EP2261758B1 - Atomic clock operated with Helium-3 - Google Patents

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

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.

The frequency of the signal of the photons restored by the gas is defined by the formula ν = ΔE / h, where ν is the frequency, ΔE the difference between the energy levels and h the Planck constant, equal to 6.62x10 ^-34 J / s.

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.

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 ).

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.

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 .

The documents WO 2009/074616 (published June 18, 2009) and WO 2009/074619 (also published on 18 June 2009) belong to the state of the art within the meaning of Article 54 (3) EPC. These documents describe gaseous helium-3 atomic clocks. It is therefore not a helium-3 plasma.

The object of the invention is to improve existing clocks.

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.

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.

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.

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, the clock comprising a second exciter device (10) for generating the helium plasma 3 from helium 3 gas.

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.

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 .

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.

• 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.

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.

There is also a second excitation device 10 for obtaining a helium plasma 3 from the gas of helium 3.

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.

The magnetic field application device 9 makes it possible to apply a magnetic field H _o 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.

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.

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.

The magnetic field application device 9 can both generate the magnetic field for the measurement of resonance if it is composed of triaxial coils.

In an improved design, the field application device 9 emits magnetic fields at radio frequencies of pulsations denoted 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).

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 the frequencies Ω / 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 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 continuously varied in amplitude and amplitude. 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.

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.

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 frequency or vector 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. 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.

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)

1. Atomic clock comprising a cell filled with a measurement medium, a first device (1) for exciting particles of the measurement medium up to a higher energy level, a system (4, 6, 7) collecting a light energy frequency returned by the measurement medium on leaving the higher energy level, said light energy frequency being exploited to give a time measurement, a device (9) for applying magnetic fields comprising at least one essentially static magnetic field and means (8) for controlling said device (9) to adjust the magnetic fields, characterised in that the measurement medium comprises helium 3 plasma, the clock comprising a second exciter device (10) to give rise to helium 3 plasma from gaseous helium 3.
2. Atomic clock according to claim 1, characterised in that the second exciter device (10) is a power radiofrequency wave generator.
3. Atomic clock according to claim 2, characterised in that the radiofrequency waves are between 20 MHz and 30 MHz.
4. Atomic clock according to claim 2, characterised in that the radiofrequency waves have a power of 1W for a quantity of helium 3 of 100 mm³ at a pressure of around 0.1 torr.
5. Atomic clock according to any of claims 1 to 4, characterised in that the higher energy level, from which the measurement medium returns the light energy frequency exploited to give the time measurement, is the metastable level 2³S₁.
6. Atomic clock according to claim 5, characterised in that the measurement medium is composed exclusively of helium 3, having a level of 1 part per million of atoms taken to the metastable level, when the second exciter device operates.
7. Atomic clock according to any of claims 1 to 6, characterised in that the first exciter device (1) comprises a laser beam, and the magnetic fields applied by the device (9) comprise at least one oscillating magnetic field.
8. Atomic clock according to claim 7, characterised in that the magnetic fields applied by the device (9) comprise two mutually perpendicular oscillating magnetic fields.
9. Atomic clock according to any of claims 7 and 8, characterised in that the essentially static magnetic field is precisely oriented in relation to the oscillating magnetic field or to the oscillating magnetic fields.
10. Atomic clock according to any of claims 1 to 9, characterised in that it comprises a magnetic shielding that surrounds it.

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