Sensing a THz Electric Field with Cold and Trapped Molecular Ions

The electromagnetic fields may be characterized with high stability and accuracy by observing their effects on quantum objects under strict control of the internal and external degrees of freedom. Electromagnetically induced transparency of Rydberg atoms enabled detection of weak electric fields at the µV/cm level with subwavelength resolution and sensitivity limited by the photon shot noise [1] . Doppler-free spectroscopy of trapped and sympathetically cooled HD + ions in a linear radiofrequency trap allowed recently significant improvements in accuracy and resolution [2] , [3] . Ab-initio quantum electrodynamics calculations provided accurate predictions for the HD + energy levels [4] , [5] and of their shifts in external fields [6] , [7] . This contribution proposes a new method to characterize completely the electric field of a THz-wave in a Cartesian reference frame based on comparison of the measurements of the lightshifts induced on a two-photon transition of HD + with the ab-initio theoretical predictions [8] .