Using radio telescopes observing distant stars, scientists have connected optical atomic clocks on different continents. The results were published in the scientific journal Nature Physics as a result of the international collaboration of 33 astronomers and experts at the National Institute of Information and Communication Technologies (NICT, Japan), the National Institute for Metrological Research (INRIM, Italy), the National Institute of Astrophysics (INAF, Italy) and the International Bureau weights and measures (BIPM, France).

The International Bureau of Weights and Measures (BIPM) in Sevres near Paris usually calculates the recommended international time for civilian use (UTC, Coordinated Universal Time), based on a comparison of atomic clocks via satellite communications. However, satellite communications, which are required to maintain synchronized global time, have lagged behind the development of new atomic clocks. It is an optical clock that uses lasers that interact with ultracold atoms to keep track of the time very accurately.

“To take full advantage of optical clocks in UTC, it is important to improve the methods of comparing world clocks,” explains Gerard Petit, a physicist in the BIPM time department.

The atomic clock is a device for measuring time, in which vibrations occurring at the level of atoms or molecules are used as a reference. The International System of Units defines one second as 9 192 631 770 periods of electromagnetic radiation arising from the transition between two levels of the ground state of the cesium-133 atom.

The atomic clock is essential in navigation. Determination of the position of spaceships, satellites, ballistic missiles, airplanes, submarines, as well as the movement of cars in the automatic mode via satellite communications (GPS, GLONASS, Galileo) is impossible without an atomic clock. Atomic clocks are also used in satellite and terrestrial telecommunications systems, including base stations for mobile communications, international and national bureaus of standards, and precision time services, which periodically broadcast temporary signals by radio.

However, atomic clocks have their own complexities – modern optical atomic clocks, which are created on the basis of lasers interacting with ultracold atoms, provide much greater accuracy than the satellite communication that links them.

In new research, high-energy extragalactic radio sources are replacing satellites as reference sources. Sekido Mamoru’s team at NICT has developed two dedicated radio telescopes, one deployed in Japan and the other in Italy, to implement very-long-baseline interferometry (VLBI) interferometry. These telescopes can observe a wide range of frequencies, and antennas with a diameter of only 2.4 meters allow them to be carried.

“We want to show that broadband VLBI can be a powerful tool not only in geodesy and astronomy but also in metrology,” explains Sekido.

The goal of the collaboration was to connect two optical clocks in Italy and Japan, separated by a base distance of 8700 km. This watch loads hundreds of ultracold atoms into an optical lattice, an atomic trap created by laser light. The watches use different atomic particles: ytterbium for watches at INRIM and strontium for NICT. Both are candidates for a future redefinition of the International System of Units (SI) seconds.

“Today, a new generation of optical watches is pushing for a revision of the definition of the second. The road to redefinition must face the challenge of comparing clocks on a global, intercontinental scale, with better performance than today, ”said David Kalonico, head of quantum metrology and nanotechnology and research coordinator at INRIM.

Communication is possible by observing quasars billions of light-years away: radio sources fed by black holes that are millions of solar masses, but so distant that they can be considered fixed points in the sky. Telescopes target a new star every few minutes to compensate for atmospheric influences. “We did not observe the signal from satellites, but from space radio sources,” commented Ido Tetsuya, director of the Space Standards Laboratory and research coordinator at NICT.

In addition to improving international timekeeping, such infrastructure also opens up new ways to study fundamental physics and general relativity, to study variations in the Earth’s gravitational field, or even variations in the fundamental constants that underlie physics.