Master and Bachelor projects in the Quantum Metrology and Laser Applications group

For up-to-date information please contact the group. The following is a list of possible masters and bachelors projects bound into the ongoing research projects (see research pages), but according to the personal interest of a candidate, we can always discuss further possibilities. Students can participate in all these projects at the Masters level (with a full year of involvement) or at the Bachelors level (6 weeks project). In any case if you are interested in conducting a masters or a bachelors research project please contact one of the scientific staff members in our group: W. Ubachs, K. Eikema, H.L. Bethlem, J. Koelemeij, or one of the Appointed special professors: I. Aben and L. Kaper.

Precision spectroscopy on H2; probing physics beyond the standard model

The rovibrational level energies of hydrogen, the smallest neutral molecule, can be calculated very accurately including all phenomena in quantum electrodynamics. The advanced calculations of molecular structure, only possible in H2 can be confronted to experiment, therewith testing QED and the possibility that forces beyond the Standard Model contribute. For the measurements dye lasers, titanium-sapphire-lasers (pulsed and continuous), and frequency-doubling techniques are used, as well as advanced calibration methods.       
Contact: Prof. Wim Ubachs (email

Masters projects at the Advanced Research Center for nanolithography (ARCNL)

Students are welcome to be involved in Masters projects at ARCNL ( We are setting up an experiment to investigate the underlying physical processes occurring in a practical Extreme Ultraviolet (EUV) laser-produced plasma source. Projects are possible in several groups. 

In the “EUV plasma dynamics” group, various studies of spectroscopic nature, and detection and characterization of highly charged ions produced in the plasma will be carried out.
Contact: Prof. Wim Ubachs (email ).

In the “EUV Generation and imaging” group advanced laser sources and methods are developed to ignite and control plasmas for EUV generation, and also methods are investigated for lensless nano-imaging.
Contact: Prof. Kjeld Eikema ( or dr. Stefan Witte (

Probing varying constants from astronomical observations

Spectra of molecules, measured in the laboratory and observed in far-distant galaxies at high redshift, can be compared to derive possible variations of the proton-electron mass ratio, a fundamental constant of nature. Astronomical spectra of H2 are obtained from the UVES spectrometer at the very large telescope; analysis of some spectra can be performed. Astronomical spectra of the methanol molecule is obtained from radio telescopes (ALMA, IRAM, EVLA, Effelsberg); analysis of spectra can be performed. There are opportunities to be involved in radio astronomical observations at Effelsberg (Germany).      
Contact: Prof. Wim Ubachs (email

Synchrotron spectroscopy; photoabsorption in CO and N2 molecules

At the DESIRS beamline of the SOLEIL synchrotron in Paris a vacuum-ultraviolet Fourier-transform spectrometer is installed to record high-resolution spectra of small molecules that exhibit narrow resonances at these short wavelengths. Our team is involved in using this unique instrument to characterize the absorption spectra of carbon monoxide and nitrogen molecules, and their isotopes. A great number of spectra still need analysis, while a visit to SOLEIL is a possibility.      
Contact: Prof. Wim Ubachs (email

Cavity Ring-down spectroscopy of carbon chain molecules

Unsaturated reactive carbon-based molecules are known to exist in the vast gas clouds of the interstellar medium, mainly through radio astronomy. Optical spectroscopy of such species is performed to identify and characterize these species via optical astronomy. The carbon based radicals are produced under low temperature consitions in a pulsed plasma discharge expansion. Detection is sensitively done via the sensitive method of cavity ring-down spectroscopy, with the use of pulsed dye lasers.      
Contact: Prof. Wim Ubachs (email

Light scattering in the atmosphere; Rayleigh-Brillouin scattering in gases

Light is scattered in gases because of the discrete nature of the substance (molecules). The resulting Rayleigh line shape is perturbed by acoustic excitations in the gas leading to Brillouin side peaks. This is a method of investigating the thermodynamic properties of gases, but the line profile is also of importance for retrieval of the wind velocity by the new ESA satellite instrument Aeolus. The studies are of fundamental nature as well as applied in the context of wind lidars. A great deal of optical instrumentation is used and a new Fabry-Perot analyzer has to be installed and tested.      
Contact: Prof. Wim Ubachs (email

Frequency-chirp measurements of nanosecond laser pulses

For ultimately accurate frequency calibration in spectroscopy using nanosecond laser pulses the time evolution of the time-dependent frequency of the laser output must be measured. We have developed a method that can measure this chirp at a GHz speed, using a number of advanced optical and electronic techniques. This setup must be calibrated, tested, and then used to perform very accurate metrology studies on atoms and molecules.      
Contact: Prof. Wim Ubachs (email

The proton radius puzzle: precision spectroscopy on helium+ ions

Until 2010 Quantum Electrodynamics (QED) was considered the best tested theory of the Standard Model, and precicion spectroscopy in atomic hydrogen was used to actually determine the size of the proton (its size shifts the 1S-2S transition in hydrogen slightly). However, in 2010 precision spectroscopy in muonic hydrogen (were the electron is replaced with the 200 times heavier muon) was performed, leading to a different proton size. The most recent experiment on muonic hydrogen already suggests a difference corresponding to 7 sigma or more. Is QED wrong, or is there an error in the Rydberg constant, or is something unknown going on? This is really significant, but up to now nobody knows where this difference comes from, despite much theoretical and experimental effort.

We want to shed light on this question by measuring the 1S-2S transition in helium+ ions very accurately (see the website, so that it can be compared to measurements recently done wit muonic helium+ at PSI in Switzerland. There are several opportunities for Master projects, as we need to trap helium+ in a Paul trap and apply several cutting edge laser methods to extract the transition frequency at an extreme ultraviolet wavelength with sub-kHz precision.
Contact: Prof. Kjeld Eikema (email )

Lensless imaging from visible to extreme ultraviolet wavelengths

We are developing methods to do imaging without using lenses for many applications. The idea is to only use diffraction patterns from objects to reconstruct what they actually look like. In this way no lenses are needed, which gives many possibilities, especialy at short wavelengths (e.g. extreme ultraviolet) where no lenses exist. We can even do this with broad bandwidth ultrafast laser pulses, which gives a wealth of new possibilities (see and,

For the projects we use several different types of lasers ranging from diodelasers for video-rate imaging at near-infrared imaging, to an ultrafast Tera-watt class laser for making light with a wavelength of only a few nanometer. Several possibilities exist for Master projects.
Contact: Prof. Kjeld Eikema (email ) or Dr. Stefan Witte (email

Ultra-stable frequency comb lasers for precision measurements

We are developing new laser methods to perform extremely accurate spectroscopy on atomic (e.g. Kr, He, He+) and molecular (H2) systems to search for physics beyond the Standard Model. An important ingradient is the frequency comb laser (see, which provides both short pulses and hundreds of thousands exactly known colors. We want to improve these frequency combs by combining it with an ultra-stable laser (sub-Hz, the only one in the Netherlands!). This should lead to an ultra-stable frequency comb. The project involves using advanced laser systems, and setting up fiber optics and amplification, nonlinear optics, and electronics.
Contact: Prof. Kjeld Eikema (email )

Cloud retrievals from GOME-2 satellite observations

Clouds affect the light path through the Earth atmosphere and thus the absorption features in the spectra measured by the GOME-2 satellite instrument. For a proper interpretation of these measurements, it is essential to infer cloud information from GOME-2 observations. The aim of the master project is to employ cloud parameters from GOME-2 measurements in the UV spectral range and around the O2 A absorption band at 760 nm. Subsequently, the cloud parameters will be used to improve an existing retrieval method of the total amount of ozone from GOME-2 measurements.
Contact: Prof.dr. Ilse Aben (

Advanced surface emissivity model to improve ozone profile retrieval from satellite observations

For trace gas retrieval from thermal infrared satellite observations, it is essential to describe the amount of light emitted by the Earth's surface with sufficient accuracy as it serves as an input to the measurement simulation of many trace gas retrieval schemes. The aim of this master project is to develop a surface emissivity model, which is based on a representative set of laboratory spectral emissivity measurements. We will infer a continuous emissivity spectrum together with surface skin temperature from IASI satellite observations. Subsequently, these spectra will be used to investigate potential improvements of ozone profile retrieval from IASI observations.
Contact: Prof.dr. Ilse Aben (

Temperature profile retrieval to facilitate methane retrieval from GOSAT satellite measurements

The Japanese Greenhouse Gases Observing Satellite (GOSAT) has the capability to observe atmospheric methane in the thermal infrared spectral ranges. In this spectral range, a proper interpretation of the satellite data depends critically on accurate knowledge of the atmospheric temperature profile. Our current GOSAT methane retrieval adopts this information from a weather prediction model. To improve on this, we propose in this master project to retrieve the atmospheric temperature profiles from GOSAT measurements at the strong CO2 absorption band at 15 μm..
Contact: Prof.dr. Ilse Aben (

Satellite observed vegetation fluorescence and regional-scale variations in the biosphere-atmosphere exchange of carbon

Climate change and increasing carbon dioxide concentrations influence the functioning of natural ecosystems. New remote sensing techniques are under development for detecting and quantifying regional scale changes. One of these techniques is to measure the fluorescence that is emitted from photosynthesizing leaves. The aim of this research project is to explore to use of satellite observed vegetation fluorescence to detect anomalies in the biosphere productivity, for example, due to droughts. The approach is to collect multi-year time series of fluorescence, soil moisture, vegetation greenness, etc. and investigate common signals in these data sets using data mining and machine learning techniques.
Contact: Prof.dr. Ilse Aben (

A molecular optical clock

Today’s most accurate atomic clocks are based on a single ion stored in a trap, which acts as a reference oscillator driven by the field of an ultrastable laser. In our group we are currently constructing a molecular optical clock, which is based on the vibrations of the molecular hydrogen ion. The vibrational frequencies of this simple molecule have been calculated with extremely high precision. A precise measurement of the molecular clock frequency may therefore give us anwers to many intriguing questions, such as: are there dark matter particles or higher dimensions hiding inside the molecule? What is the most precise value of the proton-electron mass ratio, and could it be varying over time? Are quantum-electrodynamics and general relativity valid theoretical frameworks? This Master project offers opportunities to work on the various subsystems of the molecular ion clock, with strong emphasis on lasers, (fiber-)optics, electronics, experiment automation, and clocks.
Contact: Dr. Jeroen Koelemeij (email