We demonstrate a precision frequency measurement using a phase-stabilized 120-km

We performed an absolute frequency measurement of the 1 S 0 -3 P 0 transition in 87 Sr with a fractional uncertainty of 1.2 × 10 −15 , which is less than one third that of our previous measurement. A caesium fountain atomic clock was used as a transfer oscillator to reduce the uncertainty of the link between a strontium optical lattice clock and the SI second. The absolute value of the transition frequency is 429 228 004 229 873.56(49) Hz.Recently, some optical clocks have reached the 10 −18 level 1, 2 in both uncertainty and stability, and these values surpass the caesium fountain microwave primary standards used to realise the SI unit of time. The high-performance of the optical clocks means that the scientific community is discussing a re-definition of the second. Therefore, there is a need for the metrology community to make a strenuous effort to determine the absolute frequencies of the optical frequency standards in relation to the current primary frequency standards, so that the length of one second remains unchanged after the re-definition.At the National Metrology Institute of Japan (NMIJ) we have developed atomic clocks based on optical transitions in an ensemble of neutral atoms trapped in Stark-shift-free optical lattices, 3-5 which are called optical lattice clocks. 6 In 2014, we measured the frequency of the 1 S 0 -3 P 0 clock transition in 87 Sr. 5 At that time the uncertainty of the absolute frequency (3.7 × 10 −15 ) was mainly limited by the uncertainty of a comparison with NMIJ coordinated ‡ These two authors contributed equally to this work.

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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.

High-contrast coherent population trapping signals were observed on the Cs D1 line by use of a bichromatic linear polarized light (lin‖lin field). A maximum absorption contrast of about 10% was obtained. This was nearly twice as high as that measured with the standard configuration of bichromatic circularly polarized light (σ-σ field). The results are compared with density matrix calculations of 4 and 5 level systems.