Phase-Coherent Measurement of the Hydrogen1S−2STransition Frequency with an Optical Frequency Interval Divider Chain

Abstract

We have measured the absolute frequency of the hydrogen 1S-2S two-photon resonance with an accuracy of 3.4 parts in 10 13 by comparing it with the 28th harmonic of a methane-stabilized 3.39 mm He-Ne laser. A frequency mismatch of 2.1 THz at the 7th harmonic is bridged with a phase-locked chain of five optical frequency interval dividers. From the measured frequency f 1S-2S 2 466 061 413 187.34͑84͒ kHz and published data of other authors we derive precise new values of the Rydberg constant, R` 10 973 731.568 639͑91͒ m 21 and of the Lamb shift of the 1S ground state, L 1S 8172.876͑29͒ MHz. These are now the most accurate values available. [S0031-9007(97)04182-3] PACS numbers: 31.30.Jv, 06.20.Jr, 21.10.FtFor almost three decades, the 1S-2S two-photon transition in atomic hydrogen with its natural linewidth of only 1.3 Hz has inspired advances in high resolution laser spectroscopy and optical frequency metrology [1]. This resonance has become a de facto optical frequency standard. More importantly, it is providing a cornerstone for the determination of fundamental physical constants and for stringent tests of quantum electrodynamic theory. In the future, it may unveil conceivable slow changes of fundamental constants or even differences between matter and antimatter.Here, we report on a new precise measurement of the absolute frequency of the 1S-2S interval which exceeds the accuracy of the best previous measurement [2] by almost 2 orders of magnitude. The 1S-2S resonance is observed by longitudinal Doppler-free two-photon spectroscopy of a cold atomic beam. The resonance frequency is compared with the frequency of a cesium atomic clock with the help of a phase-coherent laser frequency chain, using a transportable CH 4 -stabilized He-Ne laser at 3.39 mm as an intermediate reference.In this way, we have determined a 1S-2S interval of f 1S-2S 2 466 061 413 187.34͑84͒ kHz with an uncertainty of 3.4 parts in 10 13 , limited by the reproducibility of the He-Ne reference laser. This represents now the most accurate measurement of any optical frequency in the ultraviolet and visible region. Together with the results of other authors, in particular, the recent precision measurements of the 2S 1͞2 -8D 5͞2 transition frequency in hydrogen by the group of Biraben [3], we derive new and more precise values for both the Rydberg constant and the 1S Lamb shift. This Lamb shift provides now the best test of quantum electrodynamics for an atom.As in our earlier experiment [2] we are taking advantage of the near coincidence between the 1S-2S interval and the 28th harmonic of the frequency of a CH 4 -stabilized 3.39 mm He-Ne laser. However, a frequency mismatch of 2.

Forty years of antiprotons

“…Even at 10 K the speed of monoatomic hydrogen is some 400 m/s, resulting in a confinement time of only ϳ0.4 ms in the 15-cm-long beam used by Andreae et al (1992) and Weitz et al (1994) in their studies of the 1S-2S transition in atomic hydrogen. Although this is short compared with the 140-ms natural lifetime of the transition, these workers have recently (Udem et al, 1997) achieved an experimental precision of 3.4 parts in 10 13 on the transition frequency, thus exceeding the precision cited above for the cold trapped atom technique.…”

“…The 1SϪ2S line center in such an experiment might ultimately be found to one part in 10 3 of its width, raising the prospect of a CPT test with antihydrogen at the 10 Ϫ18 precision. For hydrogen, the cold-beam result of Udem et al (1997) referred to in Sec. IV.C.2 is 1SϪ2S ͑ H ͒ϭ2466 061 413.18734͑84͒ MHz.…”

Absolute frequency measurements on the 2S→3S transition of lithium-6,7

“…A similar approach has been used for hydrogen [24] for which QED calculations and transition frequency measurements have already reached a much higher accuracy. Here, we report the measurement of a lithium transition frequency with a femtosecond laser frequency comb that could lead to a direct charge radius determination, once theory has reached comparable precision.…”

Cold Collision Frequency Shift of the1S-2STransition in Hydrogen

“…1S-2S spectroscopy of cold trapped hydrogen can be used for optical frequency metrology and precision measurements. Linewidths of about 1 kHz have been achieved in an atomic beam [23], but with the long coherence time possible in a trap [19] it should be feasible to approach the 1.3 Hz natural linewidth of the transition. To reduce the effects of the cold collision shift below this level, it is desirable to work at densities of ≤ 10 10 cm −3 .…”