Experimental Demonstration of a Terahertz Frequency Reference based on Coherent Population Trapping. (arXiv:1903.05386v2 [quant-ph] UPDATED)

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.

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