For the first time, signals from distant stars from Earth connect optical atomic clocks.


Used antennas and optical lattice clocks. Upper left: 2.4 meter antenna installed at INAF, Italy. Upper Middle: A 2.4 m antenna is installed at NICT, Japan. Upper right: 34 m antenna located at NICT, Japan. Left below: ytterbium optical lattice watch operated in INRIM, Italy. Bottom right: Strontium optical lattice clock located at NICT, Japan. Credit: National Institute of Information and Communications Technology (NICT), excluding left. Credit: Istituto Nazionale di Raisarca Metrologica (INRIM))

Using radio telescopes that look at distant stars, scientists have connected optical atomic clocks on different continents. The results were published in the journal Scientific Nature physics Istituto Nazionale di Rysarca Metroglica (INRIM, Italy), Istituto Nazionale di Astrofisica (INAF, Italy), and Bureau Internationale des, by an international collaboration between 33 astronomers and clock experts from the National Institute of Information and Communications Technology (NICT, Japan). Poydes et Messrs (BIPM, France).


The BIPM in Svvres near Paris calculates the recommended international time for civilian use (UTC, coordinated universal time) by comparing atomic clocks via regular satellite communications. However, the satellite connections needed to maintain a global global scale have not kept up with the development of new atomic clocks: optical clocks that use lasers interacting with ultraviolet atoms to give a very sophisticated tick. “To get the full benefit of optical clocks at UTC, it is important to improve clock comparison methods worldwide,” said Gerard Petit, physicist at the Time Department at BIPM.

In this new research, highly energetic superimposed radio sources replace satellites as sources of reference signals. Saikido Momaru’s group at NICT deployed two specialized radio telescopes, one in Japan and the other in Italy, to realize connections using the technology of Very Long Baseline Interferometry (VLBI). These telescopes are capable of observation over a large bandwidth, while only 2.4-meter diameter antenna dishes keep them transportable. “We would like to point out that broadband VLBI has the potential to be a powerful tool not only for geodesy and astronomy, but also for metrology.” Commented Sekido. To reach the required sensitivity, the small antennas teamed up with a large 34-meter radio telescope in Kashima, Japan during measurements from 14 October 2018 to 14 February 2019. For the Kashima Radio Telescope, these preceded the final observations. The telescope suffered irreparable damage in September 2019 by Typhoon Faikai.

The goal of the collaboration was to connect two optical clocks in Italy and Japan, separated by a baseline distance of 8700 km. These clocks carry hundreds of loads of ultra-cold atoms in an optical lattice, an atomic trap engineered with laser light. Watches use different atomic species: ytterbium for clock at INRIM and strontium at NICT. Both are candidates for future rescheduling of the other in the International System of Units (SI). “Today, the new generation of optical clocks is insisting on reviewing the definition of the second. The road to redistribution will face the challenge of comparing clocks globally, on the intercontinental scale, to better performance than today. With, “Davis Calonico said. Quantum Metrology and Nanotechnology Division and Research Coordinator at INRIM.

This connection is possible because it is billions of billions of light years away: radio sources powered by black holes weigh millions of solar masses, but far enough away that they can be considered fixed points in the sky. Telescopes create a different star every few minutes to compensate for the effects of the atmosphere. IDO Tetsuya, director of “Space-Time Standards Laboratory” and coordinator of research at NICT, commented, “We observed signals not from satellites, but from cosmic radio sources.” “VLBI can allow us to use UTC in Asia, which we can create by ourselves,” said VLO.

Transportable antennas such as those used in these measurements can be installed directly in laboratories developing optical clocks worldwide. According to Sekido, “a global optical clock network connected by VLBI can be realized by collaboration between the international communities of metrology and Geodesy, such as the broadband VLBI network of the VLBI Global Observing System (VGOS) already established.” While Petit commented, “Looking forward to the long-range optical link, this research shows there is still much to be gained from radio links, where VLBI Global Navigation Satellite System with transportable antennas and complementary telecommunications satellites Can. ”

In addition to improving international timekeeping, such infrastructure has also opened up new ways to study fundamental physics and general relativity to explore the diversity of Earth’s gravitational field, or even basic physical To find out the variation of science infrastructure. INAF’s research coordinator Federico Perini commented, “We are proud to be part of this collaboration that helped to advance a major step forward in helping to develop such technology, the farthest radio in the universe Using sources makes it possible. ” Measurement of frequencies generated by the two most accurate clocks here on Earth. “Our comparisons using VLBI gives a new approach to improving and investigating new methods for clock comparison, also looking at contamination between different disciplines,” concludes Calonico.


Optical clocks begin international atomic time calibration


more information:
Marco Pizocaro et al, Intercontinental Comparison of Optical Atomic Clocks via Very Long Baseline Interferometry, Nature physics (2020). DOI: 10.1038 / s41567-020-01038-6

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