Positions for PhD and Postdocs available
Based on an ERC Advanced Grant ("QED-PROTONSIZE") and in addition NWO/FOM Program funding,
we have 2 PhD and one Post-doc positions available.
There is no deadline for applications: you can always apply until this
notice is removed. Each application is evaluated individually.
There are two projects, both based on testing fundamental physics using precision metrology.
The aim of the first project A1 (Postdoc and PhD positions funded by the ERC Advanced GRant): is to measure the 1S-2S
transition frequency of the helium+ ion for the first time using
advanced laser techniques, including frequency comb lasers, ion
trapping, and (sympathetic) laser cooling. This will enable us to
explore an exciting new system for tests of Quantum-Electrodynamics
(QED), and contribute to the solution of the so-called Proton-Size
Panoramic view of the new vacuum setup for 1S-2S spectroscopy in He+ (December 2017).
Laura working on the Time-of-flight detector attached to the He+ setup (1 March 2018).
what is this about? Quantum electrodynamics is arguably the best-tested
theory in physics. Based on e.g. the anomalous magnetic moment and
precision spectroscopy on e.g. the 1S-2S transition in atomic hydrogen,
it was thought that QED is basically correct. However, to compare the
spectroscopy with QED calculations, one also has to take into account
the effect of the finite proton size. Assuming that QED (and the
independently measured Rydberg constant) is correct, the spectroscopy
and theory can be used to figure out how large the proton size is. This
gave a number of about 0.88 fm, which agrees well with
electron-scattering experiments. However, in 2010 the 2S-2P transition
was measured by the CREMA collaboration in muonic hydrogen, where the
electron is replaced with a muon (which is just like the
electron, but 200 times heavier). The effects of QED and proton size
are much bigger in muonic hydrogen, and from the spectroscopy a 10
times more accurate proton size could be derived [2,3] (and now also the deuteron size from muonic deuterium ). However, the
proton appeared to be 4% smaller (the radius is approximately 0.84 fm). So far there is no
real solution to this "proton size puzzle", despite a lot of theoretical and
experimental effort. One possibility could be that the Rydberg constant
now in use is wrong (based on measured energy ratio's in atomic
hydrogen, such as 1S-2S vs. 2S-4S). If this happens to be the case, and
it needs adjustment, then the fine structure constant derived from
hydrogen spectroscopy will be off.
In 2017 a result for the proton radius was published  based on 1S-4P
spectroscopy of normal hydrogen (Munich) that agreed with the muonic
results obtained before. One might think this solved the pulzzle, but
then in early 2018 the proton radius from 1S-3S spectroscopy  of
hydrogen (Paris) agreed with the previous electronic hydrogen results
(therefore not with the muonic results). So the plot thickens...
One way to find
new clues to solve this puzzle would be to test QED in different
systems, such as Helium+ ions. The CREMA collaboration has measured the
2S-2P transition in muonic helium+ ions (both 3He and 4He), and
evaluation of the results is in progress. What we like to do is to
measure the 1S-2S transition in normal helium+ for the first time so
that it can be compared to the muonic helium+ measurements. Potentially
this could lead to a better test of QED than the current atomic
hydrogen spectroscopy, or it can be used to see if the size of the
alpha particle (the nucleus of Helium+) is consistent with muonic
will use our newly developed Ramsey-comb
method [7-9] that combines high-energy (mJ-level) ultrafast laser
pulses with kHz or better frequency precision. One of the things to
develop is extreme ultraviolet Ramsey-comb spectroscopy by combining it
with high-harmonic generation. The project is quite involved, and
includes frequency comb lasers, ultrafast lasers and amplifiers,
nonlinear optics such as high-harmonic generation, electronics,
ion-trapping in a linear Paul trap, ultra-high vacuum, fiber-laser
technology, an ultra-stable laser (sub-Hz linewidth), laser cooling and
sympathetic ion cooling, non-destructive ion-state readout, and much
more. We have already a dedicated ULN (ultra low noise) Frequency Comb
and and Ultrastable Laser from Menlo Systems available, a working
Ramsey-comb laser (and we build a second one for improved performance),
a source of 313 nm for laser cooling, and we are currently building up
a vacuum system for the high-harmnic generation and ion-trap.
In the project we work together with prof. Piet Schmidt and dr. Tanja
Mehlstaeubler of the PTB Braunschweig, Germany, for ion-trapping
technology and methods for ion trapping and spectroscopy.
The aim of the second project A2 (PhD position, funded by the NWO/FOM Program) is
to perform precision metrology on
various transitions between the X-EF states in H2 to test molecular QED
and also the proton size. So part of the description above is also
valid for this project. Molecular hydrogen is the simplest neutral
molecule, and rapidly becoming an interesting test ground for
fundamental physics too (see e.g. ). The idea is to determine the
dissociation energy with such a precision (we aim for about 10 kHz)
that we can use it for testing QED and the proton size.
Above: The Ramsey-comb laser setup for H2
transition (using two-photon 202 nm Ramsey-comb spectroscopy in a
molecular beam) is one step in this determination. By measuring
different vibrational and rotational states we can also put new limits
on e.g. hypothecial 5th forces. This project is a colaboration with
prof. W. Ubachs and dr. E. Slumbides of our own group in Amsterdam, and
with the group of prof. F. Merkt at the ETH Zurich, and theoretical
support comes from the group of prof. K. Pachucki of the University of
Warsaw. We have a working Ramsey-comb laser setup and vacuum setup for
this project, with many opportunities to improve it!
Given that many techniques are the same for both projects, there is also the possibility to be involved in both of them.
Applicants for the PhD positions (either He+ or H2) should have a Master in Physics, be
ambitious and highly motivated, and have significant affinity with optics, lasers, and
complex experimental setups. You will be working in a team of 3 PhD students and 1-2 postdocs.
The salary for PhD students will be in accordance with university
regulations for academic personnel, and ranges from € 2.174 gross per
month in the first year (salary scale 85.0) to € 2.779 gross per month
in the fourth year (salary scale 85.3) based on fulltime employment (these are 2016 values, current salaries are a bit higher).
The appointment will initially be for a period of 12 months with an
extension of another 36 months that is conditional upon assessment of adequate
functioning. You can be asked to spend a maximum of 10% of you time on
we have 1 position for ambitious Post-docs for the He+ project. Experience in ion
trapping (ideally for precision spectroscopy) is particularly valued, but is experience with ultrafast lasers /
optical parametric amplification
and laser cooling is also very welcome.
The salary for Post-doc employees will be in accordance with university
regulations for academic personnel, and depending on experience is
offered from € 2920 gross per month and onwards (for fulltime
employment). The values are those for 2016, so now the salary is a bit higher.
General conditions of employment
You can find information about our side benefits of employment at www.workingatvu.nl, like
• remuneration of 8.3% end-of-year bonus and 8% holiday allowance;
• solid pension scheme (ABP);
• discounts on collective insurances (health care and car insurance);
• generous contribution (70%) commuting allowance based on public transport.
Vrije Universiteit Amsterdam is a leading, innovative and growing
university that is at the heart of society and actively contributes to
new developments in teaching and research. Our university has ten
faculties which span a wide range of disciplines, as well as several
institutes, foundations, research centers, and support services. Its
campus is located in the fastest-growing economic region in the
Netherlands (the Zuidas district of Amsterdam), and provides work for
over 4,500 staff and scientific education for more than 23,000 students.
Send your applications (please indicate for which project, He+ or H2) to prof. dr. Kjeld Eikema, email: firstname.lastname@example.org
Please include a letter of motivation, CV, and 2 names with email addresses of references.
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 R. Pohl et al, Nature, vol. 466, pp. 213-216 (2010)
 A. Antognini, et. al., Science 339, 417-420 (2013)
 R. Pohl et al., Science 353, 669-673 (2016)
 A. Beyer et al., Science 358, 79-85 (2017)
 H. Fleurbaey et al., ArXiv 1801.08816v1 (January 2018)
 J. Morgenweg, I. Barmes, K.S.E. Eikema, Nature Physics 10, 30-33 (2014)
 J. Morgenweg, K.S.E. Eikema, Phys. Rev. A 89, 052510 (2014)
 R.K. Altmann, S. Galtier, L.S. Dreissen, and K.S.E. Eikema, Phys. Rev. Lett. 117, 173201 (2016)
 J. Liu et al., J. Chem Phys. 130, 174306 (2009)