Phase-Sensitive Vector Terahertz Electrometry from Precision Spectroscopy of Molecular Ions

Abstract

This article proposes a new method for sensing THz waves that can allow electric field measurements traceable to the International System of Units and to the fundamental physical constants by using the comparison between precision measurements with cold trapped HD+ ions and accurate predictions of molecular ion theory. The approach exploits the lightshifts induced on the two-photon rovibrational transition at 55.9 THz by a THz wave around 1.3 THz, which is off-resonantly coupled to the HD+ fundamental rotational transition. First, the direction and the magnitude of the static magnetic field applied to the ion trap is calibrated using Zeeman spectroscopy of HD+. Then, a set of lightshifts are converted into the amplitudes and the phases of the THz electric field components in an orthogonal laboratory frame by exploiting the sensitivity of the lightshifts to the intensity, the polarization and the detuning of the THz wave to the HD+ energy levels. The THz electric field measurement uncertainties are estimated for quantum projection noise-limited molecular ion frequency measurements with the current accuracy of molecular ion theory. The method has the potential to improve the sensitivity and accuracy of electric field metrology and may be extended to THz magnetic fields and to optical fields.

Two-Photon Rotational Spectroscopy of Cold Trapped HD+ Ions for Improved Characterization of THz Electric Fields

“…The comparison between the theoretical and the experimental frequencies was exploited for a test of the quantum electrodynamics, to search for a deviation of the interaction between nuclei from the Coulomb force and for improving the determination of a set of fundamental constants. The comparison experiment-theory was extended to the frequency shifts induced on HD + transitions by external fields in order to calibrate them against the values of the fundamental constants and the SI second [6,7]. Double-resonance two-photon spectroscopy has potential to increase the fractional resolution and accuracy beyond the 10 -12 level [8].…”

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

“…A double-resonance twophoton spectroscopy scheme may increase the fractional resolution and accuracy with HD + ions beyond the 10 -12 level [11]. The comparison experiment-theory for the lightshifts of a rovibrational transition induced by a THz-wave slightly detuned to the HD + energy levels enabled phase-sensitive THz vector electrometry [12]. This contribution discuses the potential of this method using a two-photon rotational transition of HD + .…”

Sensing a THz Electric Field with Cold and Trapped Molecular Ions

“…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].…”