Precision metrology for fundamental research
Atomic hydrogen
We are determined to measure a series of transition frequencies in atomic hydrogen with the utmost precision. Because the 2S level is metastable, the 1S-2S transition is the gold standard in high precision measurements. This is to test Quantum Electrodynamics (QED). Since QED contains adjustable parameters, one needs more measurements of independent frequencies than there are parameters. Using the fine structure constant and the electron to proton mass ratio from other experiments that are more sensitive to these parameters, we are left with the Rydberg constant Ry and the proton charge radius rp. This means that the second most accurate measurement dominates the uncertainty of the two parameters. Additional measurements then set a limit on the accuracy of the QED test.
From Hydrogen to Helium+
Spectroscopy of the 1S-2S transition in He+ offers a solution for most experimental and theoretical limitations. As a charged particle, He+ can be stored in the favorable environment of an ion trap (see figure) for spectroscopy. The uncertainty of the charge radius of the nucleus is far less problematic - in fact, interesting higher order QED corrections not accessible elsewhere could be determined for the first time! However, a detailed analysis shows that the experiment is not only extremely promising, but also rather challenging for several reasons: Driving the 1S-2S transition requires two XUV photons at 60.8nm where no continuous wave lasers and refractive optics are available and light propagates in vacuum only.
Strategy
A careful analysis of the excitation dynamics reveals that excitation is accompanied by a significant ionization probability. We therefore operate the ion trap such that the resulting He++ ions remain stored as signature for successful excitations. We further co-store auxiliary Be+ ions with the He+ ions that serve as coolant (sympathetic cooling) and for detection (secular excitation). Based on the insight that frequency combs can excite two-photon transitions much like a continuous wave laser of the same average power, we will use a high repetition rate XUV frequency comb for excitation. The XUV comb is generated by high-harmonic generation (HHG) of a NIR frequency comb. About 1µW average power focused to 1µm spot size yields an ionization rate of 1Hz.