Direct Excitation of the Forbidden Clock Transition in Neutral ${}^{174}\mathrm{Yb}$ Atoms Confined to an Optical Lattice

We report direct single-laser excitation of the strictly forbidden clock transition in atoms confined to a 1D optical lattice. A small () static magnetic field was used to induce a nonzero electric dipole transition probability between the clock states at 578.42 nm. Narrow resonance linewidths of 20 Hz (FWHM) with high contrast were observed, demonstrating a resonance quality factor of . The previously unknown ac Stark shift-canceling (magic) wavelength was determined to be . This method for using the metrologically superior even isotope can be easily implemented in current Yb and Sr lattice clocks and can create new clock possibilities in other alkaline-earth-like atoms such as Mg and Ca.

Figure 1

Magnetic-field-induced lattice spectroscopy. Approximately 10 000 ${}^{174}\mathrm{Yb}$ atoms were cooled, trapped, and loaded into a lattice at the Stark-free (magic) wavelength of 759.35 nm. A pair of current-carrying coils generated a static magnetic field ${\Omega }_B=⟨{}^{3}P_1,m_J=0|\overrightarrow{\mu }·\mathbf{B}|{}^{3}P_0⟩/\hslash$) to mix a small portion of the ${}^{3}P_1$ state into the ${}^{3}P_0$ clock state split by a frequency $\Delta$. A single 578.42 nm spectroscopic probe laser beam, with Rabi frequency ${\Omega }_L$ on the ${}^{1}S_0\leftrightarrow {}^{3}P_1$ transition, was aligned collinear to the lattice with a dichroic beam splitter. The tight confinement of the atoms in the probe direction provided for Doppler- and recoil-free excitation of the clock transition. Excitation was measured by ground-state fluorescence of the atoms on the 398.9 nm transition.

Figure 2

Sideband spectrum of the magnetic-field-induced clock transition of ${}^{174}\mathrm{Yb}$ atoms confined to an optical lattice ($|B|\approx 10\mathrm{mT}$). The sideband spacing of ${\omega }_0/2\pi =90\mathrm{kHz}$ implies an average lattice depth of 1 MHz ($\approx 50\mu \mathrm{K}$). The mean vibrational quantum number can be found from the ratio of the lower to the upper sideband amplitude, which is $⟨n⟩/(⟨n⟩+1)$. This yields a value of $⟨n⟩=2.4$, which indicates an average lattice temperature of $15\mu \mathrm{K}$.

Figure 3

Spectrum of the magnetic-field-induced ${}^{1}S_0\leftrightarrow {}^{3}P_0$ resonance with the lattice laser tuned close to the Stark-free wavelength. A single 400 ms measurement cycle consists of the two successive MOT stages, loading the lattice, applying a 64 ms spectroscopic pulse ($|B|=1.29\mathrm{mT}$, $I\approx 280\mathrm{mW}/{\mathrm{cm}}^2$), then measuring the ground-state population with a 398.9 nm pulse. The frequency of the probe laser was stepped 50 times in 2 Hz intervals for a total scan time of 20 s. A Gaussian fit to the line yields a FWHM of 20.3 Hz. The depletion of 40% was achieved with single pulse excitation.