Vincent Tugayé

We describe and experimentally implement a single-ion local thermometry technique with absolute sensitivity adaptable to all laser-cooled atomic ion species. The technique is based on the velocity-dependent spectral shape of a quasi-dark resonance tailored in a J $\rightarrow$ J transition such that the two driving fields can be derived from the same laser source leading to a negligible relative phase shift. We validated the method and tested its performances in an experiment on a single 88 Sr + ion cooled in a surface radio-frequency trap. We first applied the technique to characterise the heating-rate of the surface trap. We then measured the stationary temperature of the ion as a function of cooling laser detuning in the Doppler regime. The results agree with theoretical calculations, with an absolute error smaller than 100 $μ$K at 500 $μ$K, in a temperature range between 0.5 and 3 mK and in the absence of adjustable parameters. This simple-to-implement and reliable method opens the way to fast absolute measurements of single-ion temperatures in future experiments dealing with heat transport in ion chains or thermodynamics at the single-ion level.

We measured the branching fractions for the decay of the 5p 2 P 1/2 state of 88 Sr + by applying a recently demonstrated photon-counting sequential method (M. Ramm et al., Phys. Rev. Lett. 111, 023004) to a single ion laser-cooled in a micro fabricated surface trap. The branching fraction for the decay into the 5s 2 S 1/2 ground level was found to be p = 0.9453 +0.0007 --0.0005. This result is in good agreement with recent theoretical calculations but disagrees with previous experimental measurements, however affected by a one order of magnitude larger uncertainty. This experiment also demonstrates the reliability and the performances of ion micro trap technology in the domain of precision measurements and spectroscopy. PACS numbers: 32.70.Cs, 06.30.Ft, 37.10.Ty