  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.
