Precision spectroscopy of atomic hydrogen provides important tests of bound-state quantum electrodynamics (QED) and searches for new physics. As an independent approach to these tests heavier one-electron atoms can be investigates, as high-order QED terms scale with powers of Z. We aim to measure the 1S-2S transition in He+ with a combination of 790 nm and 32 nm photons using the Ramsey-comb spectroscopy method in combination with high-harmonic generation (HHG). As an important first step towards this goal, we present the first precision measurements using both these techniques on a transition in xenon at 110 nm. The relative accuracy of the measurement of 2.3x10-10 was not hampered by the HHG process and is the highest ever achieved with light generated through this process. A second important step was recently taken with the first observation of laser excitation of the 1S-2S transition in He+ using our unequal photon scheme. In this case, a single 150 fs pulse was sent through a neutral beam of He atoms to ionize them to He+, then excite the 1S-2S transition and ionize the excited ions further to He2+. This paves the way to a precision measurement in a single trapped He+ ion with Ramsey-comb spectroscopy.

We perform broadband cavity ring-down spectroscopy (CRDS) relying on the near-infrared frequency comb as the excitation source and a time-resolved mechanical Fourier transform spectrometer as detection device. The many decays corresponding to each spectral element are recorded simultaneously and sorted after Fourier transformation to yield the CRDS spectrum of CO in Ar contained in a 20’000-finesse cavity.

We present here an experimental demonstration of dual-comb spectroscopy performed around 2 μm, with the use of a new design of dispersion-controlled highly nonlinear fibre. The latter allows us to efficiently convert light via a four-wave mixing process from 1.55 μm to 2 μm, which is a spectral region suited for the detection of pollutants such as CO2 or N2O. Experimental measurements have been performed with these two molecules and show an excellent agreement with the HITRAN database.