Cyril Chatou

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.

Stringent conditions on the phase relation of multiple photons are a prerequisite for novel protocols of high-resolution coherent spectroscopy. In a recent experiment we have implemented an interrogation process of a Ca$^+$-ion cloud based on three-photon coherent population trapping, with the potential to serve as a frequency reference in the THz-range. This high-resolution interrogation has been made possible by phase-locking both laser sources for cooling and repumping of the trapped ions to a clock laser at 729~nm by means of an optical frequency comb. The clock laser, a titanium-sapphire laser built in our lab locked onto two high-finesse cavities reaches a linewidth of a few Hertz and a frequency stability below 10$^{-14}$ at one second, performances which can be copied onto the two other sources. In this paper we discuss the performances of the phase-transfer between the three involved lasers via the optical frequency comb.