Traceable Characterization of THz Electric Fields by Precision Spectroscopy of Cold Trapped HD⁺ Ions

Abstract

Terahertz electrometry may be performed by comparing the measurement of the lightshift of a molecular ion transition with the prediction provided by the quantum theory. This contribution proposes to probe the transition (v,L)=(0,0)→(0,2) of cold trapped HD + ions in order to characterize a THz electric field slightly detuned to the (v,L)=(0,0)→(0,1) transition of HD + . A static magnetic field applied to the ion trap is exploited as a directional reference and to optimize the molecular response to the THz electric field. Weak THz electric fields with amplitudes at the µV/m level may be detected by performing frequency measurements at the limit of molecular ion quantum projection noise. In addition, the exploitation of six independent lightshift measurements performed for two orientations of the magnetic field enables to characterize conjointly the amplitudes and phases of the Cartesian components of a THz-wave with µV/m-level and mrad-level uncertainties, respectively.