The dissemination of atomic clocks with fiber-based techniques finds application in the fields of metrology, fundamental physics, navigation, and spectroscopy but is a challenge in terms of reliability, maintenance, and performance. Here, we describe the realization of a 1023-km-long fiber link between the metrological institutes of Italy and France that shares the infrastructure with the Internet traffic and exploits segmentation in shorter, cascaded spans to fight optical losses exceeding 280 dB. With four months of quasicontinuous operation of this link, we compared the $\mathrm{Cs}$, $\mathrm{Rb}$, and $\mathrm{Yb}$ atomic clocks at our laboratories, highlighting the potential of this tool to assess the clock uncertainty budgets, characterize advanced satellite techniques, and develop optical timescales. The integration of the metrological, fiber-based infrastructures in the two countries, connecting photonics and spectroscopy laboratories as well as telescope facilities, provides the research community with a physical layer over which applications can be built on.

Laser intensity noise is currently recognized as one of the main factors limiting the short-term stability of vapor-cell clocks. In this paper, we propose a signal theory approach to estimate the contribution of the laser intensity fluctuations to the short-term stability of vapor-cell clocks working in pulsed regime. Specifically, given a laser intensity noise spectrum, an analytical expression is derived to evaluate its impact onto the clock Allan deviation. The theory has been tested for two intensity noise spectra of interest in clock applications: white frequency noise and flicker noise. The predicted results turn out in good agreement with experiments performed with a prototype of pulsed optically pumped (POP) Rb cell clock and can be extended to other compact clocks.

We present the new absolute frequency measurement of ytterbium (¹⁷¹Yb) obtained at INRiM with the optical lattice clock IT-Yb1 against the cryogenic caesium (¹³³Cs) fountain IT-CsF2, evaluated through a measurement campaign that lasted 14 months. Measurements are performed by either using a hydrogen maser as a transfer oscillator or by synthesizing a low-noise microwave for Cs interrogation using an optical frequency comb. The frequency of the ¹⁷¹Yb unperturbed clock transition 1 S 0 → 3 P₀ results to be 518 295 836 590 863.44(14) Hz, with a total fractional uncertainty of 2.7 × 10 − 16 that is limited by the uncertainty of IT-CsF2. Our measurement is in agreement with the Yb frequency recommended by the Consultative Committee for Time and Frequency. This result confirms the reliability of Yb as a secondary representation of the second and is relevant to the process of redefining the second in the International System of Units on an optical transition.

Optically pumped Rb vapor cell clocks are by far the most used devices for timekeeping in all ground and space applications. The compactness and the robustness of this technology make Rb clocks extremely well fit to a large number of applications including GNSS, telecommunication and network synchronization. Many efforts are devoted to improve the stability of Rb clocks and reduce their environmental sensitivity. In this paper, we investigate the use of a novel mixture of buffer gas based on Kr and N₂, capable of reducing by more than one order of magnitude the barometric and temperature sensitivities of the clock, with possible improvement of their long-term stability.

We describe a method to stabilize the amplitude of the interrogating microwave field in compact atomic clocks working in a Ramsey approach. In this technique, we take advantage of the pulsed regime to use the atoms themselves as microwave amplitude discriminators. Specifically, in addition to the dependence on the microwave detuning, the atomic signal after the Ramsey interrogation acquires a dependence on the microwave pulse area (amplitude times duration) that can be exploited to implement an active stabilization of the microwave field amplitude, in a similar way in which the Ramsey clock signal is used to lock the local oscillator frequency to the atomic reference. The stabilization allows us to reduce the microwave field-amplitude fluctuations, which in turn impact the clock frequency through cavity pulling. The proposed technique has shown to be effective to improve our clock frequency stability on medium and long term. We demonstrate the method for a vapor-cell clock working with a hot sample of atoms, but it can be extended to cold-atom compact clocks.

Vapor-cell devices based on microwave interrogation provide a stable frequency reference with a compact and robust setup. Further miniaturization must focus on optimizing the physics package, containing the microwave cavity and atomic reservoir. In this paper, we present a compact cavity-cell assembly based on a dielectric-loaded cylindrical resonator. The loaded cavity resonating at 6.83 GHz has an external volume of only 35 cm3 and accommodates a vapor cell with 0.9 cm3 inner volume. The proposed design aims at strongly reducing the core of the atomic clock, maintaining at the same time high-performing short-term stability (σy(τ)≤5×10-13 τ-1/2 standard Allan deviation). The proposed structure is characterized in terms of microwave field uniformity and atom-field coupling with the aid of finite-elements calculations. The thermal sensitivity is also analyzed and experimentally characterized. We present preliminary spectroscopy results by integrating the compact cavity within a rubidium clock setup based on the pulsed optically pumping technique. The obtained clock signals are compatible with the targeted performances. The loaded-cavity approach is thus a viable design option for miniaturized microwave clocks.