In a standing wave of light, a difference in spatial distributions of multipolar atom-field interactions may introduce atomic-motion dependent clock uncertainties in optical lattice clocks. We show that the magic wavelength can be defined so as to eliminate the spatial mismatch in electric dipole, magnetic dipole, and electric quadrupole interactions for specific combinations of standing waves by allowing a spatially constant light shift arising from the latter two interactions. Experimental prospects of such lattices used with a blue magic wavelength are discussed.

We report three-stage laser cooling of Sr atoms using the $5s^2{}^{1}S_0$ ground state and the $5s5p{}^{3}P_2$ metastable state. ${}^{87}\mathrm{Sr}$ atoms are precooled in a magneto-optical trap simultaneously operated on the $5s^2{}^{1}S_0$–$5s5p{}^{1}P_1$ transition at 461 nm and on the $5s5p{}^{3}P_2$–$5s5d{}^{3}D_3$ transition at 496 nm. Atoms optically pumped to the $5s5p{}^{3}P_2(F=13/2)$ state are magneto-optically trapped and cooled down to 2.5(2) $\mu \mathrm{K}$ on the $5s5p{}^{3}P_2$–$5s4d{}^{3}D_3$ transition at $2.9\mu \mathrm{m}$. Using this transition, we investigate one dimensional polarization gradient cooling of ${}^{88}\mathrm{Sr}$ atoms, which have simple energy levels without hyperfine structure, down to 0.22(18) $\mu \mathrm{K}$ or 17 times the recoil temperature. We discuss prospects for continuous generation of ultracold Sr atoms combined with a state transfer technique.

We introduced frequency-control and temperature-control systems in wavelength-meter-controlled cavity ring-down spectroscopy. The frequency-control system shifted the wavelength of the probe laser from 1393 nm to 696.5 nm where no strong absorption lines of water exist, and therefore, it could avoid measurement errors in laser frequency due to residual moisture in the built-in Fizeau interferometer of a wavelength meter. We verified the hypothesis that the nonuniform reflectivity of the mirror surface contributes to fluctuations in the ring-down time observed with multi-transverse-mode CRDS signals. The fluctuations due to this effect were greatly suppressed by the temperature-control system. Using this system, we could improve the minimum detectable absorption coefficient by three times on average and also improve the experimental standard deviations of the averages by nine times compared with those without the system. We measured near-infrared spectra of residual moisture in dry nitrogen at an approximately 1 nmol/mol (1 ppb) level and performed least-squares fitting of the averaged spectrum. The standard deviation of the residuals of the fitting was 6.6 × 10⁻¹² cm⁻¹, corresponding to a mole fraction of water of 6.3 pmol/mol (6.3 ppt).

Cavity ring-down spectroscopy (CRDS) is a highly sensitive technique that enables the measurement of trace species in gases at or lower than the nmol · mol ⁻¹ (ppb) level in amount-of-substance fraction (mole fraction). In particular, CRDS is a very powerful tool for trace-moisture measurement. In real-time, long-term measurement of trace moisture using CRDS, the accuracy is limited by undesired changes in ring-down time over a period of time (background drift). In this study, we introduced a dual-diode laser system in CRDS for trace-moisture measurements to compensate for the background drift. The frequency of the first laser was tuned to the on-peak position of a water absorption line to measure trace moisture, while that of the second laser was tuned to an off-peak position to track the background drift. The two laser diodes were alternately used to measure the ring-down times at on- and off-peak positions by rapidly switching the two laser beams. The effectiveness of dual-laser CRDS for real-time, long-term measurements of trace moisture was demonstrated by experiments based on a primary trace-moisture standard in the range of 8 nmol · mol ⁻¹ –630 nmol · mol ⁻¹ . The use of the primary trace-moisture standard in the experiments guaranteed the metrological traceability of the measurement results to the international system of units (SI), making the measurement results reported in this study highly reliable. The expanded uncertainty obtained using dual-laser CRDS at 8 nmol · mol ⁻¹ was more than four times better than that of the primary trace-moisture standard, implying the potential of a trace-moisture standard based on dual-laser CRDS whose uncertainty in a low range is better than that of other generator-based trace-moisture standards.

We have experimentally evaluated the accuracy of the frequency measured using a commonly used wavelength meter in the near-infrared region, which was calibrated in the visible region. An error of approximately 50 MHz was observed in the frequency measurement using the wavelength meter in the near-infrared region although the accuracy specified in the catalogue was 20 MHz. This error was attributable to residual moisture inside the Fizeau interferometer of the wavelength meter. A simple method to avoid the error is proposed.