The International Bureau of Weights and Measures (BIPM) in Sèvres near Paris usually calculates international time,
“To take full advantage of optical clocks in UTC, it is important to improve the methods of comparing world clocks,” explains Gerard Petit, physicist in the BIPM time department.
An atomic clock is a device for measuring time, inwhich uses vibrations occurring at the level of atoms or molecules as a standard. 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 automatic mode via satellite communications (GPS, GLONASS, Galileo) are 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 based on lasers interacting with ultracold atoms, provide much greater accuracy than the satellite communication that links them.
Antennas and clocks on an optical array are used. Top left: 2.4m antenna installed at INAF, Italy. Top middle: 2.4m antenna installed at NICT, Japan. Top right: 34 m antenna located at NICT, Japan. Bottom left: An ytterbium clock on an optical grating operating at INRIM, Italy. Bottom right: A clock on a strontium-based optical array located at NICT, Japan. Credit: National Institute of Information and Communication Technology (NICT), except for the lower left corner. Credit: National Institute of Metrological Medicine (INRIM).
In a new study, high-energyextragalactic radio sources are replacing satellites as sources of reference signals. Sekido Mamoru's group at NICT has developed two dedicated radio telescopes, one deployed in Japan and the other in Italy, to achieve the connection using Very-long-baseline interferometry (VLBI). 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 become a powerful tool not only in geodesy and astronomy, but also in metrology,” explains Sekido.
The goal of the collaboration was to connect the twooptical clocks in Italy and Japan, separated by a base distance of 8700 km. These clocks load hundreds of ultra-cold atoms into an optical lattice, an atomic trap created using laser light. The clocks use different atomic particles: ytterbium for the clocks in INRIM and strontium in NICT. Both are candidates for a future redefinition of the second in the International System of Units (SI).
“Today is a new generation of optical watchespushes to revise 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 onbillions of light years away: radio sources powered by black holes with a mass of millions of solar masses, but so distant that they can be considered fixed points in the sky. Telescopes aim at the new star every few minutes to compensate for the influence of the atmosphere. “We observed the signal not from satellites, but from space radio sources,” commented Ido Tetsuya, director of the Space Standards Laboratory and research coordinator at NICT, on the project.
In addition to improving international timing,such infrastructure also opens up new ways to study fundamental physics and general relativity, study variations in the Earth's gravitational field, or even variations in the fundamental constants underlying physics.
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