A novel absorption resonance for atomic clocks

We report an experimental study of an all-optical three-photon-absorption resonance (known as an "N-resonance") and discuss its potential application as an alternative to atomic clocks based on coherent population trapping (CPT). We present measurements of the N-resonance contrast, width and light shift for ⁸⁷Rb under various conditions including both D₁ and D₂ optical transitions. Under conditions such that first-order light shifts cancel, we observe promising short-term frequency stability (∼ 1.5 × 10^-11 τ_{- 1}2 /) for an N-resonance on the Di transition of ⁸⁷ Rb vapor.