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The comparison of distant atomic clocks is foundational to international timekeeping, global positioning and tests of fundamental physics. Optical-fibre links allow the most precise optical clocks to be compared, without degradation, over intracontinental distances up to thousands of kilometres, but intercontinental comparisons remain limited by the performance of satellite transfer techniques. Here we show that very long baseline interferometry (VLBI), although originally developed for radio astronomy and geodesy, can overcome this limit and compare remote clocks through the observation of extragalactic radio sources. We developed dedicated transportable VLBI stations that use broadband detection and demonstrate the comparison of two optical clocks located in Italy and Japan separated by 9,000 km. This system demonstrates performance beyond satellite techniques and can pave the way for future long-term stable international clock comparisons. Very long baseline interferometry is used to compare two optical clocks located in Japan and Italy through the observation of extragalactic radio sources. This approach overcomes limitations of the performance of satellite transfer techniques.

For the aim to enable precise frequency transfer over earth radius scale distance, NICT developed a broadband VLBI system observing 3-14 GHz radio frequency range, and that was implemented in a pair of transportable 2.4m diameter VLBI stations and Kashima 34m diameter VLBI station. The small diameter VLBI stations installed in Medicina observatory of INAF (Italy) and NICT headquarters in Koganei (Japan) were used compare the atomic frequency standards. The poor sensitivity of VLBI observation with the small antenna pair was compensated by participation of the large diameter antenna for boosting the signal to noise ratio (SNR) of the VLBI observation. Owing to the broad observing frequency range, a few pico seconds precision of group delay is achieved on intercontinental baseline even with small diameter antenna. Advanced digital data acquisition with highspeed 16 GHz sampling enabled stable group delay measurement. A series of frequency transfer experiments was conducted during Oct. 2018 – Feb. 2019 by using the broadband VLBI system between ytterbium (Yb) optical lattice clock of INRiM (Italy) and strontium (Sr) lattice clock of NICT(Japan) separated by 9000km. The deviation of fractional frequency ratio between Yb and Sr was obtained by VLBI observation as y(Yb/Sr)=2.5(2.8)x10-16. The uncertainty in this experiment is evaluated not to be limited by the VLBI link itself, and further improvement is expected. Prospect of VLBI frequency link and uncertainty budget in VLBI observation is discussed in this report.

We report the absolute frequency measurement of the unperturbed transition – at 578 nm in 171Yb realized in an optical lattice frequency standard relative to a cryogenic caesium fountain. The measurement result is 518 295 836 590 863.59(31) Hz with a relative standard uncertainty of . This value is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the International System of Units.

We present an extended model for the lattice-induced light shifts of the clock frequency in optical lattice clocks, applicable to a wide range of operating conditions. The model extensions cover radial motional states with sufficient energies to invalidate the harmonic approximation of the confining potential. We reevaluate lattice-induced light shifts in our Yb optical lattice clock with an uncertainty of 6.1E-18 under typical clock operating conditions., Comment: 12 pages, 10 figures

We performed the first direct measurement of the frequency ratio between a mercury (199Hg) and an ytterbium (171Yb) optical lattice clock to find νHg/νYb = 2.177 473 194 134 565 07(19) with the fractional uncertainty of 8.8 × 10−17. The ratio is in excellent agreement with expectations from the ratios νHg/νSr and νYb/νSr obtained previously in comparisons against a strontium (87Sr) optical lattice clock. The completed closure (νHg/νYb)(νYb/νSr)(νSr/νHg) − 1 = 0.4(1.3) × 10−16 tests the frequency reproducibility of the optical lattice clocks beyond what is achievable in comparison against the current realization of the second in the International System of Units (SI).

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 report the absolute frequency measurement of the unperturbed optical clock transition 1S0–3P0 in 171Yb performed with an optical lattice frequency standard. Traceability to the International System of Units is provided by a link to International Atomic Time. The measurement result is 518 295 836 590 863.61(13) Hz with a relative standard uncertainty of , obtained operating our 171Yb optical frequency standard intermittently for 5 months. The 171Yb optical frequency standard contributes with a systematic uncertainty of .

We discuss the relation between atomic clocks and gravity from two perspectives: gravity potential measurements for optical clock comparisons and contributions to international timescales and, conversely, the measurement of gravity potential differences using optical clocks.

We introduce a description of Ramsey spectra under atomic interactions as a sum of decomposed components with differing dependence on interaction parameters. This description enables intuitive understanding of the loss of contrast and asymmetry of Ramsey spectra. We derive a quantitative relationship between the asymmetry and atomic interaction parameters, which enables their characterization without changing atom density. The model is confirmed through experiments with a Yb optical lattice clock.

The advent of novel measurement instrumentation can lead to paradigm shifts in scientific research. Optical atomic clocks, due to their unprecedented stability and uncertainty, are already being used to test physical theories and herald a revision of the International System of units (SI). However, to unlock their potential for cross-disciplinary applications such as relativistic geodesy, a major challenge remains. This is their transformation from highly specialized instruments restricted to national metrology laboratories into flexible devices deployable in different locations. Here we report the first field measurement campaign performed with a ubiquitously applicable $^{87}$Sr optical lattice clock. We use it to determine the gravity potential difference between the middle of a mountain and a location 90 km apart, exploiting both local and remote clock comparisons to eliminate potential clock errors. A local comparison with a $^{171}$Yb lattice clock also serves as an important check on the international consistency of independently developed optical clocks. This campaign demonstrates the exciting prospects for transportable optical clocks.

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.

We implemented a compact, robust, and stable device for simultaneous frequency stabilization of lasers with different wavelengths used for the cooling and trapping of Yb atoms in an optical lattice clock. The lasers at 399, 556, and 759 nm are locked to a single ultra-stable cavity using the offset sideband locking technique, a modified version of the Pound-Drever-Hall method. For the most demanding stabilization here, the 556 nm laser, this system exhibits a 300 Hz linewidth for an integration time of 80 ms. We observed a long-term drift of less than 20 kHz per day at 759 nm that is suitable for operating the lattice laser with a light shift uncertainty below 1×10sup-18/sup. We successfully tested the system for operating the clock during a typical working day by simultaneously locking the three lasers to the cavity.

Nowadays atomic optical lattice clocks can perform frequency measurements with a fractional uncertainty at the 10-18 level in few hours of measurement, outperforming the best caesium (Cs) standards operated in the world. Since the definition of the unit of time is based on Cs, a worldwide debate about the need to promote the redefinition of the second on a optical reference is under-way. At INRIM (Istituto Nazionale di Ricerca Metrologica) we developed an optical lattice clock based on ytterbium atoms and compared it against a Cs fountain. These results are an important contribution to the debate.

We present the results of a preliminary metrological characterization of our neutral 171Ytterbium-atoms lattice clock at INRIM. During one clock cycle we cool and trap the atoms utilizing 399 nm laser light for the first stage magneto-optical trap (MOT), followed by a cooler 556 nm 2nd stage MOT. Atoms are then transferred in a one-dimensional, horizontal lattice adjusted to the magic wavelength (759 nm). Subsequently the clock transition at 578 nm is excited by a laser stabilized to an ultra-stable cavity, followed by repumping and state-detection. We obtain 2 · 103 atoms in the lattice after 150 ms of loading time. The laser radiation at 399 nm, 556 nm and 578 nm is produced via infrared lasers and non-linear crystals, whereas the lattice features a Titanium-Sapphire laser. A detailed description of the laser light generation and stabilization as well as of the the physics package is given. We locked the clock laser to the atoms and achieved a first metrological characterization through optical fiber-comb based comparison with INRIM's primary frequency standard, the IT-CsF2 cryogenic fountain clock [1]. The reported frequency of the clock transition agrees to the value recommended by the BIPM [2] within the stated uncertainty.

We present an optical lattice clock based on ytterbium 171Yb atoms developed in the laboratories of INRIM. In the experiment, we cool and trap ytterbium atoms in a two stage magneto-optical trap (MOT) (at 399nm and 556nm for the first and second stage, respectively). Atoms are then transferred in a horizontal, one-dimensional optical lattice at the magic wavelength (759 nm). Here the clock transition at 578nm is probed by a laser stabilized on an ultra-stable cavity. We describe the generation of all the laser sources, the physic package and the operation of the clock. Lasers at 399 nm, 556nm and 578nm are obtained, with different techniques, using non-linear crystals starting from infrared sources. The clock laser is stabilized using a high finesse notched ULE cavity. The lattice is made with a titanium-sapphire laser. The aluminum vacuum chamber is designed for wide optical access and its temperature is measured by 8 thermistors for blackbody shift evaluation. Our system allows for fast loading of the lattice with 1 × 104 atoms trapped in the lattice in 250 ms. We obtained preliminary spectroscopy results and we locked the clock laser to the atomic line. Future perspectives are discussed.