Test of fundamental physics via laser precision measurements of hydrogen

The hydrogen molecule (H2) is the smallest molecular quantum entity. Since the early days of quantum mechanics (1920s) it has been used as a benchmark system to test theoretical descriptions in the quantum framework by ever more precise experiments.

07/12/2018 | 11:55 AM

Nowadays that is the theory of quantum electrodynamics (QED), encompassing quantum mechanics, relativistic effects and the spurious interactions with particles popping out of the vacuum. Ultraprecise measurements of optical transitions can test these advanced QED-model descriptions, but also form a way of probing new physics: Are there forces beyond the four forces of nature that we know (electromagnetism, gravity, the strong nuclear force, and the weak force of radioactivity), and are there more than the 3+1 space-time dimensions that we know from relativity ? To mention one of the conceptual outcomes of such laboratory studies: If the 10+1 dimensional world predicted by M-string theory is a true description of nature, than the 7 extra dimensions must by compactified to less than micrometer-size. In addition the precision studies aim at resolving the recently detected inconsistencies on the size of the proton, known as the proton radius puzzle.

The VU-LaserLaB team with senior scientists Wim Ubachs, Kjeld Eikema, Edcel Salumbides and Rick Bethlem, with junior scientists and international collaborators, have performed laser precision measurements, that have led to four papers published in the prestigious journal Physical Review Letters in the first half of 2018. All these studies rely on the advanced laser infrastructure, involving frequency comb lasers and atomic clocks, developed in the past decade at LaserLaB.

1)      PhD students Robert Altmann and Laura Dreissen have performed an experiment in which the output of a frequency-comb laser was controlled to attosecond (10-18s) precision. This laser was used to follow the time evolution of the quantum wave function of an H2 molecule in its excited state. Besides the ingenuity of this novel measurement approach the experiment led to the most precise determination of an electronically excited state in the H2molecule (accuracy 73 kHz).

2)      PhD student Frank Cozijn, in collaboration with Dr. Patrick Dupre of the University Littoral (Dunkerque, France), performed an ultraprecise measurement in the HD molecular isotope, where one of the protons is replaced by a deuterium nucleus. By coupling laser light into an optical resonator the light intensity was enhanced such that a so-called Lamb-dip could be measured – for the first time a vibrational transition could be measured in a hydrogenic molecule without Doppler effect, leading to a precision of 20 kHz.

3)      PhD student Madhu Trivikram extended the studies in hydrogen to heavier isotopes and performed the first laser precision experiment in the radio-activ tritium molecule (T2). Vibrational splittings in T2 were measured by coherent Raman spectroscopy. This work was performed in collaboration with Dr. Magnus Schloesser from the tritium laboratory of the Karlsruhe Institute of Technology.

4)      PostDoc Cunfeng Cheng and PhD student Joël Hussels performed a measurement with a specially designed laser system delivering well-controlled long pulses. With the aid of a KBBF crystal, provided by collaborators from the University of Science and Technology China (Hefei), the laser light could be upconverted to vacuum ultraviolet wavelengths to perform a precision measurement under Doppler-free conditions. Combining the measurement results with those of a team at ETH Zurich, leads to a new determination of the dissociation energy of the H2molecule at an accuracy of a part-per-billion (10-9), by far the most accurate measurement of a chemical bond, and a challenge to QED theory.

Reference to the publications:

1)      R.K. Altmann, L.S. Dreissen, E.J. Salumbides, W. Ubachs, K.S.E. Eikema
Deep-ultraviolet frequency metrology of H2 for tests of molecular quantum theory
[Link: http://www.nat.vu.nl/~wimu/Publications/Altmann-PRL-2018.pdf]
Phys. Rev. Lett. 120, 043204 (2018)

2)      F.M.J. Cozijn, P. Dupre, E.J. Salumbides, K.S.E. Eikema, W. Ubachs
Sub-Doppler frequency metrology in HD for tests of fundamental physics
[Link: http://www.nat.vu.nl/~wimu/Publications/HD-PRL-2018.pdf]
Phys. Rev. Lett. 120, 153002 (2018)

3)      M.T. Trivikram, M. Schloesser, W. Ubachs, E.J. Salumbides
Relativistic and QED effects in the fundamental vibration of T2
[Link: http://www.nat.vu.nl/~wimu/Publications/PRL-T2.pdf]
Phys. Rev. Lett. 120, 163002 (2018)

4)      C. Cheng, J. Hussels, M. Niu, H.L. Bethlem, K.S.E. Eikema, E.J. Salumbides, W. Ubachs, M. Beyer, N. Hoelsch, J. A. Agner, F. Merkt, L.G. Tao, S.M. Hu, C. Jungen
Dissociation energy of the hydrogen molecule at 10-9 accuracy
[Link: http://www.nat.vu.nl/~wimu/Publications/H2-diss-2018.pdf]
Phys. Rev. Lett. 121, 013001 (2018)

This work was funded by the FOM programs “Broken Mirrors and drifting Constants” and “The Mysterious size of the proton” and by the ERC-Advanced grants of Ubachs and Eikema.


Experimental setup of the frequency comb laser used for calibration of frequency measurements