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Signals from distant stars connect optical atomic clocks across Earth for first time

Christian Fernsby |
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 by an international collaboration between astronomers and clock experts at the National Institute of Information and Communications Technology (NICT, Japan), the Istituto Nazionale di Ricerca Metrologica (INRIM, Italy), the Istituto Nazionale di Astrofisica (INAF, Italy), and the Bureau International des Poids et Mesures (BIPM, France).

The BIPM in Sèvres near Paris routinely calculates the international time recommended for civil use (UTC, Coordinated Universal Time) from the comparison of atomic clocks via satellite communications.

However, the satellite connections that are essential to maintaining a synchronized global time have not kept up with the development of new atomic clocks: optical clocks that use lasers interacting with ultracold atoms to give a very refined ticking.

"To take the full benefit of optical clocks in UTC, it is important to improve worldwide clock comparison methods." said Gérard Petit, physicist at the Time Department at BIPM.

In this new research, highly energetic extragalactic radio sources replace satellites as the source of reference signals.

The group of SEKIDO Mamoru at NICT designed two special radio telescopes, one deployed in Japan and the other in Italy, to realize the connection using the technique of Very Long Baseline Interferometry (VLBI).

These telescopes are capable of observations over a large bandwidth, while antenna dishes keep them transportable.

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 load hundreds of ultra-cold atoms in an optical lattice, an atomic trap engineered with laser light.

The clocks use different atomic species: ytterbium for the clock at INRIM and strontium at NICT.

Both are candidates for a future redefinition of the second in the International System of Units (SI).

Radio sources powered by black holes weighing millions of solar masses, but so distant that they can be considered fixed points in the sky.

The telescopes aim at a different star every few minutes to compensate for the effects of the atmosphere.

Besides improving international timekeeping, such an infrastructure also opens new ways to study fundamental physics and general relativity, to explore variations of Earth's gravitational field, or even the variation of fundamental constants underlying physics.

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