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Time makers


Franco Zecchin

THE STRUGGLE FOR TIME MEASUREMENT AND PRECISION HAS BEEN AS LONG AS THE HISTORY OF HUMAN KIND. When the very first second of the new year 2005, January 1st ticks off in the Southern Pacific islands of Fijis and, from there, around the world 24 time zones, few people will realise that the exact time is being calculated in Paris using the Global Positioning Satellite network to compare atomic clocks from different continents. "We are constantly using GPS satellites to work out the average of approximately 200 atomic clocks located in about 60 countries around the world," says Felicitas Arias, the head of the Time Section at the BIPM in Paris which is in charge of setting the official time for 50 leading countries, including the USA. The precision with which the second is calculated today is in the order of 10-15. That is to say it can lose or gain one second in 30 million years! Since 1967, Cesium atomic clocks have revolutionized time measurement standards. Before them, time was based on astronomical Earth revolution. In 1967, the atomic second was defined as the interval of time taken to complete 9,192,631,770 oscillations of the atom of Cesium-133. The atomic time is so exact that, since 1961, 32 additional seconds have been introduced to bring universal Time into compliance with real Earth's time. Because of the slowing down of the Earth rotation, days get longer by several millionths of a second. The last leap second has been introduced on 1 January 1999, at 0 h and none will be added next year. For the past 15 years, half a dozen of leading laboratories around the world, like the Observatoire de Paris, have been developing new techniques to elaborate a new generation of atomic clocks, using different atoms maintained at ultra-low temperatures to improve time measurement precision. History of the time measurement instruments shows that the same efforts were constant over centuries. In the 16th century, the best sundials had a precision of a minute. From 1967 to 2001, the second accuracy has improved by a factor of 10 000 to reach the actual precision of 10-15. The research and the competition for new technical tools to gain time measurement accuracy are far from over. In 1995, the first fountain clock, built by the BNM-SYRTE of the Observatoire de Paris was first to get the best performances in the world in terms of stability with an accuracy of 10-15. But more promising is the next generation of atomic clocks, the so-called “optical clocks”, with the Strontium atom. The gain in precision would be so great that the science of horology could undergo another revolution that may well require that the second be once again redefined as in 1967. But serious obstacles remain for the “optical clocks”. Because of the effects of Einstein relativity, optical clocks are a nightmare to set up. But why do time makers want clocks accurate to one second every 30 billion years? Because super-accurate clock technology will be used immediately in new communications technologies, in testing fundamental theories in physics and in probing human diseases. They should also improve navigation with GPS to the centimetre accuracy. © text : Frédéric Castel


 

Franco Zecchin / Picturetank ZEF0023159

François Arago, a famous scientist and director of the Observatoire de Paris, in 1843 initiated the construction of the huge cupola on the roof of this 17th century building. The whole cupola, which protects a 38 cm diameter telescope, is a rotating struc

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Franco Zecchin / Picturetank ZEF0023160

World time is calculated in Paris. Felicitas Arias (in the center), the head of the Time Section at the International Bureau of Weight and Measurements in Paris (BIPM), with the team in charge of setting the international uniform time scale for the world

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Franco Zecchin / Picturetank ZEF0023161

Pierre Arman's assemblages of clocks in front of the Saint Lazare railway station (1985) in Paris. This public sculpture, which ironically shows different times in front of the train station’s main clock, is named "Everybody’s time" (“Le Temps de Tous”).

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Franco Zecchin / Picturetank ZEF0023024

In order to constantly improve the accuracy and go some day beyond the 10-15 range of accuracy, experts at the Observatoire de Paris laboratory have constructed in 1998 a dual fountain -- still in use -- that can operate with both Rubidium and Cesium. Lui

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Franco Zecchin / Picturetank ZEF0023155

On December 9, 1874, Captain Ernest Mouchez led a major astronomical expedition in the Indian Ocean to study and to take photographs of Venus passing in front of the Sun. Later on, he was appointed admiral and was assigned in 1878 as director of the Obser

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Franco Zecchin / Picturetank ZEF0023157

The small cupola on the top of the Observatory of Paris with the Eiffel tower in the skyline of Paris. This cupola with its telescope was used in the 18th and 19th century before a larger cupola was erected in 1850. The Observatoire, built in 1672, is the

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Franco Zecchin / Picturetank ZEF0023027

Laser are used as tools to compare different atomic clocks. Daniele Rovera, a researcher of the BNM, is the lab where stabilized lasers with a stability below 10-14 at 1 second. These lasers are used as secondary standards and local oscillators in freque

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Franco Zecchin / Picturetank ZEF0023019

A few years ago, the BNM-SYRTE lab of the Observatoire de Paris, like other famous labs around the world, initiated work on “optical clocks” by using the Strontium atom. Even if the performances of this new atomic clock are not yet as precise as the curre

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Franco Zecchin / Picturetank ZEF0023022

At the BNM, one can see an optical fiber carrying some visible light of femtosecond laser and its spectrum on a piece of paper just in front. The “femtosecond” laser enables to compare state-of-the-art next generation of optical atomic clocks to Cesium at

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Franco Zecchin / Picturetank ZEF0023018

Optical fibers, protected in flexible metallic pipe, are used to polarize the laser light, and then to send it to the vacuum chamber to cool the atoms with lasers.

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Franco Zecchin / Picturetank ZEF0023156

The huge cupola on the roof of the Observatoire protects a 38 cm objective telescope with a focal length of 9 m, is an amazing rotating structure which enables -- up to now -- astronomical observations of 360 degrees in the sky of Paris. This masterpiece

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Franco Zecchin / Picturetank ZEF0023016

This is the optics table where several laser beams are going through a fiber-connected network with diodes, fiber ports and Rubidium cells that control the laser beam. This is a Rubidium table part of the dual atomic fountain clock which is also a Cesium

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Franco Zecchin / Picturetank ZEF0023162

D’Ons-en-Bray's astronomical clock. The nicely decorated astronomical pendulum was built by the famous 18th century Paris clockmaker P. Fardoil in 1710 for Louis-Leon Pajot d’Ons-en-Bray, the director of the French postal service at the time. Its main di

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Franco Zecchin / Picturetank ZEF0023017

On the left side, part of the Rubidium table of a dual atomic fountain clock which is also Cesium atomic clock. On the right, in the vertical metallic container are a vacuum system and the surrounding magnetic shields, where the cold atoms are produced in

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Franco Zecchin / Picturetank ZEF0023153

The simplest sundial, built with a vertical stick rising from a flat horizontal surface, has been used for at least 3000 years. As the Sun rises, then passes the highest point in its path and finally sets, the shadow rotates around the stick in a clockwis

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Franco Zecchin / Picturetank ZEF0023163

This is a mobile quadrant of the early 18th century made out of metal and equipped with four legs and two refractors. The first instrument of this type was built in 1668 by Abbé Jean Picard. One can measure time based on the rotation of the Earth by obser

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Franco Zecchin / Picturetank ZEF0023164

To use this kind of 18th century mobile telescope, called a transit instrument, its base axle has to be put in East-West direction in order to observe the transit of the stars at the meridian according to the astronomical star time schedule. With this mod

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Franco Zecchin / Picturetank ZEF0023020

Rodolphe Le Targat, a PhD student at the Observatoire, working on the optical clock using Strontium.

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Franco Zecchin / Picturetank ZEF0023021

An optical fiber carries some visible light of femtosecond laser.

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Franco Zecchin / Picturetank ZEF0023025

Daniele Rovera working on the femtosecond laser. The “femtosecond” lasers are used to compare state-of-the-art optical atomic clocks between them, and with the Cesium clocks.

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Franco Zecchin / Picturetank ZEF0023023

The technical improvements made to this fountain, together with new measuring procedures could allow to explore some day the 10-16 range of accuracy for both kinds of atoms. From left to right: Harold Marion, a PhD. student from Paris VI/ P & M. Curie, Lu

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Franco Zecchin / Picturetank ZEF0023026

Rodolphe Le Targat working on the optical clock using Strontium. In the middle, one can see the vacuum chamber where the Strontium atoms are cooled by laser technique. The research on the optical clocks is very promising to reach an incredible good precis

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Franco Zecchin / Picturetank ZEF0023151

This chronograph, used in the early part of the XXth century, was connected to an electrical meridian instrument, in order to print the exact time on a roll of paper, when a star was observed. The transit instruments are still used around the world for as

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Franco Zecchin / Picturetank ZEF0023154

Because of too many calls to the Observatoire asking for the official time and making the phone line constantly busy, its director, Ernest Esclangon, in 1933, decided to build the first speaking clock in the world. On February 13, the new technique combin

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Franco Zecchin / Picturetank ZEF0023028

In 1952, at the Observatory workshop, André Danjon, built this new electric astrolabe which works like a marine sextant with a mobile prism and determines time with an incredible accuracy of 1/125th second. The innovation of Danjon’s prismatic astrolabe,

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Franco Zecchin / Picturetank ZEF0023152

This is a mobile atomic clock of the 60’s enclosed in a trunk which was transported by plane to compare time in several observatories before the GPS network which enables clock synchronizing today. This heavy piece of luggage from the Observatoire de Pari

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Franco Zecchin / Picturetank ZEF0023158

These two clocks are set up on the base of the 38 cm telescope of the Observatoire de Paris and they are crucial to locate and target stars in the sky during astronomical observations. This state of the art telescope in 1854 was an essential tool for sky

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Franco Zecchin / Picturetank ZEF0023165

In front of this statue of Pierre-Simon Laplace (1749-1827), famous astronomer and mathematician, the container of a Cesium atomic clock whose atomic tube has been removed. Historians credit L. Essen and J. Parry in England with the first successful atomic clock in 1955 using Cesium. Since 1967, Cesium atomic clocks have revolutionized time measurement standards. Before them, time was based on astronomical rotation and revolution of Earth. In 1967, the atomic second was defined as the interval of time taken to complete 9,192,631,770 oscillations of the isotopic atom of Cesium-133 which is non-radioactive. The precision with which the atoms are "vibrating" make them perfect natural clocks with a very accurate frequency.

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