainty, the sensitivity to higher order QED corrections and the newly available calculations, we expect that Li+ will allow an independent and precise test of QED. In summary, the goal of this project is to perform high-precision spectroscopy of transitions in Li+ ions and to gain new insights into the QED theory describing simple atomic systems.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Thomas Udem