Optical clocks have achieved remarkable estimated fractional frequency uncertainties reaching the 10^-18 level and below, enabling applications in fundamental physics, general relativity, and geodesy. However, the challenge of verifying the international consistency of optical clocks remains critical as efforts intensify toward redefining the SI second based on an optical transition or transitions. We report on a two-month international clock comparison campaign involving seven optical clocks in four national metrology institutes (INRIM, LNE-OP, NPL, and PTB) connected via the optical fiber network established in Europe. The campaign resulted in optical frequency ratios with uncertainties ranging from 7.7×10^-18 to 6.1×10^-17. Among the results, the ^171Yb^+(E3) clocks at NPL and PTB demonstrated agreement within an uncertainty of 7.7×10^-18, marking the first international verification of two independently developed optical clocks below one part in 10^17. The operation of the ^199Hg clock at LNE-OP (formerly LNE-SYRTE) resulted in frequency ratios with improved uncertainties with ^171Yb^+(E3), ^171Yb, and ^87Sr optical clocks. These results provide input for the redefinition of the second and underscore how fiber-linked clock networks can advance metrology and scientific applications.

Optical clocks provide ultra-precise frequency references that are vital for international metrology as well as for tests of fundamental physics. To investigate the level of agreement between different clocks, we simultaneously measured the frequency ratios between ten optical clocks in six different countries, using fiber and satellite links. This is the largest coordinated comparison to date, from which we present a subset of 38 optical frequency ratios and an evaluation of the correlations between them. Four ratios were measured directly for the first time, while others had significantly lower uncertainties than previously achieved, supporting the advance towards a redefinition of the second and the use of optical standards for international time scales.

Enormous progress has been made by time and frequency laboratories around the globe on a new generation of optical frequency standards based on narrow optical transitions in laser-cooled trapped atoms or ions. Some of these optical frequency standards, including those at NPL, have significantly surpassed the performance of the best caesium fountain primary frequency standards in both systematic frequency uncertainty and frequency instability. This situation has led to the international community developing a roadmap towards a future redefinition of the second [1], which gives a set of mandatory criteria to be met before redefinition, as well as some auxiliary conditions related to work that should be well advanced by that time. NPL has contributed to worldwide progress towards meeting the various mandatory criteria in the roadmap. Our optical frequency standard based on the electric octupole transition in an ion of ytterbium-171 recently reached an evaluated systematic uncertainty of 2.2E-18, very close to the target in the roadmap [2]. The uncertainty published for our strontium optical lattice clock in 2020 was 1E-17 [3], but this has since been improved to within a factor of two of the target through a series of technical improvements to reduce leading sources of systematic uncertainty. The absolute frequency of both clocks has been measured against caesium primary frequency standards. Our optical clocks have participated in various international comparisons, using satellite links, optical fibre links and transportable optical clocks. Notable recent examples include a large scale comparison of ten optical clocks in six countries, performed as part of the Robust Optical Clocks for International Timescales (ROCIT) project [4], and the first intercontinental comparison using transportable optical clocks as part of the International Clock Oscillator Networking (ICON) project [5]. We have also performed local optical frequency ratio measurements between the two clocks. Our strontium optical lattice clock was approved for steering of International Atomic Time (TAI) in March 2023 and has so far contributed data in seven months since then. A significant body of data has been gathered from our ytterbium ion optical clock and we are close to submitting this system to the BIPM for consideration for inclusion in TAI. Locally to NPL, we have also demonstrated a prototype real-time optically steered UTCx(k) time scale using our two optical clocks, and compared this against a similar prototype time scale generated simultaneously at LNE-SYRTE [6]. Looking further ahead, we are developing an ytterbium optical lattice clock targeted at routine contributions to TAI, and investigating the possibility of intercontinental comparisons via optical fibre by adapting techniques originally conceived for seismic detection [7]. [1] N. Dimarcq et al., Metrologia 61, 012001 (2024) [2] A. Tofful et al., Metrologia 61, 045001 (2024) [3] R. Hobson et al., Metrologia 57, 065026 (2020) [4] H. S. Margolis et al., Journal of Physics: Conference Series 2889, 012022 (2024) [5] International Clock Oscillator Networking (ICON) collaboration, arXiv:2410.22973 (2024) [6] M. Abgrall et al., Journal of Physics: Conference Series 2889, 012024 (2024) [7] G. Marra et al., Science 376, 874 (2022)

We present the frequency steering of two continuously-running hydrogen masers using NPL's state-of-the-art optical atomic clocks. The resulting optically steered time scales are aligned with UTC(NPL) and UTC at the nanosecond and sub-nanosecond level, respectively.

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

A full evaluation of the uncertainty budget for the ytterbium ion optical clock at the National Physical Laboratory (NPL) was performed on the electric octupole (E3) $^2\mathrm{S}_{1/2}\,\rightarrow\, ^2\mathrm{F}_{7/2}$ transition. The total systematic frequency shift was measured with a fractional standard systematic uncertainty of $2.2\times 10^{-18}$. Furthermore, the absolute frequency of the E3 transition of the $^{171}$Yb$^+$ ion was measured between 2019 and 2023 via a link to International Atomic Time (TAI) and against the local caesium fountain NPL-CsF2. The absolute frequencies were measured with fractional standard uncertainties between $3.7 \times 10^{-16}$ and $1.1 \times 10^{-15}$, and all were in agreement with the 2021 BIPM recommended frequency.

Source identification and accurate measurements of methane gas concentration are key tools necessary for climate change management. Here we present a method for methane isotopologue ratio measurement using a laser-based frequency modulation (FM) spectroscopy technique in the near infrared at 1661 nm. We provide line shape analysis and discuss a fitting algorithm for accurate isotopologue ratio metrology and investigate and minimize the effects of residual amplitude modulation on the experimentally produced FM signal line shape. This FM technique is further evaluated for future development of a cavity-enhanced and noise-immune system capable of isotopologue ratio as well as ultrasensitive trace gas measurements, all accessible using a single distributed feedback laser.

We present a new framework to study the time variation of fundamental constants in a model-independent way. Model independence implies more free parameters than assumed in previous studies. Using data from atomic clocks based on 87 Sr, 171 Yb + and 133 Cs, we set bounds on parameters controlling the variation of the fine-structure constant, α , and the electron-to-proton mass ratio, µ . We consider variations on timescales ranging from a minute to almost a day. In addition, we use our results to derive some of the tightest limits to date on the parameter space of models of ultralight dark matter and axion-like particles.

The detection of variations of fundamental constants of the Standard Model would provide us with compelling evidence of new physics, and could lift the veil on the nature of dark matter and dark energy. In this work, we discuss how a network of atomic and molecular clocks can be used to look for such variations with unprecedented sensitivity over a wide range of time scales. This is precisely the goal of the recently launched QSNET project: A network of clocks for measuring the stability of fundamental constants. QSNET will include state-of-the-art atomic clocks, but will also develop next-generation molecular and highly charged ion clocks with enhanced sensitivity to variations of fundamental constants. We describe the technological and scientific aims of QSNET and evaluate its expected performance. We show that in the range of parameters probed by QSNET, either we will discover new physics, or we will impose new constraints on violations of fundamental symmetries and a range of theories beyond the Standard Model, including dark matter and dark energy models.

We report on longevity studies of fully sealed gas-filled Kagome hollow-core fiber cells for applications in compact laser frequency stabilization systems, and investi-gate and address baseline offsets due to electronic and optical components.

We report the development of a portable NICE-OHMS device with the aim of ultra-sensitive ( < 1 nmol/mol), real-time ( < 1 min) detection of HCl and monitoring of water vapor for applications in microelectronic fabrication cleanrooms.

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.

Real-time measurement of trace gases using NICE-OHMS shows great promise in delivering the required sensitivity to meet the needs of many sectors. We present progress on NICE-OHMS-based gas sensing devices for industrial and other applications.

Microwave generation using optical frequency comb technology hits new milestones

An atom exposed to an electric field will experience Stark shifts of its internal energy levels, proportional to their polarizabilities. In optical frequency metrology, the Stark shift due to background black-body radiation (BBR) modifies the frequency of the optical clock transition, and often represents a large contribution to a clock's uncertainty budget. For clocks based on singly-ionized ytterbium, the ion's complex structure makes this shift difficult to calculate theoretically. We present a measurement of the differential polarizabilities of two ultra-narrow optical clock transitions present in ^171Yb^+, performed by exposing the ion to an oscillating electric field at a wavelength in the region of room temperature BBR spectra. By measuring the frequency shift to the transitions caused by a laser at λ=7.17 μ m, we obtain values for scalar and tensor differential polarizabilities with uncertainties at the percent level for both the electric quadrupole and octupole transitions at 436nm and 467nm respectively. These values agree with previously reported experimental measurements and, in the case of the electric quadrupole transition, allow a 5-fold improvement in the determination of the room-temperature BBR shift. However, we note significant concerns over the validity of the uncertainty charactarization presented and draw the reader's attention to the Note on applicability section for a discussion.

We describe a compact, all fiber, frequency stabilized diode laser system at 2051 nm using CO2 gas-filled Kagome Hollow Core Fiber (HCF), capable of tuning continuously over four transitions in 12C16O2: R(24), R(26), R(28), and R(30). This laser system has been designed for use in future space-based atmospheric monitoring using differential absorption lidar (DIAL). The fully spliced Kagome HCF gas cell is filled to 2 kPa CO2 partial pressure and we compare the observed CO2 lineshape features with those calculated using HITRAN, to quantify the properties of the CO2-filled fiber cell. In this first demonstration of Kagome HCF used in a fully sealed gas cell configuration for spectroscopy at 2 µm, we characterize the frequency stability of the locked system by beat frequency comparison against a reference laser. Results are presented for the laser locked to the center of the 12C16O2 R(30) transition, with frequency stability of ∼40 kHz or better at 1 s, and a frequency reproducibility at the 0.4-MHz level over a period of > 1 month. For DIAL applications, we also demonstrate two methods of stabilizing the laser frequency ~3 GHz from this line. Furthermore, no pressure degradation was observed during the ~15-month period in which frequency stability measurements were acquired.

There is increasing industrial demand, particularly within the clean room community, for rapid online measurement of a number of airborne molecular contaminants. These measurements are usually made with commercially available cavity ring-down spectrometers, but more recently developed techniques offer potentially better sensitivity and more rapid sampling. In this paper we present ammonia spectroscopy data from a newly developed noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) system at 1532 nm, which is designed to be transportable and operate in an industrial environment. We demonstrate this using an optical cavity of 9 kHz fringe width (similar to 1.5 GHz free spectral range and finesse of 169,000) and a distributed-feedback diode laser with a relatively broad similar to 1 MHz free-running linewidth. The variation in NICE-OHMS ammonia signal amplitude is presented at different concentrations in one atmosphere of nitrogen, generated from traceable reference standards, with a flow rate of 1-2 l/min. We derive a calibration curve for our device for concentrations in the range from 100 nmol/mol to 10 mu mol/mol. (C) 2017 Optical Society of America