1. 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

2. 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

3. 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, (2016)

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

5. 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?

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

7. Ramsey(-comb) method

8. Ramsey-comb spectroscopy

9. Ramsey-comb spectroscopy

10. Ramsey-comb spectroscopy

11. Ramsey-comb spectroscopy

12. Ramsey-comb spectroscopy

13. 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

14. Ramsey-comb laser system

15. 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

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

17. Excitation setup
Counter propagating configuration BS

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

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

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

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

22. 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

23. 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

24. 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+