The details of fiber Brillouin amplifier modules developed for optical frequency dissemination and installed on the fiber link between Braunschweig and Strasbourg for performing optical clock comparisons are described and documented. The performance of these fiber Brillouin amplification modules is characterized in terms of gain, one-way phase shift, and signal-to-noise ratio across a wide parameter space of signal and pump powers. Finally, upper estimates are derived for the frequency transfer uncertainty contributions resulting from the non-reciprocity induced by the fiber Brillouin amplifier modules based on the characterized one-way phase shift.

We have measured the geopotential difference between two locations separated by 457 km by comparison of two optical lattice clocks via an interferometric fiber link, utilizing the gravitational redshift of the clock transition frequency. The 87Sr clocks have been compared side-by-side before and after one of the clocks was moved to the remote location. The chronometrically measured geopotential difference of 3918.1(2.6)m2s−2 agrees with an independent geodetic determination of 3915.88(0.30)m2s−2. The uncertainty of the chronometric geopotential difference is equivalent to an uncertainty of 27cm in height. Published by the American Physical Society 2024

For improving the performance of optical frequency dissemination and the resolution of its out-of-loop (OOL) characterization, we investigate a compact free-space interferometer design in which a monolithic assembly forms the reference arm. Two interferometer designs are realized, and their environmental sensitivity is analyzed based on the properties of the materials involved. We elucidate that in these designs the temperature sensitivities of the out-of-loop signal paths are greater than for the reference arm. As the estimated temperature-variation-induced frequency transfer errors are observed to be the relevant limitation, the out-of-loop characterization signal can be regarded as a trustworthy upper limit of the frequency transfer error to a remote place. We demonstrate a fractional frequency transfer uncertainty and OOL characterization resolution of ≤2.7×10−21 over many measurement runs. With a value of (0.23±1.07)×10−22 the weighted mean offset is significantly below the best reported results so far.

Ultrastable lasers are essential tools in optical frequency metrology enabling unprecedented measurement precision that impacts on fields such as atomic timekeeping, tests of fundamental physics, and geodesy. To characterise an ultrastable laser it needs to be compared with a laser of similar performance, but a suitable system may not be available locally. Here, we report a comparison of two geographically separated lasers, over the longest ever reported metrological optical fibre link network, measuring 2220 km in length, at a state-of-the-art fractional-frequency instability of 7 × 10 −17 for averaging times between 30 s and 200 s. The measurements also allow the short-term instability of the complete optical fibre link network to be directly observed without using a loop-back fibre. Based on the characterisation of the noise in the lasers and optical fibre link network over different timescales, we investigate the potential for disseminating ultrastable light to improve the performance of remote optical clocks.

We investigate the validation of fiber-based optical frequency transfer for frequency comparison applications. We specifically consider the frequency transfer validation for remote optical clock comparisons and want to ensure interferometric fiber link uncertainty contributions below the combined uncertainty of the clocks under test. The validation is based on signals obtained via looping back from the remote end to the sender site and comparing the input with the output of the loop. These loop-back data need to be averaged over intervals for reaching the validation goal, as the short-term instability of long-distance interferometric fiber links is typically higher than that of optical clocks. We introduce a two-step validation approach and address the finding of a compromise between opposing aspects of averaging: reaching low uncertainties versus achieving a high data coverage of the validated data set via a high temporal resolution of the fault identification. We discuss the impact of different averaging types and of the tolerance of filtering criteria on the achievable estimated uncertainty and on the coverage of the validated data set. Data from four multiple-week-long measurement campaigns on the fiber link between Physikalisch-Technische Bundesansanstalt and University of Strasbourg are used for this assessment.

Here we share the relevant data of the manuscript “Comparing ultrastable lasers at 7×10-17 fractional frequency instability through a 2,220 km optical fibre network”. Raw data was acquired using multiple synchronised, dead-time free frequency counters in Lambda-mode [1]. The integration time for each data point was 1 s. The data provided here have been processed to reflect the fractional frequency difference between the ultrastable lasers at NPL and PTB, scaled to 1542 nm. Specifically, \(y=(\nu_{\text{NPL(ULE)}}\frac{777327}{1126090}-\frac{767233}{767235}\nu_{\text{PTB(Si)}})/194.4 \ \text{THz}\) where \(y\) is the value recorded in the data files, \(\nu_{\text{NPL(ULE)}}\) and \(\nu_{\text{PTB(Si)}}\) are the optical frequencies of the ultrastable lasers at NPL (referenced to a ULE cavity) and PTB (referenced to Si cavity), respectively. The numerators and the denominators of the scaling factors correspond to mode numbers of the optical frequency comb at NPL and PTB, respectively. The expression for \(y\) corresponds to the fractional transfer beat [2] between the NPL and PTB ultrastable lasers. The file “833000_s_874000_s_data_for_fig_2.txt” contains the timeseries data used to compute the modified Allan deviation reported in Fig. 2a. The “0” values correspond to invalid data due to glitches in the operation of the optical fibre link. A linear drift of 40 mHz s-1 has been removed in these data. The file “432000_s_912077_s_data_for_fig_3.txt” contains the timeseries data used in Fig. 3. The “0” values correspond to invalid data due to glitches in the operation of the optical fibre link. These data have additionally been high pass filtered with a cut off frequency of 1 mHz to decouple the short-term instability of the optical fibre link from the drift of the ultrastable lasers (with a characteristic time >1000 s), as described in the manuscript. The files “222000_s_232000_s_data_for_supp_fig_1.txt”, “270000_s_288000_s_data_for_supp_fig_1.txt”, “754000_s_765000_s_data_for_supp_fig_1.txt”, “832000_s_890000_s_data_for_supp_fig_1.txt”, contain the timeseries data used to compute the modified Allan deviation reported in Supplementary Fig. 1. The “0” values correspond to invalid data due to glitches in the operation of the optical fibre link. A linear drift of 40 mHz s-1 has been removed in these data. The temporal starting point is displayed in seconds in the title of the files relative to 00:00 UTC of 2019/07/06. References [1] Dawkins, S. T., McFerran, J. J. & Luiten, A. N. Considerations on the Measurement of the Stability of Oscillators with Frequency Counters. IEEE Transactions on ultrasonics, ferroelectrics, and frequency control 54, 918-925 (2007). [2] Telle, H.R., Lipphardt, B. & Stenger, J. Kerr-lens, mode-locked lasers as transfer oscillators for optical frequency measurements. Appl. Phys. B 74, 1-6 (2002).

We search for transient variations of the fine structure constant using data from a European network of fiber-linked optical atomic clocks. By searching for coherent variations in the recorded clock frequency comparisons across the network, we significantly improve the constraints on transient variations of the fine structure constant. For example, we constrain the variation in alpha to <5*10^-17 for transients of duration 10^3 s. This analysis also presents a possibility to search for dark matter, the mysterious substance hypothesised to explain galaxy dynamics and other astrophysical phenomena that is thought to dominate the matter density of the universe. At the current sensitivity level, we find no evidence for dark matter in the form of topological defects (or, more generally, any macroscopic objects), and we thus place constraints on certain potential couplings between the dark matter and standard model particles, substantially improving upon the existing constraints, particularly for large (>~10^4 km) objects.

We have extended the existing interferometric fibre link PTB-Strasbourg with a branch connecting the node at Karlsruhe Institute of Technology (KIT) to the Max Planck Institute of Quantum Optics (MPQ) in Garching/Munich. This fibre connection enables chronometric levelling experiments between PTB and MPQ, separated by a line-of-sight distance of about 460 km. Here, we study the performance of the PTB-MPQ fibre link which mainly employs stimulated Brillouin scattering as means of amplification. We consider two configurations: a single-span loop configuration and an antiparallel configuration with frequency transfer over the two concatenated, individually stabilized forward and backward links.

We investigate optical frequency dissemination over a 1400 km long fiber link in looped configuration over a pair of underground fibers between Braunschweig and Strasbourg. This fiber link is the first to combine fiber Brillouin amplifiers with a repeater laser station. Phase-coherent operation over more than five days is demonstrated. We analyze the repeatability of the performance over four campaigns and present results of 65 d in total. The weighted mean of the fractional frequency offset of the transferred optical frequency over the complete data set is (-1.1 +/- 0.4) x 10(-20). By analyzing the stabilization signals of the two individual fibers, the correlation of the phase noise on the two fibers is shown to be > 98%.

Chronometrie levelling [1-3] is a technique of deriving gravitational potential differences from clock frequency comparisons allowing the determination of height differences. Interferometric optical fibre links (IFL) [4] enable chronometric levelling between distant labs housing optical clocks [5]. An IFL is a phase stabilized telecom fibre link connection establishing a fixed optical phase relationship between two locations up to ~1000 km apart [6]. To perform a chronometric levelling campaign between the stationary optical clocks of the Physikalisch-Technische Bundesanstalt (PTB) and PTB's transportable Strontium clock [7] placed at the Max Planck Institute of Quantum Optics (MPQ) we have established a new 940 km long IFL between the two locations. At both link ends the frequencies of the clock lasers are compared to the 1,5 pm link transfer laser via fs-combs. To enable chronometric levelling experiments at the level of 10 cm height resolution, we aim a fractional frequency uncertainty of the frequency transfer of <; 1×10 -18 for averaging times of ~40 ks. To monitor the frequency transfer over the complete 940 km long fibre link the signal is sent back from the receiving end to the sender on a second fibre, which allows an out-of-loop characterization using data of the 1880 km loop. We study two different link setups, cf. Fig. 1a: i) a single-span loop configuration with the option to tap off the signal at MPQ with a passive frequency extraction unit [8]. This setup requires monitoring at the sending end only and allows us to investigate an ultralong IFL, even longer than the previous record [9]. And ii) an antiparallel configuration of two concatenated, individually stabilized interferometric fibre links. This setup offers lower instabilities due to a decreased delay-limit [10] but requires setting up and maintaining a stabilization system at both link ends. To compensate the attenuation of the fibres we used fibre Brillouin amplifier modules (FBAM) developed inhouse providing a typical gain of > 40 dB.

Methods for long-distance time and frequency transfer over optical fibres have demonstrated excellent performances and are evolving rapidly. CLONETS is a new European Union-funded coordination and support action intended to accelerate the transfer of these technologies to industry and to strengthen the coordination between research infrastructures and research and education network providers, in order to prepare the deployment of this technology for a sustainable, pan-European fibre network, providing high-performance clock services to European research infrastructures and supporting wider services to industry and society.

Dynamic development of long-distance methods of time and frequency signals transmission over optical fibers is giving the opportunity to create optical network with dedicated clock services. In order to prepare the deployment of this technology for a sustainable, pan-European fibre network, providing high-performance clock services to European research infrastructures and supporting wider services to industry and society, the CLONETS project, funded by the European Commission in H2020 program, is a coordination and support action intended to accelerate the transfer of these technologies to industry and to strengthen the coordination between research infrastructures and research and education network providers.. The optical network will cover a European area, and will be providing highest available performance for research infrastructures, commercial entities and other organisations.

CLONETS is a new, European Union funded project which aims to prepare the transition toward a permanent, pan-European, optical fiber-based network providing time and frequency comparisons and distribution at the highest performance levels for research infrastructures, as well as support to a wide range of services for industry and society. The project started in January 2017 and is scheduled for 30 months. The project consortium is formed by 19 organizations from 7 European countries. This paper provides information about the project in progress and briefly describes its results achieved during the first year. INTRODUCTION The project CLONETS (Clock Network Services) is motivated by recent progress in time and frequency (T&F) metrology and novel applications in fundamental physics, geodesy, telecommunication, industry and society that require time and frequency reference signals with significantly higher performance than is currently mediated by satellite techniques. Very high performance time and frequency reference signals are also moving from radio signal broadcasting to transport over optical fiber networks as the development of relevant technologies for T&F transfer on optical links progresses rapidly. However, widespread utilization of such signals is currently hampered by a lack of sustainable and reliable infrastructure. Nevertheless optical fiber links for T&F are being actively developed and operated by several countries in Europe, including some cross-border links. CLONETS aims to prepare the conditions in which these links may be combined and completed to form a pan-European network, with a sustainable organisation allowing it to operate as a long term service to research infrastructures, industry and society. CLONETS receives funding from the European Union's Horizon 2020 research and innovation programme (2014-2020). The project brings together a diversified group of actors: National Metrology Institutes (NMIs), academic research groups, National Research and Education Network providers (NRENs), an internet exchange and small and medium-sized high-technology companies, who concentrate a wide range of expertise and activities in this area. The proposed network is intended to be open to the participation of all relevant entities in Europe.

We review experimental progress on optical atomic clocks and frequency transfer, and consider the prospects of using these technologies for geodetic measurements. Today, optical atomic frequency standards have reached relative frequency inaccuracies below 10(-17), opening new fields of fundamental and applied research. The dependence of atomic frequencies on the gravitational potential makes atomic clocks ideal candidates for the search for deviations in the predictions of Einstein's general relativity, tests of modern unifying theories and the development of new gravity field sensors. In this review, we introduce the concepts of optical atomic clocks and present the status of international clock development and comparison. Besides further improvement in stability and accuracy of today's best clocks, a large effort is put into increasing the reliability and technological readiness for applications outside of specialized laboratories with compact, portable devices. With relative frequency uncertainties of 10(-18), comparisons of optical frequency standards are foreseen to contribute together with satellite and terrestrial data to the precise determination of fundamental height reference systems in geodesy with a resolution at the cm-level. The long-term stability of atomic standards will deliver excellent long-term height references for geodetic measurements and for the modelling and understanding of our Earth.

Time and frequency transfer based on optical fiber links techniques have demonstrated excellent performances. CLONETS is EU project intended to accelerate transfer of related technologies, in order to prepare deployment of technology for sustainable network providing high-performance clock services.

Phase compensated optical fiber links enable high accuracy atomic clocks separated by thousands of kilometers to be compared with unprecedented statistical resolution. By searching for a daily variation of the frequency difference between four strontium optical lattice clocks in different locations throughout Europe connected by such links, we improve upon previous tests of time dilation predicted by special relativity. We obtain a constraint on the Robertson-Mansouri-Sexl parameter |α|≲1.1×10^{-8}, quantifying a violation of time dilation, thus improving by a factor of around 2 the best known constraint obtained with Ives-Stilwell type experiments, and by 2 orders of magnitude the best constraint obtained by comparing atomic clocks. This work is the first of a new generation of tests of fundamental physics using optical clocks and fiber links. As clocks improve, and as fiber links are routinely operated, we expect that the tests initiated in this Letter will improve by orders of magnitude in the near future.

We report on the first comparison of distant caesium fountain primary frequency standards (PFSs) via an optical fiber link. The 1415 km long optical link connects two PFSs at LNE-SYRTE (Laboratoire National de metrologie et d'Essais-SYstsme de References TempsEspace) in Paris (France) with two at PTB (Physikalisch-Technische Bundesanstalt) in Braunschweig (Germany). For a long time, these PFSs have been major contributors to accuracy of the International Atomic Time (TAI), with stated accuracies of around 3 x 10(-16). They have also been the references for a number of absolute measurements of clock transition frequencies in various optical frequency standards in view of a future redefinition of the second. The phase coherent optical frequency transfer via a stabilized telecom fiber link enables far better resolution than any other means of frequency transfer based on satellite links. The agreement for each pair of distant fountains compared is well within the combined uncertainty of a few 10(-16) for all the comparisons, which fully supports the stated PFSs' uncertainties. The comparison also includes a rubidium fountain frequency standard participating in the steering of TAI and enables a new absolute determination of the Rb-87 ground state hyperfine transition frequency with an uncertainty of 3.1 x 10(-16.)This paper is dedicated to the memory of Andre Clairon, who passed away on 24 December 2015, for his pioneering and long-lasting efforts in atomic fountains. He also pioneered optical links from as early as 1997.

In the past few years, optical atomic clocks have made spectacular progress. They have become 100 times more precise than the best cesium clocks. So far, their precision has been available only locally, since frequency transfer via satellite cannot provide sufficient resolution. This has now changed thanks to a novel 1,400 km optical fiber link between Braunschweig and Paris. This link allows frequencies to “travel” and optical atomic clocks to be compared across national borders. In the first comparison between PTB’s optical strontium clocks and those of the French LNE-SYRTE, an unrivaled fractional uncertainty of 5 ⋅ 10−17 was achieved. Especially interesting for • geodesy • fundamental research in physics

Leveraging the unrivalled performance of optical clocks as key tools for geo-science, for astronomy and for fundamental physics beyond the standard model requires comparing the frequency of distant optical clocks faithfully. Here, we report on the comparison and agreement of two strontium optical clocks at an uncertainty of 5 × 10 −17 via a newly established phase-coherent frequency link connecting Paris and Braunschweig using 1,415 km of telecom fibre. The remote comparison is limited only by the instability and uncertainty of the strontium lattice clocks themselves, with negligible contributions from the optical frequency transfer. A fractional precision of 3 × 10 −17 is reached after only 1,000 s averaging time, which is already 10 times better and more than four orders of magnitude faster than any previous long-distance clock comparison. The capability of performing high resolution international clock comparisons paves the way for a redefinition of the unit of time and an all-optical dissemination of the SI-second.