Comparison of 87Rb N-resonances for D1 and D2 transitions.

We report an experimental comparison of three-photon-absorption resonances (N-resonances) for the D1 and D2 optical transitions of thermal (87)Rb vapor. We find that the D2 N-resonance has better contrast, a broader linewidth, and a more symmetric line shape than the D1 N-resonance. Taken together, these factors imply superior performance for frequency standards operating on alkali D2 N-resonances, in contrast with coherent population trapping resonances, for which the D2 transition provides poorer frequency standard performance than the D1 transition.

Cancellation of light shifts in an N-resonance clock.

We demonstrate that first-order light shifts can be canceled for an all-optical, three-photon-absorption resonance (N-resonance) on the D1 transition of 87Rb. This light-shift cancellation facilitates improved frequency stability for an N-resonance clock. For example, by using a tabletop apparatus designed for N-resonance spectroscopy, we measured a short-term fractional frequency stability (Allan deviation) of approximately/= 1.5 x 10(-11) tao1/2 for observation times of 1 s < or = tao < or = 50 s. Further improvements in frequency stability should be possible with an apparatus designed as a dedicated N-resonance clock.