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The determination of the proton rms charge radius with an accuracy of 10−3 is the main goal of our experiment, opening the way to check bound‐state QED predictions in hydrogen to a level of 10−7. The principle is to measure the 2S1/2(F = 1) − 2P3/2(F = 2) energy difference in muonic hydrogen (μ−p) by infrared laser spectroscopy to a precision of 30 ppm. Very low‐energy negative muons are stopped in 0.6 mbar of hydrogen gas, where, following the μ− atomic capture and cascade, 1% of the muonic hydrogen atoms form the metastable 2S state with a lifetime of 1.3 μs. A 6 μm laser pulse is used to drive the 2S → 2P transition. When on resonance, the laser induces the transition, and the subsequent muonic deexcitation to the 1S state emits a 1.9 keV x ray which is detected by avalanche photodiodes. The resonance frequency, and hence the Lamb shift and the proton charge radius, is determined by measuring the rate of laser‐induced x rays as a function of the laser wavelength. Some details of the experiment, recent measurements and improvements will be presented.

We present the optical frequency measurements of the 2S-8S/D and 2S-12D two-photon transitions in hydrogen and deuterium. From an analysis taking into account these results and the very accurate measurements of the 1S-2S transition, we show that the optical frequency measurements have superseded the microwave determination of the 2S Lamb shift and we deduce optimized values for the Rydberg constant and for the 1S and 2S Lamb shifts. We report also the recent development of our 1S-3S experiment.

A measurement of the 2S Lamb shift (2S−2P energy difference) in muonic hydrogen (μ−p) is being prepared at the Paul Scherrer Institute (PSI). The goal of the experiment is to measure the Lamb shift with 30 ppm precision and to deduce the root mean square (rms) proton charge radius with 10−3 relative accuracy, 20 times more precise than presently known. The experiment is based on the availability of long-lived metastable muonic hydrogen atoms in the 2S state which has been investigated in a recent series of experiments at PSI. From the low-energy part of the initial kinetic energy distribution of μp(2S) atoms we determined the fraction of long-lived μp(2S) to be ∼1.5% for H2 gas pressures between 1 and 64 hPa. Another analysis involving μp(1S) with a kinetic energy of 0.9 keV originating from quenching of thermalized μp(2S) via the resonant process μp(2S)+H2→{[(ppμ)+]*pee}*→μp(1S)+p+…+2 keV gives the same result. This is the first direct observation of long-lived μp(2S) atoms. The realization of the μp Lam...

We have studied the hyperfine structure of the 5S1/2‐5D3/2,5/2 two‐photon transition in rubidium. Stabilizing a titanium‐sapphire laser on some of these transitions we have carefully investigated their metrological potential. The development of a new frequency standard at λ=778 nm may be expected soon. This will have performance similar to that of the (He‐Ne)/CH4 laser at λ=3.39 μm. With these two references at one’s disposal it will be possible to measure, in a simple way, the absolute frequency of the (He‐Ne)/I2 laser at λ=633 nm with an accuracy of some parts in 1012: this is of great interest for length metrology.

We have performed a new measurement of the Rydberg constant by frequency comparison of the 2S–8S and 2S–8D two‐photon transitions in hydrogen with the difference of two optical standards connected to a frequency chain. Our result is: R∞=109 737.315 6830 (31) cm−1.