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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.

Twin-field quantum key distribution on optical fibers, a promising approach to long-distance secure communication, can take advantage from atomic clocks technologies such as narrow linewidth lasers and phase-coherent distribution of optical pulses. We will describe the integration of these technologies in a TF-QKD setup implemented on an ex-tended metropolitan fiber network and report on the expected QKD performances.

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.

Quantum entanglement is one of the most extraordinary effects in quantum physics, with many applications in the emerging field of quantum information science. In particular, it provides the foundation for quantum key distribution (QKD), which promises a conceptual leap in information security. Entanglement-based QKD holds great promise for future applications owing to the possibility of device-independent security and the potential of establishing global-scale quantum repeater networks. While other approaches to QKD have already reached the level of maturity required for operation in absence of typical laboratory infrastructure, comparable field demonstrations of entanglement-based QKD have not been performed so far. Here, we report on the successful distribution of polarization-entangled photon pairs between Malta and Sicily over 96 km of submarine optical telecommunications fiber. We observe around 257 photon pairs per second, with a polarization visibility above 90%. Our results show that QKD based on polarization entanglement is now indeed viable in long-distance fiber links. This field demonstration marks the longest-distance distribution of entanglement in a deployed telecommunications network and demonstrates an international submarine quantum communication channel. This opens up myriad possibilities for future experiments and technological applications using existing infrastructure., peer-reviewed

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.

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.

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).

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.

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.

This paper presents a digital instrumentation for coherent frequency transfer over fiber links. It detects the phase and the amplitude of the beat notes at the two ends and uses them for compensating the fiber phase noise and the polarization rotation. The system is implemented on the commercial platform Red-Pitaya, which integrates the Zynq System-on-Chip with fast Analog to Digital and Digital to Analog Converters, and provides great flexibility for on-site reconfiguration and remote operation. The system we propose can be used for Doppler and two-way compensations, both in open and closed loop. Additionally, it features a detection bandwidth of 10 MHz which is of particular importance for minimizing cycle slips in thousand kilometer links. The electronics exhibits residual frequency stability of the order of 10-18 at 1 s exceeding 10-21 at 10000 s. We verified the system performance on a 300 km loop link for the two-way scheme.

We realized a coherent fiber link, 1739 km long, to connect a facility of the Italian Space Agency devoted to Space Geodesy. We describe here the motivations, the architecture of the link, the Time and Frequency station at the remote end and the first results.

Timing over fiber achieves an accuracy at the subns level. We describe here the distribution over fiber implemented for industrial and scientific users, using two of the most common techniques, related to the IEEE-1588 standard, over different links spanning from 140 to 1700 km, and their performance.

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.

The Risk Based Inspection (RBI), proposed by the American Petroleum Institute, is actually the highest benefit maintenance guidelines in the chemical industry, but data collection that characterises this analysis is very complex and time-consuming. The support of the RBI module of the Inspection Manager software helps in overcoming such issues as it encloses all functionalities for an easier management of technical data in the Plant's Inspection. The objective of this work is to test the newer version of the RBI module, to achieve this aim some quantitative examples were examined. In addition, by means of the support of such a module, results of the consequence assessment were also compared with those obtained by two commercial softwares in order to verify the Inspection Manager 's support in risk-based decisions also when methods for frequency and consequence assessment, different than those of API RBI, are applied and to comment about the available approaches for the consequence analysis.

The Italian Institute of Metrology (INRIM) realized a fibre-optic backbone, 1850 km long, for the dissemination of accurate time and frequency standards. Moreover, we implemented a dedicated service for time distribution with traceability to UTC for industrial users. Here we describe techniques, architectures and results.

A new measurement system (in the following indicated as switch) had been introduced at INRIM Time Laboratory (TL), to grant to UTC(IT) time scale better characteristics in terms of robustness and reliability, especially when a change from primary and backup time scale realizations, is required. A complete description of the system implementing such a switching process is described in the present paper, together with the analysis of the results and the performed functional tests.

The Italian Institute of Metrology (INRIM) developed an optical fibre backbone for the dissemination of accurate time and frequency standards to several research institutions of the Country. A network of research facilities, involved in areas ranging from atom and molecular spectroscopy to Very Long Baseline Interferometry (VLBI) for geodesy and radioastronomy, has been established. The time transfer over fibre has also been implemented for industrial users. We will describe the status of this 1800 km long facility, the most recent implementations and the research activities where fiber-based frequency dissemination is being exploited.

This paper describes the application of a novel active disturbance rejection control (ADRC) to the stabilization of the temperature of two ultra-stable Fabry-Perot cavities. The cavities are 10 cm long and entirely made of ultralow- expansion glass. The control is based on a linear extended state observer that estimates and compensates the disturbance in the system in real time. The resulting control is inherently robust and easy to tune. A digital implementation of ADRC gives a temperature instability of 200 μK at one day of integration time.

In this paper, we describe the development of an ultrastable laser source at 578 nm, realized using frequency sum generation. This source will be used to excite the clock transition (1)S(0) - (3)P(0) in an ytterbium optical lattice clock experiment. Two independent ultrastable lasers have been realized, and the laser frequency noise and stability have been characterized.

We stabilized the frequency of a compact planar-waveguide external cavity laser (ECL) on a Fabry-Perot cavity (FPC) through a Pound-Drever-Hall scheme. The residual frequency stability of the ECL is 10(-14), comparable to the stability achievable with a fiber laser (FL) locked to an FPC through the same scheme. We set up an optical link of 100 km, based on fiber spools, that reaches 10(-19) relative stability, and we show that its performances using the ECL or FL are comparable. Thus ECLs could serve as an excellent replacement for FLs in optical links where cost-effectiveness and robustness are important considerations.

Bidirectional phase-stabilized optical fiber links allow state-of-the-art optical clocks to be compared on a continental scale. However, intercontinental comparisons are still based on satellite techniques, preventing optical clocks from being compared without degradation. We performed the first optical frequency transfer experiment over submarine links, measuring levels of environmentally induced noise substantially lower than for on-land links. From these measurements, we predict the transfer stability that can be achieved over transoceanic links. We also performed the first optical frequency transfer over fibers installed in power cables, observing optical perturbations caused by the high-voltage field. Finally, we show that the low background noise of fibers on the seafloor allows applications in geophysical sensing.

Distributed Raman amplification (DRA) is widely exploited for the transmission of broadband, modulated signals used in data links, but not yet in coherent optical links for frequency metrology, where the requirements are rather different. After preliminary tests on fiber spools, in this paper we deeper investigate Raman amplification on deployed in-field optical metrological links. We actually test a Doppler-stabilized optical link both on a 94 km-long metro-network implementation with multiplexed ITU data channels and on a 180 km-long dedicated fiber haul connecting two cities, where DRA is employed in combination with Erbium-doped fiber amplification (EDFA). The performance of DRA is detailed in both experiments, indicating that it does not introduce noticeable penalties for the metrological signal or for the ITU data channels. We hence show that Raman amplification of metrological signals can be compatible with a wavelength division multiplexing architecture and that it can be used as an alternative or in combination with dedicated bidirectional EDFAs. No deterioration is noticed in the coherence properties of the delivered signal, which attains frequency instability at the 10(-19) level in both cases. This study can be of interest also in view of the undergoing deployment of continental fiber networks for frequency metrology.

In this work we report the results of a study on phase-coherent transfer of narrow linewidth optical frequencies in single-mode fiber links employing bi-directional distributed Raman amplification (DRA). Both co- and counter-propagating amplification pump schemes have been investigated and with optimized input parameters, a coherent optical frequency at 1542 nm was transferred on a 305 km fiber. We also describe the optical frequency transfer on a 94-km long metro fiber link in presence of two ITU WDM telecom data channels with the help of DRA, allowing for an attained fractional frequency instability of 3·10-19 at 1000 s for the transferred frequency signal.

We realized a coherent optical fiber link in Italy, with a fiber haul of 642 km. To characterize the link, we doubled the link to 1284 km, demonstrating a characterization technique based on the double round trip on a single fiber. The link is intended to significantly improve the frequency references used in radio-astronomy and precision measurements in atomic physics. The data analysis is based on the Allan deviation, whose expression is theoretically derived from the noise power spectrum. The demonstrated resolution is 3×10−19 in 1000 s and the uncertainty of the transfer is 5×10−19. The arming of a second fiber is avoided, and this could be rather beneficial to long hauls realizations for a continental fiber network for frequency and time metrology.

We describe the realization of a coherent optical fiber link for the metrological frequency dissemination on the national scale. This infrastructure will improve the frequency references used in radio-astronomy and in atomic physics and will benefit several laboratories in Italy involved in high resolution spectroscopy, matter physics and radioastronomy. The present infrastructure will be part of a forthcoming European network of optical links. This paper describes the haul implementation, the characterization and the future applications of this backbone. Keywords-coherent fiber links, frequency metrology, optical fibers

We realized a 650-km optical fiber link in Italy for frequency dissemination and remote clocks comparison. We present its implementation and characterization at the 5e-19 uncertainty level, discussing its future applications.

We analyze the benefits of distributed Raman amplification in metrology transfer of phase-coherent ultra-narrowband optical frequencies in a long fiber link (200 km with additional 16 dB loss), providing high-gain bi-directional amplification with limited impact on phase noise, attaining a fractional frequency instability of 3 × 10-19 over 1000 s.

This work describes the use of Distributed optical Raman Amplification for the realization of coherent optical fiber links for frequency dissemination. The main advantages of this technique are high gain and feasibility of long fiber spans with a simple apparatus, without degrading the link stability. © 2013 IEEE.

The Italian Institute of Metrology (INRIM), the European Laboratory for NonLinear Spectroscopy (LENS), the Physics and Astronomy Department of the University of Firenze (UNIFI) and the Politecnico of Torino started a project to establish a 650 km optical fiber link that will enable remote frequency comparisons between high accuracy microwave and optical clocks of INRIM and UNIFI-LENS and an absolute measurement of the UNIFI-LENS Sr clock versus INRIM Cesium fountains. We report the overall architecture of the project, the present status of the experiment and the preliminary tests performed on the equipments.

We describe a fiber-optic gyroscope based on the Sagnac effect, realized on a multiplexed telecom fiber network. Our loop encloses an area of 20 km² and coexists with Internet data traffic. This Sagnac interferometer is capable of detecting signals that are larger than 10(-8) (rad/s)/√Hz, thus approaching ring laser gyroscopes without using a narrow-linewidth laser or sophisticated optics. The proposed gyroscope could be useful for seismic applications, opening new possibilities for this kind of optical fiber sensor.

In this paper we described the development and the characterization of a 578 nm laser source to be the clock laser for an Ytterbium Lattice Optical clock. Two indipendent laser sources have been realized and the characterization of the stability with a beat note technique is presented.

At the beginning of 2009 INRIM, the Italian metrological laboratory, started a research activity in order to realize a low noise optical link. Two telecom fiber lasers at 1550 nm have been locked on two independent notched ultralow expansion glass (ULE) Fabry-Perot cavities. We present the thermal stabilization system, and report the finite-element analysis (FEA) made to precisely determinate the positions of the cavity supports. The sensitivity of optical axes length to acceleration is found to be less than 4.5 pm/g. Two different commercial lasers are stabilized by a Pound-Drever-Hall (PDH) technique and a stability of 6×10-14 is achieved at 1 s. Finally, the experimental set-up for the optical link is described and a first noise cancellation result on a 25 km long fiber is reported.