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

Florin Lucian Constantin

doi:10.3390/atoms8040070

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.

Highlights

The sensitive detection of electromagnetic fields has a broad range of fundamental and technological applications, spanning from the search for physics beyond the Standard Model and tests of fundamental symmetries to time-keeping, navigation, modern communications, geophysics, and medical imaging
The System of Units (SI) traceability of the electric field magnitude was ensured by the frequency measurement of the AT splitting, which was assumed as a linear ac-Stark effect depending on the Planck constant and the known dipole moment of the transition between the atomic Rydberg states
An electric field oscillating by 1.3 THz is off-resonantly coupled to the electric-dipole allowed (v, L) = (0, 0)→(0, 1) rotational transition of HD+

Summary

Introduction

The sensitive detection of electromagnetic fields has a broad range of fundamental and technological applications, spanning from the search for physics beyond the Standard Model and tests of fundamental symmetries to time-keeping, navigation, modern communications, geophysics, and medical imaging. The measurement standards based on atoms and molecules were used for the determination of a number of physical quantities [1,2,3]—for example, the time (s), the length (m), and the mass (kg)—and of many fundamental constants [4], including the Rydberg constant, the fine structure constant, and various particle masses. The SI traceability of the electric field magnitude was ensured by the frequency measurement of the AT splitting, which was assumed as a linear ac-Stark effect depending on the Planck constant and the known dipole moment of the transition between the atomic Rydberg states. Comparing to the previous radiofrequency calibrations performed with standard electromagnetic fields and antenna probes [12,13], which display sensitivities at the 1 mV/(Hz1/2 .cm) level, and fractional accuracies in the range of 5–20%, the method using the

Methods

Results

Conclusion