Patent ID: 12189345

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, preferred embodiments of an atomic cell, a method for manufacturing an atomic cell, and a quantum interference device according to the present disclosure will be described in detail with reference to the accompanying drawings.

1. Quantum Interference Device

First, an atomic oscillator200which is a quantum interference device according to an embodiment will be described. In addition to the atomic oscillator200, the quantum interference device according to the present disclosure can also be applied to, for example, a magnetic sensor, a quantum memory, an atomic gyroscope, or the like.

FIG.1is a functional block diagram showing the atomic oscillator200which is the quantum interference device according to the embodiment.FIG.2is a diagram showing an energy state of an alkali metal in an atomic cell100of the atomic oscillator200shown inFIG.1.FIG.3is a graph showing a relationship between a frequency difference in two types of light emitted from a light emitting unit and a detection intensity detected by a light detection unit in the atomic oscillator200shown inFIG.1.FIG.4is a cross-sectional view schematically showing the atomic cell100shown inFIG.1.

1.1. Atomic Oscillator

The atomic oscillator200is an atomic oscillator using a coherent population trapping. The atomic oscillator200using the coherent population trapping can be reduced in size as compared with an atomic oscillator using a double resonance effect.

As shown inFIG.1, the atomic oscillator200includes the atomic cell100, a light emitting unit210, optical components220,222,224, and226, a light detection unit230, a heater240, a temperature sensor250, a magnetic field generating unit260, and a control unit270.

An alkali metal in a gaseous state is sealed in the atomic cell100. Examples of the alkali metal include rubidium, cesium, and sodium.

As shown inFIG.2, the alkali metal has three-level energy levels, and has three states, that is, an excited state and two ground states (ground states1and2) having different energy levels. Here, the ground state1is an energy state lower than that of the ground state2.

With respect to the alkali metal in the gaseous state, when the alkali metal in the gaseous state is irradiated with the two types of resonance light L1and L2having different frequencies, a light absorptance (light transmittance) in the alkali metal of the resonance light L1or L2changes according to a difference (ω1-ω2) between a frequency ω1of the resonance light L1and a frequency ω2of the resonance light L2. When the difference (ω1-ω2) between the frequency ω1of the resonance light L1and the frequency ω2of the resonance light L2coincides with a frequency corresponding to an energy difference between the ground state1and the ground state2, the excitation from the ground states1and2to the excited state is stopped. At this time, the resonance light L1and L2are both transmitted without being absorbed by the alkali metal. Such a phenomenon is called a CPT phenomenon or an electromagnetically induced transparency (EIT) phenomenon.

Here, for example, when the frequency ω1of the resonance light L1is fixed and the frequency ω2of the resonance light L2is changed, when the difference (ω1-ω2) between the frequency ω1of the resonance light L1and the frequency ω2of the resonance light L2coincides with a frequency ω0corresponding to the energy difference between the ground state1and the ground state2, a detection intensity of the light detection unit230increases rapidly as shown inFIG.3. Such a steep signal is referred to as an EIT signal. The EIT signal has an eigenvalue determined by the type of the alkali metal. Therefore, by using the EIT signal as a reference, it is possible to implement the atomic oscillator200with high accuracy. Hereinafter, units of the atomic oscillator200will be described in order.

1.1.1. Light Emitting Unit

The light emitting unit210emits an excitation light L for exciting the alkali metal in the atomic cell100. Specifically, the light emitting unit210emits two types of light (resonance light L1and resonance light L2) having different frequencies as described above as the excitation light L.

The resonance light L1excites the alkali metal in the atomic cell100from the above-described ground state1to the excited state. On the other hand, the resonance light L2excites the alkali metal in the atomic cell100from the above-described ground state2to the excited state.

The light emitting unit210is not particularly limited as long as the light emitting unit210can emit the excitation light as described above, and for example, a semiconductor laser such as a vertical cavity surface emitting laser (VCSEL) is used.

The light emitting unit210is coupled to an excitation light control unit272of the control unit270, which will be described later, and is driven and controlled based on a detection result of the light detection unit230. A temperature of the light emitting unit210is adjusted to a predetermined temperature by a temperature control element (not shown) such as a heating resistor and a Peltier element.

1.1.2. Optical Component

The optical components220,222,224, and226are provided on an optical path of the excitation light L between the light emitting unit210and the atomic cell100. In the example shown inFIG.1, the first optical component220, the second optical component222, the third optical component224, and the fourth optical component226are disposed in this order from the light emitting unit210toward the atomic cell100.

The first optical component220is a collimator lens. The first optical component220can irradiate the atomic cell100with the excitation light L without waste. The first optical component220has a function of converting the excitation light L into parallel light. Therefore, it is possible to easily prevent the excitation light L from being reflected by an inner wall of the atomic cell100. Accordingly, the resonance of the excitation light in the atomic cell100is suitably generated, and as a result, oscillation characteristics of the atomic oscillator200can be improved.

The second optical component222is a polarizing plate. The second optical component222can adjust the polarization of the excitation light L from the light emitting unit210in a predetermined direction.

The third optical component224is a neutral density filter (ND filter). The third optical component224can adjust (decrease) an intensity of the excitation light L incident on the atomic cell100. Therefore, even when an output of the light emitting unit210is large, the excitation light incident on the atomic cell100can be set to a desired light amount.

The fourth optical component226is a λ/4 wave plate. The fourth optical component226can convert the excitation light L, which is from the light emitting unit210, from linearly polarized light to circularly polarized light (right-handed circularly polarized light or left-handed circularly polarized light).

1.1.3. Light Detection Unit

The light detection unit230detects the intensity of the excitation light L (resonance light L1and L2) transmitted through the atomic cell100. The light detection unit230is not particularly limited as long as the light detection unit230can detect the excitation light L, and for example, a light detector (light receiving element) such as a solar cell or a photodiode is used. The light detection unit230is coupled to the excitation light control unit272of the control unit270, which will be described later.

1.1.4. Heater

The heater240heats the atomic cell100. Accordingly, the alkali metal in the atomic cell100can be maintained in the gaseous state at an appropriate concentration. The heater240generates heat by energization, and includes, for example, two heating resistors (not shown) provided on an outer surface of the atomic cell100. The heater240is electrically coupled to a temperature control unit274of the control unit270, which will be described later.

As will be described later, the atomic cell100shown inFIG.4has window portions14and16through which the excitation light L passes. One of the heating resistors is provided on the window portion14, which is an incident side window portion, and the other heating resistor is provided on the window portion16, which is an emission side window portion. Such a heating resistor is made of a material having transparency to the excitation light L, for example, a transparent electrode material of an oxide such as indium tin oxide (ITO), indium zinc oxide (IZO), In3O3, SnO2, Sb-containing SnO2, or Al-containing ZnO.

The heating resistor can be formed by, for example, a CVD method such as plasma CVD or thermal CVD, a dry plating method such as vacuum deposition, or a sol-gel method.

The heater240is not limited to the above-described form as long as the heater240can heat the atomic cell100, and various types of heaters can be used. In addition, the heater240may be in non-contact with the atomic cell100. The atomic cell100may be heated using the Peltier element instead of the heater240or in combination with the heater240.

1.1.5. Temperature Sensor

The temperature sensor250detects a temperature of the heater240or the atomic cell100. Then, a heat generating amount of the heater240is controlled based on a detection result of the temperature sensor250. Accordingly, the alkali metal in the atomic cell100can be maintained at a desired temperature. An installation position of the temperature sensor250is not particularly limited, and may be, for example, on the heater240or on the outer surface of the atomic cell100.

The temperature sensor250is not particularly limited, and various known temperature sensors such as a thermistor and a thermocouple can be used. The temperature sensor250is electrically coupled to the temperature control unit274of the control unit270, which will be described later, via a wiring (not shown).

1.1.6. Magnetic Field Generating Unit

The magnetic field generating unit260generates a magnetic field for Zeeman splitting a plurality of energy levels in which the alkali metal in the atomic cell100is degraded. Accordingly, a gap between different energy levels in which the alkali metal is degraded can be expanded, and a resolution can be improved. As a result, accuracy of an oscillation frequency of the atomic oscillator200can be improved.

The magnetic field generating unit260is implemented by, for example, a Helmholtz coil that sandwiches the atomic cell100or a solenoid coil that covers the atomic cell100. Accordingly, a uniform magnetic field in one direction can be generated in the atomic cell100. The magnetic field generated by the magnetic field generating unit260is a constant magnetic field (DC magnetic field), and an AC magnetic field may be superimposed. The magnetic field generating unit260is electrically coupled to a magnetic field control unit276of the control unit270, which will be described later, and is energized and controlled.

1.1.7. Control Unit

The control unit270controls operations of the light emitting unit210, the heater240, and the magnetic field generating unit260. The control unit270includes the excitation light control unit272that controls the frequencies of the resonance light L1and L2of the light emitting unit210, the temperature control unit274that controls the temperature of the alkali metal in the atomic cell100, and the magnetic field control unit276that controls the magnetic field from the magnetic field generating unit260.

The excitation light control unit272controls, based on the detection result of the light detection unit230, the frequencies of the resonance light L1and L2emitted from the light emitting unit210. Specifically, the excitation light control unit272controls the frequencies of the resonance light L1and L2emitted from the light emitting unit210such that the difference (ω1-ω2) between the frequency ω1and the frequency ω2detected by the light detection unit230is the frequency ω0specified by the above-described alkali metal. In addition, the excitation light control unit272controls center frequencies of the resonance light L1and L2emitted from the light emitting unit210. Further, although not shown, the excitation light control unit272includes a voltage-controlled crystal oscillator (oscillation circuit), and outputs an oscillation frequency of the voltage-controlled crystal oscillator as an output signal of the atomic oscillator200while synchronizing and adjusting the oscillation frequency based on the detection result of the light detection unit230.

The temperature control unit274controls the energization of the heater240based on the detection result of the temperature sensor250. Accordingly, the temperature of the atomic cell100can be maintained within a desired range.

The magnetic field control unit276controls the energization of the magnetic field generating unit260such that the magnetic field generated by the magnetic field generating unit260is constant.

Such a control unit270is provided in, for example, an integrated circuit (IC) chip mounted on a substrate.

2. Atomic Cell

Next, the atomic cell100according to the embodiment will be described.

FIG.4is a cross-sectional view schematically showing the atomic cell100according to the embodiment.FIG.5is an enlarged view of a region A shown inFIG.4.FIG.6is a perspective view schematically showing the atomic cell100according to the embodiment. In the drawings of the present application, an X axis, a Y axis, and a Z axis are set as three axes orthogonal to one another. Each axis is represented by an arrow, and a tip end side is referred to as a “plus side” and a base end side is referred to as a “minus side”. In addition, for example, both a plus side direction and a minus side direction of the X axis are referred to as an “X axis direction”. The same applies to a Y axis direction and a Z axis direction.

The atomic cell100shown inFIG.4includes a substrate10and a coating layer20. As shown inFIG.5, the coating layer20includes a first coating layer22, a second coating layer24, and a third coating layer26.

2.1. Substrate

The substrate10includes a body portion12and a pair of the window portions14and16provided with the body portion12sandwiched therebetween. As shown inFIG.4, the body portion12has a columnar through hole13having an axis extending in the Z axis direction. The through hole13forms a part of an inner wall of an internal space S of the atomic cell100. A cross-sectional shape of the through hole13is not limited to a circle, and may be, for example, a polygon such as a quadrangle or a pentagon, and an ellipse. Examples of a constituent material of the body portion12include glass, crystal, metal, resin, and silicon.

The window portions14and16sandwich the body portion12in a manner of closing the through hole13. The window portions14and16each have a plate shape extending along an X-Y plane. The window portions14and16form another portion of the inner wall of the internal space S. That is, the inner wall of the internal space S is formed by the through hole13of the body portion12and the window portions14and16. The internal space S is filled with the alkali metal in the gaseous state. A part of the alkali metal filling the internal space S may be present as a gas, and the remaining part may be present as a surplus component in a form of a liquid or a solid. In addition, in the internal space S, as necessary, a rare gas such as argon or neon, or an inert gas such as nitrogen may be sealed as a buffer gas together with the alkali metal.

The window portions14and16each have a through hole17communicating with the internal space S. The through hole17is sealed with a sealing material18. Accordingly, the internal space S can be hermetically sealed. Examples of a constituent material of the sealing material18include a metal material such as a silver solder and an Au/Sn alloy, and low melting point glass such as vanadium-based glass. The through hole17may be formed only in one of the window portions14and16.

The constituent material of the window portions14and16is a material through which the excitation light L emitted from the light emitting unit210of the atomic oscillator200passes. The excitation light L excites the gaseous alkali metal. In addition, the constituent material of the window portions14and16includes a compound having a polar group. Specifically, examples of the constituent material of the window portions14and16include quartz glass and borosilicate glass. Examples of the polar group include a hydroxyl group.

2.2. Coating Layer

FIG.7is a cross-sectional view schematically showing an inner wall10aof the substrate10before the coating layer20inFIG.5is formed.FIG.8is a cross-sectional view schematically showing the first coating layer22and the second coating layer24that are formed on the inner wall10ashown inFIG.7.

As shown inFIG.7, the inner wall10amade of a material containing a compound having the above-described polar group has, for example, a hydroxyl group. Since the constituent material of the window portions14and16described above contains silicon and oxygen, such a hydroxyl group is easily present on the inner wall10a, which is useful in the formation of the coating layer20.

A thickness of the coating layer20is not particularly limited, and is preferably 10 nm or more and 3000 nm or less, and more preferably 50 nm or more and 1000 nm or less. When the thickness of the coating layer20is less than the lower limit value, relaxation of an electron spin state of the alkali metal may not be sufficiently prevented depending on the composition or the like of the coating layer20. On the other hand, when the film thickness of the coating layer20exceeds the upper limit value, a long-term stability of the coating layer20may decrease depending on the composition or the like of the coating layer20.

The thickness of the coating layer20can be measured based on an observation result by, for example, a microscope such as a transmission electron microscope (TEM), a scanning tunneling microscope (STM), or an atomic force microscope (AFM).

2.2.1. First Coating Layer

The first coating layer22is provided on a surface of the inner wall10aof the substrate10. The first coating layer22may be provided at least on the inner walls of the window portions14and16in the inner wall10a, and as shown inFIG.4, the first coating layer22is preferably provided on the inner walls of the window portions14and16and an inner wall of the body portion12. Although not shown, the first coating layer22may be provided on an inner wall of the sealing material18. A thickness of the first coating layer22is, for example, preferably 1 nm or more and 1000 nm or less, and more preferably 10 nm or more and 100 nm or less.

The first coating layer22is formed of a first molecule that is a metal oxide. That is, the first coating layer22is formed of a compound derived from the first molecule. As the metal oxide, an oxide of any metal may be used, and in particular, a tantalum oxide (TaOx), a zirconium oxide (ZrOx), a hafnium oxide (HfOx), or a titanium oxide (TiOx) is preferably used. As shown inFIG.8, oxygen of the first molecule substitutes the hydroxyl group present on the inner wall10a, and bonds a metal atom of the first molecule and silicon present on the inner wall10a. InFIG.8, tantalum atoms are shown as examples of metal atoms.

The metal oxide described above is a metal oxide that hardly chemically reacts with an alkali metal. That is, the oxygen of the first molecule bonding the metal atom of the first molecule and the silicon of the constituent material of the inner wall10ais not substituted with the alkali metal. Accordingly, it is possible to reduce the chance that the coating layer20is peeled off from the inner wall10a.

2.2.2. Second Coating Layer

The second coating layer24is provided on a surface of the first coating layer22(laminated on the first coating layer22). A thickness of the second coating layer24is, for example, preferably 1 nm or more and 1000 nm or less, more preferably 5 nm or more and 500 nm or less, and still more preferably 10 nm or more and 300 nm or less.

The second coating layer24is formed of a second molecule having a non-polar group and a reactive group that undergoes a desorption reaction with the first coating layer22. Specifically, the second coating layer24is formed by chemically reacting the reactive group of the second molecule with the first coating layer22. That is, the second coating layer24is formed of a compound derived from the second molecule.

Examples of the second molecule forming the second coating layer24include a coupling agent, a metal alkoxide, an alcohol, and a polyimide.

As the second molecule, various coupling agents are preferably used, and a silane coupling agent is more preferably used. The coupling agent generally has a reactive group and a functional group, and is used for the purpose of introducing the functional group into an object to be treated. The coupling agent used as the second molecule has a non-polar group as the functional group. By using the coupling agent, various non-polar groups can be introduced into the second coating layer24at high density. Accordingly, chemical bonding or physical adsorption can be caused between the second coating layer24and the third coating layer26, and peeling between the second coating layer24and the third coating layer26can be particularly prevented. As a result, it is possible to improve a heat resistance of the atomic cell100, and it is possible to implement the atomic oscillator200that stably operates without using an expensive heat dissipation mechanism even in a high-temperature environment in which, for example, direct sunlight is incident.

In the coupling agent, the reactive group and the non-polar group are located at opposite end portions of a molecular chain. Therefore, when the reactive group is bonded to the first coating layer22, a probability of the non-polar group being oriented to an opposite side from the first coating layer22increases. Accordingly, when a third molecule is supplied to a surface of the second coating layer24, orientation and adhesion of the third molecule can be easily improved. As a result, the third coating layer26having a high degree of crystallinity and adhesion force is finally obtained.

When the second molecule is, for example, a silane coupling agent, the second molecule is represented by the following Formula (I). Other coupling agents are the same as those of the silane coupling agent described below.

In Formula (I), each of R1, R2and R3independently represents a hydrogen atom, an alkoxy group, a halogen atom, or an alkyl group. At least one of R1, R2and R3is an alkoxy group or a halogen atom which is a reactive group. R1, R2and R3may be the same as or different from one another. n is, for example, an integer of 1 or more and 20 or less, preferably 4 or more and 18 or less, and more preferably 6 or more and 12 or less. In the present specification, —(CH2)n— bond is referred to as a spacer. In addition, X is a non-polar group.

When the reactive group is, for example, an alkoxy group, the alkoxy group is substituted with the oxygen derived from the first molecule by a desorption reaction such as a dehydration reaction or a dealcoholization reaction. As a result, as shown inFIG.8, the oxygen derived from the first molecule binds to, for example, a metal atom derived from the first molecule and a silicon atom of the second molecule. In addition, when the reactive group is, for example, a halogen atom, the halogen atom is substituted with the oxygen derived from the first molecule.

The number of reactive groups present in one molecule is preferably 2 or 3, and more preferably 3. The larger the number of the reactive groups, the higher the adhesion force when the second molecule is bonded to the first coating layer22. Accordingly, the second coating layer24is less likely to peel off.

Examples of the non-polar group X include a linear alkyl group, a cycloalkyl group, a vinyl group, an alkenyl group, and a phenyl group. Among these groups, the non-polar group of the coupling agent is preferably a linear alkyl group, a vinyl group, or a phenyl group. Accordingly, a particularly strong intermolecular attractive force between the non-polar group and the third coating layer26derived from the non-polar third molecule is generated. Accordingly, the adhesion force between the second coating layer24and the third coating layer26can be particularly increased.

When the non-polar group X is a long-chain alkyl group, the non-polar group X and the spacer can be regarded as the long-chain alkyl group. In this case, the number of carbon atoms of the long-chain alkyl group is preferably 6 or more and 24 or less, and more preferably 10 or more and 20 or less. Accordingly, non-polarity of the second coating layer24is particularly significant, and the second coating layer24can impart high orientation to the third molecule, so that the crystallinity of the third coating layer26can be particularly improved. As a result, it is possible to effectively prevent the electron spin state of the alkali metal filling the atomic cell100from being relaxed. The second coating layer24shown inFIG.8illustrates an example in which the non-polar group X is a long-chain alkyl group.

Examples of the silane coupling agent containing such a long-chain alkyl group include octadecyltrimethoxysilane (ODS, CH3(CH2)17Si(OCH3)3) or octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3).

When the non-polar group X is a long-chain alkyl group, the third molecule forming the third coating layer26is physically adsorbed to the second molecule forming the second coating layer24.FIG.9is a schematic view showing a state in which, when the silane coupling agent containing the long-chain alkyl group is used as the second molecule and paraffin is used as the third molecule, the third molecule is physically adsorbed to the second molecule. Since the paraffin is a non-polar molecule but has a large molecular weight, a strong intermolecular attractive force is generated between the paraffin and the long-chain alkyl group of the second molecule. Accordingly, as shown inFIG.9, the paraffin which is the third molecule is physically adsorbed to the long-chain alkyl group of the second molecule.

When the non-polar group X is a vinyl group, the non-polarity of the second coating layer24is particularly significant. Accordingly, the second coating layer24can impart the high orientation to the third molecule, and the crystallinity of the third coating layer26can be particularly improved. The third molecule forming the third coating layer26is physically adsorbed to the second molecule forming the second coating layer24.

When the non-polar group X is a vinyl group, for example, by a graft reaction of the vinyl group, at least a part of the third molecule forming the third coating layer26may be chemically bonded to the second molecule forming the second coating layer24. Since the second molecule and the third molecule are chemically bonded to each other, the adhesion force between the second coating layer24and the third coating layer26can be further increased.

When the non-polar group X is a vinyl group, the number of carbon atoms of the spacer is preferably 2 or more and 14 or less, and more preferably 6 or more and 12 or less. Accordingly, since a degree of freedom of the vinyl group is improved, a reactivity of the vinyl group is particularly high, and the degree of crystallinity and the adhesion force of the third coating layer26can be particularly improved.

When the non-polar group X is a phenyl group, the non-polarity of the second coating layer24is particularly significant. Accordingly, the second coating layer24can impart the high orientation to the third molecule, and the crystallinity of the third coating layer26can be particularly improved. The third molecule forming the third coating layer26is physically adsorbed to the second molecule forming the second coating layer24.

When the non-polar group X is a phenyl group, for example, by a CH/π interaction or a π/π interaction, at least a part of the third molecule forming the third coating layer26can be chemically bonded to the second molecule forming the second coating layer24. Accordingly, the adhesion force between the second coating layer24and the third coating layer26can be further increased.

When the non-polar group X is a phenyl group, the number of carbon atoms of the spacer is preferably 2 or more and 14 or less, and more preferably 6 or more and 12 or less. Accordingly, since a degree of freedom of the phenyl group is improved, a reactivity of the phenyl group is particularly high, and the degree of crystallinity and the adhesion force of the third coating layer26can be particularly improved.

When the second molecule forming the second coating layer24is a metal alkoxide, examples of the second molecule include a titanium coupling agent, an aluminum coupling agent, and a zirconium coupling agent.

When the second molecule forming the second coating layer24is an alcohol, examples of the second molecule include linear alcohols such as decyl alcohol and octadecyl alcohol.

2.2.3. Third Coating Layer

The third coating layer26is provided on a surface of the second coating layer24(laminated on the second coating layer24). The third coating layer26is formed of the non-polar third molecule. That is, the third coating layer26is formed of a compound derived from the third molecule. A thickness of the third coating layer26is, for example, preferably 1 nm or more and 1000 nm or less, more preferably 5 nm or more and 500 nm or less, and still more preferably 10 nm or more and 300 nm or less.

The degree of crystallinity of the third coating layer26is 70% or more due to the action of the second coating layer24. According to the third coating layer26having such a degree of crystallinity, it is possible to obtain the coating layer20which is particularly hard to adsorb the alkali metal atom. As a result, the effect of preventing the relaxation of the electron spin state of the alkali metal filling the atomic cell100can be sufficiently increased.

The degree of crystallinity of the third coating layer26may be 70% or more, preferably 75% or more, and more preferably 80% or more.

The degree of crystallinity of the third coating layer26can be obtained based on an X-ray diffraction spectrum of the third coating layer26obtained by an X-ray diffraction method. Specifically, first, an X-ray diffraction spectrum of the coating layer20is obtained by the X-ray diffraction method. In the X-ray diffraction spectrum, a diffraction peak derived from a crystal component contained in the third coating layer26and a halo pattern derived from an amorphous component are mixed. Therefore, profile fitting is performed on the diffraction peak or the halo pattern based on data of a standard sample and a database. Accordingly, the diffraction peak and the halo pattern can be separated, and an area of the diffraction peak derived from the crystal component and an area of the halo pattern derived from the amorphous component can be obtained. Then, the degree of crystallinity [%] is obtained by the following calculation formula.

degree⁢of⁢crystallinity[%]=area⁢of⁢diffraction⁢peakarea⁢of⁢diffraction⁢peak+area⁢of⁢halo⁢pattern

Depending on an incident angle of X-rays in the X-ray diffraction method, the X-ray diffraction spectrum may be affected by the second coating layer24and the first coating layer22. In such a case, by reducing the incident angle of the X-rays with respect to the surface of the coating layer20, these influences can be reduced.

Examples of the third molecule forming the third coating layer26include polypropylene (PP), polyethylene (PE), polymethylpentene (PMP), paraffin, diacetylene, and diene. Since these third molecules are non-polar molecules, the non-polar groups are easily arranged with respect to the second coating layer24oriented outward. As a result, the third coating layer26having a high degree of crystallinity is obtained. In addition, a direction in which the third molecule is arranged is a direction intersecting the inner wall10a. That is, a major axis of the third molecule is arranged in a direction intersecting the surface of the inner wall10a. The arrangement in the intersecting direction refers to a state in which an angle formed by an extending direction of the major axis of the third molecule and the surface of the inner wall10ais 45° or more and 90° or less. In addition, the angle is preferably 60° or more and 90° or less.

The third molecule is particularly preferably paraffin or diacetylene. By the action of the second coating layer24, it is possible to form the third coating layer26having a particularly high crystallinity as compared with when PP, PE, PMP, or the like is used as the third molecule. Therefore, by using these third molecules, it is possible to form the third coating layer26which is particularly hard to adsorb the alkali metal atom.

In the present specification, an alkane having 20 or more carbon atoms is referred to as paraffin. The paraffin is represented by the following Formula (II).
CnH2n+2(II)

In Formula (II), n is preferably 20 or more, and more preferably 25 or more and 100 or less.

As represented by Formula (II), since the paraffin is a non-polar and inactive long-chain saturated hydrocarbon, the paraffin is particularly, densely, and easily aligned with respect to the non-polar group of the second coating layer24. Therefore, by using the paraffin as the third molecule, the third coating layer26which is particularly hard to adsorb the alkali metal atom is obtained. Therefore, by using the paraffin as the third molecule, the effect of preventing the relaxation of the electron spin state of the alkali metal is more significant.

When the third coating layer26is derived from the paraffin, a rocking curve of a paraffin (110) plane can be obtained from the X-ray diffraction spectrum of the third coating layer26. A half-value width of the rocking curve is preferably 10° or less, and more preferably 5° or less. At this time, it can be said that a degree of paraffin orientation in the third coating layer26is high. Therefore, the third coating layer26that satisfies the above condition contributes to the implementation of the coating layer20which is particularly hard to adsorb the alkali metal atom.

The diacetylene is represented by the following Formula (III).
R4C≡C—C≡CR5(III)

In Formula (III), R4and R5are each preferably an alkyl group. R4and R5may be the same alkyl group or different alkyl groups. The alkyl group may be branched, and is preferably linear from the viewpoint of further reducing surface free energy of the coating layer20. The number of carbon atoms of the alkyl group is preferably 6 or more and 24 or less, more preferably 8 or more and 20 or less, and still more preferably 10 or more and 18 or less. By setting the number of carbon atoms within the above range, the degree of crystallinity of the third coating layer26can be easily increased.

A triple bond contained in the diacetylene is cleaved by energy application such as heating to cause solid phase polymerization. Accordingly, a polydiacetylene compound is obtained. The polydiacetylene compound is a polymer obtained by polymerizing and crosslinking adjacent diacetylene monomers. In the polydiacetylene compound, a site derived from a substituent of the diacetylene is a side chain extending from a crosslinking site. The side chain is particularly, densely, and easily aligned with respect to the non-polar group of the second coating layer24. Accordingly, the third coating layer26having an excellent film strength due to the crosslinking site and having particularly small surface free energy is obtained. As a result, it is possible to implement the coating layer20, which is difficult to be peeled off and in which the effect of preventing the relaxation of the electron spin state of the alkali metal is particularly significant.

The third coating layer26derived from the diacetylene has an excellent heat resistance. That is, by forming a film by solid phase polymerization, the third coating layer26, which is less likely to be peeled off from the second coating layer24even under a high temperature, is obtained. Accordingly, it is possible to implement the atomic cell100and the atomic oscillator200which are excellent in resistance at a high temperature.

2.3. Effects According to Embodiment

As described above, the atomic cell100according to the embodiment is filled with the alkali metal and includes the substrate10, the first coating layer22, the second coating layer24, and the third coating layer26. The first coating layer22is provided on the inner wall10aof the substrate10and is derived from the first molecule. The second coating layer24is provided on the first coating layer22, and is derived from the second molecule having the non-polar group and the reactive group that undergoes the desorption reaction with the first molecule. The third coating layer26is provided on the second coating layer24and is derived from the non-polar third molecule. The third coating layer26has a degree of crystallinity of 70% or more.

According to such a configuration, since the degree of crystallinity of the third coating layer26is high, the alkali metal atom is particularly hard to be adsorbed to the third coating layer26. Therefore, it is possible to obtain the atomic cell100in which the effect of preventing the relaxation of the electron spin state of the alkali metal filling the atomic cell100is sufficiently high. By using such an atomic cell100, for example, in the atomic oscillator200, an intensity of the EIT signal can be increased, and a line width of the EIT signal (a half-value width of the EIT signal) can be reduced. As a result, the atomic oscillator200having an excellent frequency stability is obtained.

As described above, the third molecule is preferably paraffin or diacetylene. The paraffin is a non-polar and inactive long-chain saturated hydrocarbon, the paraffin is particularly, densely, and easily aligned with respect to the non-polar group of the second coating layer24. Therefore, when the third molecule is the paraffin, the third coating layer26which is particularly hard to adsorb the alkali metal atom is obtained. The diacetylene forms the third coating layer26having an excellent film strength due to solid phase polymerization, and is particularly, densely, and easily aligned with respect to the non-polar group of the second coating layer24. Therefore, when the third molecule is the diacetylene, the third coating layer26, which is particularly hard to adsorb the alkali metal atom and is less likely to be peeled off, is obtained.

As described above, the first molecule is preferably a tantalum oxide, a zirconium oxide, a hafnium oxide, or a titanium oxide. Since these metal oxides hardly chemically react with the alkali metal, the chance of the coating layer20being peeled off from the inner wall10acan be reduced.

As described above, the second molecule is preferably a coupling agent. By using the coupling agent as the second molecule, various non-polar groups can be introduced into the second coating layer24at high density. Accordingly, peeling between the second coating layer24and the third coating layer26can be particularly prevented. In addition, since the non-polar group can be oriented on the opposite side from the first coating layer22using self-organization of the coupling agent, the third molecule can be easily aligned. Accordingly, the third coating layer26having a particularly high degree of crystallinity is obtained.

The non-polar group of the second molecule is preferably a linear alkyl group. Accordingly, the non-polarity of the second coating layer24is particularly significant. As a result, the second coating layer24can impart the high orientation to the third molecule, and the crystallinity of the third coating layer26can be particularly improved.

The non-polar group of the second molecule is preferably a vinyl group. Accordingly, the non-polarity of the second coating layer24is particularly significant. As a result, the second coating layer24can impart the high orientation to the third molecule, and the crystallinity of the third coating layer26can be particularly improved. In addition, by a graft reaction of the vinyl group, the third molecule forming the third coating layer26can be bonded to the second molecule forming the second coating layer24.

The non-polar group of the second molecule is preferably a phenyl group. Accordingly, the non-polarity of the second coating layer24is particularly significant. As a result, the second coating layer24can impart the high orientation to the third molecule, and the crystallinity of the third coating layer26can be particularly improved. In addition, by a CH/n interaction or a n/n interaction of the phenyl group, the third molecule forming the third coating layer26can be bonded to the second molecule forming the second coating layer24.

3. Method for Manufacturing Atomic Cell

Next, a method for manufacturing the atomic cell100will be described.

FIG.10is a process diagram showing the method for manufacturing the atomic cell100shown inFIG.4.FIGS.11and12are cross-sectional views showing the method for manufacturing the atomic cell100shown inFIG.10.

The method for manufacturing the atomic cell100shown inFIG.10includes a first coating layer forming step S102, a second coating layer forming step S104, a third coating layer forming step S106, an alkali metal filling step S108, and a through hole sealing step S110. Hereinafter, the steps will be described.

3.1. First Coating Layer Forming Step

In the first coating layer forming step S102, the first molecule is supplied to the inner wall10aof the substrate10shown inFIG.11. Accordingly, the first coating layer22shown inFIG.12is formed. Examples of a method for supplying the first molecule include a chemical vapor deposition (CVD) method, an atomic layer deposition (ALD) method, a sputtering method, an ion plating method, and a sol-gel method. Among these methods, when the first coating layer22is formed by the CVD method, the first molecule in the gaseous state is deposited on the surface of the inner wall10athrough the through holes17.

3.2. Second Coating Layer Forming Step

In the second coating layer forming step S104, the second molecule is supplied to the first coating layer22. The second molecule has a reactive group that undergoes the desorption reaction with the first molecule, and a non-polar group. Accordingly, the second coating layer24shown inFIG.12is formed. Examples of the method for supplying the second molecule include a coating method and a CVD method. Among these methods, when the second coating layer24is formed by the coating method, the second molecule is dispersed in a predetermined dispersion medium, and the obtained dispersion liquid is applied to the surface of the first coating layer22through the through holes17, and then dried. When the second coating layer24is formed by the CVD method, the second molecule in the gaseous state is deposited on the surface of the first coating layer22through the through holes17.

3.3. Third Coating Layer Forming Step

In the third coating layer forming step S106, the third molecule is supplied to the second coating layer24. Accordingly, the third coating layer26shown inFIG.12is formed. Examples of the method for supplying the third molecule include a coating method and a vacuum deposition method. Among these methods, when the third coating layer26is formed by the coating method, the third molecule is dispersed in a predetermined dispersion medium, and the obtained dispersion liquid is applied to the surface of the second coating layer24through the through holes17, and then dried. When the third coating layer26is formed by the vacuum deposition method, the third molecule in the gaseous state is deposited on the surface of the second coating layer24through the through holes17. As described above, the coating layer20shown inFIG.12is obtained.

If necessary, the supplied third molecule is heated at a temperature equal to or higher than a melting point of the third molecule. Accordingly, the supplied third molecule moves and is oriented, and is self-organized. Accordingly, the degree of crystallinity of the obtained third coating layer26can be further increased. For example, when the third molecule is paraffin, a melting point of the paraffin is about 80° C. to 100° C. Therefore, it is preferable to heat the third molecule at a temperature higher than 100° C. and preferably at a temperature higher than the melting point by 10° C. or higher. An upper limit value may vary depending on other conditions such as a heating time, and may be set to a temperature, for example, about 200° C., at which the third molecule is not modified by heat.

The heating time may be appropriately set according to a heating temperature, and may be set while checking a state of the orientation. As an example, the heating time is set to about 30 seconds or more and 60 minutes or less. A heating atmosphere may be an air atmosphere, and is preferably an inert gas atmosphere in consideration of oxidation of the third molecule or the like.

On the other hand, when the third molecule is diacetylene, it is preferable to perform a heating treatment of heating the third molecule at a temperature of 100° C. or higher and 120° C. or lower after supplying the third molecule to the second coating layer24. By such a heating treatment, the third molecule can be subjected to solid phase polymerization, and can be easily polymerized in a state in which the crystallinity is high. Since the obtained polydiacetylene has a particularly high degree of crystallinity, the particularly high-quality third coating layer26is obtained.

When the heating temperature is less than the lower limit value, the solid phase polymerization of the third molecule may not sufficiently proceed, and the degree of crystallinity of the third coating layer26may not be sufficiently increased. On the other hand, when the heating temperature exceeds the upper limit value, the crystallized third molecule may be modified to lower the degree of crystallinity.

A time of the heating treatment is not particularly limited, and is preferably 1 minute or more and 180 minutes or less, and more preferably 5 minutes or more and 120 minutes or less.

Further, an atmosphere of the heating treatment is not particularly limited, and is preferably an inert gas atmosphere such as a nitrogen atmosphere or an argon atmosphere. Accordingly, the degree of crystallinity of the third coating layer26can be further increased.

3.4. Alkali Metal Filling Step

In the alkali metal filling step S108, the internal space S is filled with the alkali metal in the gaseous state through the through holes17. The alkali metal fills the internal space S under a condition that the coating layer20does not melt.

3.5. Through Hole Sealing Step

In the through hole sealing step S110, the through hole17is sealed with the sealing material18. Specifically, a sealing material member in a form of a ball fills the through hole17, and then is melted by a laser or the like. Accordingly, the sealing material18that seals the through hole17can be formed. As a result, the internal space S filled with the alkali metal can be hermetically sealed.

The atomic cell100can be manufactured as described above.

3.6. Effects According to Embodiment

As described above, the method for manufacturing the atomic cell100according to the embodiment is a method for manufacturing an atomic cell filled with an alkali metal, and includes the first coating layer forming step S102, the second coating layer forming step S104, and the third coating layer forming step S106. In the first coating layer forming step S102, the first coating layer22is formed by supplying the first molecule to the inner wall10aof the substrate10. In the second coating layer forming step S104, the second coating layer24is formed by supplying, to the first coating layer22, the second molecule having the non-polar group and the reactive group that undergoes the desorption reaction with the first coating layer22. In the third coating layer forming step S106, the third coating layer26is formed by supplying the non-polar third molecule to the second coating layer24. The third coating layer26has a degree of crystallinity of 70% or more.

According to such a configuration, since the degree of crystallinity of the third coating layer26is high, the alkali metal atom is particularly hard to be adsorbed to the third coating layer26. Therefore, it is possible to manufacture the atomic cell100in which the effect of preventing the relaxation of the electron spin state of the alkali metal filling the atomic cell100is sufficiently high.

A water contact angle of the second coating layer24is preferably 70° or more and 120° or less, and more preferably 80° or more and 120° or less. The fact that the water contact angle of the second coating layer24is within the above range supports that the non-polar group of the second molecule is oriented in a more densely and more highly aligned state. Therefore, when the water contact angle of the second coating layer24is within the above range, the degree of crystallinity of the third coating layer26can be particularly increased.

When the water contact angle is less than the lower limit value, the degree of crystallinity of the third coating layer26may decrease. On the other hand, when the water contact angle exceeds the upper limit value, it is difficult to efficiently form the second coating layer24, and the difficulty of formation may increase.

The water contact angle of the second coating layer24is measured by a θ/2 method. Measurement conditions are a temperature of 25° C. and a relative humidity of 50%±5%. A water dropping amount is 3 μL, and the measurement is performed 5 seconds after drop adhesion. Examples of a measuring device of a contact angle include a contact angle measuring device Drop Master500manufactured by Kyowa Interface Science Co., Ltd.

When the third molecule is diacetylene, the third coating layer forming step S106preferably includes a heating treatment of heating the third molecule at a temperature of 100° C. or higher and 120° C. or lower after supplying the third molecule to the second coating layer24.

According to the third coating layer forming step S106, the third molecule can be subjected to solid phase polymerization, and can be easily polymerized in a state in which the crystallinity is high. Since the obtained polydiacetylene has a particularly high degree of crystallinity, the particularly high-quality third coating layer26is obtained.

As described above, the atomic oscillator200(quantum interference device according to the embodiment) includes the atomic cell100, the light emitting unit210, and the light detection unit230. The light emitting unit210emits the excitation light L for exciting the alkali metal. The light detection unit230detects the excitation light L transmitted through the atomic cell100.

According to such a configuration, a quantum interference device such as the atomic oscillator200capable of attaining the effects of the atomic cell100can be obtained. In addition, in the light detection unit230, it is possible to detect the EIT signal having a higher intensity and a smaller line width (half-value width of the EIT signal). Accordingly, the atomic oscillator200(quantum interference device) having an excellent frequency stability is obtained.

4. Electronic Device

Next, an electronic device including the atomic oscillator200will be described. Hereinafter, a positioning system including the atomic oscillator200as the electronic device will be described.FIG.13is a diagram showing a positioning system300that includes the atomic oscillator200and that uses a global positioning system (GPS) satellite.

As shown inFIG.13, the positioning system300includes a GPS satellite310, a base station device320, and a GPS reception device330.

The GPS satellite310transmits positioning information (GPS signal).

The base station device320includes, for example, a reception device324that receives the positioning information from the GPS satellite310with high accuracy via an antenna322disposed on an electronic reference point (GPS continuous observation station), and a transmission device328that transmits, via an antenna326, the positioning information received by the reception device324. The reception device324includes the atomic oscillator200as a reference frequency oscillation source. The positioning information received by the reception device324is transmitted in real time by the transmission device328.

The GPS reception device330includes a satellite reception unit334that receives the positioning information from the GPS satellite310via an antenna332, and a base station reception unit338that receives positioning information from the base station device320via an antenna336.

Since the positioning system300as described above includes the atomic oscillator200, the positioning system300has excellent accuracy and reliability.

The electronic device is not limited to the positioning system, and may be, for example, a mobile phone, a digital still camera, an ink jet discharging device, a personal computer, a television, a video camera, a video tape recorder, a car navigation device, a pager, an electronic notebook, an electronic dictionary, a calculator, an electronic game device, a word processor, a workstation, a video phone, a security television monitor, electronic binoculars, a POS terminal, a medical device, a fish finder, various measuring devices, meters, a flight simulator, a terrestrial digital broadcasting, and a mobile phone base station.

5. Vehicle

Next, a vehicle including the atomic oscillator200will be described. Hereinafter, an automobile including the atomic oscillator200as the vehicle will be described.FIG.14is a diagram showing an automobile400including the atomic oscillator200.

As shown inFIG.14, the automobile400includes a vehicle body410and four wheels420, and is configured to rotate the wheels420by a power source (engine) (not shown) provided in the vehicle body410. The automobile400includes the atomic oscillator200.

Since the automobile400includes the atomic oscillator200, the automobile400has excellent accuracy and reliability.

The vehicle is not limited to the automobile, and may be, for example, an aircraft such as a jet machine or a helicopter, a ship, a rocket, and an artificial satellite.

Although the atomic cell, the method for manufacturing the atomic cell, and the quantum interference device according to the present disclosure have been described above based on the illustrated embodiment, the present disclosure is not limited thereto.

For example, in the atomic cell and the quantum interference device according to the present disclosure, a configuration of each part of the above embodiment may be replaced with any configuration having the same function, and any constituent may be added to the above embodiment.

In the method for manufacturing the atomic cell according to the present disclosure, any desired step may be added to the embodiment.