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A High-frequency Geodetic VLBI Experiment for Optical Clock Comparison

This paper demonstrates that standard high-frequency geodetic VLBI observations can viably perform intercontinental clock comparisons between Italy and South Korea with an accuracy of $10^{-15}$ s/s, matching satellite link and optical clock methods while offering a pathway for further improvement through broadband receivers and advanced phase transfer techniques.

Original authors: Monia Negusini, Myoung-Sun Heo, Cecilia Clivati, Shuangjing Xu, Roberto Ricci, Taehyun Jung, Buseung Cho, Matteo Stagni, Claudio Bortolotti, Giuseppe Maccaferri, Federico Perini, Mauro Roma, Do-Young

Published 2026-03-25

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CC BY 4.0

Original authors: Monia Negusini, Myoung-Sun Heo, Cecilia Clivati, Shuangjing Xu, Roberto Ricci, Taehyun Jung, Buseung Cho, Matteo Stagni, Claudio Bortolotti, Giuseppe Maccaferri, Federico Perini, Mauro Roma, Do-Young Byun, Do-Heung Je, Marco Pizzocaro, Davide Calonico, Elena Cantoni, Giancarlo Cerretto, Stefano Condio, Giovanni A. Costanzo, Simone Donadello, Irene Goti, Michele Gozzelino, Alberto Mura, Filippo Levi, Matias Risaro, Huidong Kim, Won-Kyu Lee, Chang Yong Park, Dai-Hyuk Yu, Young Kyu Lee, Joon Hyo Rhee, Chanjin Park, Minseong Lee, Hyo Ryoung Kim, Sung-Moon Yoo, Jungho Cho, Jongsoo Kim, Sang-Oh Yi, Ha Su Yoon, Pablo de Vicente, Javier González, Cristina García Miró

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have two incredibly precise clocks, one in Italy and one in South Korea. You want to know if they are ticking at exactly the same speed. Usually, to compare them, you'd have to send a signal via satellite (like a phone call across the ocean) or physically fly one clock to the other. But satellites aren't perfect, and flying clocks is expensive and slow.

This paper describes a clever experiment where scientists used radio telescopes to compare these clocks without moving them. They treated the telescopes like giant, ultra-sensitive ears listening to the universe to see if the "time" they heard matched up.

Here is the story of how they did it, broken down into simple concepts:

1. The Cast of Characters

The Clocks (Hydrogen Masers): These are the "clocks" at the radio telescopes in Medicina (Italy) and Sejong (Korea). They are very good, but not perfect.
The Gold Standard (Optical Clocks): Deep in the labs of the National Metrology Institutes, scientists have "super-clocks" made of lasers and atoms (Optical Clocks). These are the most accurate timekeepers on Earth.
The Messengers (Fiber Optics): To make sure the telescopes' clocks were close to the "Gold Standard," the scientists ran fiber optic cables (like super-fast internet cables) from the labs to the telescopes. This let the telescopes "listen" to the super-clocks.
The Universe's Lighthouse (Quasars): These are super-bright, distant galaxies that act like fixed lighthouses in space. They never move from our perspective.

2. The Experiment: Listening to the Universe

The scientists set up a "listening party" that lasted 24 hours. They pointed six radio telescopes (in Italy, Spain, and Korea) at the same lighthouses (quasars) in the sky.

The Goal: When a radio signal from a quasar hits the Italian telescope, it arrives a tiny fraction of a second before or after it hits the Korean telescope. This difference is called a time delay.
The Trick: By measuring this delay with extreme precision, the scientists could calculate exactly how fast the Italian clock was ticking compared to the Korean clock. It's like two people clapping to the beat of a song; if one is slightly off-beat, you can tell exactly how much by listening to the echo between them.

3. The "Three-Way" Check

To make sure their radio telescope method worked, they compared it against two other methods:

The Satellite Method: They used GPS satellites to compare the clocks (the old, standard way).
The Optical Clock Method: They compared the telescopes' clocks directly against the "Gold Standard" super-clocks in the labs via the fiber optic cables.

The Result: All three methods agreed! The radio telescope method was accurate to within one part in a quadrillion (that's a 1 with 15 zeros after it). This proves that radio telescopes can be used to compare time across continents just as well as satellites, but potentially even better in the future.

4. Why This Matters (The "So What?")

Redefining the Second: Scientists are thinking about changing how we define a "second." Instead of using the vibration of a cesium atom (the current standard), they might switch to these super-accurate optical clocks. To do this, they need to prove that these super-clocks agree with each other no matter where they are in the world.
The Satellite Problem: Satellites are good, but they have a "noise floor" that limits how precise they can be. Radio telescopes, if upgraded, could be even more precise.
The Future Upgrade: Right now, this experiment used "K-band" radio waves (like a standard radio station). The paper suggests that in the future, they will upgrade the telescopes to listen to much higher frequencies (like a high-pitched whistle). This is like upgrading from a standard-definition TV to 8K Ultra HD. It will cut through atmospheric "static" (like rain or wind) and make the time comparison even sharper.

The Analogy: The Orchestra

Think of the two radio telescopes as musicians in an orchestra, one in Italy and one in Korea. They are trying to play the same note (the time signal) at the exact same moment.

The Quasar is the conductor standing far away.
The Fiber Optics are the musicians checking their sheet music against the conductor's baton to make sure they aren't drifting.
The Satellite is a radio broadcast of the conductor, which sometimes has static.
The VLBI (Radio Telescope) Method is the musicians listening to the sound of the conductor's baton hitting the air across the ocean. By analyzing the tiny delay in the sound, they can tell if they are perfectly in sync.

Conclusion

This paper is a "pilot test." It showed that using standard radio telescopes to compare ultra-precise clocks across the ocean works. It's a proof of concept. Now, the scientists are planning to upgrade their equipment to "High-Definition" radio waves to make this method the new gold standard for global timekeeping.

In short: They used the stars to check if two clocks on opposite sides of the world were ticking in sync, and they found a new, highly accurate way to do it that doesn't rely on satellites.

1. Problem Statement

Intercontinental comparisons of atomic clocks are essential for fundamental metrology, particularly for the potential redefinition of the SI second and tests of fundamental physics. Currently, these comparisons rely primarily on Global Navigation Satellite Systems (GNSS), which often lack the accuracy required for next-generation optical clocks (uncertainty levels around $10^{-16}$ to $10^{-18}$ ). While fiber-optic links offer superior stability, they are limited to regional distances.

Geodetic Very Long Baseline Interferometry (VLBI) has historically been used for Earth orientation and geodesy but has not been fully exploited for high-precision clock comparison. Previous attempts using legacy S/X-band (2–10 GHz) systems achieved uncertainties around $1.5 \times 10^{-15}$ , which is insufficient for comparing modern optical clocks. The challenge lies in determining if standard, high-frequency (K-band and above) geodetic VLBI campaigns, using routine instrumentation and scheduling, can achieve metrological-grade clock comparison accuracy comparable to or better than satellite links.

2. Methodology

The study conducted a pilot experiment comparing Hydrogen Masers (H-masers) at the Medicina (Italy) and Sejong (South Korea) radio telescopes. The experiment utilized a multi-faceted approach to validate the VLBI results:

Experimental Setup:
- Network: A 24-hour geodetic VLBI session involving six stations: Medicina (Italy), Yebes (Spain), and four Korean stations (Sejong, Yonsei, Ulsan, Tamna).
- Frequency: K-band observations (21–22 GHz) with a total bandwidth of 512 MHz (16 channels).
- Clock Calibration: The local H-masers at both sites were calibrated against high-precision Ytterbium (Yb) optical lattice clocks (IT-Yb1 at INRIM, Italy; KRISS-Yb1 at KRISS, Korea).
- Fiber Distribution: Optical clock signals were delivered to the telescopes via phase-stabilized fiber links (535 km for Italy, 50 km for Korea) to ensure the H-masers were referenced to the optical standards during the VLBI session.
- GNSS Validation: A GPS Precise Point Positioning (PPP) link was maintained between the two National Metrology Institutes (NMIs) to provide an independent time/frequency transfer reference.
Data Analysis:
- Correlation: Data was correlated using the DiFX software.
- Fringe Fitting: Performed using the Haystack Observatory's fourfit system.
- Geodetic Modeling: Two independent software packages were used to estimate clock parameters (offset, rate, and drift):
  1. $\nu$ Solve: Standard geodetic analysis (no ionospheric correction for single-band K-band).
  2. VieVS: Utilized advanced tropospheric modeling (VMF3 mapping function) and Global Ionospheric Maps (GIMs) for ionospheric correction, even in single-band sessions.
- Metric: The fractional frequency difference ( $y$ ) between the two H-masers was derived by extrapolating clock rates from the VLBI group delay residuals.

3. Key Contributions

Feasibility of Standard VLBI for Metrology: The paper demonstrates that standard geodetic VLBI campaigns, using existing K-band infrastructure and routine scheduling, can perform intercontinental clock comparisons with an accuracy of $\sim 10^{-15}$ s/s.
Multi-Method Validation: It provides a rare "closure" test by comparing three independent methods for the same clock pair:
1. VLBI-derived frequency difference.
2. Optical clock comparison (via fiber).
3. GNSS (GPS) comparison.
Software Comparison: It highlights the consistency between different geodetic analysis software ( $\nu$ Solve vs. VieVS) and the specific impact of ionospheric modeling on single-band K-band data.
Roadmap for Future Improvements: The study outlines a clear path toward sub-picosecond group delay accuracy by deploying new-generation Compact Triple-band Receivers (CTRs) operating across K, Q, and W bands (18–115 GHz) and integrating Frequency Phase Transfer (FPT) and Photonic PCAL systems.

4. Results

The experiment yielded the following fractional frequency differences ( $y$ ) between the Sejong and Medicina H-masers:

Method	Frequency Difference ( $y$ )	Uncertainty ( $10^{-15}$ )
VLBI (VieVS)	$-212.5 $\|$ 1.1$
Optical Clocks	$-213.29 $\|$ 0.14$
GPS (PPP)	$-215.7 $\|$ 2.0$

Agreement: The VLBI result agreed with the optical clock result within $2\sigma$ (difference of $0.8 \times 10^{-15}$ ). The VLBI and GPS results also showed consistency within their respective uncertainties.
Group Delay Residuals: The experiment achieved a weighted root-mean-square (WRMS) of group delay residuals of 25.55 ps ( $\nu$ Solve) and 26.41 ps (VieVS).
Closure Tests: The closure differences between the three methods were consistent with zero within the combined uncertainties, confirming the reliability of the VLBI approach.
Atmospheric Effects: Ionospheric corrections using GIMs in VieVS showed negligible impact on the final clock parameter for this specific baseline, though the authors note this may change for longer baselines or different solar activity.

5. Significance and Future Outlook

Metrological Impact: This study proves that VLBI is a viable, independent alternative to satellite links for intercontinental clock comparisons. It offers a pathway to validate optical clocks globally without relying solely on GNSS.
Scalability: The current uncertainty ( $\sim 10^{-15}$ ) is limited by the K-band bandwidth and source structure effects. The authors project that upgrading to broadband K/Q/W-band receivers (CTRs) will reduce group delay uncertainties to the sub-picosecond level ( $\sim 0.2$ ps), potentially reaching $10^{-16}$ or better.
Technological Synergy: The integration of optical fiber links for clock delivery and future photonic Phase Calibration (PCAL) systems will allow VLBI to fully exploit the stability of optical clocks, bridging the gap between astronomical geodesy and fundamental metrology.
Next Steps: A follow-up campaign is planned for 2026 involving upgraded telescopes with CTRs to test the limits of high-frequency, wideband VLBI for clock comparison.

In conclusion, this pilot study successfully validates the use of standard high-frequency geodetic VLBI for intercontinental clock comparison, establishing a foundation for a future global network capable of supporting the redefinition of the second and high-precision relativistic geodesy.