# Experimental Demonstration of a Terahertz Frequency Reference based on Coherent Population Trapping

## TL;DR

This paper demonstrates a new method for creating a terahertz frequency reference using coherent population trapping in an atomic system, achieving high-contrast spectral lines with potential sub-kHz resolution.

## Contribution

It introduces a novel interrogation protocol based on coherent population trapping involving three photons in an N-level atomic scheme.

## Key findings

- High-contrast spectral lines referenced to atomic transitions in the THz domain.
- Uncertainty sources and limitations are analyzed.
- Sub-kHz resolution is experimentally feasible.

## Abstract

A novel protocol of interrogation based on coherent population trapping in an N-level scheme atomic system leads to dark resonances involving three different photons. An ensemble of several hundreds of radiofrequency-trapped ions is probed by three lasers simultaneously locked onto the same optical frequency comb, resulting in high-contrast spectral lines referenced to an atomic transition in the THz domain. We discuss the cause of uncertainties and limitations for this method and show that reaching a sub-kHz resolution is experimentally accessible via this interrogation protocole.