High-precision Ramsey-comb spectroscopy in the deep-ultraviolet for testing QED R.K. Altmann, L.S. Dreissen, S. Galtier and K.S.E. Eikema VU University Amsterdam Good morning Last month already two extensive talks on the subject Therefore I will not repeat the story again but will talk (relatively short) about some specific details that we have been working on in the recent past. PSAS conference 2016-05-25

Motivation/background Results from (muonic) hydrogen spectroscopy cannot be explained by current theory. He+ could provide new insights but… Why is it interesting to do this kind of spectroscopy? Well as we all know high precision spectroscopy can be used as a tool to test QED theory and since this theory plays an important role in modern physics we want to rigorously test this theory to make sure that it is correct. Thus far it has preformed remarkably well but under even the most stringent tests

Current goals High resolution Ramsey-comb spectroscopy in the deep-ultraviolet Spectroscopy of krypton Possible tests of QED at the molecular level QED tests in H2 Current status of the EF state: σ = 3.5 MHz J. Komasa, J. Chem. Theory Comp. 7, 3105 (2011) W. Ubachs, J. Mol. Spect. 320, 1 (2016) K. Pachucki, J. Chem. Phys. 144,164306 (2016)

How to do high-precision spectroscopy in the UV? Start with a frequency comb Frequency comb ̴ 4 nJ @ 800 nm We want ̴ 2 mJ!! for frequency conversion

Frequency comb amplification Frequency comb typically at 100 MHz Amplification to 1 mJ → 100 kW laser Full repetition rate amplification to μJ realized How to combine mJ pulses with high-resolution spectroscopy?

Ramsey(-comb) method Using two pulses resembles a Ramsey excitation Depends on time delay and phase

Ramsey(-comb) method

Ramsey-comb spectroscopy

Ramsey-comb spectroscopy

Ramsey-comb spectroscopy

Ramsey-comb spectroscopy

Ramsey-comb spectroscopy

What are the consequences? Combining Ramsey-fringes: Rejection of common phase effects (including AC-Stark effect) Less sensitive to phase errors Increased resolution Analysis directly in time domain (phase only) Enables to resolve multiple transitions

Ramsey-comb laser system

Spectral interferometry results Amplification can induce phase shifts and therefore frequency shifts! To be sure that this does not influence the frequency measurement we measure the differential phase shift Phase stable amplification over 300 ns within 5 mrad! Measurements at longer time delays influenced by switching electronics

Frequency conversion to the UV Deep-UV spectroscopy on Kr (212 nm) Deep-UV spectroscopy on H2 (202 nm)

Excitation setup Counter propagating configuration BS

Results in krypton at 212 nm Spectrally split pulses to reduce background excitation

Results in krypton at 212 nm Ramsey-interference signals Account for all systematic effects Contribution Theory [kHz] Experiment [kHz] σ [kHz] Statistical 2820833101688 58 AC-stark shift -13 72 Optical phase shift 1 35 Gain depletion 25 Zeeman shift 3 13 Total 2820833101679 103

H2: Signal and challenges Excitation from one side Slowing down by mixing

Preliminary result: Ramsey fringes in H2 at 202 nm One measurement → 15 kHz statistical uncertainty

Prospect for precision measurement Accuracy increases with time delay Lifetime Doppler broadening Laser linewidth Beam size Wavefront Single-side background Systematic effects (Residual) Doppler shift Residual AC-stark shift Zeeman shift … One measurement → 15 kHz statistical uncertainty Goal: 25 kHz absolute accuracy

Prospects for QED tests Theory not at experimental accuracy yet Ionization energy Dissociation energy Possible future work Improved results for the Ryd.(n=54)←EF value EF←X(υ=1) transition (~210 nm) GK ← X transition

Summary and outlook Demonstrated Ramsey-comb spectroscopy in the deep UV with 3.7*10-11 relative uncertainty Efficient production of 202 nm and first Ramsey-signals H2 Future goal: <1*10-11 relative uncertainty Future progress: Determination of systematic effects for H2 Extension to isotopes and different transitions Extension to shorter wavelengths for He+