Method for quantum-limited detection of narrow-linewidth transitions in cold atomic ensembles

A scheme is presented that allows quantum-noise-limited, background-free signal detection in high-resolution spectroscopy, atom-light interferometers, and optical clocks. The method combines the electron-shelving technique with a removal of all absorbers not participating in the excitation scheme. Application to the S-1(0)-P-3(1) transition of Ca demonstrated a more than fourfold improvement of the signal-to-noise ratio with respect to the previous photon shot-noise limit. [S1050-2947(98)50412-X].