Atomic clock

A atomic clock is a Horloge which uses one of the physical characteristics of the atoms to ensure the exactitude and the stability of the oscillating signal that it produces. This characteristic is the perenniality and the immutability of the Fréquence of the electromagnetic Rayonnement emitted by a electron at the time of the passage of an energy level to another. One their principal uses is the maintenance of the international atomic Time and the distribution of the universal Time coordinated which are the scales of time of reference.

History

In 1948, Louis Essen created the first atomic clock while using as reference a absorption line of the Ammoniac located in the spectral field to 24 GHz on which the oscillations of an oscillator with quartz were controlled. However, the Doppler effect very present tended to shift the absorption line and, the precision of this clock not being better than that of the simple oscillator with quartz, the idea was initially abandoned.

It is in 1955 that Essen and Parry, thanks to work of Ramsey on a method allowing the improvement of the interaction wave electromagnetic-atoms in 1950, carried out the control of an oscillator with quartz by the resonance of cesium: the first cesium jet atomic clock had been born.

Zacharias, raises of Rabi Thomas Hick, developed industrial prototypes of clocks with cesium jet which were marketed starting from 1956 and, following that, it proposed a new method using of the cold atoms which could be born only in 1967 because of technical constraints.

Consequently, the inaccuracy of the standards of frequency was reduced to $10^ {- 12}$ relative. It is at this time that it was decided to define time compared to an atomic reference: it is the birth of the international atomic time. The second becomes the duration of 9 then; 192 631 770 periods of radiation corresponding to the transition enters the two hyperfine levels of the fundamental state of the atom of Césium (natural cesium: stable Isotope ¹³³Cs).

Principle of operation

The general principle of the atomic clocks is to control the vibrations of an oscillator with quartz on an atomic transition between two levels from a hyperfine Structure.

Introduction to the transition processes between atomic levels

atomic energy Process

A Atome passer by of an excited state of energy E2 in another state of weaker E1 energy (with $E2>E1$ and $\ Delta E = E2 - E1$ ) emits a Photon Fréquence ν such as $\ naked = \ frac {\ Delta E} {H}$ . It is the process of spontaneous emission. Contrary, an atom in a state of E1 energy will pass to a excited state of E2 energy by the Absorption of a photon of frequency ν and energy $\ Delta E=E_ {2} - E_ {1} =h \ nu$ , H being the Constante of Planck such as $h=6.62.10^ {- 34} J.s$ . One knows also the consistent principle of emission stimulated for an atom to pass from an excited state of energy towards a fundamental state after the meeting of another photon. The energy of the atom will then be dissipated by the emission of another photon which will have the same characteristics as the initiating photon. There exists also a nonnull probability so that an atom being in an excited state of energy $E_ {2}$ goes down again in state of a more stable and weaker energy by a process of nonradiative de-energizing, i.e. without emitting photon. The system then having to satisfy the relation of conservation of energy, it will result either a heating from it from the atom or a transfer of momentum.

These elementary atomic processes, whose theory was developed partly by Einstein, will be at the base of all the interaction making it possible to provide the atomic standard of measurement of time.

Concept of fine and hyperfine structure

The observation with high-resolution of the luminous lines of an absorption or emission spectrum highlights the presence of a superposition of several components within the same line.

A principal line is given by quantum number principal N characterizing the clean states of the functions of wave of its orbital electronic. In the same principal quantum level, the theory will give for the same quantum number N a series of under-levels corresponding in degenerated quantum states which will be created by the various interactions physique' within the atom (spin-orbit interaction, effects of volume, effects of mass…). These under-levels are in fact the cause of the structure made up of the principal line observed in the spectrum. One speaks then about even hyperfine fine structure for certain atoms under particular conditions of magnetic field.

Example of the jet atomic clock of Cesium 133

A physical system makes it possible to create a jet of atoms, in which only the atoms corresponding at the desired state (here, 1) are kept (selection by magnetic fields).
On another side, an oscillator with quartz emits a wave of frequency close to ν which allows as we saw the passage of the state (1) to the state (2).
The jet of atoms in the state (1) passes in the oscillator to quartz and a certain number arises in the state (2). The exit of the atomic jet is analyzed (one checks the number of atoms in the state (2) by a system similar to that which selected the atoms in the state (1)).
The closer the frequency of the oscillator with quartz is to ν, the larger the number of atoms leaving in the state (2) is, and thus, the larger the number of atoms (2) counted at exit is.

A system of Asservissement permanently adjusts the frequency of the oscillator with quartz to have the maximum of atoms in the state (2) at exit.

The counting of time is thus provided by the oscillations of quartz (as in a quartz watch), but its frequency is adjusted at the frequency standard ν which is that of the passage of the atom of the state (1) to the state (2), or the passage of the electron of the energy level E1 to that of E2 energy.

In the case of the cesium, the frequency ν is of 9.192.631 770 Hz. This frequency is, moreover, the base of the legal definition of the second.

Applications

The international atomic Time is the world reference founded on the definition of the second atomic, calculated with the International office of the weights and measures with Sevres, by making the average of 250 atomic clocks throughout the world. In France, legal time rests on the readings of a score of atomic clocks.

In addition to being used to define a universal chronological reference, the atomic clocks are also used in technologies of geographical positioning. The satellite of the NavStar constellation (used in technology GPS) or those of the program Galileo, embark each one several atomic clocks (typically 4 for satellites GPS).

The December 28th 2005, an atomic clock was placed on the orbit envisaged, with 23 000 km of altitude, by ESA and GJU, on board first of two experimental satellites named GIOVE-A (GSTB-2A), intended for the European system of navigation by satellite Galileo, since a Russian rocket Soyuz launched Cosmodrome de Baïkonour with the Kazakhstan.

The atomic clocks are also used in the networks of Télécommunications to provide a signal of reference to the internal oscillators of the equipment, in order to ensure a quality of transmission of the services in agreement with the international standards. One uses either the signals directly produced by atomic clocks or the signals worked out starting from the emissions of the satellites of the constellation GPS which have the stability of the embarked atomic clocks.

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