A clock transition for future optical frequency standard with trapped atoms

Direct observation and frequency measurement of ¹S₀→³P₀ line within an accuracy of 20 kHz is reported. The resonance frequency with an accuracy of ∼ 100 kHz is reported by indirect measurement of the energy difference between ¹S₀ and ³P₀ states. It is performed by successive frequency measurements of atomic transitions with higher linewidth, easier to observe: ¹S₀ - ³P₁, ³P₁ - ³S₁ and ³S₁ - ³P₀ transitions. To perform direct observation of the clock transition, atoms are cooled to a temperature of ∼ 2 mK in a magneto-optical trap (MOT). Although the saturation broadening of the transition is orders of magnitude smaller than the Doppler width (1.5 MHz), a detectable signal is achieved by inducing a small leak in the MOT with a laser tuned to resonance. Atoms accumulate in the metastable ³P₀ state where they escape the trap. The resonance is detected by a decrease in the MOT fluorescence signal with a contrast of 1%.