Our group is conducting high resolution laser spectroscopy on simple atomic systems such as atomic hydrogen or hydrogen like helium ions. The primary goal is to compare experimental results with calculations in the framework of quantum electrodynamics (QED). The latter includes constants that cannot be determined theoretically but have to be fixed by measurements. To test whether QED is consistent with observations more measurements than constants are required. Within the CODATA least squares adjustment, two constants, the Rydberg constant and the proton charge radius are obtained from 14 high precision measurements in atomic hydrogen. The values obtained in this way are in disagreement with the measured Lamb shift in muonic hydrogen. We are working on shedding light on this so called proton radius puzzle by improving previous values of experimental transition frequencies in atomic hydrogen. In addition we are working towards laser spectroscopy of hydrogen like helium ions and helium like lithium ions. The latter is coming up for high precision tests of QED as calculations of two electron systems are constantly improving.
The only metrologically relevant transition in atomic hydrogen that comes with a very narrow natural linewidth occurs between the 1S ground and the metastable 2S state. Its transition frequency has been measured in our lab with an uncertainty of 5 parts in 1015. Any of the other metrologically relevant transitions have natural line widths of the order of MHz. At least one of these broad transitions has to be used in order to fix the values of the Rydberg constant and the proton charge radius. Another one is required to test QED. Therefore the only path to improve QED tests on hydrogen is to precisely understand and model the large line width transitions.
For this reason we have been re-measuring the 2S-4P transition frequency using the previous 1S-2S apparatus that is now employed as a source of cold, laser excited 2S atoms. The result is as accurate as all previous hydrogen data combined and confirms the muonic value of the proton charge radius. Hence it is in significant contradiction with the previous hydrogen data. The cause of this discrepancy is as of yet unknown. One may speculate about yet unknown systematic errors of the measurements, errors in QED calculations or even new physics.
Another route followed by our group is to determine the 1S-3S transition frequency with improved accuracy. For this experiment we use a frequency comb laser that has the advantage of being insensitive to the notorious frequency chirp of the traditional pulsed lasers. In fact, using the comb structure results in an observational line width given by a single mode of the comb, while the transition rate is determined by the power of all modes combined. An apparatus including a mode locked laser, reference cavity, pulsed frequency quadrupling to 205 nm and a vacuum system including a cryostat that generates an atomic hydrogen beam has been designed and is generating the data.
To extend comb spectroscopy to even shorter wavelengths we started a project to perform spectroscopy on a single trapped helium ion. This system is as simple as hydrogen but eliminates its main experimental problem, the thermal motion of the atoms through the laser beam. In addition, the scaling with large powers of the nuclear charge makes the helium ion 10 times more sensitive to high order QED contributions. Its nucleus, the alpha particle, is much better understood than the proton. In addition the experience with muonic hydrogen urges that systems different from regular atomic hydrogen need to investigated. The helium ion is the perfect candidate because its energy levels can be computed as accurately as atomic hydrogen and muonic hydrogen. For this project we received an ERC Advanced Grant in 2017.