A High-frequency Geodetic VLBI Experiment for Optical Clock Comparison

TL;DR

This study demonstrates that high-frequency geodetic VLBI can effectively compare optical and microwave clocks across continents, offering a promising alternative to satellite links for precise timekeeping.

Contribution

It introduces a novel method using geodetic VLBI for intercontinental clock comparison, achieving high accuracy comparable to satellite-based methods.

Findings

VLBI-based clock comparison agrees with satellite and optical methods at 10^{-15} s/s.

Standard high-frequency VLBI can be a viable alternative for intercontinental clock comparisons.

Planned upgrades will further improve the accuracy and reliability of VLBI-based clock comparisons.

Abstract

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. 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 (FPT) 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.