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Ultrafast Laser Physics and Precision Metrology For Fundamental Tests

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Jobs: Two PhD Positions Available

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Application deadline 31 January 2020.
Evaluations of the candidates are done continuously (so early application is beneficial).
As long as this message is visible, the positions are still open.

Based on an NWO/FOM MYSTP Program and Projectruimte funding, we have 2 PhD positions available.
Each application is evaluated individually.

Both projects are based on testing fundamental physics using advanced precision spectroscopy.

The first aim of project 1 (PhD positions funded by our NWO Program MYSTP):
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 Puzzle [1-4] with a new determination of the charge radius of the alpha and helion particle.

New vacuum setup for 1S-2S He+ spectroscopy
Panoramic view of the first version of new vacuum setup for 1S-2S spectroscopy in He+

He+ setup
                with Laura
Laura working on the Time-of-flight detector attached to the He+ setup; soon we replace this with a top flange holding the ion-trap
.

So 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 [4]). However, the proton appeared to be 4% smaller (the radius is approximately 0.84 fm). For a long time there was no real solution to this "proton size puzzle", despite a lot of theoretical and experimental effort. This also has great repercussions for the Rydberg constant.
In 2017 a result for the proton radius was published [5] based on 1S-4P spectroscopy of normal hydrogen (Munich) that agreed with the muonic results obtained before. One might think this solved the puzzle, but then in early 2018 the proton radius from 1S-3S spectroscopy [6] of hydrogen (Paris) agreed with the previous electronic hydrogen results (therefore not with the muonic results). Most recently electron scattering now confirmed the smaller radius.

One way to find new clues for possible new physics is to test QED & nuclear size influences 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 measurements.

We 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 a challange, but we great progress recently, and it 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 (Quantum-logic spectroscopy), 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 the high-harmonic generation also works now. We are currently converting the vacuum system for the ion-trap and its special mounting.
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 second aim of project 1 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. [10]). 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.

Ramsey-comb
              setup for H2
Above: The Ramsey-comb laser setup for H2


The X-EF 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.

Project 2 (PhD position, funded by the NWO Projectruimte): precision spectroscopy in metastable helium (He*)
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 to reach either a Bose condensate (4He) or a degenerate Fermi gas (3He). The challenge is to measure the absorption frequencies with an accuracy of potentially 10 Hz (!), which corresponds to a 13 digits accuracy at the transition wavelength of 1557 nm.

In the experiment (see [11-13] for more information) an atomic beam of helium atoms, in the long-lived 1s2s triplet state, is decelerated in a Zeeman slower and the slow atoms are laser-cooled and trapped in a Magneto Optical Trap.  Subsequently the atoms, transferred to a cloverleaf magnetic trap, are further cooled towards Bose-Einstein condensation (4He) or Fermi degeneracy (3He) and loaded into a 320-nm crossed dipole trap, where the spectroscopy takes place. Ultimately we want to use a standing wave ('lattice') 320 nm magic wavelength trap to reach 10 Hz accuracy. The experiments are performed at ultrahigh vacuum pressure (<10^-10 mbar). The narrow linewidth spectroscopy laser is locked to an ultrastable laser (linewidth <2 Hz) using a femtosecond frequency comb laser and cesium atomic clock, allowing the required accuracy for wavelength measurements.

He metastable
              setup
Above: The quantum-gas meta-stable helium spectroscopy setup; on the right you see a Zeeman slower.


Applicants for the 2 PhD positions (either He+/H2 or He*)
:
You should have a Master in Physics, be enthusiastic for these (great!) projects, and
should have significant affinity with optics, lasers, and complex experimental setups. You will be working in a team of several PhD students and 1-2 postdocs.
The salary for PhD students will be in accordance with university regulations for academic personnel, and ranges from (as an indication) � 2.325 gross per month in the first year (salary scale 85.0) to approximately 2.972 gross per month in the fourth year (salary scale 85.3) based on fulltime employment.
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 teaching duties.

General conditions of employment

You can find information about the side benefits for PhD project 1 at
https://www.nwo-i.nl/personeelsinformatie/arbeidsvoorwaarden/algemene-informatie

and for PhD project 2 at www.workingatvu.nl.

Both projects are based in the LaserLaB Vrije Universiteit Amsterdam:
http://www.vu.nl/en/index.asp
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.

Application deadline 31 January 2020.
Evaluations of the candidates are done continuously (so early application is beneficial).
As long as this message is visible, the positions are still open.

 
Send your application (please indicate for which project: no. 1: He+/H2, or no. 2: He*) to prof. dr. Kjeld Eikema, email: k.s.e.eikema@vu.nl.
Please include a letter of motivation, CV, and 2 names with email addresses of people as a reference.

[1] C.G. Parthey et al., PRL 107, 203001 (2011)
[2] R. Pohl et al, Nature, vol. 466, pp. 213-216 (2010)
[3] A. Antognini, et. al., Science 339, 417-420 (2013)
[4] R. Pohl et al., Science 353, 669-673 (2016)
[5] A. Beyer et al., Science 358, 79-85 (2017)
[6] H. Fleurbaey et al., ArXiv 1801.08816v1 (January 2018)
[7] J. Morgenweg, I. Barmes, K.S.E. Eikema, Nature Physics 10, 30-33 (2014)
[8] J. Morgenweg, K.S.E. Eikema, Phys. Rev. A 89, 052510 (2014)
[9] R.K. Altmann, S. Galtier, L.S. Dreissen, and K.S.E. Eikema, Phys. Rev. Lett. 117, 173201 (2016)
[10] J. Liu et al., J. Chem Phys. 130, 174306 (2009)
[11] R. van Rooij et al., Science 333, 198 (2011);
[12] R.P.M.J.W. Notermans et al., Phys. Rev. Lett. 117, 213001 (2016);
[13] R.J. Rengelink et al., Nature Physics 14, 1132 (2018)