Doppler-Free Spectroscopy of the ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_0$ Optical Clock Transition in Laser-Cooled Fermionic Isotopes of Neutral Mercury

We report direct laser spectroscopy of the ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_0$ transition at 265.6 nm in fermionic isotopes of neutral mercury in a magneto-optical trap. Measurements of the frequency against the LNE-SYRTE primary reference using an optical frequency comb yield $1128575290808.4\pm 5.6\mathrm{kHz}$ in ${}^{199}\mathrm{Hg}$ and $1128569561139.6\pm 5.3\mathrm{kHz}$ in ${}^{201}\mathrm{Hg}$. The uncertainty, allowed by the observation of the Doppler-free recoil doublet, is 4 orders of magnitude lower than previous indirect determinations. Mercury is a promising candidate for future optical lattice clocks due to its low sensitivity to blackbody radiation.

Figure 1

Relevant energy levels of mercury. The ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_1$ transition at 253.7 nm is used for magneto-optical trapping. The clock transition studied in this Letter is the ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_0$ transition at 265.6 nm. The mentioned natural linewidth of $\sim 100\mathrm{mHz}$ is for ${}^{199}\mathrm{Hg}$ based on [23].

Figure 2

Generation of the probe light at 265.6 nm and referencing to the primary frequency reference. AOM2 frequency depends on the isotope.

Figure 3

Spectrum of ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_0$ transition in ${}^{199}\mathrm{Hg}$ observed with a retro-reflected probe beam. Sharp Doppler-free features appear at the top of the 580 kHz wide Doppler profile.

Figure 4

Recoil doublet observed in the ${}^{199}\mathrm{Hg}$ (top) and ${}^{201}\mathrm{Hg}$ (bottom) spectra when a retro-reflected probe beam is used. Data are taken at a rate of one point per second. Spectra have been averaged 4 times.