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Ultrafast Laser Physics and Precision Metrology For Fundamental Tests

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Research / News

Update February 2020:
The new ion trap for He+ 1S-2S spectroscopy has been constructed with the generous help of the group of Prof. Tanja Mehlstaeubler at PTB Germany, and it is now in Amsterdam.

Update October 2019: our paper about the xenon Ramsey-comb+HHG demonstration experiment described below has been published in Phys. Rev. Lett. 123, 143001 (2019)

April 2019: One of our main targets: precision spectroscopy of the 1S-2S He+ transition for fundamental tests, has just come a lot closer. With an experiment in xenon we demonstrated that the Ramsey-comb method of excitation, which we will use for He+, works very well indeed with high-harmonic generation (HHG). HHG is  required to produce the wavelength for 1S-2S excitation near ~30 nm. For the demonstration xenon was excited in an atomic beam using the 7th harmonic at 110 nm, and we obtained a fractional accuracy better than any other experiment involving the HHG process. This is a vital step towards He+ 1S-2S spectroscopy!

Older news:

Together with the collegues of the AML group, Prof. dr. Kjeld Eikema was awarded a NWO/FOM Program of 1.8 MEuro in December 2016 to investigate the Proton Radius Puzzle using precision metrology in several systems.

Moreover, he also received an European Research Council (ERC) Advanced Grant of 2.5 MEuro in March 2016 to  investigate QED and the proton radius puzzle by precision spectroscopy on the 1S-2S transition in trapped helium+ ions. You can find a short description below, and at the section about open PhD and Postdoc positions.
     ERC logonwo logo

Research Themes

The research of the Ultrafast Laser Physics and Precision Metrology Group is based on precision spectroscopy using frequency comb lasers and ultrafast laser technology, to test fundamental physics. The main projects of the group are listed below with a short explanation.

Ramsey-comb laser

Ramsey-Comb Excitation - Precision Spectroscopy for Fundamental Tests

We developed the Ramsey-comb spectroscopy method to enable precision spectroscopy of calculable systems, such a singly-ionized helium or molecular hydrogen, for fundamental tests. These systems have transitions in the deep-UV, or even extreme ultraviolet, for which no precision lasers exist. One can make these wavelengths by nonlinear upconversion in crystals and by high-harmonic generation. We obtain the required power for this by amplification of two (ultrashort) frequency comb laser pulses. Phase-coherent excitation based on these pulses then enables to measure transition frequencies of atoms and molecules at short wavelengths with unprecedented accuracy.

More information


1S-2S Spectroscopy of He+ Ions Trapped in a Paul Trap

Quantum Electrodynamics (QED) is so far the best tested and most successful theory within the Standard Model. We want to improve tests of QED (and e.g. determine the alpha particle charge radius, or the Rydberg constant) by a precision measurement of the 1S-2S transition in singly-ionized helium. By comparing the results with measurements of muonic hydrogen and 'normal' electronic hydrogen, one can search for new physics. We are currently building up an experiment for this purpose (funded by an ERC Advanced Grant) and we will use the Ramsey-Comb Spectroscopy method to excite the 1S-2S transition in helium+ ions. The scheme requires 32 nm light (extreme ultraviolet), which we generate by high-harmonic generation of amplified frequency comb laser pulses. The initial target is a spectroscopic accuracy of 13 decimal places (1 kHz), enabling a several times improved test of QED, and shed light on the 'proton-radius puzzle'. Ultimately an accuracy approaching 15 decimal places might become possible!

Recently we demonstrated for the first time the combination of high-harmonic generation and Ramsey-comb spectroscopy. This is an important milestone for the He+ project, and we achieved a record high accuracy of 0.23 parts per billion with light produced by high-harmonic generation. A paper about this will be published soon in Phys. Rev. Lett (2019).
 
More information (under construction)
precision spectroscopy


H2 setup

X-EF Spectroscopy of Molecular Hydrogen

The theory of the simplest neutral molecule, H2, is improving so rapidly that it has become very interesting to perform precision spectroscopy to test QED and molecular theory with it. The target is to measure the ionization or dissociation energy, as that can be calculated best. Our part in it is the measurement of the X-EF transition (X=ground state) using Ramsey-comb spectroscopy in the deep-UV, which we improved for ortho H2 by 100 times (Phys. Rev. Lett.  120, 043204 (2018)).
We are now working on measuring the same transition for para-H2 and to a measurement of the fundamental ground tone (V=0 to V=1 energy difference).

A page with more information is currently under construction




2 3S - 2 1S Spectroscopy of Quantum-Degenerate Helium

This project aims to measure the size of the nucleus of the 4He and 3He atom (the alpha particle resp. helion) by performing high-resolution laser spectroscopy on the 2 3S - 2 1S transition at 1557 nm. To reach the highest accuracy the atoms are trapped at ultralow temperatures in the focus of a laser beam at a 'magic wavelength' of 320 nm. Helium is then trapped either as a Bose condensate (4He) or a degenerate Fermi gas (3He). The challenge is to measure the transition frequencies with an accuracy of potentially 10 Hz (!), which corresponds to a 13 decimal places at the transition wavelength of 1557 nm.
This project was originated by Wim Vassen, and is now continued in our group.

A page with more information is currently under construction

Meta-stable helium
                                spectroscopy


X-ray
                                imaging

Soft X-ray Generation and Lens-less Imaging

This project is based on a collaboration with the group of Stefan Witte at the ARCNL. The resolution of imaging, such as microscopy, ultimately depends on the wavelength that is used. With shorter wavelengths one can resolve smaller objects. We are developing methods using 'lensless' imaging techniques combined with high-harmonic generation of soft-X-rays to explore the new possibilities in this wavelength range. One interesting property is that materials become partly transparent at soft-X-rays, enabling to look through materials.
The extreme ultraviolet / soft-X-ray light is produced via high-harmonic generation using ultra-short laser pulses (<20 fs) at a repetition rate of 300 Hz. The harmonic generating process is rather inefficient, but it leads to highly coherent X-rays. If an object is illuminated with such a beam, then clear diffraction patterns are produced (see the picture) from which the object can be reconstructed via iterative procedures. The ultimate goad is 3D imaging with element selectivity.

More information


Direct Frequency Comb Excitation and Coherent Control

In the past we have performed full-reprate direct frequency comb excitation for the first time of trapped ions (Ca+) published in Phys. Rev. Lett., and used coherent control to suppress the background in two-photon direct frequency comb spectroscopy (published in Nature Photonics).

A page with more information is under construction
.
Older
                                projects



Below you see a picture of the lab a few years ago. Since then the lab has doubled in size. In October 2016 we re-arranged the lab for building up the new vacuum system and ultra-stable Ramsey-comb laser setup for the He+ 1S-2S experiment.

Have a look at the pictures shown in the "Positions" page.

lab




We gratefully acknowledge financial support from the following organizations:

NWO NWO Laser Europe VU STW

Questions? Contact: k.s.e.eikema@vu.nl