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We performed a frequency comparison of lattice clocks in Tokyo and Torino for the first time using broadband VLBI technique. Two portable antennas with 2.4 m diameters were installed at NICT Koganei headquarters and INAF Medicina Radio Observatory, close to Bologna, Italy, realizing a VLBI network together with Kashima large-aperture antenna of 34 m diameter. The clock signal at INAF was evaluated by INRIM using an optical fiber link between Medicina and Torino. The fractional frequency difference that VLBI evaluated was consistent with the difference between two HM frequencies which were separately calibrated by a Sr lattice clock at NICT and a Yb lattice clock at INRIM.

We designed and implemented a simple and robust optical system for the frequency stabilization of lasers at different wavelength, used for the cooling and trapping of atoms in a Yb optical lattice clock. We used a single ultra-stable cavity to lock the frequency of three different lasers at 399 nm, 556 nm and 759 nm exploiting the offset sideband locking technique, derived from the common Pound-Drever-Hall method. A linewidth of less than 300Hz is obtained at 556 nm with a fractional frequency stability of 3 × 10−14 at 1s. At 759 nm we measured a long term drift less than 20kHz per day, which is sufficient to keep the lattice light shift fractional uncertainty under 1 × 10−18. The system was tested by simultaneously locking the three lasers to the cavity and operating the clock without any significant reduction in number of atoms.