Matias Risaro

We report the development of the first optical frequency comb (OFC) based on an in-house Ti:Sa femtosecond laser in Argentina, implemented at the National Metrology Institute (INTI). The system operates at a 1 GHz repetition rate with a broadband spectrum spanning from 500 to 1100 nm. The carrier-envelope offset frequency and the repetition rate are both phase-locked to a cesium atomic clock, establishing a direct link between the national frequency standard and the optical domain. This development represents a major step forward for national metrology and enables frequency-instability measurements of stable lasers within the country. To demonstrate its capabilities, the performance of a Nd:YAG Mephisto laser at 1064 nm was evaluated by measuring the beat note between its second harmonic and the OFC, allowing a direct assessment of its frequency noise and stability.

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.

An intercontinental metrological clock comparison between Italy and the Republic of Korea was performed by means of geodetic K -band VLBI observations. The comparison involved the hydrogen masers (H-masers) used at Medicina and Sejong radio telescopes. The same clocks were simultaneously compared by a satellite link and by high-precision optical clocks maintained at the National Metrology Institutes, KRISS in Korea and INRIM in Italy, and delivered to VLBI antennas via optical fiber. The H-masers frequency difference was estimated by extrapolating the clock rate from VLBI data using two geodetic VLBI software. This was subsequently compared with clock differences derived by satellite link and by local optical clocks. Results obtained with different approaches were in agreement at the level of 10 ⁻¹⁵ s s ⁻¹ . This pilot study demonstrates that standard high-frequency ( K -band) geodetic VLBI campaigns could be a viable approach to conduct intercontinental clock comparisons, now only possible via satellite links. This uncertainty can be reduced thanks to the planned installation of new-generation, broadband, high-frequency receivers on the involved telescopes. K / Q / W -band geodetic observations will allow an improvement of the accuracy of the resulting group delays through broad bandwidth synthesis from 20 to 100 GHz. Furthermore, the Frequency Phase Transfer method will also be explored together with the use of PCAL systems installed at the radio telescopes to improve phase stability and thus allow a better estimation of the station clock parameters.

An atomic time scale is a method for marking events and the passage of time by using atomic frequency standards. Thanks to the superior performance of atomic clocks based on optical transitions, time scales generated with optical clocks have the potential to be more accurate and stable than those based on microwave clocks. In this work, we demonstrate an experimental optical time scale based on the INRiM Yb optical lattice clock and a hydrogen maser as a flywheel oscillator, showing sub-nanosecond accuracy over months-long periods and nanosecond accuracy over a 1-year period. The obtained results show that optical time scales have competitive performances even when the optical clock has a limited and non-uniformly distributed up-time. Consequently, we are working to include the Yb clock within the ensemble of clocks routinely used for the generation of the Italian time scale. Furthermore, these results represent a crucial step towards the future redefinition of the second of the International System of Units based on an optical transition.

We present the new absolute frequency measurement of ytterbium (¹⁷¹Yb) obtained at INRiM with the optical lattice clock IT-Yb1 against the cryogenic caesium (¹³³Cs) fountain IT-CsF2, evaluated through a measurement campaign that lasted 14 months. Measurements are performed by either using a hydrogen maser as a transfer oscillator or by synthesizing a low-noise microwave for Cs interrogation using an optical frequency comb. The frequency of the ¹⁷¹Yb unperturbed clock transition 1 S 0 → 3 P₀ results to be 518 295 836 590 863.44(14) Hz, with a total fractional uncertainty of 2.7 × 10 − 16 that is limited by the uncertainty of IT-CsF2. Our measurement is in agreement with the Yb frequency recommended by the Consultative Committee for Time and Frequency. This result confirms the reliability of Yb as a secondary representation of the second and is relevant to the process of redefining the second in the International System of Units on an optical transition.

We perform low phase noise, efficient serrodyne modulation for optical frequency control and spectral purity transfer between two ultrastable lasers. After characterizing serrodyne modulation efficiency and its bandwidth, we estimate the phase noise induced by the modulation setup by developing a novel, to the best of our knowledge, composite self-heterodyne interferometer. Exploiting serrodyne modulation, we phase locked a 698 nm ultrastable laser to a superior ultrastable laser source at 1156 nm by means of a frequency comb as a transfer oscillator. We show that this technique is a reliable tool for ultrastable optical frequency standards.