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The Proton Size Puzzle: Testing QED at Extreme Wavelengths

Periodic Reporting for period 2 - QED-PROTONSIZE (The Proton Size Puzzle: Testing QED at Extreme Wavelengths)

Reporting period: 2018-03-01 to 2019-08-31

With lasers it is possible to investigate at which wavelengths light is absorbed by atoms and molecules, when the energy of the photon matches an internal energy interval of the electrons. For the simplest systems, such as atomic hydrogen, it can then be compared to theoretical predictions with extreme precision to test our current understanding of physics, called the Standard Model. In particular, Quantum Electrodynamics (QED) is tested in this way, which is in fact the best tested and most successful part of the Standard Model. Precision spectroscopy also enables to determine some of the fundamental constants of Nature that are required to do accurate calculations with the theory. Any deviation found experimentally from theory could hint at new physics, which we know must be there, e.g. given the existence of 'dark matter' and 'dark energy'. Previous measurements based on e.g. atomic hydrogen, and exotic varieties of it such as muonic-hydrogen, have shown contradicting results.
The aim of this project is to enable precision measurements for the first time on singly-ionised helium on the 1S-2S transition, to contribute to solving these issues. Singly-ionized helium is like hydrogen but has much bigger QED effects. A major challenge is that its internal energy structure requires light at extreme wavelengths for which no lasers exist. We are developing a new laser spectroscopy method, called Ramsey-Comb spectroscopy, that combines the precision of low power frequency comb lasers (which get their accuracy from an atomic clock) with high-power laser pulses that can be converted to the extreme ultraviolet. It requires development of a complex laser system, a state-of-the-art ion trap (provided through a collaboration with PTB in Germany), laser cooling via a different ion to the lowest quantum-mechanical energy state, and special quantum-logic methods to detect that the transition was made. We also do measurements on other species to demonstrate the new possibilities and to do new tests of QED in the search for a better understanding of the laws of physics.
We recently demonstrated for the first time that the proposed spectroscopy method to measure the energy structure of a single helium+ ion really works very well, enabling unprecedented accuracy for spectroscopy at short wavelengths. It is based on a technique called Ramsey-comb spectroscopy (developed in our lab), that now can be combined with the process of high-harmonic generation (HHG) to produce the required extreme ultraviolet for excitation. In a demonstration experiment, we performed precision spectroscopy of xenon atoms at 110 nm, which improved the accuracy of the measured transition by 10 000 times. It presents the best measurement ever performed with HHG by a factor 3.6 (an accuracy of 0.2 parts per billion) and it has the potential to become many orders of magnitude more precise. A publication about this achievement is accepted (2019) by Physical Review Letters.

To enable this key step we have designed and built a 7-chamber vacuum setup for spectroscopy at vacuum- and extreme-ultraviolet light. In this setup two powerful infrared pulses from a Ramsey-comb laser system are focused in a gas jet of argon atoms, and a small fraction of the light is upconverted through the HHG process to much shorter wavelengths (therefore higher energy) for spectroscopy. The generated light is then refocused by special mirrors into an interaction chamber where later the helium-ion trap will be installed. For the experiment with xenon we produced a beam of xenon instead in this chamber.
We improved also our current Ramsey-comb laser system in several ways for the initial demonstration experiment with xenon, and are preparing another experiment with molecular hydrogen for QED tests with a new high-density cryogenic molecular beam setup that was constructed in collaboration with the group of F. Merkt of the ETH in Zurich.
With the same group at the ETH we collaborated in a project to determine the dissociation energy of H2 by combining measurements done with the Ramsey-comb laser method in our lab with measurements done at the ETH, as a benchmark test of theory of molecules.

A second Ramsey-comb laser system is now under construction in an advanced state, that will be ultra-low phase noise and dedicated to the measurements of singly-ionized helium.
For the laser cooling of helium+ we constructed a laser system in the UV at 313 nm, and made the design for additional laser beams to enable ground-state cooling of He+ via a single Be+ ion. We have also completed the design of the ion trap for helium+ in a collaboration with T. Mehlstäubler and P.O. Schmidt of the PTB in Braunschweig, Germany. This ion trap is going to be built at PTB and then transferred to Amsterdam for the first measurements. The ion-trap will be installed on a special mechanical construction we put together, that was designed to enable careful alignment of the ion trap from outside of the ultra-high vacuum chamber.
We demonstrated recently the most accurate spectroscopic measurement with light produced via high-harmonic generation (HHG). A fractional accuracy of 0.2 parts per billion was reached, which is 3.6 times better than demonstrated before using HHG. We showed this on a transition in xenon atoms at 110 nm (vacuum-ultraviolet) with an excited state lifetime of only 23 ns.

Applying this method to molecular hydrogen, we expect to improve the accuracy of the ground-state to first-excited state energy interval by an order of magnitude compared to the best previous measurement (also from our lab), with the potential to improve the dissociation energy and ionization energy of this molecule (for ortho and para states) in a collaborative effort with the ETH in Zurich, and thereby also test the size of the proton to a level of 1%.

The final goal of the project is to measure the 1S-2S transition in a single He+ ion with two-photon excitation involving 32 nm (extreme ultraviolet) and 790 nm (near-infrared). This would be the first coherent excitation of He+ from the ground state, and will enable us to perform one of the best tests of Quantum Electrodynamic theory (QED) to date. Based on a pulse delay of at least 10 microseconds, we expect an accuracy of about 1 kHz (or 1 part in 10 trillion), or better. For the extreme ultraviolet spectral region this would be 3 orders of magnitude better than ever demonstrated before, leading to an unprecedented accuracy in a spectral region that has hardly been explored so far. Moreover, by combining these results with other measurements and more advanced theory, we also expect to be able to extract an improved alpha-particle charge radius, and to obtain a new measurement of the fundamental Rydberg constant in a system other than hydrogen. Both provide a direct contribution to the ongoing debate over conflicting proton charge-radii (and resulting Rydberg constant) obtained from precision spectroscopy in normal atomic hydrogen and muonic hydrogen, which has been one of the main motivations for this research.
The approach we take to make the helium+ ion spectroscopy possible for the first time consist of several non-conventional ideas, which will be explained as the project progresses.
Helium+ experiment with the HHG chamber in the foreground
Installing the time-of-flight detector for the xenon experiment
View from the spectroscopy vacuum chamber of the helium+ experiment
Background: Ramsey-comb laser no. 1, foreground: H2 experiment