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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.

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.

Quantum key distribution (QKD) based on entangled photon pairs holds the potential for repeater-based quantum networks connecting clients over long distance. We demonstrate long-distance entanglement distribution by means of polarisation-entangled photon pairs through two successive deployed 96 km-long telecommunications fibres in the same submarine cable. One photon of each pair was detected directly after the source, while the other travelled the fibre cable in both directions for a total distance of 192 km and attenuation of 48 dB. The observed two-photon Bell state exhibited a fidelity 85% $\pm$ 2% and was stable over several hours. We employed neither active stabilisation of the quantum state nor chromatic dispersion compensation for the fibre., 7 pages, 3 figures

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.

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 describe a VLBI experiment in which, for the first time, the clock reference is delivered from a National Metrology Institute to a radio telescope using a coherent fibre link 550 km long. The experiment consisted of a 24-hours long geodetic campaign, performed by a network of European telescopes; in one of those (Medicina, Italy) the local clock was alternated with a signal generated from an optical comb slaved to a fibre-disseminated optical signal. The quality of the results obtained with this facility and with the local clock is similar: interferometric fringes were detected throughout the whole 24-hours period and it was possible to obtain a solution whose residuals are comparable to those obtained with the local clock. These results encourage further investigation of the ultimate VLBI performances achievable using fibre dissemination at the highest precision of state-of-the-art atomic clocks.

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.

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.

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.

Optical fiber links offer the best performance to transfer time and frequency. They are beneficial to primary metrology, and there is a strong effort to develop a European network of T&F fiber links. In Italy, we realized a coherent optical fiber link from the National Metrological Institute INRIM to the INAF radio-antennas in Medicina, Bologna and the physics laboratories in Sesto Fiorentino, for a total haul of 642 km. We demonstrated the transfer of primary frequency references to the radio-antenna in Medicina, with potential impact for future development in radioastronomy and VLBI. The extension of this link targets to arrive to Matera, at the Space Geodesy Center of ASI. We describe here the space metrology interests of the optical fiber link, the present set-up of the experiment the preliminary results and the future developments.

Optical fiber links are beneficial not only for frequency metrology, but also for other fields of physical research, such as radioastronomy and geodesy. We realized the first optical fiber link from a National Institute of Metrology to a Radiotelescope used for Very Long Baseline Interferometry and for geodetic measurements, over a distance of 544 km. We performed a remote calibration of the hydrogen-maser used as a frequency reference at Medicina Radiotelescopes, in central Italy, against the Italian Cs fountain primary frequency standard. The comparison was limited by the statistical uncertainty of the hydrogen-masers. This experiment demonstrates that optical links can provide radioastronomical facilities with very accurate and stable frequency references, in perspective better than the currently used hydrogen-masers. This opens new perspectives in the ultimate limits of VLBI and a more precise space geodesy.

We realize a coherent fiber link for application in very long baseline interferometry (VLBI) for radio astronomy and geodesy. A 550-km optical fiber connects the Italian National Metrological Institute (INRIM) to a radio telescope in Italy and is used for the primary Cs fountain clock stability and accuracy dissemination. We use an ultrastable laser frequency- referenced to the primary standard as a transfer oscillator; at the radio telescope, an RF signal is generated from the laser by using an optical frequency comb. This scheme now provides the traceability of the local maser to the SI second, realized by the Cs fountain at the 1.7 × 10(-16) accuracy. The fiber link never limits the experiment and is robust enough to sustain radio astronomical campaigns. This experiment opens the possibility of replacing the local hydrogen masers at the VLBI sites with optically-synthesized RF signals. This could improve VLBI resolution by providing more accurate and stable frequency references and, in perspective, by enabling common- clock VLBI based on a network of telescopes connected by fiber links.

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

To significantly improve the frequency references used in radio-astronomy and precision measurements in atomic physics, we provide frequency dissemination through a 642 km coherent optical fiber link, that will be also part of a forthcoming European network of optical links. We obtained a resolution of 3e-19 at 1000 s on the frequency transfer, and an accuracy of 5e-19. The ultimate link performance has been evaluated by doubling the link to 1284 km, demonstrating a new characterization technique based on the double round-trip on a single fiber. The arming of a second fiber is avoided: this is beneficial to long hauls realizations in view of a continental fiber network for frequency and time metrology. The data analysis is based on the Allan deviation; its expression is theoretically derived for the observed noise power spectrum, which is seldom found in the literature.

We describe the application of Raman Optical-fiber Amplification (ROA) for the phase coherent transfer of optical frequencies in an optical fiber link. ROA uses the transmission fiber itself as a gain medium for bi-directional coherent amplification. In a test setup we evaluated the ROA in terms of on-off gain, signal-to-noise ratio, and phase noise added to the carrier. We transferred a laser frequency in a 200 km optical fiber link with an additional 16 dB fixed attenuator (equivalent to 275 km of fiber on a single span), and evaluated both co-propagating and counter-propagating amplification pump schemes, demonstrating nonlinear effects limiting the co-propagating pump configuration. The frequency at the remote end has a fractional frequency instability of 3e-19 over 1000 s with the optical fiber link noise compensation.

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.

The Unit of Time in the International Systems of Units is realized at the relative frequency accuracy level of few parts in 10-16 by laser cooled Cesium fountains [1]; since 2003 INRIM has operated a laser cooled Cesium fountain with relative frequency accuracy at the level of 5×10-16 and has contributed with this frequency standard to the generation of the International Atomic Timescale of Bureau International des Poids et des Mesures (BIPM). Since 2009 a cryogenic Cesium fountain is under operation and its metrological characterization is in progress, with a target accuracy of 1×10{-16}.

An experiment aimed at the determination of the constant G was started in 1995 at the Politecnico di Torino, Torino, Italy, based on measurement of the period of a simple pendulum. Two source masses are periodically moved, orthogonally to the swinging plane, between two fixed positions. An optical system detects the pendulum perigee, and an electronic circuit measures the period. This paper describes recent progress and results, in which an unexpected effect, which is currently under investigation, limits the desired 10-4 uncertainty level to 3.2%

This paper presents a fiber optic gyroscope (FOG) on a 47 km single mode commercial fiber located in the urban area around the city of Turin, enclosing an area of 20 km2 with a resolution of about 10-8 (rad/s)/√Hz, without using complex instrumentation. The fiber is used for the Internet data traffic and is implemented on a dense wavelength division multiplexing (DWDM) architecture, in which the channel 44 was dedicated to the experiment. The setup is the same as for standard FOGs: laser radiation is split into two beams, that are injected in the telecom loop through optical add d the two beams travel over the loop in opposite directions, accumulating a phase difference due to Earth rotation, as predicted by the Sagnac effect. After a round trip, the two beams are extracted and recombined to detect the Sagnac phase. A well known technique of nonreciprocal phase modulation was used to this purpose. A closed loop system in which the nonreciprocal phase is compensated inserting a small frequency offset between the two beams is developed. This experiment shows the feasibility of a network of these type of sensors for seismic detection, placed side by side with traditional instrumentation, supported by the increasing activity on fiber networks for frequency metrology.