Peter Fendel

Two-photon frequency comb spectroscopy of the 6s-8s transition in cesium

We report a new absolute frequency measurement of the Cs 6s-8s two-photon transition measured using frequency comb spectroscopy. The fractional frequency uncertainty is 5x10(-11), a factor of 6 better than previous results. The comb is derived from a stabilized picosecond laser and referenced to an octave-spanning femtosecond frequency comb. The relative merits of picosecond-based frequency combs are discussed, and it is shown that the AC Stark shift of the transition is determined by the average rather than the much larger peak intensity.

Precision spectroscopy of hydrogen and femtosecond laser frequency combs

Precision spectroscopy of the simple hydrogen atom has inspired dramatic advances in optical frequency metrology: femtosecond laser optical frequency comb synthesizers have revolutionized the precise measurement of optical frequencies, and they provide a reliable clock mechanism for optical atomic clocks. Precision spectroscopy of the hydrogen 1S–2S two-photon resonance has reached an accuracy of 1.4 parts in 1014, and considerable future improvements are envisioned. Such laboratory experiments are setting new limits for possible slow variations of the fine structure constant α and the magnetic moment of the caesium nucleus μCs in units of the Bohr magneton μB.

A deep-UV optical frequency comb at 205 nm.

By frequency quadrupling a picosecond pulse train from a Ti:sapphire laser at 820 nm we generate a frequency comb at 205 nm with nearly bandwidth-limited pulses. The nonlinear frequency conversion is accomplished by two successive frequency doubling stages that take place in resonant cavities that are matched to the pulse repetition rate of 82 MHz. This allows for an overall efficiency of 4.5 % and produces an output power of up to 70 mW for a few minutes and 25 mW with continuous operation for hours. Such a deep UV frequency comb may be employed for direct frequency comb spectroscopy in cases where it is less efficient to convert to these short wavelengths with continuous wave lasers.

2S hyperfine structure of atomic deuterium

We have measured the frequency splitting between the (2S,F=1/2) and (2S,F=3/2) hyperfine sublevels in atomic deuterium by an optical differential method based on two-photon Doppler-free spectroscopy on a cold atomic beam. The result f{sub HFS}{sup (D)}(2S)=40 924 454(7) Hz is the most precise value for this interval to date. In comparison to previous radio-frequency measurements we have improved the accuracy by a factor of 3. The specific combination D{sub 21}=8f{sub HFS}{sup (D)}(2S)-f{sub HFS}{sup (D)}(1S) of metastable and ground state hyperfine frequency intervals in deuterium derived from our measurement agrees well with the value for D{sub 21} calculated from quantum electrodynamics.

Photoionization Broadening of the 1S-2S Transition in a Beam of Atomic Hydrogen

We consider the excitation dynamics of the two-photon 1S-2S transition in a beam of atomic hydrogen by 243 nm laser radiation. Specifically, we study the impact of ionization damping on the transition line shape, caused by the possibility of ionization of the 2S level by the same laser field. Using a Monte Carlo simulation, we calculate the line shape of the 1S-2S transition for the experimental geometry used in the two latest absolute frequency measurements [M. Niering et al., Phys. Rev. Lett. 84, 5496 (2000) and M. Fischer et al., Phys. Rev. Lett. 92, 230802 (2004)]. The calculated line shift and linewidth are in excellent agreement with the experimentally observed values. From this comparison we can verify the values of the dynamic Stark shift coefficient for the 1S-2S transition for the first time on a level of 15%. We show that the ionization modifies the velocity distribution of the metastable atoms, the line shape of the 1S-2S transition, and has an influence on the derivation of its absolute frequency.

2s Hyperfine Splitting in Light Hydrogen-like Atoms: Theory and Experiment

Since the combination D21 = 8fHFS(2s)-fHFS(1s) of hyperfine intervals in hydrogen and light two-body hydrogen-like atomic systems weakly depends on the nuclear structure, comparison between theory and experiment can be sensitive to high order QED corrections. New theoretical and experimental results are presented. Calculations have been performed for the hydrogen and deuterium atoms and for the helium-3 ion. Experiments on the 2s hyperfine splitting (responsible for the dominant contribution to the error in D21) have been conducted for hydrogen and deuterium. The theory and experiment are in good agreement, and their accuracy is comparable to that attained in verifying the QED theory of the hyperfine splitting in leptonic atoms (muonium and positronium).

Towards laser spectroscopy of antihydrogen

Cold antihydrogen atoms in a magnetic trap will open up a fascinating field of very precise CPT tests by ultrahigh-resolution laser spectroscopy. Equally exciting is the prospect for experiments on the gravitational acceleration of antimatter. For both types of experiment it is of great importance to have antihydrogen as cold as possible. Laser cooling of antihydrogen can be done on the strong 1S–2P transition at Lyman-α (121.56 nm). The highest cooling efficiency, lowest temperature, and best magnetic sublevel selectivity is expected for continuous coherent radiation. We present an account of the first source for continuous coherent radiation at Lyman-α and discuss possible applications in experiments with antihydrogen.

Study of hyperfine structure in simple atoms and precision tests of the bound state QED

We consider the most accurate tests of bound state QED, precision theory of simple atoms, related to the hyperfine splitting in light hydrogen-like atoms. We discuss the HFS interval of the 1 s state in muonium and positronium and of the 2 s state in hydrogen, deuterium and helium-3 ion. We summarize their QED theory and pay attention to involved effects of strong interactions. We also consider recent optical measurements of the 2 s HFS interval in hydrogen and deuterium.