The recently concluded collaborative European project "Robust optical clocks for international timescales" (ROCIT) tackled some of the key challenges on the roadmap towards a redefinition of the SI second. This paper gives an overview of progress made on improving the robustness and automation of optical clocks and verifying their uncertainty budgets through coordinated international comparison campaigns. It also presents work on the incorporation of optical clocks into time scales, covering both their use to steer local physical time scales and their use for evaluations of hydrogen masers contributing data for the computation of International Atomic Time (TAI). The overall objective of the project was to bring European optical clocks to the stage where they could be operated routinely as secondary frequency standards, regularly contributing to TAI.

We perform low phase noise, efficient serrodyne modulation for optical frequency control and spectral purity transfer between two ultrastable lasers. After characterizing serrodyne modulation efficiency and its bandwidth, we estimate the phase noise induced by the modulation setup by developing a novel, to the best of our knowledge, composite self-heterodyne interferometer. Exploiting serrodyne modulation, we phase locked a 698 nm ultrastable laser to a superior ultrastable laser source at 1156 nm by means of a frequency comb as a transfer oscillator. We show that this technique is a reliable tool for ultrastable optical frequency standards.

SummaryWe report the result of the new absolute frequency measurement of 171 Yb performed at INRIM. The frequency of our optical lattice clock IT-Yb1 is measured against the local cryogenic Cs fountain IT-CsF2 during a campaign that lasted 14 months. The frequency measurements were performed with two different techniques, i.e. using a hydrogen maser as a transfer oscillator or by synthesizing a low-noise microwave for the Cs interrogation from the Yb ultrastable laser with an optical frequency comb. The frequency of the Yb unperturbed clock transition $^1{S_0}{ \to ^3}{P_0}$ is 518 295 836 590 863.44(14) Hz, with a total fractional uncertainty of 2.7 × 10 –16 , limited by the uncertainty of IT-CsF2. This is the absolute frequency measurement of a Yb clock against a Cs fountain with the lowest uncertainty. The result agrees with the Yb frequency value recommended by the Consultative Committee for Time and Frequency (CCTF). Moreover, the Yb data acquired during this campaign have been submitted to the Bureau of Weights and Measures (BIPM) to contribute to the calibration of the International Atomic Time (TAI). This work confirms the reliability of Yb as a secondary representation of the second.

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 Cs, Rb, and 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.

espanolLa Disociacion Multifotonica InfrarRoja (DMFIR) es un metodo de separacion de isotopos con laser altamente selectivo el cual consiste en la absorcion secuencial de fotones resonantes con un modo de vibracion de la molecula que contiene el isotopo de interes, desde el estado fundamental hasta sobrepasar el umbral de disociacion. Las moleculas con umbrales de disociacion alto requieren laseres de alta energia. Esta desventaja puede superarse mediante la DMFIR con dos frecuencias. En esta tecnica, un laser de baja energia, resonante con los primeros niveles vibracionales garantiza la selectividad isotopica del proceso y otro, menos restrictivo en sintonia, pero de alta fluencia produce la disociacion molecular. En este trabajo se investigo la posibilidad de obtener enriquecimiento isotopico utilizando SiF4 como molecula de trabajo. Se estudio la DMFIR de SiF4 con dos frecuencias provenientes de dos laseres de CO2 TEA monomodo transversal en un jet molecular. Como sistema de deteccion se utilizo un espectrometro de masas de tiempo de vuelo con ionizacion multifotonica UV. Se determino el efecto de la variacion de la fluencia y longitud de onda de los laseres de excitacion y de disociacion sobre los valores de los estimadores α y β asociados a la eficiencia de la disociacion y al grado de enriquecimiento isotopico obtenido, respectivamente, y se comparo con los resultados obtenidos en la DMFIR con una sola frecuencia. EnglishInfraRed Multi-Photon Dissociation (IRMPD) is a highly selective laser isotope separation technique. This process consists of a sequential IR photon absorption from the ground vibrational state up to dissociation by a molecule that contains the isotope of interest. High dissociation threshold molecules require large intensity radiation fields. This drawback could be overcome by two-frequency IRMPD. In this technique, a low energy laser resonant with the first energy levels guarantees isotopic selectivity excitation and a second non-resonant large energy laser achieves molecular dissociation. The possibility of obtaining silicon laser isotopic enrichment using SiF4 as working molecule was investigated in this work. Two-frequency IRMPD of SiF4 with two TEA CO2 single transverse mode lasers was studied in a molecular jet. The dissociation process was monitored with a Time-of-Flight mass spectrometer with UV multi-photon ionization. The excitation laser fluence and wavelength dependence of the isotopic dissociation estimator, α, and the enrichment factor estimator, β, were determined and compared to those obtained in single-frequency IRMPD.