Fourier transform detection of weak optical transitions in atoms undergoing cyclic routines

We demonstrate a means of detecting weak optical transitions in cold atoms that undergo cyclic (preparation, probing, and detection) routines with improved sensitivity. The gain in sensitivity is made by probing atoms on alternate cycles of a repeated experimental sequence, leading to regular modulation of the ground state atom population when at the resonance frequency. The atomic transition is identified by conducting a fast Fourier transform via an algorithm or instrument. We find an enhancement of detection sensitivity compared to more conventional scanning methods of $∼20$ for the same sampling time, and can detect contemporary clock lines with fewer than 10³ atoms in a magneto-optical trap. We apply the method to the $(6s2) 1S0−(6s6p) 3P0$ clock transition in ¹⁷¹Yb and ¹⁷³Yb. In addition, the ac-Stark shift of this line in ¹⁷¹Yb is measured to be 0.19(3) kHz $W−1$ m² at 556 nm.