Observation and Absolute Frequency Measurements of the ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_0$ Optical Clock Transition in Neutral Ytterbium

We report the direct excitation of the highly forbidden optical transition in two odd isotopes of neutral ytterbium. As the excitation laser frequency is scanned, absorption is detected by monitoring the depletion from an atomic cloud at in a magneto-optical trap. The measured frequency in () is . The measured frequency in () is . Measurements are made with a femtosecond-laser frequency comb calibrated by the National Institute of Standards and Technology cesium fountain clock and represent nearly a -fold reduction in uncertainty. The natural linewidth of these to transitions is calculated to be , making them well suited to support a new generation of optical atomic clocks based on confinement in an optical lattice.

Figure 1

Ytterbium atomic energy levels. Wavelengths and natural linewidths are indicated for the relevant cooling, trapping, and clock transitions. The fine-structure splitting is not to scale. The hyperfine structure for the ${}^{171}\mathrm{Yb}$ ($\mathrm{I}=1/2$) and ${}^{173}\mathrm{Yb}$ ($\mathrm{I}=5/2$) isotopes is ignored for clarity.

Figure 2

Line shape data for the ${}^{171}\mathrm{Yb}$ ${}^{1}S_0\leftrightarrow {}^{3}P_0$ and ${}^{173}\mathrm{Yb}$ ${}^{1}S_0\leftrightarrow {}^{3}P_0$ transitions, averaged 4 times and taken with 50 traveling-wave pulses at each 7.5 kHz step. Each step lasts $\sim 200\mathrm{ms}$. The Doppler-broadened widths correspond to temperatures of $84\mu \mathrm{K}$ for ${}^{171}\mathrm{Yb}$ and $48\mu \mathrm{K}$ for ${}^{173}\mathrm{Yb}$.