Experimental demonstration of three-photon Coherent Population Trapping in an ion cloud

An interrogation protocol based on coherent population trapping in an N-level scheme atomic system leads to dark resonances involving three different photons. An ensemble of several hundred radiofrequency-trapped Ca+ 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 terahertz domain. We discuss the cause of uncertainties and limitations of this method and show that reaching subkilohertz resolution is experimentally possible by this interrogation protocol.