Precision spectroscopy for fundamental tests

Determination of the ground state energy of helium with an XUV frequency comb We have demonstrated high-precision spectroscopy from 85-51 nm using a Ramsey-XUV frequency comb. This "Ramsey comb" is based on harmonic upconversion of two amplified frequency comb pulses. With this system we determined the energy position of the ground state of helium  with an accuracy of 6 MHz (an 8-fold improvement over the previous determination with conventional single nanosecond pulse excitation). One issue with the previous system (used for the helium measurement) was the need to calibrate the phase shift induced between the two pulses due to the parametric amplifier and in the harmonic generation process. Also the time delay between the pulses was limited to 10 ns. We have now developed a new system where a phase measurement is no longer necessary. Moreover, the pulses can be selected under computer control over more than a microsecond. We performed measurements with this system on two-photon transitions in Rb, Kr and H2 (Spring 2017, paper in preparation). With this we have shown that kHz-level accuracy is possible (see here) and the prospects are very good to go down to wavelengths as short as 30 nm with the Ramsey-comb technique. Currently the measurements of the helium ground state is 6 times more accurate (6 MHz) than current theory (36 MHz). Once the theory becomes better, we pick this up again, and will measure the 1s^2 - 1s2s 1S0 transition  at 2x120 nm. We expect to be able to improve it by 100x to reach 50 kHz. OLD Papers: D.Z. Kandula, C. Gohle, T.J. Pinkert, W. Ubachs, K.S.E. Eikema,  "XUV frequency-comb metrology on he ground state of helium" Phys. Rev. A 84, 062512 (2011) T.J. Pinkert, D.Z. Kandula, C. Gohle, I. Barmes, J. Morgenweg, K.S.E. Eikema, "Widely tunable XUV frequency comb generation" Opt. Lett. 36, 2026-2028 (2011) D.Z. Kandula, C. Gohle, T.J. Pinkert, W. Ubachs, K.S.E. Eikema, "Extreme Ultraviolet Frequency Comb Metrology", Phys. Rev. Lett. 105, 063001 (2010) see also Physics Synopsys, and Nature Research Highlights

Direct frequency comb spectroscopy on Ca ions to test for a possible variation of the fine-structure constant alpha. With the incredible accuracy of frequency comb lasers, one can now test whether the fundamental constants such as the fine-structure constant is actually constant. We do this by comparing transitions in ions with observations done with telescopes. With a telescope you can effectively look back into the past over many billions of years. If you take care of the red-shift (due to the expanding universe), one can check whether alpha has changed on a cosmological timescale. Our part of the comparison is in providing accurate laboratory values for the principle transitions that are interesting for such a comparison. To this end we have performed direct frequency comb spectroscopy in Ca+ ions in the UV by frequency doubling a full-reprate frequency comb. The calcium ions are trapped in a RF Paul-trap. Laser cooling on the 397 nm transition reduced the temperature to 13 mK for precision spectroscopy. Also a 'forbidden' clock transition could be excited directly with the frequency comb at 729 nm. Recently (Spring 2017) we took the Ca+ trap apart to recycle it partly for the new He+ 1S-2S spectroscopy experiment currently under construction. OLD Papers: A.L. Wolf et al., "Direct frequency-comb spectroscopy of a dipole-forbidden clock transition in trapped 40Ca+ ions" Opt. Lett., 36, 49-51 (2011) A.L. Wolf, S.A. van den Berg, W. Ubachs, K.S.E. Eikema, "Direct Frequency Comb Spectroscopy of Trapped Ions" Phys. Rev. Lett. 102, 223901 (2009)

Projects in collaboration with other groups: He*, H2, Ca, CO In a collaboration with the group of dr. Wim Vassen (PI) we determined the transition frequency of the highly forbidden 2 3S1 - 2 1S0 transition in meta-stable helium at the 1 kHz level. From this measurement a nuclear charge radius difference could be derived. We intend to improve this measurement by an order of magnitude in the near future. Other projects involve molecular hydrogen and simple atomic systems via a collaboration with prof. W. Ubachs and dr. Edcel Salumbides, and the CO molecule in collaboration with dr. H. Bethlem. Papers: W. Ubachs, J.C.J. Koelemeij, K.S.E. Eikema, E.J. Salumbides, "Physics beyond the Standard Model from hydrogen spectroscopy", J. Mol. Spectr. 320, 1-12 (2016) J. Biesheuvel, J. Ph. Karr, L. Hilico, K.S.E. eikema, W. Ubachs, J.C.J. Koelemeij, "Probing QED and the fundamental constants through laser spectroscopy of vibrational transitions in HD", Nature Communications 7, 10385 (2016) R. van Rooij, J.S. Borbely, J. Simonet, M.D. Hogerland, K.S.E. Eikema, R.A. Rozendaal, W. Vassen,  "Frequency metrology in quantum degenerate helium: direct measurement of the 2 3S1 - 2 1S0 transition", Science 333, 196-198 (2011) A.J. de Nijs, E.J. Salumbides, K.S.E. Eikema, W. Ubachs, H.L. Bethlem, "UV-frequency metrology on CO a 3Π: isotope effects and sensitivity to a variation of the proton-to-electron mass ratio", Phys. Rev. A 84, 052509 (2011) Salumbides, E.J., Maslinskas, V., Dildar, I.M., Wolf, A.L., Duijn, E.J. van, Eikema, K.S.E. & Ubachs, W. "High precision frequency measurement of the 423 nm Ca I line", Phys. Rev. A 83, 012502 (2011). J.J. Liu, E.J. Salumbides, U. Hollenstein, J.C.J. Koelemaij, K.S.E. Eikema, W. Ubachs, F. Merkt, "Determination of the ionization and dissociation energies of the hydrogen molecule", J. Chem. Phys. 130, 174306 (2009) E.J. Salumbides, D. Bailly, A. Khramov, A.L. Wolf, K.S.E. Eikema, M. Vervloet, W. Ubachs Improved Laboratory Values of the H-2 Lyman and Werner Lines for Constraining Time Variation of the Proton-to-Electron Mass Ratio Phys. Rev. Lett. 101, 223001 (2008)

We gratefully acknowledge financial support from:

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