Wei Huang

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 Yb + 171 ( E 3 ) 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 Hg 199 clock at LNE-OP (formerly LNE-SYRTE) resulted in frequency ratios with improved uncertainties with Yb + 171 ( E 3 ) , Yb 171 , and Sr 87 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.

Ultrastable lasers play a key role in optical frequency metrology, setting the measurement speed, and ultimately impacting both the stability and accuracy of optical frequency standards. Here, we demonstrate a laser fractional frequency instability at 4×10 ⁻¹⁷ and a laser frequency linewidth of 12 mHz full width at half-maximum, employing a 68 cm long optical reference cavity operating at room temperature. To the best of our knowledge, both the frequency instability and the linewidth are the lowest ever reported for a room temperature system. This work highlights that state-of-the-art frequency stability and spectral purity are achievable at room temperature, making them accessible to a broader range of users.

The H2020 DEMETRA project provides short latency clock monitoring services to the time users using the Atomium precise point positioning (PPP) software developed by the Royal Observatory of Belgium. In this paper, three recent updates of the current Atomium software are introduced: adding Galileo signals in the PPP computation; the option to constrain the receiver clock; PPP with integer ambiguity resolution. The advantages of these updates are demonstrated: Combining the Galileo and global positioning system (GPS) signals for PPP time transfer will further improve the frequency stability inside the computation batch; PPP with receiver clock constraint is not only used to reduce the short-term noise of the clock measurements but can also be used for some specific applications to a keep continuous clock solution in the computation batch or retrieve correct clock measurements from extremely noisy environments; the integer PPP allows a continuous clock solution, and improves the mid-term and long-term stability of the frequency transfer compared to the current PPP frequency transfer techniques.

When calibrating Global Positioning System (GPS) stations dedicated to timing, the hardware delays of P 1 and P 2, the P ( Y )-codes on frequencies L 1 and L 2, are determined separately. In the international atomic time (TAI) network the GPS stations of the time laboratories are calibrated relatively against reference stations. This paper aims at determining the consistency between the P 1 and P 2 hardware delays (called d P 1 and d P 2) of these reference stations, and to look at the stability of the inter-signal hardware delays d P 1–d P 2 of all the stations in the network. The method consists of determining the d P 1–d P 2 directly from the GPS pseudorange measurements corrected for the frequency-dependent antenna phase center and the frequency-dependent ionosphere corrections, and then to compare these computed d P 1–d P 2 to the calibrated values. Our results show that the differences between the computed and calibrated d P 1–d P 2 are well inside the expected combined uncertainty of the two quantities. Furthermore, the consistency between the calibrated time transfer solution obtained from either single-frequency P 1 or dual-frequency P 3 for reference laboratories is shown to be about 1.0 ns, well inside the 2.1 ns uB uncertainty of a time transfer link based on GPS P 3 or Precise Point Positioning. This demonstrates the good consistency between the P 1 and P 2 hardware delays of the reference stations used for calibration in the TAI network. The long-term stability of the inter-signal hardware delays is also analysed from the computed d P 1–d P 2. It is shown that only variations larger than 2 ns can be detected for a particular station, while variations of 200 ps can be detected when differentiating the results between two stations. Finally, we also show that in the differential calibration process as used in the TAI network, using the same antenna phase center or using different positions for L 1 and L 2 signals gives maximum differences of 200 ps on the hardware delays of the separate codes P 1 and P 2; however, the final impact on the P 3 combination is less than 10 ps.

The R2CGGTTS software tool developed at the Royal Observatory of Belgium (ROB) to provide clock solutions in the standard Common GNSS Generic Time Transfer Standard (CGGTTS) has been extended to BeiDou Navigation Satellite System (BDS). The BDS includes satellites in three different orbits: 1) Medium Earth Orbit (MEO); 2) Inclined Geosynchronous Satellite Orbit (IGSO); and 3) Geostationary Earth Orbit (GEO). This paper presents first results obtained with this upgraded software, and a comparison between common view (CV) time transfer solutions obtained with either BDS, or GPS or Galileo. These preliminary results indicate that the BeiDou MEO satellites give time transfer results with a higher noise than the GPS results. This additional noise is shown to be due to some elevation-dependent delay in the BDS code measurements. Some biases were furthermore pointed out between the CV results obtained with the different BeiDou MEO satellites when the receivers used in the two stations are of different make. These biases may reach some nanoseconds, and find most probably their origin in the receiver hardware or firmware. It is shown additionally that using the BeiDou IGSO satellites and the GEO satellites, although increasing the number of observations, especially in the Asia-Pacific region, introduces a significant time transfer noise in the CV results.