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Sediment gravity flows are common processes in the submarine environment. They are important for the global sediment transport, but can destroy offshore infrastructure and may even contribute to tsunami generation. These flows, however, remain poorly understood. There is a lack of direct observations due to difficulties with deploying appropriate instruments and predicting the occurrence and route of these flows, especially on open continental slopes. Deployed instruments are further often destroyed as a result of the gravity flows. Submarine fibre cables are present along almost all continental margins worldwide. They are economically important for telecommunication and internet data transfer. Historic records, however, have shown that submarine gravity flows affect and even severe these cables. Recent studies successfully tested the usage of fibre optic cables to detect earthquakes and other processes such changes in the wave height associated with storm events. The aim of this study will be to test whether fibre optic cables can also detect submarine gravity flows using laser interferometry. The study is based on a cooperation between the University of Malta and the Istituto Nazionale di Ricerca Metrologica (INRiM) in Italy and is part of the European funded project “Modern and recent sediment gravity flows offshore eastern Sicily, western Ionian Basin (MARGRAF, ID 101038070)”. The University of Malta has been granted permission to collect data from a 260-km long optical fibre cable that connects Malta and Sicily through the western Ionian Basin. INRiM provided the measurement system and technical support needed to carry out the experiment. The western Ionian Basin is an ideal study site, as it is characterised by many earthquakes, tsunamis and submarine sediment gravity flows. The cable crosses known pathways of these gravity flows and thus provides a high possibility to detect modern sediment flows. The laser interferometry data will be analysed to detect disturbances (e.g., twists, expansions, contractions) on the cable. Any detected disturbances will be compared with oceanographic and seismometer data, both from onshore stations and Ocean Bottom Seismometers (OBS). This comparison will allow us to infer the source of the cable disturbance. In addition, we plan to collect gravity cores in vicinity of the event to assess whether the event was based on a gravity flow or not. Initial results showed earthquakes and various storm events recorded by the cable. The findings are expected to improve our current understanding of gravity flows in the region in terms of potential trigger mechanisms and reoccurrence rate. Eastern Sicily is densely populated and hosts touristic and industrial infrastructure, which makes it important to constrain the geohazard implication of these flows. A successful test will further allow to use this application on cables in other regions worldwide.

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.

The Falcon Dipole (Future Accelerator post-LHC Cos-theta Optimized $\rm {Nb_3Sn}$ Dipole) is a single aperture $\rm {Nb_3Sn}$ cos-theta dipole short model in the framework of the Future Circular Collider (FCC) project. The Italian Institute of Nuclear Physics (INFN), in collaboration with CERN, is in charge of designing and constructing the magnet, which is a crucial step towards the construction of High Field $\rm {Nb_3Sn}$ magnets suitable for a post LHC collider. This paper recalls the electromagnetic design, the field quality and performances of the Falcon Dipole. The coil ends design has been implemented in a 3D FEM to study the peak field distribution on the magnet and influence on field quality. A special focus is given to the 2D analysis to study the effect of superconductor magnetization on field quality from the injection to the final energy and then to investigate the effectiveness of compensation methods.

The recombination dipoles D2 (MBRD) for the luminosity upgrade of the Large Hadron Collider (LHC) are double aperture magnets to be placed on each side of ATLAS and CMS experiments, generating 4.5 T along a magnetic length of 7.78 m and a bore diameter of 105 mm. Its development plan foresees the construction of a short model 1.6 m long, followed by a prototype and by the series of 6 magnets. The magnet design was carried out at INFN Genova in a collaboration framework with CERN and the construction is ongoing in the industry (ASG Superconductors, Italy). The short model activities have just been accomplished after a successful power test performed at CERN, while the prototype is in its construction phase. In this contribution, the main features of the D2 magnet will be described, underlining the improvements implemented in the prototype with respect to the short model design. Then, the main results of the power test will be presented, focusing on training performance, protection scheme effectiveness and magnetic measurements.

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.

Detecting ocean-floor seismic activity is crucial for our understanding of the interior structure and dynamic behavior of the Earth. However, 70% of the planet’s surface is covered by water and seismometers coverage is limited to a handful of permanent ocean bottom stations. We show that existing telecommunication optical fiber cables can detect seismic events when combined with state-of-the-art frequency metrology techniques by using the fiber itself as the sensing element. We detected earthquakes over terrestrial and submarine links with length ranging from 75 to 535 km and a geographical distance from the earthquake's epicenter ranging from 25 to 18,500 km. Implementing a global seismic network for real-time detection of underwater earthquakes requires applying the proposed technique to the existing extensive submarine optical fiber network., peer-reviewed

Recently, the informatics infrastructure of INRiM Time and Frequency Laboratory has been completely renewed with particular attention to network security and software architecture aspects, with the aims to improve the reliability, robustness and automation of the overall set-up. This upgraded infrastructure has allowed, since January 2020, a fully automated generation and monitoring of the Italian time scale UTC(IT), based on dedicated software developed in-house [1]. We focus in this work on the network and software aspects of our set-up, which enable a robust and reliable automatic time scale generation with continuous monitoring and minimal human intervention.

We report on the development of a new compact strontium optical lattice clock employing a 2D-MOT as cold atomic source. The clock system reliability is currently studied using the most abundant isotope, the bosonic 88Sr. More than 105 88Sr atoms are loaded in the optical lattice at 813 nm at temperatures of few microKelvin. All the needed optical frequencies are stabilized to a zero-CTE high finesse optical resonator, and can be operated without frequency adjustments for weeks. The system shows the advantage of optical guiding and switching the atomic source to the clock spectroscopy region by demonstrating more than 17 s lifetime. First spectroscopy tests on the clock transition on 88Sr atoms has been performed by magnetic-field induced spectroscopy.

Extreme Energy Events (EEE) is an extended Cosmic Rays (CRs) Observatory, composed of about 60 tracking telescopes spread over more than 10 degrees in Latitude and Longitude. We present the metrological characterization of a representative set of actually installed EEE GPS receivers, their calibration and their comparison with respect to dual-frequency receivers for timing applications, as well as plans for a transportable measurement system to calibrate the currently deployed GPS receivers. Finally, the realization of an INRIM Laboratory dedicated to EEE, aimed at hosting reference telescopes and allowing timing studies for Particle Physics/Astrophysics experiments, is presented, as well as the possibility of synchronizing already deployed telescopes utilizing White Rabbit Technique, over optical fiber links, directly with the Universal Time Coordinated time scale, as realized by INRIM (UTC(IT)).

A new recombination dipole D2 for the luminosity upgrade of the Large Hadron Collider at CERN (LHC) is under development since 2016 at INFN in a collaboration framework with CERN. These twin aperture magnets will be installed on both sides of LHC main multipurpose experiments, ATLAS and CMS, interaction points. The target integral magnetic field in an aperture of 105 mm is 35 T m, obtained with a magnetic field of 4.5 T and a magnetic length of 7.78 m. The coils in both apertures are asymmetric for canceling the unwanted multipoles caused by the magnetic field cross-talk. The asymmetric design and the demanding requirements on the field quality made preferable an independent collaring, which allows for a more accurate assembly: the mechanical stability is ensured by a novel approach of coil integration in an Al-alloy mechanical structure. After a design phase, the construction of a short model has been planned followed by the construction of a full-length prototype. This contribution is focused on the aspects of the short model assembly and the implications on the prototype design in terms of mechanical tolerances, manufacturing processes, and materials involved in the construction.

We present the results of 10 years of research related to the development of a Rubidium vapor cell clock based on the principle of pulsed optical pumping (POP). Since in the pulsed approach, the clock operation phases take place at different times, this technique demonstrated to be very effective in curing several issues affecting traditional Rb clocks working in a continuous regime, like light shift, with a consequent improvement of the frequency stability performances. We describe two laboratory prototypes of POP clock, both developed at INRIM. The first one achieved the best results in terms of frequency stability: an Allan deviation of σy(τ) = 1.7 × 10−13 τ−1/2, being τ the averaging time, has been measured. In the prospect of a space application, we show preliminary results obtained with a second more recent prototype based on a loaded cavity-cell arrangement. This clock has a reduced size and exhibited an Allan deviation of σy(τ) = 6 × 10−13 τ−1/2, still a remarkable result for a vapor cell device. In parallel, an ongoing activity performed in collaboration with Leonardo S.p.A. and aimed at developing an engineered space prototype of the POP clock is finally mentioned. Possible issues related to space implementation are also briefly discussed. On the basis of the achieved results, the POP clock represents a promising technology for future GNSSs.

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 on the realization of a new compact strontium optical clock using a 2-D magneto-optical-trap (2D-MOT) as cold atomic source and a multi-wavelength cavity as the frequency stabilization system. All needed optical frequencies are stabilized to a zero-thermal expansion high-finesse optical resonator and can be operated without frequency adjustments for weeks. We present the complete characterization of the apparatus. Optical control of the atomic source allows us to perform low-noise clock operation without atomic signal normalization. Long- and short-term stability tests of the clock have been performed for the 88 Sr bosonic isotope by means of interleaved clock operation. Finally, we present the first preliminary accuracy budget of the system.

Recently, the official Italian time scale UTC(IT) has been successfully automatically generated in real-time, with a steering algorithm using Rapid UTC (UTCr) as steering reference [1]. This work aims to present the results of an on-line test of other two steering algorithms: one using the INRIM cesium fountain (ITCsF2) [2], and another one using an ensemble of various commercial clocks, as steering references alternative to UTCr [3].

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 article, 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 realized a UV laser spectrometer at 253.7 nm for Doppler broadening thermometry on the 1 S 0 − 3 P 1 intercombination line in mercury vapors. Our setup is based on the two-stage duplication of a 1014.8 nm diode laser in a fiber-coupled periodically poled lithium niobate waveguide crystal and a beta-barium borate crystal in enhancement cavity, and we exploit injection locking of a 507.4 nm diode laser to boost the available optical power after the first duplication. Our setup addresses spectroscopic features that allow the thermodynamic temperature determination of the atomic sample from the absorption profile with 10 − 6 accuracy. The realized UV laser source has 1 × 10 − 4 relative intensity stability, Gaussian shape, and over 10 GHz mode-hop-free tunable range. These features are crucial for the practical realization of the kelvin in the new International System of Units through a spectroscopic technique.

In the last decade, fiber networks for telecommunication have started to be exploited in a metrological context, for the distribution of accurate time-frequency signal to both research institutes and commercial partners. Time and frequency distribution over fiber is nowadays enabling most of high-resolution comparisons of atomic clocks between distant Metrological Institutes all over Europe and has been adopted in other fields both in fundamental and applied science. Recently, we demonstrated that the same technique can be used to detect weak earthquakes and other acoustic phenomena. This raised strong interest in the geophysical community, especially in view of its exploitation in seas and oceans, where currently no sensors are available. Our contribution describes the basics of technique and achieved results, with particular attention to geophysical applications, and future perspectives.

In view of high-precision molecular spectroscopy in the infrared, we realized a narrow-linewidth laser source at 5.8 $$\upmu$$m traceable to the second in the International System of Units by mixing two near-infrared lasers in an orientation-patterned GaP crystal. The generated radiation is traceable to a Cs fountain through an optical fiber-delivered carrier; its estimated linewidth is 2.3 kHz at 1 s integration time. A quantum cascade laser emitting at the same wavelength is used to boost the power of the mid-IR radiation up to few tens of mW. Thanks to a phase-locking loop, about 70% of the quantum cascade laser output power is within the linewidth of the difference-frequency radiation. This apparatus is used for absolute molecular spectroscopy in a region, above 5 $$\upmu$$m, where high-precision traceability and reliable operation are still challenging. In this paper, we provide the basic information to implement a powerful and metrological grade source in the mid-infrared for precision measurements.

The INRIM Time Laboratory is in charge of different important tasks. In this paper the Laboratory and related activities description will be provided. Some improvements have been introduced in the Laboratory during the last year, and the most relevant ones will be hereafter explained. An overview of all the Laboratory institutional, scientific and contractual activities will be also addressed.

The automated generation of a robust real-time time scale with high accuracy, stability, availability and reliability is of crucial importance for several applications. Among them, the local realizations of the Coordinated Universal Time (UTC) by national laboratories, UTC(k), or the generation of the reference time of a Global Navigation Satellite System (GNSS). In this work, we describe the current status and the developments of the generation of the Italian national time scale, UTC(IT), by reporting on the results of a 5-month experiment and on the recent implementation of the new automated procedure for the generation of UTC(IT).

The linearity of the responsivity of a silicon carbide photodetector in the deep-ultraviolet has been studied using a variant of the so-called intensity-ratio method in view of the implementation of low-uncertainty Doppler-broadening thermometry in mercury vapours. To this purpose, coherent radiation at the wavelength of 254 nm has been produced by means of nonlinear mixing of two visible laser beams in a β-barium borate crystal. The linearity has been quantified as a function of the exposure power, variable in the range between 50 and 300 nW. It is shown that possible deviations from linearity are within the uncertainty of 4 10-5 (corresponding to one standard deviation). Such a performance makes the detector well suitable for the thermometric application.

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 structure accommodates a clock cell with $0.9 \, \mathrm{cm^3}$ inner volume and has an outer volume of only $35 \, \mathrm{cm^3}$. The proposed design aims at strongly reducing the core of the atomic clock, maintaining at the same time high-performing short-term stability ($\sigma_y(\tau) \leq 5\times 10^{-13} \,\tau^{-1/2}$ standard Allan deviation). The proposed structure is characterized in terms of magnetic 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., Comment: Submitted to IEEE-UFFC

We characterized the phase noise and instability of submarine optical fibers and estimate achievable performances of trans-oceanic fiber-based optical frequency transfer, in view of intercontinental atomic clock comparisons. We also show that the low noise of the submarine environment allows exploitation of long-haul telecom fibers as distributed sensors for underwater earthquakes.

We realize a phase-stabilised optical fiber backbone that connects the Italian National Metrology Institute with two radio telescopes over a 600 km baseline. This allows referencing of Very Long Baseline Interferometry (VLBI) facilities with the best atomic frequency standards available today and the implementation of a common-clock architecture, which we are now using to assess VLBI ultimate performances.

High-resolution spectroscopy in the 1–10 μm region has never been fully tackled for the lack of widely-tunable and practical light sources. Indeed, all solutions proposed thus far suffer from at least one of three issues: they are feasible only in a narrow spectral range; the power available for spectroscopy is limited; the frequency accuracy is poor. Here, we present a setup for high-resolution spectroscopy, whose approach can be applied in the whole 1–10 μm range. It combines the power of quantum cascade lasers (QCLs) and the accuracy achievable by difference frequency generation using an orientation patterned GaP crystal. The frequency is measured against a primary frequency standard using the Italian metrological fibre link network. We demonstrate the performance of the setup by measuring a vibrational transition in a highly-excited metastable state of CO around 6 μm with 11 digits of precision.

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.

Laser intensity noise is currently recognized as one of the main factors limiting the short-term stability of vapor-cell clocks. In this article, 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 a pulsed regime. Specifically, given the laser intensity noise spectrum, an analytical expression is derived to evaluate its impact on the clock Allan deviation (ADEV). 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 to be in good agreement with experiments performed with a prototype of pulsed optically pumped Rb-cell clock, and can be extended to other compact clocks.

We demonstrate the enhancement and optimization of a cold strontium atomic beam from a two-dimensional magneto-optical trap (2D MOT) transversely loaded from a collimated atomic beam by adding a side-band frequency to the cooling laser. The parameters of the cooling and sideband beams are scanned to achieve the maximum atomic beam flux and are compared with Monte Carlo simulations. We obtain a 2.3 times larger total atomic flux and a brightness increase of a factor 4 compared with a conventional, single-frequency 2D MOT for a given total power of 200 mW. We show that the sideband-enhanced 2D MOT can reach the loading-rate performances of space-demanding Zeeman-slower-based systems, while it can overcome systematic effects due to thermal-beam collisions and hot-black-body-radiation shift, making it suitable for both transportable and accurate optical lattice clocks. Finally, we numerically study possible extensions of the sideband-enhanced 2D MOT to other alkaline-earth species.

The generation of a real-time time scale with high accuracy, stability, availability and reliability is a very challenging and demanding task. Here, we describe a method applied for the generation of the Italian national time scale UTC(IT), based on the realization of two independent time scales (master and backup) which are then aligned at a high level of accuracy, so that it is possible to switch between the two at any time without discontinuities. This work is mostly focused on the alignment strategy and on the methodology adopted for monitoring and preserving the quality of the final output real-time time scale. Results of a preliminary test in real-time, based on a fully automated process, are also reported and discussed.

The existing optical fiber network for telecommunication, together with coherent laser interferometry techniques such as narrow-linewidth lasers and heterodyne detection, is proposed as a tool for the detection of earthquakes. Using this infrastructure, we were able to detect several earthquakes, with moment magnitude as small as 3.4 and distances to the epicenter ranging between 25 and 18500 km. Such capability is extremely relevant for the detection of earthquakes in the seas and oceans, which are today poorly covered by bulk seismometers. We propose this technique as complementary to traditional detection and show how it can be established on the existing fiber network.