The second-order susceptibility d(36) of ammonium dihydrogen phosphate (ADP) was determined from phase-matched second-harmonic generation (SHG) at two wavelengths. A cw single-mode He-Ne laser (λ= 633 nm) and a cw single-mode Nd:YAG laser (λ= 1064 nm) were used as fundamental beam sources. The results were d(36)(ADP, 633 nm) =(1.31 ± 0.05) ×10(-9) esu = 0.55 ± 0.02 pm/V and d(36)(ADP, 1064 nm) = (1.10 ± 0.06) × 10(-9) esu = 0.46 ± 0.03 pm/V. The d(11) values of α-quartz were determined relative to d(36)(ADP) to be d(11)(α-quartz, 633 nm) = (7.4 ± 0.3) × 10(-10) esu = 0.31 ± 0.01 pm/V and d(11)(α-quartz, 1064 nm) = (7.1 ± 0.3) × 10(-10) esu = 0.30 ± 0.01 pm/V by the use of the Maker fringe method. The Miller's delta ofADP and α-quartz is in good agreement at the two wavelengths.

The frequency stability of an atomic fountain clock was significantly improved by employing an ultra-stable local oscillator and increasing the number of atoms detected after the Ramsey interrogation, resulting in a measured Allan deviation of 8.3 × 10(-14)τ(-1/2)). A cryogenic sapphire oscillator using an ultra-low-vibration pulse-tube cryocooler and cryostat, without the need for refilling with liquid helium, was applied as a local oscillator and a frequency reference. High atom number was achieved by the high power of the cooling laser beams and optical pumping to the Zeeman sublevel m(F) = 0 employed for a frequency measurement, although vapor-loaded optical molasses with the simple (001) configuration was used for the atomic fountain clock. The resulting stability is not limited by the Dick effect as it is when a BVA quartz oscillator is used as the local oscillator. The stability reached the quantum projection noise limit to within 11%. Using a combination of a cryocooled sapphire oscillator and techniques to enhance the atom number, the frequency stability of any atomic fountain clock, already established as primary frequency standard, may be improved without opening its vacuum chamber.

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A remote synchronization system for the on‐board crystal oscillator (RESSOX) of the QZSS is proposed. Aimed at offering an alternative architecture for the classic GPS time‐keeping system (TKS), the proposed system consists of a synchronization framework where an ultrastable atomic clock, localized at the ground station, is kept synchronized to a second time reference, a voltage‐controlled crystal oscillator (VCXO), on‐board the QZSS satellite. Based on orbit calculations and delay predictions, feedback and feed‐forward control guarantee the correct synchronization of the on‐board time reference and the ground‐station atomic clock. Compared with the classic GPS TKS, the RESSOX scheme offers several advantages in terms of accuracy, cost, weight, and power consumption. An extensive explanation of this novel design is presented. Differences and advantages of the proposed system are compared with the classic atomic clock scheme.

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