EP2261758A1 - Atomic clock operated with Helium-3 - Google Patents

Atomic clock operated with Helium-3 Download PDF

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Publication number
EP2261758A1
EP2261758A1 EP10165184A EP10165184A EP2261758A1 EP 2261758 A1 EP2261758 A1 EP 2261758A1 EP 10165184 A EP10165184 A EP 10165184A EP 10165184 A EP10165184 A EP 10165184A EP 2261758 A1 EP2261758 A1 EP 2261758A1
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Prior art keywords
atomic clock
helium
magnetic field
clock according
measuring medium
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EP10165184A
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German (de)
French (fr)
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EP2261758B1 (en
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Matthieu Le Prado
Jean-Michel Leger
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Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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Commissariat a lEnergie Atomique CEA
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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    • G—PHYSICS
    • G04—HOROLOGY
    • G04F—TIME-INTERVAL MEASURING
    • G04F5/00—Apparatus for producing preselected time intervals for use as timing standards
    • G04F5/14—Apparatus for producing preselected time intervals for use as timing standards using atomic clocks

Definitions

  • the subject of the invention is an atomic clock operating with helium 3.
  • Atomic clocks comprise an often alkaline gaseous medium, a device for exciting the atoms of this gas such as a laser, able to pass them to higher energy states, and a means for measuring a frequency signal emitted by atoms returning to the usual energy level, using the photons from the laser.
  • Energy levels are sensitive to the surrounding magnetic field. This sensitivity is low (of the second order) for the sub-level of angular momentum equal to 0, but much stronger (of the first order) for the other sub-levels: the transitions made from or up to them produce photons whose the frequency is variable and can not be used as a reference, and only the portion of the signal corresponding to the transition between the two sub-levels of zero angular momentum is used for the measurement, which affects its quality.
  • the alkaline gases have hitherto been preferred as measuring medium in atomic clocks since they generally comprise stable and excited states each having a zero angular momentum sub-level which therefore ensures a measurement at a frequency of stable resonance.
  • These bodies have the disadvantage of being able to present several physical states under ordinary operating conditions and to be chemically very reactive.
  • the object of the invention is to improve existing clocks.
  • helium 3 is chemically inert, no reaction with the surrounding material is to be feared; and since only a reduced portion is usually brought to the state of plasma, most of it remains gaseous and serves as a buffer gas in order to limit the shocks between the atoms of helium 3 in the metastable level, said atoms being carriers of magnetic information.
  • the radio frequency waves can be between 20 MHz and 30 MHz, and their power can be 1 W for a quantity of helium gas 3 of 100 mm 3 at a pressure of 0.1 Torr approximately.
  • the chemical stability of this element which makes it all the more interesting as a buffer gas that being of the same chemical nature as the element used for the measurement, it does not react chemically with it, which does not is not the case with alkaline gases, which often have to be mixed with buffer gases to give a stable state. It is consistent with a preferred embodiment of the invention that the measuring medium is, therefore, composed exclusively of helium 3, the metastable state being the level 2 3 S 1 .
  • the first exciter device may comprise a laser beam; and the magnetic fields applied by the device, which are intended for the stabilization of the energy levels of the measuring medium, may comprise at least one essentially static and enslaved magnetic field, and possibly one or two oscillating magnetic fields perpendicular to the preceding one.
  • the heart of the clock ( figure 2 ) is a cell 1 filled with a measuring medium.
  • An exciter 2 transmits energy to this medium in the form of a flux of photons polarized by a quarter wave plate 3.
  • the exciter may be a laser injecting a light beam to detect the resonances of the medium.
  • a photodetector 4 collects the light energy restored by the excited medium of the cell 1 and transmits a signal to a counting device 5, the photodetector 4 being advantageously arranged in the extension of a laser beam emanating from the exciter 2.
  • a frequency separator 6 collects the signal at the output of the counting device 5 and transmits its results to an operating device 7 of the clock and a servo-control device 8, which governs the exciter 2 and a magnetic field application device 9.
  • the first exciter 2 is a laser diode of wavelength 1083 nm for a power of 100 mW, with a pump current modulated at approximately 3.37 GHz in order to induce an optical intensity modulation responsible for generating the resonance of Microwave of the hyperfine transition of helium 3.
  • the quarter-wave plate 3 imposes a left circular polarization for the photons.
  • Cell 1 is filled with helium 3 subjected to a pressure of approximately 0.1 torr. It is cylindrical, in Pyrex, and its volume is 100 mm 3 .
  • the second exciter device 10 comprises two electrodes contiguous to the cell 1 on either side of it and which are connected to a radiofrequency power generator at 25 MHz (between 20 MHz and about 30 MHz) and 1W. It creates the helium plasma, which is necessary to populate the metastable level 2 3 S 1 with the hyperfine structure.
  • the magnetic field application device 9 makes it possible to apply a magnetic field H 0 of 500 ⁇ T parallel to the laser beam to block the sub-levels at constant energies.
  • a pair of Helmholtz coils is used.
  • This magnetic field is slaved to a constant value by measuring the Larmor frequency within the hyperfine structure. Thus, variations in the ambient magnetic field are prevented from disturbing the microwave transition defining the resonance frequency fo.
  • the magnetic field application device 9 again generates a low-frequency oscillating magnetic field component applied perpendicularly to the static magnetic field and which is controlled by the servo-control device 8 to the Zeeman transition at about 12 MHz. .
  • This oscillating field makes it possible to induce a resonance within the Zeeman sub-levels that will give the aforementioned measurement for evaluating the resulting ambient magnetic field and enslaving it to a constant value.
  • helium 3 is devoid of sub-levels with zero angular momentum index, it is necessary to operate the device with a constant magnetic field, which can be obtained by an artificial field controlled with or without a magnetic shield.
  • the enslavement of the magnetic field can be accomplished in a scalar or vector manner by the Larmor or vector frequency by a zero total magnetic field search.
  • the magnetic field application device 9 can both generate the magnetic field for the measurement of resonance if it is composed of triaxial coils.
  • the field application device 9 emits magnetic fields at radio frequencies of pulsations ⁇ , and ⁇ , which are perpendicular to each other and of direction dependent on the polarization (for example perpendicular to the light rays emitted by the exciter 2 in the case of a circular polarization).
  • the magnetic field application device 9 applies both the substantially static magnetic compensation field and the radio frequency magnetic fields.
  • the enslavement is accomplished by any known hardware including a computing unit.
  • the coils are driven by current or voltage.
  • the excitation at the resonance frequency f 0 is accomplished by amplitude modulation of the laser diode at the frequency f 0/2 or by a microwave cavity resonating at the frequency f 0 .
  • An exciter comprising two lasers, the frequency deviation is f 0 can also be envisaged.
  • helium 3 is devoid of sub-levels with zero angular momentum index, it is necessary to operate the device with a constant magnetic field, which can be obtained by an artificial field controlled with or without a magnetic shield.
  • the enslavement of the magnetic field can be accomplished in a scalar or vector manner by the Larmor or vector frequency by a zero total magnetic field search.
  • the magnetic field application device 9 can both generate the magnetic field for the measurement of resonance if it is composed of triaxial coils.
  • the instrument measuring the laser flux may be an InGaAs type photodiode.
  • This embodiment comprising a magnetic field stabilizing device, does not include magnetic shielding.
  • a magnetic shield in addition to the magnetic field servo device as previously described.
  • the magnetic shielding may be composed of, for example, a soft iron cylinder and a nested metal cylinder.
  • Exciter 2 could include a lamp or VCSEL (for Variation capacity surface emitting light). In the absence of a device for stabilizing the ambient magnetic field, excitation at the resonance frequency could also be provided by a resonant microwave cavity or by two lasers whose frequency difference is the resonance frequency.
  • VCSEL Variation capacity surface emitting light

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)
  • Stabilization Of Oscillater, Synchronisation, Frequency Synthesizers (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)
  • Lasers (AREA)

Abstract

The clock has a cell (1) filled with a measuring medium, and an exciter (2) i.e. laser diode, for exciting particles of the medium until a higher energy level. A photodetector (4) collects frequency of light energy reproduced by the medium, and transmits a signal to a counting device (5). An application device (9) applies static magnetic field parallel to a laser beam. A control unit (8) controls the application device to adjust the magnetic field. The medium includes helium-3 plasma. Another exciter (10) provokes the helium-3 plasma from gaseous helium-3.

Description

Le sujet de l'invention est une horloge atomique fonctionnant à l'hélium 3.The subject of the invention is an atomic clock operating with helium 3.

Des horloges atomiques comprennent un milieu gazeux souvent alcalin, un dispositif d'excitation des atomes de ce gaz tel qu'un laser, apte à les faire passer à des états d'énergie supérieurs, et un moyen de mesure d'un signal fréquentiel émis par les atomes en revenant au niveau d'énergie habituel, en utilisant les photons provenant du laser.Atomic clocks comprise an often alkaline gaseous medium, a device for exciting the atoms of this gas such as a laser, able to pass them to higher energy states, and a means for measuring a frequency signal emitted by atoms returning to the usual energy level, using the photons from the laser.

La fréquence du signal des photons restitués par le gaz est définie par la formule V =ΔE/h, où V est la fréquence, ΔE la différence entre les niveaux d'énergie et h la constante de Planck, égale à 6,62x10-34 J/s.The frequency of the signal of the photons restored by the gas is defined by the formula V = ΔE / h, where V is the frequency, ΔE the difference between the energy levels and h the Planck constant, equal to 6.62 × 10 -34 J / s.

Il est connu que cette fréquence est très stable et qu'elle peut donc servir d'unité de référence au temps. Cela n'est toutefois plus vrai quand on considère la structure Zeeman de la matière : les niveaux d'énergie apparaissent alors comme composés de sous-niveaux correspondant à des états un peu différents, qu'on distingue par leur indice de moment angulaire mF, 0 pour un état de référence du niveau d'énergie et -1, -2, etc. ou +1, +2, etc. pour les autres. Cela est illustré par la figure 1 dans le cas de l'élément 87Rb, dont on a figuré la décomposition des deux premiers niveaux d'énergie (de moments angulaires F=1 et F=2).It is known that this frequency is very stable and can therefore serve as a unit of reference for time. This is however more true when you consider the Zeeman structure of matter: the energy levels appear as compounds sublevels corresponding to slightly different conditions, we distinguished by their angular momentum index m F , 0 for a reference state of the energy level and -1, -2, etc. or +1, +2, etc. for the others. This is illustrated by the figure 1 in the case of element 87 Rb, the decomposition of the first two energy levels (of angular moments F = 1 and F = 2) has been figured out.

Les niveaux d'énergie sont sensibles au champ magnétique ambiant. Cette sensibilité est faible (du second ordre) pour le sous-niveau de moment angulaire égal à 0, mais beaucoup plus forte (du premier ordre) pour les autres sous-niveaux : les transitions faites depuis ou jusqu'à eux produisent des photons dont la fréquence est variable et ne peut donc pas servir de référence, et seule la portion du signal correspondant à la transition entre les deux sous-niveaux de moment angulaire nul est exploitée pour la mesure, ce qui nuit à sa qualité. La fréquence de référence donnée par l'horloge est alors fo=E0/h, où E0 est la différence d'énergie entre les sous-niveaux à mF=0 des deux états (F=1 et F=2 de l'exemple de la figure 1).Energy levels are sensitive to the surrounding magnetic field. This sensitivity is low (of the second order) for the sub-level of angular momentum equal to 0, but much stronger (of the first order) for the other sub-levels: the transitions made from or up to them produce photons whose the frequency is variable and can not be used as a reference, and only the portion of the signal corresponding to the transition between the two sub-levels of zero angular momentum is used for the measurement, which affects its quality. The reference frequency given by the clock is then fo = E 0 / h, where E 0 is the energy difference between the sub-levels at m F = 0 of the two states (F = 1 and F = 2 of the example of figure 1 ).

Les gaz alcalins ont été préférés jusqu'à présent comme milieu de mesure dans les horloges atomiques puisqu'ils comprennent en général des états stables et excités dotés chacun d'un sous-niveau à moment angulaire nul qui assure donc une mesure à une fréquence de résonnance stable. Ces corps présentent toutefois l'inconvénient de pouvoir présenter plusieurs états physiques aux conditions ordinaires de fonctionnement et d'être chimiquement très réactifs.The alkaline gases have hitherto been preferred as measuring medium in atomic clocks since they generally comprise stable and excited states each having a zero angular momentum sub-level which therefore ensures a measurement at a frequency of stable resonance. These bodies, however, have the disadvantage of being able to present several physical states under ordinary operating conditions and to be chemically very reactive.

S'il est possible de maintenir le champ magnétique ambiant à une valeur fixe, tous les sous-niveaux sont fixés et peuvent contribuer à la mesure. Plusieurs techniques de stabilisation du champ magnétique ambiant ont été développées et exposées dans certaines publications, telles que le brevet américain US2007/0247241 .If it is possible to maintain the ambient magnetic field at a fixed value, all sub-levels are fixed and can contribute to the measurement. Several techniques for stabilizing the ambient magnetic field have been developed and described in some publications, such as the US patent US2007 / 0247241 .

L'objet de l'invention est de perfectionner des horloges existantes.The object of the invention is to improve existing clocks.

Elle est fondée sur l'emploi comme milieu de mesure de l'hélium 3, mais qui a été porté à l'état de plasma par un dispositif excitateur distinct du dispositif classique servant à l'excitation des particules en vue de la mesure.It is based on the use as a measuring medium of helium 3, but which has been brought to the plasma state by an exciter device distinct from the conventional device used for excitation of the particles for the purpose of measurement.

Seuls des milieux de mesures gazeux sont généralement considérés pour les mesures dans les horloges atomiques. L'emploi d'un plasma, et plus particulièrement celui d'hélium 3, permet de peupler un niveau métastable muni d'une structure hyperfine dont la fréquence est élevée et fournit donc une base de mesure de temps appréciable pour sa précision.Only gaseous measurement media are generally considered for measurements in atomic clocks. The use of a plasma, and more particularly that of helium 3, makes it possible to populate a metastable level provided with a hyperfine structure whose frequency is high and thus provides a basis for appreciable time measurement for its accuracy.

De plus, comme l'hélium 3 est chimiquement inerte, aucune réaction avec le matériel environnant n'est à redouter ; et comme seule une portion réduite est usuellement portée à l'état de plasma, la plus grande part reste gazeuse et sert de gaz tampon afin de limiter les chocs entre les atomes de l'hélium 3 dans le niveau métastable, lesdits atomes étant porteurs de l'information magnétique.Moreover, since helium 3 is chemically inert, no reaction with the surrounding material is to be feared; and since only a reduced portion is usually brought to the state of plasma, most of it remains gaseous and serves as a buffer gas in order to limit the shocks between the atoms of helium 3 in the metastable level, said atoms being carriers of magnetic information.

Une définition synthétique de l'invention est une horloge atomique comprenant une cellule emplie d'un milieu de mesure, un premier dispositif excitateur (1) de particules du milieu de mesure jusqu'à un niveau d'énergie supérieur, un système (4, 6, 7) recueillant une fréquence d'énergie lumineuse restituée par le milieu de mesure en quittant le niveau d'énergie supérieur, ladite fréquence bande d'énergie lumineuse étant exploitée pour donner une mesure du temps, un dispositif (9) d'application de champs magnétiques comprenant au moins un champ magnétique essentiellement statique et des moyens d'asservissement (8) dudit dispositif (9) pour ajuster les champs magnétiques, caractérisée en ce que le milieu de mesure comprend du plasma d'hélium 3, et il est prévu un second dispositif excitateur (10) pour susciter le plasma d'hélium 3 à partir d'hélium 3 gazeux.A synthetic definition of the invention is an atomic clock comprising a cell filled with a measuring medium, a first exciter device (1) of particles of the measuring medium up to a higher energy level, a system (4, 6, 7) collecting a luminous energy frequency restored by the measurement medium by leaving the upper energy level, said light energy band frequency being used to give a measurement of the time, a device (9) for applying magnetic fields comprising at least one essentially static magnetic field and servocontrol means (8) for said device (9) for adjusting the magnetic fields, characterized in that the measuring medium comprises plasma of helium 3, and there is provided a second exciter device (10) for generating the helium plasma 3 from helium 3 gas.

Le second dispositif excitateur est éventuellement un générateur d'ondes de radiofréquences « de puissance ». L'expression signifie que la puissance que ce second dispositif instaure dans le milieu de mesure est nettement supérieure à celle qui est instaurée par le premier dispositif excitateur, responsable de l'excitation à l'origine de la mesure.The second exciter device is optionally a "power" radiofrequency wave generator. The expression means that the power that this second device establishes in the measuring medium is significantly greater than that which is established by the first exciter device, responsible for the excitation at the origin of the measurement.

Les ondes de radiofréquences peuvent être comprises entre 20 MHz et 30 MHz, et leur puissance peut être de 1 W pour une quantité de gaz d'hélium 3 de 100 mm3 à une pression de 0,1 Torr environ. Il suffit en réalité d'ioniser une partie seulement du milieu de mesure, ayant par exemple une teneur de 1 partie par million des atomes portés au niveau métastable, le reste de l'hélium 3 restant à l'état gazeux et étant alors sans utilité directe pour la mesure ; il sert cependant de gaz tampon aux atomes de l'hélium 3 dans le niveau métastable. On a déjà mentionné la stabilité chimique de cet élément, qui le rend d'autant plus intéressant comme gaz tampon qu'étant de même nature chimique que l'élément servant à la mesure, il ne réagit pas chimiquement avec lui, ce qui n'est pas le cas avec les gaz alcalins, qui doivent souvent être mêlés à des gaz tampons pour donner un état stable. Il est conforme à une réalisation privilégiée de l'invention que le milieu de mesure soit, en conséquence, composé exclusivement d'hélium 3, l'état métastable étant le niveau 23S1.The radio frequency waves can be between 20 MHz and 30 MHz, and their power can be 1 W for a quantity of helium gas 3 of 100 mm 3 at a pressure of 0.1 Torr approximately. In reality, it suffices to ionize only a part of the measuring medium, having, for example, a content of 1 part per million of the atoms carried at the metastable level, the remainder of the helium 3 remaining in the gaseous state and then being of no use direct for measurement; however, it serves as a buffer gas for the helium 3 atoms in the metastable level. We have already mentioned the chemical stability of this element, which makes it all the more interesting as a buffer gas that being of the same chemical nature as the element used for the measurement, it does not react chemically with it, which does not is not the case with alkaline gases, which often have to be mixed with buffer gases to give a stable state. It is consistent with a preferred embodiment of the invention that the measuring medium is, therefore, composed exclusively of helium 3, the metastable state being the level 2 3 S 1 .

Parmi d'autres solutions, le premier dispositif excitateur peut comprendre un faisceau de laser ; et les champs magnétiques appliqués par le dispositif, qui sont destinés à la stabilisation des niveaux d'énergie du milieu de mesure, peuvent comprendre au moins un champ magnétique essentiellement statique et asservi, et éventuellement un ou deux champ magnétiques oscillants perpendiculaires au précédent.Among other solutions, the first exciter device may comprise a laser beam; and the magnetic fields applied by the device, which are intended for the stabilization of the energy levels of the measuring medium, may comprise at least one essentially static and enslaved magnetic field, and possibly one or two oscillating magnetic fields perpendicular to the preceding one.

L'invention sera maintenant décrite en liaison aux figures :

  • la figure 1 illustre un diagramme d'énergie d'un élément de mesure dans une horloge atomique ;
  • la figure 2 est une représentation de l'horloge atomique selon l'invention ;
  • et les figures 3 et 4 illustrent un mode d'asservissement de champ magnétique de stabilisation.
The invention will now be described with reference to the figures:
  • the figure 1 illustrates an energy diagram of a measuring element in an atomic clock;
  • the figure 2 is a representation of the atomic clock according to the invention;
  • and the Figures 3 and 4 illustrate a stabilization magnetic field servo mode.

Le coeur de l'horloge (figure 2) est une cellule 1 remplie d'un milieu de mesure. Un excitateur 2 transmet de l'énergie à ce milieu sous forme d'un flux de photons polarisés par une lame quart d'onde 3. L'excitateur peut être un laser injectant un faisceau lumineux pour détecter les résonances du milieu. Un photodétecteur 4 recueille l'énergie lumineuse restituée par le milieu excité de la cellule 1 et transmet un signal à un dispositif de comptage 5, le photodétecteur 4 étant disposé avantageusement dans le prolongement d'un faisceau laser émanant de l'excitateur 2. Un séparateur de fréquences 6 recueille le signal à la sortie du dispositif de comptage 5 et transmet ses résultats à un dispositif d'exploitation 7 de l'horloge et un dispositif d'asservissement 8, qui gouverne l'excitateur 2 et un dispositif d'application de champ magnétique 9.The heart of the clock ( figure 2 ) is a cell 1 filled with a measuring medium. An exciter 2 transmits energy to this medium in the form of a flux of photons polarized by a quarter wave plate 3. The exciter may be a laser injecting a light beam to detect the resonances of the medium. A photodetector 4 collects the light energy restored by the excited medium of the cell 1 and transmits a signal to a counting device 5, the photodetector 4 being advantageously arranged in the extension of a laser beam emanating from the exciter 2. A frequency separator 6 collects the signal at the output of the counting device 5 and transmits its results to an operating device 7 of the clock and a servo-control device 8, which governs the exciter 2 and a magnetic field application device 9.

On trouve également un second dispositif d'excitation 10 pour obtenir un plasma d'hélium 3 à partir du gaz de l'hélium 3.There is also a second excitation device 10 for obtaining a helium plasma 3 from the gas of helium 3.

Voici quelques éléments de construction d'une réalisation possible de l'invention. Le premier excitateur 2 est une diode laser de longueur d'onde 1083 nm pour une puissance de 100 mW, avec un courant de pompe modulé à 3,37 GHz environ afin d'induire une modulation d'intensité optique chargée de générer la résonance de micro-onde de la transition hyperfine de l'hélium 3. La lame quart d'onde 3 impose une polarisation circulaire gauche pour les photons. La cellule 1 est emplie d'hélium 3 soumis à une pression de 0,1 torr environ. Elle est cylindrique, en Pyrex, et son volume est de 100 mm3. Le second dispositif excitateur 10 comprend deux électrodes accolées à la cellule 1 de part et d'autre d'elle et qui sont branchées à un générateur de puissance de radiofréquences à 25 MHz (entre 20MHz et 30 MHz environ) et 1W. Il crée le plasma d'hélium, qui est nécessaire pour peupler le niveau métastable 23S1 disposant de la structure hyperfine.Here are some building elements of a possible embodiment of the invention. The first exciter 2 is a laser diode of wavelength 1083 nm for a power of 100 mW, with a pump current modulated at approximately 3.37 GHz in order to induce an optical intensity modulation responsible for generating the resonance of Microwave of the hyperfine transition of helium 3. The quarter-wave plate 3 imposes a left circular polarization for the photons. Cell 1 is filled with helium 3 subjected to a pressure of approximately 0.1 torr. It is cylindrical, in Pyrex, and its volume is 100 mm 3 . The second exciter device 10 comprises two electrodes contiguous to the cell 1 on either side of it and which are connected to a radiofrequency power generator at 25 MHz (between 20 MHz and about 30 MHz) and 1W. It creates the helium plasma, which is necessary to populate the metastable level 2 3 S 1 with the hyperfine structure.

Le dispositif d'application de champ magnétique 9 permet d'appliquer un champ magnétique H0 de 500 µT parallèlement au faisceau du laser pour bloquer les sous-niveaux à des énergies constantes. On utilise pour cela une paire de bobines de Helmholtz. Ce champ magnétique est asservi à une valeur constante par la mesure de la fréquence de Larmor au sein de la structure hyperfine. Ainsi, on empêche que les variations du champ magnétique ambiant ne perturbent la transition de micro-ondes définissant la fréquence de résonnance fo.The magnetic field application device 9 makes it possible to apply a magnetic field H 0 of 500 μ T parallel to the laser beam to block the sub-levels at constant energies. For this purpose a pair of Helmholtz coils is used. This magnetic field is slaved to a constant value by measuring the Larmor frequency within the hyperfine structure. Thus, variations in the ambient magnetic field are prevented from disturbing the microwave transition defining the resonance frequency fo.

Le dispositif de d'application de champ magnétique 9 engendre encore une composante de champ magnétique oscillant à basse fréquence, appliquée perpendiculairement au champ magnétique statique et que l'on asservit grâce au dispositif d'asservissement 8 à la transition de Zeeman à 12 MHz environ. Ce champ oscillant permet d'induire une résonnance au sein des sous-niveaux de Zeeman qui donnera la mesure précitée pour évaluer le champ magnétique ambiant résultant et l'asservir à une valeur constante.The magnetic field application device 9 again generates a low-frequency oscillating magnetic field component applied perpendicularly to the static magnetic field and which is controlled by the servo-control device 8 to the Zeeman transition at about 12 MHz. . This oscillating field makes it possible to induce a resonance within the Zeeman sub-levels that will give the aforementioned measurement for evaluating the resulting ambient magnetic field and enslaving it to a constant value.

L'hélium 3 étant dépourvu de sous-niveaux à indice de moment angulaire nul, il est nécessaire de faire fonctionner le dispositif à champ magnétique constant, ce qui peut être obtenu par un champ artificiel asservi avec ou sans un blindage magnétique. L'asservissement du champ magnétique peut être accompli de façon scalaire ou vectorielle par la fréquence de Larmor ou vectorielle par une recherche de champ magnétique total nul.Since helium 3 is devoid of sub-levels with zero angular momentum index, it is necessary to operate the device with a constant magnetic field, which can be obtained by an artificial field controlled with or without a magnetic shield. The enslavement of the magnetic field can be accomplished in a scalar or vector manner by the Larmor or vector frequency by a zero total magnetic field search.

Le dispositif d'application de champ magnétique 9 peut à la fois générer le champ magnétique servant à la mesure de la résonnance s'il est composé de bobines triaxiales asservies.The magnetic field application device 9 can both generate the magnetic field for the measurement of resonance if it is composed of triaxial coils.

Dans une conception perfectionnée, le dispositif d'application de champ 9 émet des champs magnétiques à des radiofréquences de pulsations notées Ω, et ω, qui sont perpendiculaires entre eux et de direction dépendant de la polarisation (par exemple perpendiculaires aux rayons lumineux émis par l'excitateur 2 dans le cas d'une polarisation circulaire).In an improved design, the field application device 9 emits magnetic fields at radio frequencies of pulsations Ω, and ω, which are perpendicular to each other and of direction dependent on the polarization (for example perpendicular to the light rays emitted by the exciter 2 in the case of a circular polarization).

On se reporte à la figure 3. Le signal issu du dispositif de comptage 5 comprend plusieurs raies lumineuses, et d'abord une qui est à la fréquence f0 utile correspondant à la restitution des photons par le milieu gazeux et qui donne la référence à la mesure de temps. Il révèle encore des raies spectrales aux fréquences Q/2π, (ω-Ω)/2π, ω/2π, et (ω+Ω)/2π. Ces raies spectrales apparaissent pour des champs magnétiques de faibles valeurs, très inférieures à 1/δ.TR , où TR est le temps de relaxation des sous-niveaux et γ est leur rapport gyromagnétique, caractéristique de l'élément chimique excité. Elles correspondent à des résonances entre les sous-niveaux. Leur amplitude est proportionnelle au champ magnétique ambiant. Il est conforme à ce mode d'asservissement d'appliquer un champ magnétique de compensation du champ magnétique ambiant essentiellement statique, mais qu'on fait varier de façon continue en amplitude et en direction si nécessaire, de façon que l'amplitude de ces raies soit réduite autant que possible, ce qui signifie que le champ de compensation a équilibré le champ magnétique ambiant. La figure 4 montre alors que les sous-niveaux de chaque niveau principal sont à une même valeur d'énergie, si bien que les photons restitués par le milieu gazeux sont tous à la fréquence f0 utile : la raie spectrale correspondante apparaît sous forme d'un pic beaucoup plus fin et haut et dont la détection est donc facilitée. Il devient envisageable d'omettre le blindage magnétique traditionnel des horloges atomiques ; toutefois, comme le blindage magnétique filtre par effet de peau le champ électrique, un blindage électrique est avantageusement ajouté afin de ne pas perturber les niveaux d'énergie des atomes si le blindage magnétique est supprimé. Les amplitudes des champs de radiofréquences sont avantageusement choisies pour maximiser l'amplitude des raies spectrales de résonance (avant l'application du champ statique de compensation). On préconise de respecter approximativement les égalités γHω/ω = 1 et γHΩ/Ω = 1, où Hω et HΩ sont les amplitudes des champs de radiofréquences de pulsations ω et Ω. Avantageusement, le dispositif d'application du champ magnétique 9 applique à la fois le champ magnétique sensiblement statique de compensation et les champs magnétiques de radiofréquences.We refer to the figure 3 . The signal coming from the counting device 5 comprises several light lines, and first one which is at the useful frequency f 0 corresponding to the restitution of the photons by the gaseous medium and which gives the reference to the measurement of time. It also reveals spectral lines at frequencies Q / 2π, (ω-Ω) / 2π, ω / 2π, and (ω + Ω) / 2π. These spectral lines appear for magnetic fields of low values, much lower than 1 / δ. T R , where T R is the relaxation time of the sub-levels and γ is their gyromagnetic ratio, characteristic of the excited chemical element. They correspond to resonances between the sub-levels. Their amplitude is proportional to the ambient magnetic field. It is consistent with this mode of servocontrol to apply a magnetic field of compensation of the essentially static ambient magnetic field, but which is varied continuously in amplitude and in direction if necessary, so that the amplitude of these lines is reduced as much as possible, which means that the compensation field has balanced the ambient magnetic field. The figure 4 then shows that the sub-levels of each main level are at the same energy value, so that the photons restored by the gaseous medium are all at the frequency f 0 useful: the corresponding spectral line appears in the form of a peak much more fine and high and whose detection is facilitated. It becomes possible to omit the traditional magnetic shielding of atomic clocks; however, as the magnetic shielding filters the electric field by skin effect, an electrical shield is advantageously added so as not to disturb the energy levels of the atoms if the magnetic shielding is removed. The amplitudes of the radiofrequency fields are advantageously chosen to maximize the amplitude of the resonance spectral lines (before the application of the static compensation field). It is recommended to respect approximately the equalities γHω / ω = 1 and γHΩ / Ω = 1, where Hω and HΩ are the amplitudes of the radio frequency fields of pulsations ω and Ω. Advantageously, the magnetic field application device 9 applies both the substantially static magnetic compensation field and the radio frequency magnetic fields.

Il peut consister en des bobines triaxiales, ou en trois bobines monoaxiales concentriques entre elles. L'asservissement est accompli par tout matériel connu comprenant une unité de calcul. Les bobines sont pilotées en courant ou en tension. L'excitation à la fréquence de résonance f0 est accomplie par une modulation en amplitude de la diode laser à la fréquence f0/2 ou par une cavité à micro-ondes résonnant à la fréquence f0. Un excitateur comprenant deux lasers dont l'écart en fréquence est f0 peut aussi être envisagé.It may consist of triaxial coils, or three monoaxial coils concentric with each other. The enslavement is accomplished by any known hardware including a computing unit. The coils are driven by current or voltage. The excitation at the resonance frequency f 0 is accomplished by amplitude modulation of the laser diode at the frequency f 0/2 or by a microwave cavity resonating at the frequency f 0 . An exciter comprising two lasers, the frequency deviation is f 0 can also be envisaged.

L'hélium 3 étant dépourvu de sous-niveaux à indice de moment angulaire nul, il est nécessaire de faire fonctionner le dispositif à champ magnétique constant, ce qui peut être obtenu par un champ artificiel asservi avec ou sans un blindage magnétique. L'asservissement du champ magnétique peut être accompli de façon scalaire ou vectorielle par la fréquence de Larmor ou vectorielle par une recherche de champ magnétique total nul.Since helium 3 is devoid of sub-levels with zero angular momentum index, it is necessary to operate the device with a constant magnetic field, which can be obtained by an artificial field controlled with or without a magnetic shield. The enslavement of the magnetic field can be accomplished in a scalar or vector manner by the Larmor or vector frequency by a zero total magnetic field search.

Le dispositif d'application de champ magnétique 9 peut à la fois générer le champ magnétique servant à la mesure de la résonnance s'il est composé de bobines triaxiales asservies.The magnetic field application device 9 can both generate the magnetic field for the measurement of resonance if it is composed of triaxial coils.

L'instrument mesurant le flux laser peut être une photodiode de type InGaAs. Ce mode de réalisation, comprenant un dispositif de stabilisation du champ magnétique, ne comprend pas de blindage magnétique. Cependant, il est également possible d'utiliser un blindage magnétique en plus du dispositif d'asservissement du champ magnétique tel que précédemment décrit. Le blindage magnétique peut être composé par exemple d'un cylindre de fer doux et de cylindre de µ métal imbriqués.The instrument measuring the laser flux may be an InGaAs type photodiode. This embodiment, comprising a magnetic field stabilizing device, does not include magnetic shielding. However, it is also possible to use a magnetic shield in addition to the magnetic field servo device as previously described. The magnetic shielding may be composed of, for example, a soft iron cylinder and a nested metal cylinder.

L'excitateur 2 pourrait comprendre une lampe ou une VCSEL (pour Variation capacity surface emitting light). En l'absence d'un dispositif de stabilisation du champ magnétique ambiant, L'excitation à la fréquence de résonnance pourrait aussi être procurée par une cavité micro-onde résonnante ou par deux lasers dont l'écart des fréquences est la fréquence de résonance.Exciter 2 could include a lamp or VCSEL (for Variation capacity surface emitting light). In the absence of a device for stabilizing the ambient magnetic field, excitation at the resonance frequency could also be provided by a resonant microwave cavity or by two lasers whose frequency difference is the resonance frequency.

Claims (10)

Horloge atomique comprenant une cellule emplie d'un milieu de mesure, un premier dispositif excitateur (1) de particules du milieu de mesure jusqu'à un niveau d'énergie supérieur, un système (4, 6, 7) recueillant une fréquence d'énergie lumineuse restituée par le milieu de mesure en quittant le niveau d'énergie supérieur, ladite fréquence d'énergie lumineuse étant exploitée pour donner une mesure du temps, un dispositif (9) d'application de champs magnétiques comprenant au moins un champ magnétique essentiellement statique et des moyens d'asservissement (8) dudit dispositif (9) pour ajuster les champs magnétiques, caractérisée en ce que le milieu de mesure comprend du plasma d'hélium 3, et il est prévu un second dispositif excitateur (10) pour susciter le plasma d'hélium 3 à partir d'hélium 3 gazeux.Atomic clock comprising a cell filled with a measuring medium, a first exciter device (1) of particles of the measuring medium up to a higher energy level, a system (4, 6, 7) collecting a frequency of light energy restored by the measuring medium by leaving the higher energy level, said light energy frequency being used to give a measurement of time, a device (9) for applying magnetic fields comprising at least one magnetic field essentially static and servo means (8) of said device (9) for adjusting the magnetic fields, characterized in that the measuring medium comprises helium plasma 3, and there is provided a second exciter device (10) for generating helium plasma 3 from gaseous helium 3. Horloge atomique suivant la revendication 1, caractérisée en ce que le second dispositif excitateur (10) est un générateur d'ondes de radiofréquences de puissance.Atomic clock according to Claim 1, characterized in that the second exciter device (10) is a power radio frequency wave generator. Horloge atomique suivant la revendication 2, caractérisée en ce que les ondes de radiofréquences sont comprises entre 20 MHz et 30 MHz.Atomic clock according to Claim 2, characterized in that the radio frequency waves are between 20 MHz and 30 MHz. Horloge atomique suivant la revendication 2, caractérisée en ce que les ondes de radiofréquences ont une puissance de 1W pour une quantité d'hélium 3 de 100 mm3 à une pression de 0,1 torr environ.Atomic clock according to Claim 2, characterized in that the radio frequency waves have a power of 1 W for a quantity of helium 3 of 100 mm 3 at a pressure of approximately 0.1 torr. Horloge atomique suivant l'une quelconque des revendications 1 à 4, caractérisée en ce que le niveau d'énergie supérieur, duquel le milieu de mesure restitue la fréquence d'énergie lumineuse exploitée pour donner la mesure du temps, est le niveau métastable 23S1.Atomic clock according to one of Claims 1 to 4, characterized in that that the higher energy level, from which the measuring medium renders the frequency of light energy exploited to give the measurement of time, is the metastable level 2 3 S 1 . Horloge atomique suivant la revendication 5, caractérisée en ce que le milieu de mesure est composé exclusivement d'hélium 3, ayant une teneur de 1 partie par million d'atomes portés au niveau métastable, quand le second dispositif excitateur fonctionne.Atomic clock according to claim 5, characterized in that the measuring medium is composed exclusively of helium 3, having a content of 1 part per million atoms carried at the metastable level, when the second exciter device operates. Horloge atomique suivant l'une quelconque des revendications 1 à 6, caractérisée en ce que le premier dispositif excitateur (1) comprend un faisceau de laser, et les champs magnétiques appliqués par le dispositif (9) comprennent au moins un champ magnétique oscillant.Atomic clock according to any one of claims 1 to 6, characterized in that the first exciting device (1) comprises a laser beam, and the magnetic fields applied by the device (9) comprise at least one oscillating magnetic field. Horloge atomique suivant la revendication 7, caractérisée en ce que les champs magnétiques appliqués par le dispositif (9) comprennent deux champs magnétiques oscillants perpendiculaires entre eux.Atomic clock according to Claim 7, characterized in that the magnetic fields applied by the device (9) comprise two oscillating magnetic fields perpendicular to one another. Horloge atomique suivant l'une quelconque des revendications 7 et 8, caractérisée en ce que le champ magnétique essentiellement statique est précisément orienté par rapport au champ magnétique oscillant ou aux champs magnétiques oscillants.An atomic clock according to any one of claims 7 and 8, characterized in that the essentially static magnetic field is precisely oriented with respect to the oscillating magnetic field or the oscillating magnetic fields. Horloge atomique suivant l'une quelconque des revendications 1 à 9, caractérisée en qu'elle comprend un blindage magnétique qui l'entoure.An atomic clock as claimed in any one of claims 1 to 9, characterized in that it comprises a surrounding magnetic shield.
EP10165184A 2009-06-11 2010-06-08 Atomic clock operated with Helium-3 Not-in-force EP2261758B1 (en)

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US8183942B2 (en) 2012-05-22
JP2010286490A (en) 2010-12-24
FR2946766A1 (en) 2010-12-17
US20100315173A1 (en) 2010-12-16
ATE536573T1 (en) 2011-12-15
FR2946766B1 (en) 2011-07-01

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