Frequency measurement of a CH4 hyperfine line at 88 THz/`optical clock´

A frequency-measurement chain was used to measure the frequency of a hyperfine line of the ν₃P(7) CH₄ multiplet, which was resolved by a linear optical Ramsey resonance technique. A frequency of 88, 376, 181, 599.7±1 kHz is found. The Ramsey experiment was used as a clock; to ~100 s the stability of this optical clock is equal to a high-performance commercial Cs clock. It is concluded that the frequency division necessary for optical transitions to be used as a clock does not necessarily introduce the noise and errors which would a priori rule out optical transitions as candidates for good clocks