Gas cell and quantum sensor

Abstract

The present invention provides a gas cell and a quantum sensor that exhibit excellent bonding performance of a bonding part and high reliability in terms of airtightness.　Provided is a gas cell in which alkali metal atoms are encapsulated. The gas cell comprises: a body part that has at least one opening; a transparent substrate that is provided to the opening; and a bonding part that is provided between the body part and the transparent substrate and that bonds the body part and the transparent substrate. The bonding part is constituted by a melt-fixed layer of glass frit and filler which has a thermal expansion coefficient smaller than that of the glass frit. When the thermal expansion coefficient of the transparent substrate is αa, the thermal expansion coefficient of the bonding part is αb, the thermal expansion coefficient of the body part is αc, and (αb / αa)+(αa / αc) is Q, 2.00≤Q≤3.10 is satisfied.

Description

Gas cells and quantum sensors 【0001】 The present invention relates to a gas cell used in quantum sensors such as atomic oscillators or optical pumping magnetic sensors, and to a quantum sensor equipped with a gas cell. 【0002】 When light acts on alkali metal vapors, phenomena such as spin polarization or quantum interference effects (CPT: Coherent Population Trapping) occur. In recent years, atomic oscillators and quantum sensors such as optical pumping magnetic sensors that utilize these phenomena have attracted attention. For example, optical pumping magnetic sensors are used as brain function imaging devices to measure and image the physiological activity and function of various parts of the brain. Therefore, improvements to optical pumping magnetic sensors can lead to the early detection and treatment of lesions, as well as an improvement in quality of life (QOL), and are expected to contribute to the realization of Goal 3 of the United Nations' Sustainable Development Goals (SDGs), "Ensure healthy lives and promote well-being for all." 【0003】 Quantum sensors such as atomic oscillators and optical pumping magnetic sensors include, for example, a gas cell containing alkali metal vapor and buffer gases such as nitrogen, helium, and argon; a light source that emits excitation light to excite the alkali metal in the gas cell; and a photodetector that detects the light that has passed through the gas cell. Alkali metal vapor is obtained by heating and vaporizing alkali metal or alkali metal compounds sealed in the gas cell. For this reason, the gas cell must operate in a high-temperature environment, and it is important to uniformly fill the inside of the gas cell with alkali vapor. 【0004】 Patent Document 1 describes an airtight container comprising a cell body having opposing first and second main surfaces and a through hole penetrating between the first and second main surfaces, a first lid member provided on the first main surface of the cell body, and a second lid member provided on the second main surface of the cell body. The cell body, the first lid member, and the second lid member are made of glass. 【0005】 Patent No. 7491112 【0006】In the hermetic container of Patent Document 1, the cell body, the first lid member, and the second lid member are joined by thermocompression bonding. In Patent Document 1, regarding the joint between the cell body and the first lid member and the joint between the cell body and the second lid member, the joint property is not sufficient, such as peeling occurring or deformation occurring in the first lid member or the second lid member. Furthermore, in Patent Document 1, heating is performed to gasify the enclosed alkali metal. In this case, the hermetic container needs to maintain airtightness. However, in the joint by the above-described thermocompression bonding, the joint portion cannot maintain sufficient airtightness against the temperature change accompanying heating, and the reliability regarding airtightness is not sufficient. An object of the present invention is to provide a gas cell and a quantum sensor having excellent joint property of a joint portion and high reliability regarding airtightness. 【0007】 As a result of intensive studies on the above problems, the present inventor has found that the above problems can be solved by the following configuration. That is, the present inventor has found that the above problems can be solved by the following configuration. (1) A gas cell in which an alkali metal atom is enclosed, having a main body portion having at least one opening, a transparent substrate provided in the opening, and a joint portion for joining the main body portion and the transparent substrate between the opening of the main body portion and the transparent substrate. The joint portion is composed of a molten and fixed layer of glass frit and a filler having a thermal expansion coefficient smaller than that of the glass frit. Let the thermal expansion coefficient of the transparent substrate be α ａ Let the thermal expansion coefficient of the joint portion be α ｂ Let the thermal expansion coefficient of the main body portion be α ｃ When (α ｂ / α ａ ) + (α ａ / α ｃ ) is Q, a gas cell in which 2.00 ≤ Q ≤ 3.10. (2) When the glass transition temperature of the transparent substrate is Tg ａ (° C) and the softening temperature of the joint portion is Ts ｂ (° C), the gas cell according to (1), wherein -30 ° C ≤ Tg ａ - Ts ｂ (3) The gas cell according to (1) or (2), wherein the main body portion is made of SiSiC. (4) The glass frit has a glass composition of B ２ O ３15-50 mol%, SiO ２ 0-30 mol%, Al ２ O ３ 0-25 mol%, Bi ２ O ３ 0-35 mol%, ZnO 0-65 mol%, TeO ２ 0-20 mol%, TiO ２ +ZrO ２ +Nb ２ O ５ 0-15 mol%, MgO + CaO + SrO + BaO 0-15 mol%, Li ２ O + Na ２ O+K ２ A gas cell according to any one of (1) to (3), which contains 0 to 10 mol% of O and has a softening temperature Ts of less than 650°C. 【0008】 (5) The gas cell according to any one of (1) to (4), wherein the transparent substrate has a first main surface that together with the main body constitutes a closed space for encapsulating alkali metal atoms, a second main surface opposite to the first main surface, and a side surface that is incorporated into the outer edges of the first and second main surfaces, and the opening of the main body and the transparent substrate are joined to two or three surfaces of the first main surface, second main surface and side surface of the transparent substrate. (6) A quantum sensor characterized by comprising the gas cell according to any one of (1) to (5), a light source that emits excitation light to excite electrons of alkali metal atoms contained in the gas cell, and a photodetector that detects light that has passed through the gas cell. (7) The quantum sensor according to (6), which is an optical pumping magnetic sensor. (8) The quantum sensor according to (6), which is an atomic oscillator. 【0009】 According to the present invention, it is possible to provide a gas cell and a quantum sensor that have excellent bonding properties at the joint and high reliability in terms of airtightness. 【0010】This is a perspective view showing an example of a gas cell according to the first embodiment of the present invention. This is a cross-sectional view showing an example of a gas cell according to the first embodiment of the present invention. This is a cross-sectional view showing a modified version of the gas cell according to the first embodiment of the present invention. This is a perspective view showing a modified version of the gas cell according to the first embodiment of the present invention. This is a cross-sectional view showing a modified version of the gas cell according to the first embodiment of the present invention. This is a cross-sectional view showing an example of a gas cell according to the second embodiment of the present invention. This is a cross-sectional view showing an example of a gas cell according to the second embodiment of the present invention. This is a cross-sectional view showing a modified version of the gas cell according to the second embodiment of the present invention. This is a perspective view showing an example of a gas cell according to the third embodiment of the present invention. This is a cross-sectional view showing an example of a gas cell according to the third embodiment of the present invention. This is a perspective view showing a modified version of the gas cell according to the third embodiment of the present invention. This is a schematic perspective view showing the shape of the main body. 【0011】 Embodiments of the present invention will be described below with reference to the drawings. However, the embodiments and figures described below are illustrative for illustrating the present invention, and the present invention is not limited to the embodiments and figures shown below. Various modifications and substitutions can be made to the embodiments below without departing from the scope of the present invention. In the following, numerical ranges expressed using "~" mean a range that includes the numerical values ​​written before and after "~" as the lower and upper limits. 【0012】 The gas cell of the present invention is characterized by having alkali metal atoms sealed inside, and comprising a main body having at least one opening, a transparent substrate provided in the opening, and a joint between the opening of the main body and the transparent substrate that joins the main body and the transparent substrate, the joint being composed of a fused layer of glass frit and a filler having a smaller coefficient of thermal expansion than the glass frit, and the coefficient of thermal expansion of the transparent substrate is α ａ The thermal expansion coefficient of the joint is set to α ｂ The thermal expansion coefficient of the main body is set to α ｃ (α ｂ / α ａ ) + (αａ / α ｃ When Q is the coefficient of thermal expansion of the transparent substrate, then 2.00 ≤ Q ≤ 3.10. ａ The thermal expansion coefficient α of the joint ｂ The thermal expansion coefficient α of the main body ｃ Because the relationship between the thermal expansion coefficients is defined, the gas cell exhibits excellent joint bonding. Furthermore, even when heated to gasify the internal alkali metals, the defined relationship between the thermal expansion coefficients, as described above, allows for sufficient airtightness against temperature changes associated with heating, resulting in high reliability regarding airtightness. Excellent joint bonding means that delamination of the joint itself is unlikely, and deformation or cracking of the transparent substrate in contact with the joint is unlikely. 【0013】 The gas cell will now be described. Figures 1 to 14 are conceptual diagrams showing a gas cell according to an embodiment of the present invention. The gas cell according to an embodiment of the present invention is a gas cell in which alkali metal atoms are sealed inside, and comprises a main body having at least one opening, a transparent substrate provided in the opening, and a joint between the opening of the main body and the transparent substrate that joins the main body and the transparent substrate. The joint will be described in detail later. 【0014】[First Embodiment] An example of a gas cell according to the first embodiment of the present invention is shown in Figures 1 and 2. Figure 1 is a perspective view showing the gas cell 1 according to this embodiment. Figure 2 is a cross-sectional view of the gas cell 1 along the line I-I in Figure 1. Hereinafter, components common to the gas cell according to the first embodiment will be denoted by the same reference numerals, and their detailed descriptions will be omitted. The gas cell 1 of this embodiment has a main body portion 10, a transparent substrate 20, and a reflective member 30 (see Figure 2). The main body portion 10 has one opening 11. The transparent substrate 20 is provided in the opening 11 of the main body portion 10. Inside the gas cell 1, a closed space 40 (see Figure 2) is formed, surrounded on all sides by the main body portion 10 and the transparent substrate 20, and alkali metal atoms and a sealed gas (buffer gas) are sealed in this closed space 40. As shown in Figure 2, the gas cell 1 has a joint portion 50 between the opening 11 of the main body portion 10 and the transparent substrate 20 that joins the main body portion 10 and the transparent substrate 20. More specifically, the main body 10 is provided with a fitting portion 14 in the opening 11. The transparent substrate 20 is fitted into the fitting portion 14 provided in the opening 11. A joint portion 50 is provided between the fitting portion 14 in the opening 11 and the transparent substrate 20. The joint portion 50 is composed of a fused layer (not shown) of glass frit (not shown) and a filler (not shown) having a lower coefficient of thermal expansion than glass frit. Glass frit and the filler having a lower coefficient of thermal expansion than glass frit will be explained later. Hereinafter, the filler having a lower coefficient of thermal expansion than glass frit will also be referred to as a low-expansion filler. 【0015】 In the gas cell 1, the thermal expansion coefficients of the main body 10, the joint 50, and the transparent substrate 20 have the following relationship: The thermal expansion coefficient of the transparent substrate 20 is α ａ The thermal expansion coefficient of the joint 50 is set to α ｂ The thermal expansion coefficient of the main body 10 is set to α ｃ (α ｂ / α ａ ) + (α ａ / α ｃWhen Q is the coefficient of thermal expansion, then 2.00 ≤ Q ≤ 3.10. Preferably, 2.00 ≤ Q ≤ 3.00, and more preferably 2.00 ≤ Q ≤ 2.80. Due to temperature changes associated with heating during the manufacturing process of the gas cell or during use of the gas cell, the main body 10, the joint 50, and the transparent substrate 20 deform based on their coefficients of thermal expansion. Here, the member with the largest volume of the gas cell is assumed to be the driving force of thermal stress, and the main body 10, the joint 50, and the transparent substrate 20 have a coefficient of thermal expansion α ｃ , α ａ , α ｂ The relationship between magnitudes is basically α ｃ ≤α ａ ≤α ｂ Assume that this is the case. In this case, in order to make the thermal expansion coefficient of the joint, which has a smaller volume than the transparent substrate, or the thermal expansion coefficient of the transparent substrate, which has a smaller volume than the main body, appropriate, 2.00 ≤ Q (= (α ｂ / α ａ ) + (α ａ / α ｃ Let )) ≤ 3.10. 2.00 ≤ Q (= (α ｂ / α ａ ) + (α ａ / α ｃ If Q ≤ 3.10, deformation and cracking of the transparent substrate 20 in contact with the joint 50 are suppressed. As a result, the gas cell can maintain sufficient airtightness and has high reliability in terms of airtightness. Even when heated to gasify the internal alkali metals, the gas cell can maintain sufficient airtightness and has high reliability in terms of airtightness. If Q < 2.00, the thermal expansion coefficient of the joint, which has a smaller volume than the transparent substrate, or the thermal expansion coefficient of the transparent substrate, which has a smaller volume than the main body, becomes too small, and as a result, defects such as delamination and cracks may occur at the joint or cracks in the transparent substrate, which may impair the airtightness of the gas cell. If 3.10 < Q, delamination may occur at the joint or cracks may occur in the transparent substrate due to temperature changes associated with heating during the manufacturing process of the gas cell or during use of the gas cell. For this reason, the joint is not sufficiently bonded and has low reliability in terms of airtightness. 【0016】 Thermal expansion coefficient α of the transparent substrate 20 ａThe thermal expansion coefficient α of the joint 50 is preferably 3.0 to 10.0 (ppm / K), and more preferably 3.2 to 8.5 (ppm / K). ｂ The thermal expansion coefficient α of the main body 10 is preferably 3.5 to 12.0 (ppm / K), and more preferably 4.0 to 9.0 (ppm / K). ｃ The coefficient of thermal expansion α of the transparent substrate 20 is preferably 2.0 to 8.0 (ppm / K), and more preferably 2.2 to 5.0 (ppm / K). ａ The thermal expansion coefficient α of the joint 50 ｂ The thermal expansion coefficient α of the main body 10 ｃ In terms of magnitude, α ａ ≤α ｂ , and α ｃ ≤α ａ It is preferable that the thermal expansion coefficient α of the transparent substrate 20 described above. ａ , the thermal expansion coefficient α of the joint 50 ｂ , and the thermal expansion coefficient α of the main body 10 ｃ This can be measured by the following method: The thermal expansion coefficient α of the transparent substrate 20. ａ and the thermal expansion coefficient α of the main body 10 ｃ This involves using a differential thermal expander to calculate the average linear expansion coefficient between 50 and 350°C when quartz glass is heated from room temperature at a rate of 5°C / min. The thermal expansion coefficient α of the joint 50 is also calculated. ｂ The thermal expansion coefficient of the glass frit measured by the above method and the catalog values ​​of the thermal expansion coefficient of the low-expansion filler were used to calculate the value from the combined volume fraction using a compounding law. 【0017】 In gas cell 1, the glass transition temperature of the transparent substrate 20 is set to Tg ａ (°C) The softening temperature of the joint 50 is set to Ts ｂ When (°C), -30°C ≤ Tg ａ -Ts ｂ Preferably, -30°C ≤ Tg ａ -Ts ｂThis ensures a process window for the firing temperature during the gas cell manufacturing process, thereby suppressing deformation of the transparent substrate, and preventing a decrease in the fluidity of the bonding material constituting the joint, which can lead to a decrease in airtightness. Glass transition temperature Tg of transparent substrate 20 ａ And the softening temperature Ts of the joint 50 ｂ Preferably, 0°C ≤ Tg ａ -Ts ｂ More preferably, 30°C ≤ Tg ａ -Ts ｂ The glass transition temperature Tg and softening temperature Ts mentioned above are measured by the following method. The glass transition temperature Tg and softening temperature Ts are determined from the thermal expansion curve obtained in the same manner as the measurement of the thermal expansion coefficient described above, at the temperature at which the transparent substrate and the joint soften and no further elongation is observed, i.e., the flexing point Ts Ａ The temperature is measured up to a certain point, and the temperature corresponding to the inflection point in the thermal expansion curve is defined as the glass transition temperature Tg. 【0018】 The transparent substrate 20 has, for example, a first main surface 21 that, together with the main body 10, constitutes a closed space 40 for encapsulating alkali metal atoms, a second main surface 22 opposite to the first main surface 21, and a side surface 23 incorporated into the outer edges of the first main surface 21 and the second main surface 22. For example, the first main surface 21 and the second main surface 22 are parallel, and the side surface 23 is perpendicular to the first main surface 21 and the second main surface 22. However, the side surface 23 is not particularly limited to being perpendicular to the first main surface 21 and the second main surface 22. As will be described later, light that excites electrons of alkali metal atoms and excitation light, etc., are transmitted through the transparent substrate 20. 【0019】The gas cell 1 is provided with a fitting portion 14 in the main body 10 into which the transparent substrate 20 is fitted, resulting in a gas cell 1 with excellent airtightness between alkali metal atoms and the enclosed gas. This is thought to be because the area in which the main body 10 (side wall portion 13) and the transparent substrate 20 are joined has increased, thereby increasing the length of the fitting portion between the main body 10 and the transparent substrate 20, that is, the distance from the closed space 40 to the outside of the gas cell 1. Furthermore, if the transparent substrate is not fitted into the main body, displacement of the transparent substrate may occur during manufacturing or use, shortening the distance from the closed space 40 to the outside in the fitting portion 14, which may make it easier for the enclosed gas to leak. In contrast, in the gas cell 1 of this embodiment, since the transparent substrate 20 is fitted by the fitting portion 14 of the main body 10, the above-mentioned displacement does not occur, and the airtightness of the gas cell 1 is further improved. The gas cell of this embodiment will be described in more detail below. 【0020】 The main body 10 has, for example, a plate-shaped base 12 with a rectangular outer shape in plan view, and a rectangular tubular side wall portion 13 having four side walls. The side wall portion 13 is provided so as to surround the entire circumference of the peripheral edge of the base portion 12. The main body 10 shown in Figures 1 and 2 has a configuration in which the base portion 12 and the side wall portion 13 are integrally molded. In the main body 10, the protruding end of the side wall portion 13 opposite to the base portion 12 is an opening 11, and a fitting portion 14 is provided on the inside (opening 11 side) of the protruding end of the side wall portion 13. As shown in Figure 2, the fitting portion 14 has, for example, two orthogonal first fitting surfaces 141 and a second fitting surface 142. A joint portion 50 is provided between the fitting portion 14 and the transparent substrate 20. The joint portion 50 is provided on the entire surface of the orthogonal first fitting surface 141 and the second fitting surface 142. The first main surface 21 of the transparent substrate 20 is joined to the first mating surface 141 by the joining portion 50. The side surface 23 of the transparent substrate 20 is joined to the second mating surface 142 by the joining portion 50. The opening 11 of the main body 10 and the transparent substrate 20 are joined to two surfaces: the first main surface 21 of the transparent substrate 20 and the side surface 23 of the transparent substrate 20. 【0021】In this embodiment, the main body 10 has one opening 11. When a gas cell having one opening in this embodiment is used for a quantum sensor or the like, a light source that emits excitation light for exciting electrons of an alkali metal atom, and light that has passed through the inside of the gas cell, for example, excitation light, a photodetector for detecting the light are arranged on the same side (the transparent substrate 20 side) with respect to the gas cell. Thereby, miniaturization of the quantum sensor including the gas cell 1 can be achieved. 【0022】 In this embodiment, a reflecting member 30 having a function of reflecting light for exciting electrons of an alkali metal atom is provided on the surface of the base portion 12 of the main body 10 facing the transparent substrate 20. Thus, in a gas cell in which the main body has one opening, by providing a reflecting member at a position facing the opening inside the main body, light that has entered the closed space through the transparent substrate provided at the opening can be made to pass through the transparent substrate 20 again after exciting the electrons of the alkali metal atom, and can be detected by an external photodetector. 【0023】 The thickness of the base portion 12 and the side wall portion 13 of the main body 10 is preferably 0.3 mm or more and less than 10 mm, and more preferably 0.5 mm or more and less than 5.0 mm. By the thickness of the base portion 12 and the side wall portion 13 being within the above range, while maintaining mechanical properties, shape stability and heat uniformity can be enhanced, and further, by securing the bonding area with the transparent substrate 20, airtightness can be further enhanced. Note that the mechanical properties refer to bending strength and impact resistance. 【0024】 The constituent material of the main body 10 is not particularly limited, and examples include inorganic crystals such as glass materials, quartz, ceramic materials, and silicon materials, and resin materials. Glass materials, ceramic materials, or silicon materials are preferred. The ceramic material is, for example, Si ３ N ４ and Al ２ O ３ and is. In addition to being able to be formed as an integrally molded product, the main body 10 is more preferably composed of SiC - Si because of its high specific strength, small thermal expansion coefficient, and high thermal conductivity. SiC - Si is, for example, SiSiC (reaction sintered SiC). 【0025】 The fitting portion into which the transparent substrate is fitted will be explained in more detail. In the gas cell 1 shown in Figure 2, as described above, the fitting portion 14 is joined to two surfaces of the transparent substrate 20, namely the first main surface 21 and the side surface 23, by the joining portion 50. The transparent substrate 20 is fitted into the fitting portion 14 and then joined to the main body portion 10 by the joining portion 50. 【0026】 In the fitting portion 14, if the length in the thickness direction of the fitting portion that joins with the side surface 23 of the transparent substrate 20 is a, it is preferable that the length a of the fitting portion 14 and the thickness t of the transparent substrate are t > a. This allows for more uniform stress to be applied to the transparent substrate by pressing when joining it to the main body, and allows the transparent substrate to be properly fitted into the fitting portion. This further improves the airtightness of the gas cell 1. In terms of further improving the airtightness of the gas cell 1, it is preferable that the difference between the thickness t of the transparent substrate and the length a is 0.1 mm or more and less than 1.0 mm. 【0027】 The length a of the fitting portion 14 is preferably 0.1 mm or more, and more preferably 0.5 mm or more. When the length a is greater than or equal to the lower limit, the contact area between the transparent substrate 20 and the main body portion 10 increases, and the airtightness of the gas cell 1 is further improved. The length a of the fitting portion 14 may also be 3.9 mm or less, or 2.0 mm or less. 【0028】 In the mating portion 14, if b is the length of the mating portion that joins the first main surface 21 of the transparent substrate 20 in a direction perpendicular to the thickness direction of the transparent substrate 20 (in Figure 2, the direction toward the central axis of the side wall portion 13 of the main body portion 10), then b is preferably 0.5 mm or more and less than 5.0 mm, and more preferably 0.7 to 3.0 mm. When b is greater than or equal to the lower limit, the joining area between the transparent substrate 20 and the main body portion 10 becomes larger, and the airtightness of the gas cell 1 is further improved. When b is less than the upper limit, the size of the main body portion is suppressed, making it easier to miniaturize and lighten the quantum sensor equipped with the gas cell, or the volume of the closed space becomes larger, and the signal sensitivity is further improved. 【0029】<Method for Manufacturing a Gas Cell of the First Embodiment> The method for manufacturing the gas cell 1 of this embodiment will be described below. The method for manufacturing the gas cell 1 of this embodiment includes, for example, a preparation step of preparing the main body portion 10 and the transparent substrate 20, a forming step of forming a bonding material layer for forming a joint portion 50, and a fitting step of fitting the transparent substrate 20 into the fitting portion 14, joining the main body portion 10 and the transparent substrate 20, and fitting the transparent substrate 20 into the fitting portion 14. 【0030】 In the preparation step, the main body 10 and the transparent substrate 20 are prepared. The main body 10, consisting of a base portion 12 and side wall portions 13, can be manufactured using known techniques such as etching, photolithography, casting, and 3D printing. A reflective member 30 is provided on the inner surface of the base portion 12 of the main body 10. In the forming step, for example, a bonding paste containing glass frit and a filler with a lower coefficient of thermal expansion than glass frit (low-expansion filler) is applied to the bonding region of the transparent substrate 20, specifically, the bonding region of the first main surface 21 and side surface 23 of the transparent substrate 20 that is joined to the fitting portion 14, using a screen printing method, for example, and then fired to form a bonding layer in the bonding region of the transparent substrate 20. The bonding paste containing the aforementioned glass frit and low-expansion filler is applied to the bonding area of ​​the main body 10, specifically the first fitting surface 141 and the second fitting surface 142 of the fitting portion 14 in Figure 2. After application, for example, it is fired to form a bonding layer on the fitting portion 14 of the main body 10. 【0031】 In the fitting process, after fitting the transparent substrate 20 into the fitting portion 14 of the opening 11 of the main body 10, a predetermined load is applied, and at a predetermined firing temperature and firing time, the bonding material layer of the transparent substrate 20 and the bonding material layer of the main body 10 are melted to form a molten bond layer and form a joint portion 50. The transparent substrate 20 is then bonded to the opening 11 of the main body 10 with the transparent substrate 20 fitted into the fitting portion 14 by the joint portion 50. 【0032】When manufacturing a gas cell in which alkali metal atoms are sealed in a closed space 40, the sealing material containing alkali metal atoms should be placed inside the main body 10 before the fitting process. Examples of sealing materials containing alkali metal atoms include liquid or solid alkali metals, ampoules containing alkali metals, wax, etc. When liquid or solid alkali metals are placed, heating the gas cell during use creates a closed space 40 containing alkali metal atoms. When ampoules containing alkali metals are placed, after joining the main body 10 and the transparent substrate 20, the ampoules are destroyed by irradiation with an external laser beam, or the wax is melted, creating a closed space 40 containing alkali metal atoms. Alternatively, a stable alkali metal compound and an azide may be placed, and then heated to generate alkali metal vapor through a chemical reaction. 【0033】 Another method for manufacturing a gas cell in which alkali metal atoms are sealed in a closed space 40 is to first provide open holes in either the base portion 12 or the side wall portion 13 of the main body portion 10, then join the main body portion 10 and the transparent substrate 20, introduce a sealing material containing alkali metal atoms, and then seal the open holes. The method for forming the closed space 40 in which alkali metal atoms are sealed by the sealing material is as described above. As for the method of sealing the open holes, one example is to fill the open holes with the above-mentioned bonding agent and then perform heat treatment. 【0034】 The gas cell of this embodiment has been described above based on the gas cell 1 shown in Figures 1 and 2, but other forms of gas cell besides the gas cell 1 are also included in this embodiment. The external shape of the gas cell 1 is a hexahedron with one face shaped like a square, but the external shape of the gas cell of this embodiment can be selected to suit the design of the quantum sensor. The external shape of the gas cell of this embodiment may be, for example, a polygonal prism, cylinder, elliptical prism, or polygon other than a rectangular parallelepiped (cube). The closed space 40 inside the main body 10 is a rectangular parallelepiped (cube), but the shape of the closed space may be a polygonal prism, cylinder, elliptical prism, or polygon other than a rectangular parallelepiped. The external shape of the gas cell and the shape of the closed space may be the same or different. 【0035】The following modifications are also possible for the gas cell of this embodiment. In each modification, elements common to the gas cell 1 shown in Figure 1 are denoted by the same reference numerals, and their descriptions are omitted. 【0036】 <Modification 1 of the First Embodiment> A modification 1 of the gas cell according to this embodiment is shown in Figure 3. The main body 10 of the gas cell 2 shown in Figure 3 is not a member in which the base 12 and the side wall 13 are integrally molded, but a member formed by joining a rectangular plate-shaped member corresponding to the base 12 and a rectangular tubular member corresponding to the side wall 13. The joining method is not particularly limited, but examples include joining using a bonding agent, anodic joining, heat compression bonding, and laser joining. In this modification, a reflective member 30 can be easily formed on the base 12. In this modification, the material constituting the base 12 and the material constituting the side wall 13 may be the same or different, but it is preferable that they be the same material in that the design regarding thermal expansion is easier. 【0037】<Modification 2 of the First Embodiment> A modification 2 of the gas cell according to this embodiment is shown in Figures 4 and 5. Figure 4 is a perspective view showing the gas cell 3 according to this modification. Figure 5 is a cross-sectional view of the gas cell 3 along the line I-I in Figure 4. As shown, the main body 10 of the gas cell 3 has, for example, a plate-shaped base 12 with a rectangular outer shape in plan view, and a rectangular tubular side wall portion 13 having four side walls. The side wall portion 13 is provided so as to surround the entire circumference of the peripheral edge of the base portion 12. On the inside of the protruding end of the side wall portion 13 opposite to the base portion 12 (on the opening 11 side), a fitting portion 14 having a frame-shaped lid portion 15 in plan view is provided. As shown, the fitting portion 14 has, for example, three orthogonal first fitting surfaces 141, a second fitting surface 142, and a third fitting surface 143. The first main surface 21 of the transparent substrate 20 is joined to the first fitting surface 141 by a joining portion 50. The side surface 23 of the transparent substrate 20 is joined to the second mating surface 142 by the joint 50. The second main surface 22 of the transparent substrate 20 is joined to the third mating surface 143 by the joint 50. In this modified gas cell 3, the mating surface 14 of the main body 10 is joined to the three surfaces of the transparent substrate 20, namely the first main surface 21, the second main surface 22, and the side surface 23, by the joint 50. This configuration increases the contact area between the main body 10 and the transparent substrate 20, further improving airtightness and resulting in a gas cell with superior robustness for use in harsher environments. 【0038】 In this modified example, if c is the length in the thickness direction of the transparent substrate 20 of the fitting portion 14 that joins the first main surface 21, the second main surface 22, and the side surface 23 of the transparent substrate 20, then it is preferable that the length c and the thickness t of the transparent substrate are t ≤ c. This is because the airtightness of the gas cell 3 can be further improved, and a gas cell with superior robustness when used in more severe environments can be obtained. In terms of further improving the airtightness of the gas cell 3, the difference obtained by subtracting the thickness t of the transparent substrate from the length c of the fitting portion 14 is preferably 0 to 0.5 mm. It should also be said that the above length c is the distance between the surface of the transparent substrate 20 facing the first main surface 21 (first fitting surface 141) in the fitting portion 14 and the surface of the transparent substrate 20 facing the second main surface 22 (third fitting surface 143) in the lid portion 15 of the fitting portion 14. 【0039】In this modified example, the length c of the fitting portion 14 is preferably 0.6 mm or more, and more preferably 1.0 mm or more. When the length c is greater than or equal to the lower limit, the contact area between the transparent substrate 20 and the main body portion 10 increases, and the airtightness of the gas cell 1 is further improved. The length c of the fitting portion 14 may also be 4.5 mm or less, or 2.1 mm or less. 【0040】 In the lid portion 15 of the fitting portion 14, if the length of the fitting portion 14 that joins with the first main surface 21 of the transparent substrate 20 is d in a direction perpendicular to the thickness direction of the transparent substrate 20 (in Figure 5, the direction toward the central axis of the side wall portion 13 of the main body portion 10), then the length d is preferably 0.5 to 6.0 mm, and more preferably 0.7 to 4.0 mm. When the length d is greater than or equal to the lower limit, the joining area between the transparent substrate 20 and the lid portion 15 is increased, and the airtightness of the gas cell 3 is further improved. When the length d is less than or equal to the upper limit, sufficient area can be secured for the excitation light that excites alkali metal atoms to enter the closed space 40 and for the excitation light to exit the closed space 40, making optical axis adjustment easier. 【0041】 In this modified example, the main body portion 10 is a member formed by joining members corresponding to the base portion 12 and the side wall portion 13 with a frame-shaped member corresponding to the lid portion 15. The joining method is not particularly limited, but the joining method described in Modified Example 1 is an example. In this modified example, the materials constituting the base portion 12 and the side wall portion 13 and the material constituting the lid portion 15 may be the same or different, but it is preferable that they be the same material in that the design regarding thermal expansion is easier. 【0042】 The base portion 12 and side wall portion 13 of the main body portion 10 may be members formed by joining a rectangular plate-shaped member corresponding to the base portion 12 and a rectangular tubular member corresponding to the side wall portion 13, similar to the first modification. 【0043】 The manufacturing method for the gas cell 3 of this modified example includes, for example, a preparation step of preparing a member A that will become the base portion 12 and the side wall portion 13, a member B that will become the lid portion 15, and a transparent substrate 20; a forming step of forming a bonding material layer for forming the joint portion 50; and a bonding step of joining member A, the transparent substrate 20, and member B. 【0044】The preparation of member A and the transparent substrate 20 in the preparation step is as described in the manufacturing method of the gas cell 1 above. Member A has an opening and a fitting portion. Member B, which becomes the frame-shaped lid portion 15, can be manufactured using known techniques such as etching, photolithography, casting, and 3D printing. In the forming step, as described above, a bonding material layer is formed in the bonding region of the transparent substrate 20 using a bonding material paste containing the glass frit and low-expansion filler. A bonding material layer is also formed in the bonding regions of member A and member B using a bonding material paste containing the glass frit and low-expansion filler. In the bonding step, the transparent substrate 20 is fitted into the fitting portion of the opening of member A, and then member B is placed over it and bonded. The bonding step, including preferred forms, is the same as the fitting step in the manufacturing method of the gas cell 1 above, so a detailed explanation is omitted. 【0045】 The method for manufacturing a gas cell in which alkali metal atoms are sealed in the closed space 40 of the gas cell 3 (the method for sealing alkali metal atoms in the closed space 40) is as described in the manufacturing method of the gas cell 1 above. 【0046】<Modification 3 of the First Embodiment> A modification 3 of the gas cell according to this embodiment is shown in Figure 6. Figure 6 is a cross-sectional view showing the gas cell 4 according to this modification. The main body 10 of the gas cell 4 shown in Figure 6 has, for example, a plate-shaped base 12 with a rectangular outer shape in plan view, and a rectangular tubular side wall portion 13 having four side walls. The side wall portion 13 is provided so as to surround the entire circumference of the peripheral edge of the base portion 12. On the inner side (opening 11 side) of the protruding end side of one of the four side walls of the side wall portion 13, a fitting portion 14 into which a transparent substrate 20 is fitted is provided. As shown in Figure 6, the fitting portion 14 has, for example, three orthogonal first fitting surfaces 141, a second fitting surface 142, and a third fitting surface 143. The first main surface 21 of the transparent substrate 20 is joined to the first fitting surface 141 by a joining portion 50. The side surface 23 of the transparent substrate 20 is joined to the second mating surface 142 by the joining portion 50. The second main surface 22 of the transparent substrate 20 is joined to the third mating surface 143 by the joining portion 50. In addition, on one side of the side wall portion 13 facing the mating portion 14, a through hole 16 is provided at the position where the mating portion 14 is located, with a shape corresponding to one of the side surfaces 23 of the transparent substrate 20. The main body portion 10 of the gas cell 4 has a structure in which the transparent substrate 20 is slid into the through hole 16 and fitted with the mating portion 14. The space outside the transparent substrate 20 within the internal space of the through hole 16 is sealed by the sealing portion 17. Thus, in this modified gas cell 4, the mating portion 14 of the main body portion 10 is joined to the three surfaces of the transparent substrate 20: the first main surface 21, the second main surface 22, and the side surface 23, by the joining portion 50. This configuration increases the contact area between the main body 10 and the transparent substrate 20, further improving airtightness and resulting in a gas cell with superior robustness for use in harsher environments. 【0047】In this modified example, if c is the length in the thickness direction of the transparent substrate 20 of the fitting portion 14 that joins the first main surface 21, the second main surface 22, and the side surface 23 of the transparent substrate 20, then the difference obtained by subtracting the thickness t of the transparent substrate 20 from the length c of the fitting portion 14 is preferably 0.1 to 0.5 mm. This is because it is possible to further improve the airtightness of the gas cell 4 and obtain a gas cell with superior robustness when used in more severe environments. Furthermore, if the above difference is 0.1 mm or more, the manufacturing of the gas cell 4, including insertion into the through hole 16 of the transparent substrate 20, becomes easier. 【0048】 In the through-hole 16 of the main body 10, if e is the length of the fitting portion that joins the first main surface 21 (or second main surface 22) of the transparent substrate 20, in a direction perpendicular to the thickness direction of the transparent substrate 20 (in Figure 6, the direction toward the central axis of the side wall portion 13 of the main body 10), then e is preferably 0.7 to 10 mm, and more preferably 2 to 7 mm. When e is greater than or equal to the lower limit, the joining area between the transparent substrate 20 and the side wall portion 13 is increased, and the airtightness of the gas cell 4 is further improved. When e is less than or equal to the upper limit, the size of the main body is suppressed, making it easier to miniaturize and lighten the quantum sensor equipped with the gas cell. 【0049】 In this modified example, the space outside the transparent substrate 20 within the through-hole 16 is sealed by the sealing portion 17. The material constituting the sealing portion 17 is preferably the same as the material of the joint portion that joins the main body portion and the transparent substrate as described above, in that it further improves the airtightness of the gas cell 4. 【0050】 In this modified example, the main body portion 10 is a member formed by integrally molding a base portion 12 and a side wall portion 13. In the gas cell 1 of this modified example, even though the main body portion 10 is integrally molded, the transparent substrate 20 can be fitted into the fitting portion 14 of the main body portion 10, and the first main surface 21, second main surface 22, and side surface 23 of the transparent substrate 20 can be joined to the main body portion 10 by the joining portion 50. The main body portion 10 may also be a member formed by joining a rectangular plate-shaped member corresponding to the base portion 12 and a rectangular tubular member corresponding to the side wall portion 13, similar to the first modified example. 【0051】The manufacturing method for the gas cell 4 of this modified example first involves preparing the main body 10 and the transparent substrate 20. Next, a forming step is carried out to form a bonding material layer for forming the joint 50. In the forming step, for example, the bonding material paste described above is applied to the bonding region of the transparent substrate 20 that will be joined to the fitting portion 14, among the three surfaces of the transparent substrate 20: the first main surface 21, the second main surface 22, and the side surface 23, by a screen printing method, and then fired, for example, to form a bonding material layer in the bonding region of the transparent substrate 20. The bonding material paste is applied to the bonding region of the main body 10, in Figure 6, to the first fitting surface 141, the second fitting surface 142, and the third fitting surface 143 of the fitting portion 14, and then fired, for example, to form a bonding material layer in the fitting portion 14 of the main body 10. Next, after fitting the transparent substrate 20 into the fitting portion 14 of the main body 10, a molten bonding layer is formed between the transparent substrate 20 and the fitting portion 14 to form a joint portion 50, and a fitting process is performed to join the main body 10 and the transparent substrate 20. In the fitting process, the transparent substrate 20 is fitted into the fitting portion 14 by inserting the transparent substrate 20 into the through hole 16 of the main body 10 and sliding it. Next, it is heated to a predetermined firing temperature and maintained for a predetermined firing time to melt the bonding material layer of the transparent substrate 20 and the bonding material layer of the main body 10 to form a molten bonding layer and form the joint portion 50. As a result, the transparent substrate 20 is joined to the opening 11 of the main body 10 with the transparent substrate 20 fitted into the fitting portion 14. 【0052】 Next, a sealing step is performed to seal the through hole 16 with the sealing portion 17. The sealing step is a step of sealing the space outside the transparent substrate 20 within the internal space of the through hole 16 with a sealing agent to form the sealing portion 17. The sealing agent can be the material that constitutes the joint portion 50 used to join the main body portion 10 and the transparent substrate 20. It is preferable to form the sealing portion 17 using the material that constitutes the joint portion 50 in order to further improve the airtightness of the gas cell 4. 【0053】 The preparation steps are as described in the manufacturing method of the gas cell 1 above. The main body 10 having a through hole 16 on one surface of the side wall 13 may be manufactured by 3D printing, or a member consisting of a base 12 and a side wall 13 without a through hole 16 may be manufactured, and then a through hole may be formed on one surface of the side wall 13 using a technique such as etching. 【0054】 The method for manufacturing a gas cell in which alkali metal atoms are sealed in the closed space 40 of the gas cell 4 (the method for sealing alkali metal atoms in the closed space 40) is as described in the manufacturing method of the gas cell 1 above. 【0055】 The gas cells of the second and third embodiments will be described below. As shown in the second and third embodiments, the gas cell of the present invention may have two or more openings in its main body. In the gas cells of the second and third embodiments, the number of transparent substrates and fitting parts differs from that of the gas cell of the first embodiment, depending on the number of openings. Unless otherwise specified, the constituent materials, shape and size of the main body and transparent substrate, as well as the enclosed gas containing alkali metal atoms, are the same in the gas cells of the second and third embodiments as in the gas cell of the first embodiment. 【0056】[Second Embodiment] An example of a gas cell according to the second embodiment of the present invention is shown in Figures 7 and 8. Figure 7 is a perspective view showing a gas cell 5 according to this embodiment. Figure 8 is a cross-sectional view of the gas cell 5 along the line I-I in Figure 7. In the following description of the gas cell according to the second embodiment, components common to the gas cell of the first embodiment are denoted by the same reference numerals, and their detailed explanation is omitted. As shown in Figure 8, in the gas cell 5 according to this embodiment, the main body portion 10 consists of a rectangular tubular side wall portion 13, and the side wall portion 13 has two openings 11a and 11b facing each other with a closed space 40 in between at both ends of the central axis. Also, fitting portions 14a and 14b are provided on the inside of the side wall portion 13 of the main body portion 10 (on the side of the opening 11a or 11b). The gas cell 5 has transparent substrates 20a and 20b, which are fitted into the fitting portions 14a and 14b, respectively. A joint portion 50 is provided between the fitting portion 14a and the transparent substrate 20. The joint portion 50 is provided across the entire surface of the orthogonal first mating surface 141a and the second mating surface 142a. The first main surface 21a of the transparent substrate 20 is joined to the first mating surface 141a by the joint portion 50. The side surface 23a of the transparent substrate 20 is joined to the second mating surface 142a by the joint portion 50. In addition, a joint portion 50 is provided between the mating portion 14b and the transparent substrate 20. The joint portion 50 is provided across the entire surface of the orthogonal first mating surface 141b and the second mating surface 142b. The first main surface 21b of the transparent substrate 20 is joined to the first mating surface 141b by the joint portion 50. The side surface 23b of the transparent substrate 20 is joined to the second mating surface 142b by the joint portion 50. In the gas cell 5 according to this embodiment, similar to the first embodiment, the transparent substrates 20a and 20b are joined to the fitting portions 14a and 14b of the main body portion 10 by the joining portions 50, thereby providing a gas cell 5 with excellent airtightness for the enclosed gas containing alkali metal atoms. 【0057】The transparent substrates 20a and 20b of the gas cell 5 according to this embodiment may differ in their constituent material, shape, and size, but it is preferable that they all be the same in terms of superior gas cell manufacturing and signal stability. The fitting portions 14a and 14b of the main body 10 are the same as the fitting portion 14 of the gas cell 1 of the first embodiment, including preferred forms. The gas cell 5 according to this embodiment can be manufactured in accordance with the manufacturing method of the gas cell 1 of the first embodiment. 【0058】 In this embodiment, the main body 10 has two openings 11a and 11b that face each other across a closed space 40. When the gas cell 5 of this embodiment is used as a quantum sensor or the like, a light source that emits excitation light to excite electrons of alkali metal atoms is placed on one opening side (for example, opening 11a), and a photodetector that detects light that has passed through the closed space 40, for example, excitation light, is placed on the other opening side (for example, opening 11b). 【0059】 The following modifications are also possible for the gas cell of this embodiment. In each modification, elements common to the gas cell 5 shown in Figure 8 are denoted by the same reference numerals, and their descriptions are omitted. 【0060】<Modification 1 of the Second Embodiment> A modification 1 of the gas cell according to this embodiment is shown in Figure 9. Figure 9 is a cross-sectional view showing the gas cell 6 according to this modification. In the gas cell 6 shown in Figure 9, the main body portion 10 has two openings 11a and 11b at positions facing each other across a closed space 40. The gas cell 6 has two transparent substrates 20a and 20b corresponding to these two openings 11a and 11b. The main body portion 10 also has a rectangular tubular side wall portion 13, and fitting portions 14a and 14b having frame-shaped lid portions 15a and 15b in a plan view are provided on the inside of both ends of the central axis of the side wall portion 13 (on the side of the opening 11a or 11b). The lid portions 15a and 15b are joined to the second main surface 22a of the transparent substrate 20a and the second main surface 22b of the transparent substrate 20b by joining portions 50, respectively. As shown in the figure, the fitting portion 14a has, for example, three orthogonal first fitting surfaces 141a, a second fitting surface 142a, and a third fitting surface 143a. The first main surface 21a of the transparent substrate 20a is joined to the first fitting surface 141a by the joining portion 50. The side surface 23a of the transparent substrate 20a is joined to the second fitting surface 142a by the joining portion 50. The second main surface 22a of the transparent substrate 20a is joined to the third fitting surface 143a by the joining portion 50. Similarly, the fitting portion 14b has, for example, three orthogonal first fitting surfaces 141b, a second fitting surface 142b, and a third fitting surface 143b. The first main surface 21b of the transparent substrate 20b is joined to the first fitting surface 141b by the joining portion 50. The side surface 23b of the transparent substrate 20b is joined to the second mating surface 142b by the joint 50. The second main surface 22b of the transparent substrate 20b is joined to the third mating surface 143b by the joint 50. Thus, in this modified gas cell 6, the mating portion 14a is joined to the three surfaces of the transparent substrate 20a, namely the first main surface 21a, the second main surface 22a, and the side surface 23a, by the joint 50, and the mating portion 14b is joined to the three surfaces of the transparent substrate 20b, namely the first main surface 21b, the second main surface 22b, and the side surface 23b, by the joint 50. With this configuration, the bonding area between the main body 10 and the transparent substrates 20a and 20b is increased, which further improves airtightness and provides a gas cell with superior robustness when used in more severe environments. 【0061】The fitting portions 14a and 14b of the main body 10 in this modified example are the same as the fitting portion 14 of the main body 10 in Modification Example 2 of the first embodiment, including the preferred form. The gas cell 6 according to this modified example can be manufactured in accordance with the manufacturing method of the gas cell 3 according to Modification Example 2 of the first embodiment. 【0062】<Modification 2 of the Second Embodiment> A modification 2 of the gas cell according to this embodiment is shown in Figure 10. Figure 10 is a cross-sectional view showing the gas cell 7 according to this modification. In the gas cell 7 shown in Figure 10, the main body portion 10 has two openings 11a and 11b at positions facing each other across a closed space 40. The gas cell 7 has two transparent substrates 20a and 20b corresponding to these two openings 11a and 11b. The main body portion 10 has a rectangular tubular side wall portion 13, and fitting portions 14a and 14b are provided on the inside of both ends of the central axis of the side wall portion 13 (on the side of the opening 11a or 11b), into which the transparent substrates 20a and 20b are fitted, respectively. Furthermore, on one side of one of the four side walls of the side wall portion 13, a through hole 16a corresponding to one of the side surfaces 23a of the transparent substrate 20a and a through hole 16b corresponding to one of the side surfaces 23b of the transparent substrate 20b are provided at the positions where the fitting portions 14a and 14b are provided. The main body portion 10 of the gas cell 7 has the transparent substrates 20a and 20b slid into the through holes 16a and 16b. The transparent substrates 20a and 20b are fitted into the fitting portions 14a and 14b, respectively, and joined by the joint portion 50. As shown in the figure, the fitting portion 14a has, for example, three orthogonal first fitting surfaces 141a, a second fitting surface 142a, and a third fitting surface 143a. The first main surface 21a of the transparent substrate 20a is joined to the first fitting surface 141a by the joint portion 50. The side surface 23a of the transparent substrate 20a is joined to the second mating surface 142a by the joining portion 50. The second main surface 22a of the transparent substrate 20a is joined to the third mating surface 143a by the joining portion 50. Similarly, the mating portion 14b has, for example, three orthogonal first mating surfaces 141b, second mating surface 142b, and third mating surface 143b. The first main surface 21b of the transparent substrate 20b is joined to the first mating surface 141b by the joining portion 50. The side surface 23b of the transparent substrate 20b is joined to the second mating surface 142b by the joining portion 50. The second main surface 22b of the transparent substrate 20b is joined to the third mating surface 143b by the joining portion 50. In addition, the space outside the transparent substrate 20a within the internal space of the through hole 16a is sealed by the sealing portion 17a. The space outside the transparent substrate 20b within the through-hole 16b is sealed by the sealing portion 17b. 【0063】In this modified gas cell 7, the fitting portion 14a is joined to the first main surface 21a, the second main surface 22a, and the side surface 23a of the transparent substrate 20a by the joint portion 50. The fitting portion 14b is joined to the first main surface 21b, the second main surface 22b, and the side surface 23b of the transparent substrate 20b by the joint portion 50. This configuration increases the contact area between the main body 10 and the transparent substrates 20a and 20b, thereby further improving airtightness and providing a gas cell with superior robustness for use in harsher environments. 【0064】 The fitting portions 14a and 14b of the main body portion 10 in this modified example are the same as the fitting portion 14 of the main body portion 10 in Modification 3 of the first embodiment, including preferred forms. The sealing portions 17a and 17b of this modified example are the same as the sealing portion 17 of the main body portion 10 in Modification 3 of the first embodiment, including preferred forms. In the gas cell 7 shown in Figure 10, the through holes 16a and 16b are provided on the same side wall of the side wall portion 13, but in this modified example, the two through holes may be provided on different side walls of the side wall portion. Whether or not to provide the two through holes on the same side wall portion is selected as appropriate depending on the structural design and manufacturing method. The gas cell 7 according to this modified example can be manufactured in accordance with the manufacturing method of the gas cell 4 according to Modification 3 of the first embodiment. 【0065】[Third Embodiment] An example of a gas cell according to the third embodiment of the present invention is shown in Figures 11 and 12. Figure 11 is a perspective view showing a gas cell 8 according to this embodiment. Figure 12 is a cross-sectional view of the gas cell 8 along the line I-I in Figure 11. In the following description of the gas cell according to the third embodiment, components common to the gas cells of the first and second embodiments are denoted by the same reference numerals, and their detailed descriptions are omitted. As shown in the figure, in the gas cell 8 according to this embodiment, the main body 10 has two base portions 12 and side wall portions 13. The two base portions 12 are plate-shaped with a rectangular outer shape in plan view and face each other across a closed space 40. The side wall portions 13 have four side walls provided so as to surround the entire circumference of each peripheral edge of the base portion 12. The main body 10 has openings 11a to 11d provided in each of the four side walls of the side wall portions 13. The gas cell 8 has four transparent substrates 20a to 20d corresponding to these four openings 11a to 11d. Each of the transparent substrates 20a to 20d has a first main surface 21a to 21d that, together with the main body 10, constitutes a closed space 40 for encapsulating alkali metal atoms, a second main surface 22a to 22d opposite to the first main surface 21a to 21d, and a side surface 23a to 23d incorporated into the outer edges of the first main surface 21a to 21d and the second main surface 22a to 22d. The base 12 and side wall 13 of the main body 10 are provided with fitting portions 14a to 14d into which the transparent substrates 20a to 20d are fitted. As shown in the figure, each of the fitting portions 14a to 14d has, for example, two orthogonal first fitting surfaces 141a to 141d and second fitting surfaces 142a to 142d. The first main surfaces 21a to 21d of the transparent substrates 20a to 20d are joined to the first mating surfaces 141a to 141d by the joining portions 50. The side surfaces 23a to 23d of the transparent substrates 20a to 20d are joined to the second mating surfaces 142a to 142d by the joining portions 50. In the gas cell 8 according to this embodiment, as in the first embodiment, the transparent substrates 20a to 20d are joined to the mating portions 14a to 14d of the main body 10 by the joining portions 50, thereby providing a gas cell 8 with excellent airtightness for the enclosed gas containing alkali metal atoms. 【0066】In the gas cell 8 of this embodiment, the four openings 11a to 11d consist of two sets: a set of openings 11a and 11b facing each other across the closed space 40, and a set of openings 11c and 11d facing each other across the closed space 40. When the gas cell 8 of this embodiment is used in a quantum sensor or the like, two different alkali metal atoms are sealed in the closed space 40, and two sets of light sources and photodetectors suitable for each alkali metal are prepared, and the same sets of light sources and photodetectors are arranged to face each other across the gas cell 8. For example, a light source corresponding to alkali metal A is placed on the opening 11a side, a photodetector corresponding to alkali metal A is placed on the opening 11b side, a light source corresponding to alkali metal B is placed on the opening 11c side, and a photodetector corresponding to alkali metal B is placed on the opening 11d side. The positions of the same sets of light sources and photodetectors may be reversed from the above embodiment depending on the design of the quantum sensor. 【0067】 The transparent substrates 20a to 20d of the gas cell 8 may differ in their constituent material, shape, and size, but it is preferable that they all be the same in terms of superior gas cell manufacturing and signal stability. The fitting portions 14a to 14d of the main body 10 are the same as the fitting portion 14 of the gas cell 1 of the first embodiment, including preferred forms. The external shape of the gas cell 8 shown in Figure 12 is a rectangular parallelepiped (cube), but the external shape of the gas cell in this embodiment can be selected to suit the design of the quantum sensor. The external shape of the gas cell in this embodiment may be, for example, a polygonal prism or polygonal shape other than a rectangular parallelepiped. The gas cell 8 according to this embodiment can be manufactured in accordance with the manufacturing method of the gas cell 1 of the first embodiment. 【0068】 The following modifications are also possible for the gas cell of this embodiment. In these modifications, elements common to the gas cell 8 shown in Figure 12 are given the same reference numerals, and their descriptions are omitted. 【0069】<Modification of the Third Embodiment> Modifications of the gas cell according to this embodiment are shown in Figures 13 and 14. Figure 13 is a perspective view showing the gas cell 9 according to this embodiment. Figure 14 is a cross-sectional view of the gas cell 9 along the line I-I in Figure 13. In the gas cell 9 according to this embodiment, the main body portion 10 has two base portions 12 and side wall portions 13 provided so as to surround the entire circumference of each peripheral edge of the base portion 12. The main body portion 10 also has openings 11a to 11d provided in each of the four side walls of the side wall portion 13. The gas cell 9 has four transparent substrates 20a to 20d corresponding to these four openings 11a to 11d. The base portions 12 and side wall portions 13 of the main body portion 10 are provided with fitting portions 14a to 14d into which the transparent substrates 20a to 20d are fitted. Each of the fitting portions 14a to 14d has a frame-shaped lid portion 15a to 15d in plan view, and the lid portions 15a to 15d are joined to the second main surfaces 22a to 22d of the transparent substrates 20a to 20d by joint portions 50. In this modified gas cell 9, each of the fitting portions 14a to 14d is joined to the first main surface, second main surface, and side surface of the transparent substrates 20a to 20d by joint portions 50. This configuration increases the contact area between the main body portion 10 and each of the transparent substrates 20a to 20d, thereby further improving airtightness and providing a gas cell with superior robustness for use in harsher environments. 【0070】 The fitting portions 14a to 14d of the main body portion 10 in this modified example are the same as the fitting portion 14 of the main body portion 10 in Modification Example 2 of the First Embodiment, including the preferred form. The gas cell 9 according to this modified example can be manufactured in accordance with the manufacturing method of the gas cell 3 according to Modification Example 2 of the First Embodiment. In any of the gas cells described above, the fitting portion 14 is not necessarily required as long as the transparent substrate 20 is joined to the main body portion 10 by the joining portion 50. 【0071】The gas cell according to the present invention, as described above, is suitably used in quantum sensors such as atomic oscillators and optical pumping magnetic sensors. An example of a quantum sensor is an embodiment comprising the gas cell according to the present invention, a light source that emits excitation light to excite electrons of alkali metal atoms sealed in the gas cell, and a photodetector that detects light that has passed through the gas cell. A known laser light source can be used as the light source that emits the excitation light. The excitation light may be linearly polarized or circularly polarized. The photodetector that detects light transmitted through the gas cell may detect the excitation light transmitted through the gas cell, or it may detect probe light different from the excitation light transmitted through the gas cell. The quantum sensor may also include a coil for adjusting the magnetic field within the gas cell, a power supply for the coil, and a magnetic shield that covers the gas cell and attenuates or blocks external magnetic fields. Furthermore, the gas cell according to the present invention can also be applied to airtight containers in devices such as wavelength conversion members. 【0072】(Transparent Substrate) The transparent substrate, together with the main body, constitutes a gas cell (airtight container). The transparent substrate is provided at the opening of the main body and is joined to the main body by a joint. The transparent substrate allows excitation light, which excites electrons of alkali metals, to be incident on the internal space of the main body. It also allows light, such as the excitation light that has passed through the internal space of the main body, to be emitted to the outside of the main body. The transparent substrate is not particularly limited as long as it is made of a material that is transparent at the wavelength used in the quantum sensor, but examples include glass, inorganic crystals such as sapphire, and translucent ceramics. Examples of glass used for the transparent substrate include alkali-free borosilicate glass, borosilicate glass, soda-lime glass, high-silica glass, and other oxide-based glasses mainly composed of silicon dioxide. The glass transition temperature Tg is preferably 500°C or higher. If the glass transition temperature Tg is less than 500°C, the transparent substrate may deform during joining. The glass transition temperature Tg is more preferably 520°C or higher, even more preferably 580°C or higher, and particularly preferably 650°C or higher. The softening temperature Ts is preferably 700°C or higher. If the softening temperature Ts is less than 700°C, the transparent substrate may deform during bonding. The softening temperature Ts is more preferably 800°C or higher, and even more preferably 900°C or higher. In addition, if necessary, an alkali barrier film may be formed on the first main surface 21 of the transparent substrate 20. In addition, an anti-reflective film may be formed on at least one of the first main surface 21 and the second main surface 22 of the transparent substrate 20. 【0073】 The thickness t of the transparent substrate in the thickness direction from the first main surface to the second main surface (see, for example, Figure 2) is preferably 0.2 to 4.0 mm, and more preferably 0.5 to 2.0 mm. When the thickness t of the transparent substrate is greater than or equal to the lower limit, the rigidity of the transparent substrate is increased, which suppresses warping of the substrate due to heating, etc., when a gas cell is used in a quantum sensor, and improves signal stability. Furthermore, when the thickness t of the transparent substrate is less than or equal to the upper limit, the size of the gas cell is suppressed, making it easier to miniaturize and lighten the quantum sensor equipped with the gas cell. 【0074】The external shape of the transparent substrate is, for example, a rectangle in plan view. However, the shape of the transparent substrate in the gas cell of this embodiment is not limited to the shape into which the transparent substrate is fitted by the fitting portion of the main body, and may or may not be the same as the shape of the closed space inside the main body. It is preferable that the shape of the transparent substrate in plan view and the shape of the closed space in plan view are the same or similar in that a sufficient contact area with the main body is secured and the airtightness of the gas cell is improved. 【0075】(Main body) The main body, together with the transparent substrate as described above, constitutes a gas cell (airtight container). The internal space of the main body is the inside of the gas cell, and alkali metal atoms are sealed in the internal space of the main body. In addition to alkali metal atoms, a sealing gas (buffer gas) is sealed in the internal space of the main body. Examples of alkali metals used include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), or cesium (Cs). The alkali metal may also be in the form of a combination of the above alkali metals with compounds such as zirconium, silicon, titanium, and aluminum, or it may be a compound of alkali metal atoms such as azides, halides, and nitrates. Examples of sealing gas (buffer gas) used include nitrogen gas, helium gas, xenon gas, neon gas, or argon gas. The alkali metal atoms and sealing gas (buffer gas) sealed in the closed space are appropriately selected according to the application of the gas cell. It is preferable that the main body does not transmit excitation light that excites the electrons of the sealed alkali metal atoms. This allows excitation light to be emitted from the transparent substrate only to the outside of the main body, making it easier to detect excitation light that has passed through the internal space of the main body (inside the gas cell). The main body also has a sealing section for encapsulating alkali metal atoms. The sealing section is provided in the main body and has an open hole that connects the internal space of the main body to the outside, and a sealing agent that seals the open hole. The sealing section encapsulates the alkali metal atoms and the encapsulated gas in the internal space of the main body (inside the gas cell). Examples of sealing agents include low-melting-point solder, low-melting-point glass molded body, or glass frit. For example, glass frit that constitutes a joint can be used. The position of the open hole is not particularly limited as long as it is not an opening in the main body. As described later, after introducing the alkali metal and encapsulated gas into the internal space of the main body, the open hole is sealed with the sealing agent. 【0076】The main body preferably has a heater to gasify the alkali metals in the closed space (internal space) of the main body. The heater is not particularly limited, but it is preferable to use a ceramic heater with high power density when it is necessary to shorten the warm-up time of the atomic oscillator or quantum sensor such as an optical pumping magnetic sensor, or when high-temperature operation is required. On the other hand, it is preferable to use a polyimide heater with low power density when it is necessary to make the temperature distribution of the main body uniform, to miniaturize and lower the position of the atomic oscillator or optical pumping magnetic sensor, or to reduce the cost of the atomic oscillator or optical pumping magnetic sensor. 【0077】 The shape of the main body is not particularly limited to the illustrated example. The shape of the main body may be, for example, a cube, a rectangular prism, a polygon, or a cylinder, and a shape suitable for the design of a quantum sensor can be selected. 【0078】 (Reflective Member) The reflective member reflects light such as excitation light from the closed space 40 (internal space) of the main body and emits it to the outside of the main body. As long as the reflective member can reflect light such as excitation light from the internal space of the main body and emit it to the outside of the main body as described above, its configuration is not particularly limited. 【0079】(Manufacturing Method for the Main Body) The manufacturing method for the main body, which is an integrally molded product, will be described below. The main body 10 can be formed as an integrally molded product using a three-dimensional printing method with a three-dimensional printer. Therefore, even if the shape of the main body 10 is complex, the main body 10 can be formed as an integrally molded product. As described above, the main body 10 can have various configurations, but any configuration can be manufactured by the manufacturing method shown below. The manufacturing method for the main body can be the manufacturing method for SiC-Si composite members having the following steps (1) to (4) described in Japanese Patent Publication No. 7235044. This makes it possible to manufacture a main body made of SiSiC. The manufacturing method for the main body includes, for example, the steps of (1) preparing a first molded body containing SiC particles by a three-dimensional printing method, (2) impregnating the pores of the first molded body with carbon particles to form a second molded body, (3) drying the second molded body, and (4) impregnating the second molded body with Si and reaction sintering the second molded body to obtain a SiC-Si composite member. The SiC-Si composite member is the main body, and the main body is manufactured in this way. However, the step of drying the second molded body is performed as needed and is not an essential step in the manufacturing method of the main body. Each step will be described in detail below. A three-dimensional printer is used for the three-dimensional printing method. 【0080】 In the process of preparing the first molded body described above, a first molded body containing SiC particles is formed using a three-dimensional printing method. The shape of the first molded body is made to be the same as the shape of the main body that is formed. If CAD (Computer Aided Design) data for the main body that is formed is available, the shape of the first molded body can be made to be the same as the shape of the main body that is formed using the CAD data. The data for the three-dimensional printer of the first molded body can also be obtained from the CAD data. 【0081】 (Joint) The joint connects the main body and the transparent substrate. As described above, the joint consists of a fused layer of glass frit and a filler with a lower coefficient of thermal expansion than the glass frit (low-expansion filler). The glass frit is, for example, made of B glass. ２ O３ is 15 to 50 mol%, SiO ２ is 0 to 30 mol%, Al ２ O ３ is 0 to 25 mol%, Bi ２ O ３ is 0 to 35 mol%, ZnO is 0 to 65 mol%, TeO ２ is 0 to 20 mol%, TiO ２ +ZrO ２ +Nb ２ O ５ is 0 to 15 mol%, MgO + CaO + SrO + BaO is 0 to 15 mol%, Li ２ O + Na ２ O + K ２ O is 0 to 10 mol%, and a glass having a softening temperature Ts of less than 650 °C is used. This glass composition consists essentially of the above components, but may contain other components up to 5 mol% as long as the object of the present invention is not impaired. Hereinafter, each component of this glass composition will be described. 【0082】 B ２ O ３ is a glass network former, a component that can stabilize the glass, and an essential component. B ２ O ３ content is 15 to 50 mol%. When the content of B ２ O ３ is less than 15 mol%, the glass becomes unstable, is likely to crystallize, and may lose its sinterability. On the other hand, when the content of B ２ O ３ exceeds 50 mol%, the chemical durability of the glass may decrease. The content of B ２ O ３ is more preferably 20 to 45 mol%, and even more preferably 25 to 40 mol%. SiO ２ is a glass network former, a component that enhances the stability of the glass, improves the bonding strength, and lowers the thermal expansion coefficient. The content of SiO ２ is 0 to 30 mol%. When the content of SiO ２ exceeds 30 mol%, the softening temperature Ts becomes too high, and the fluidity during firing may decrease. SiO ２The content is preferably 25 mol% or less, more preferably 20 mol% or less. ２ O ３ Al is a component that improves chemical durability, lowers the coefficient of thermal expansion, and further improves bonding strength. ２ O ３ The content is 0 to 25 mol%. ２ O ３ If the content exceeds 25 mol%, the stability of the glass will decrease, and the softening temperature Ts may become too high, potentially reducing fluidity during firing. ２ O ３ The content is preferably 20 mol% or less, and more preferably 10 mol% or less. ２ O ３ This is a component that lowers the softening temperature Ts without reducing the chemical durability of the glass. ２ O ３ The content is 0 to 35 mol%. ２ O ３ If the content exceeds 35 mol%, the glass may become unstable, prone to crystallization, and impair sinterability. Furthermore, the coefficient of thermal expansion may increase, potentially reducing the bonding strength. ２ O ３ The content of is more preferably 30 mol% or less, and even more preferably 25 mol% or less. 【0083】 ZnO lowers the softening temperature Ts, and Bi ２ O ３ It is a component that does not significantly increase the coefficient of thermal expansion compared to [another component]. The ZnO content is 0 to 65 mol%. If the ZnO content exceeds 65 mol%, the glass becomes unstable, prone to crystallization, and may impair sinterability. The ZnO content is preferably 60 mol% or less, and more preferably 50 mol% or less. TeO ２ This is an ingredient that lowers the softening temperature Ts and improves chemical durability. TeO ２ The content is 0-20 mol%. TeO ２ If the content exceeds 20 mol%, it may impair sinterability or reduce bonding strength. The TeO2 content is more preferably 16 mol% or less, and even more preferably 12 mol% or less. ２, ZrO ２ , Nb ２ O ５ TiO is a component that lowers the coefficient of thermal expansion and improves the chemical durability of glass. ２ , ZrO ２ , and Nb ２ O ５ The total amount of these components may be 0 to 15 mol%. If this total amount exceeds 10 mol%, the stability of the glass may decrease, the softening temperature Ts may become too high, and the fluidity during firing may decrease. More preferably, the total amount is 10 mol% or less, and even more preferably 5 mol% or less. Alkaline earth metal oxides CaO, SrO, MgO, and BaO are components that enhance the stability of the glass and improve its sinterability. The content of each of these alkaline earth metal oxide components is 0 to 15 mol%, that is, the content of MgO is 0 to 15 mol%, the content of CaO is 0 to 15 mol%, the content of SrO is 0 to 15 mol%, and the content of BaO is 0 to 15 mol%, and the total amount of these alkaline earth metal oxides is preferably 0 to 15 mol%. If this total amount exceeds 15 mol%, the stability of the glass may decrease and the chemical durability may decrease. More preferably, the total amount is 16 mol% or less, and even more preferably 10 mol% or less. 【0084】 Li ２ O, Na ２ O and K ２ Alkali metal oxides in O are components that lower the softening temperature Ts. The content of each of the above alkali metal oxides is 0 to 10 mol%, i.e., Li ２ O content is 0-10 mol%, Na ２ O content is 0-10 mol%, K ２ The O content is preferably 0 to 10 mol%, and the total amount of these alkali metal oxides is preferably 0 to 10 mol%. If the total amount exceeds 10 mol%, the chemical durability of the glass may decrease and the coefficient of thermal expansion may increase. This total amount is more preferably 0 to 8 mol%, and even more preferably 0 to 5 mol%. Unless there is a particular reason such as wanting to lower the softening temperature Ts, it is preferable not to include them. The glass composition contains CeO as an oxidizing agent in the glass. ２ MoO３ , SnO ２ MnO ２ Alternatively, to remove internal bubbles, the mixture may contain metal compounds with oxidation catalytic properties, such as copper chloride, or elements that can have multiple oxidation states depending on the valence, such as antimony oxide. The content of these components is preferably 0 to 5 mol%. The softening temperature Ts is less than 650°C. If the softening temperature Ts is 650°C or higher, the firing temperature during bonding may become too high, potentially causing deformation of the transparent substrate. The softening temperature Ts is preferably 620°C or lower, more preferably 600°C or lower, and even more preferably 570°C or lower. 【0085】 A filler with a lower coefficient of thermal expansion than the glass frit constituting the fused layer (low-expansion filler) plays a role in reducing the coefficient of thermal expansion of the joint. Examples of low-expansion fillers include silica, alumina, zirconia, zirconium silicate, cordierite, mullite, lead titanate, β-eucryptite, β-spodumene, β-quartz solid solution, zirconium phosphate compounds, soda-lime glass, or borosilicate glass. An example of a zirconium phosphate compound is (ZrO) ２ P ２ O ７ NaZr ２ (PO ４ ) ３ , KZr ２ (PO ４ ) ３ Ca ０．５ Zr ２ (PO ４ ) ３ , NbZr(PO ４ ) ３ , or Zr ２ (WO ３ ) (PO ４ ) ２ These composite compounds are examples. The content of the low-expansion filler is related to the thermal expansion coefficient α of the joint. ｂ The thermal expansion coefficient α of the transparent substrate ａ and the thermal expansion coefficient α of the main body ｃ The relationship Q = (α ｂ / α ａ ) + (α ａ / α ｃThe coefficient of thermal expansion of the low-expansion filler is set appropriately so that it is between 2.00 and 3.10. Therefore, although it depends on the thermal expansion coefficients of the glass frit, transparent substrate, and main body, it is preferable to include the low-expansion filler in a range of 0.1 to 50 volume percent relative to the glass frit. If the low-expansion filler content exceeds 50 volume percent, the molten-bonded layer may not obtain sufficient fluidity during firing, which may reduce airtightness. 【0086】 The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. In Examples 1 to 10, a gas cell was manufactured using a main body 100 having the shape shown in Figure 15. In Examples 1 to 10, Examples 4, 8, and 9 are examples, and Examples 1 to 3, 5 to 7, and 10 are comparative examples. The main body 100 shown in Figure 15 was manufactured so that the sizes of each part D1 to D8 are the values ​​shown in Table 1 below. The main body 100 shown in Figure 15 has the same configuration as the main body shown in Figure 8, so a detailed explanation is omitted. The main body 100 shown in Figure 15 has two openings 101 facing each other. Parts D4 to D8 indicate the sizes of the openings. Note that only one of the two openings 101 is shown in Figure 15. 【0087】 【0088】<Manufacturing Method of the Main Body> The main body 100, having the shape shown in Figure 15, was formed from SiSiC according to the manufacturing method of the main body described above. The manufacturing method of the main body will now be explained. First, a first molded body with the same shape as the main body was prepared by a three-dimensional printing method. The data for the three-dimensional printer of the first molded body was obtained from the CAD data of the main body 100 shown in Figure 15. The "binder jet molding method" was adopted as the three-dimensional printing method, and a binder was sprayed from an inkjet nozzle toward the powder layer. Phenolic resin was used as the binder. The powder layer was a mixture of SiC particles and a hardening agent. The hardening agent content was approximately 0.1% by mass. As the SiC particles, α-SiC powder (manufactured by Shinano Electric Refining Co., Ltd.) with an average particle size of 80 μm was used. The thickness of each powder layer was approximately 200 μm, and the binder spraying was repeated each time a powder layer was stacked. As a result, a first molded body with the same shape as the main body was prepared. The bulk density of the first molded body is 1.14 g / cm³. ３ The porosity of the first molded body was 64.6%. The average pore size M1 of the first molded body was 46.9 μm. 【0089】 Next, a second molded body was formed from a first molded body having the same shape as the main body. First, the first molded body was immersed in a dispersion containing carbon particles. The concentration of carbon particles in the dispersion was 40% by mass, and the carbon particles were nanoparticles. The average particle size M2 of the secondary carbon particles was approximately 120 nm. Therefore, M2 / M1 was approximately 0.00256. The dispersion medium was water. 【0090】 The first molded body was immersed in a dispersion under reduced pressure. This impregnated the pores of the first molded body with carbon particles, forming a second molded body with the same shape as the main body. Subsequently, the second molded body was dried. Vacuum freeze-drying was used for drying. Specifically, the second molded body was frozen by passing through a temperature range of 0°C to -10°C in less than 20 minutes, and then the moisture was sublimated from the second molded body by evacuating it under vacuum. The bulk density of the second molded body was 1.37 g / cm³. ３The porosity was 53.2%. Furthermore, when the concentration of carbon particles contained in the second molded body was measured using thermal analysis, the carbon particle content relative to SiC in the molded body was 20% by mass. 【0091】 Next, metallic Si was impregnated into the second molded body. For the metallic Si impregnation process, a heated body, constructed by placing metallic Si on top of the second molded body, was placed in the reaction furnace. The amount of metallic Si was 12.7 g. Next, the heated body was heated to 1550°C under reduced pressure in the reaction furnace. This process caused the metallic Si to melt, and the molten material impregnated the second molded body. Simultaneously, a reaction sintering process proceeded. That is, SiC was formed by the reaction between the carbon in the pores and the impregnated Si, and a sintering reaction occurred between the SiC contained in the second molded body, including these newly formed SiC, resulting in a sintered body (SiC-Si composite member), and the main body of an integrally molded product made of SiSiC was obtained. The main body made of SiSiC was measured using the above method and found to have a thermal expansion coefficient of 2.3 (ppm / K). 【0092】Glass substrates a1 to a4, shown in Table 2, were prepared as transparent substrates. Glass frits b1 and b2, shown in Table 3, were prepared as glass frits to form the bonding area. Glass substrates a1 to a4 were sized 50 mm × 50 mm × 1.1 mm (thickness). The average particle size (D50) of glass frits b1 and b2 was measured using a laser diffraction / scattering particle size analyzer (Nikkiso Co., Ltd.: Microtrac HRA). Glass frit b1 had a particle size of 2.3 μm, and glass frit b2 had a particle size of 2.6 μm. Cordierite powder with an average particle size (D50) of 2.8 μm (thermal expansion coefficient: 1.5 (ppm / K)) was used as a low-expansion filler to form the bonding area. The average particle size (D50) was measured using the laser diffraction / scattering particle size analyzer (Nikkiso Co., Ltd.: Microtrac HRA) as described above. Cordierite powder was mixed in the predetermined proportions (volume %) shown in Table 4 to obtain the solid component of the bonding material. Next, the solid component and vehicle were mixed in a ratio of 83% by mass of solid and 17% by mass of vehicle to prepare a bonding material paste. The vehicle was prepared by dissolving 5% by mass of ethylcellulose as a binder component in 95% by mass of a solvent consisting of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate. Using the obtained bonding material paste, it was applied to the bonding regions of the prepared glass substrates a1 to a4 by screen printing, then placed in a firing furnace and dried at 80°C for 10 minutes, followed by deashification at 300°C for 30 minutes, and finally calcined at a temperature 20°C lower than the firing temperature shown in Table 4 (i.e., firing temperature - 20°C) for 10 minutes to form the bonding material layer. Glass substrates a1 to a4, each with a bonding layer formed on it, were diced to an outer dimension of 11.5 mm x 11.5 mm to obtain 16 transparent substrates with a bonding layer formed on them, each with an opening of 7 mm x 7 mm in the main body and a bonding layer thickness of 10 μm. Note that the aforementioned outer dimension of 11.5 mm includes two bonding allowances with a width of 2.25 mm. 【0093】A solvent was added to the bonding paste to adjust its viscosity and dilute it to a paste with a bonding paste:solvent ratio of 4:1 (mass%). The prepared paste was applied to the bonding area of ​​the openings in the main body by dip coating. Next, the main body was placed in a firing furnace and dried at 80°C for 10 minutes, then deashed at 300°C for 30 minutes to obtain a main body with a bonding layer thickness of 20 μm. Transparent substrates with the bonding layer formed thereon were fitted into each opening of the obtained main body with a bonding layer, sandwiched between porous alumina setters, and then weighted down with a load of approximately 2 g / mm while firing at the firing temperature and firing time of 2 hours shown in Table 4 to form the joints and obtain a gas cell. Based on the above manufacturing method, gas cells were obtained using the glass substrate type, glass frit type, glass frit content, low-expansion filler content, and firing temperature shown in Table 4. The glass transition temperature Tg, thermal expansion coefficient α, and softening temperature Ts of the glass substrates a1 to a4 shown in Table 2 were measured using the method described above. The glass transition temperature Tg, thermal expansion coefficient α, and softening temperature Ts of the joints were measured using the same method as the glass substrates described above. The densities of the glass substrates a1 to a4, and the densities of the glass frits b1 and b2 were measured using the Archimedes method. 【0094】 The bonding properties of gas cells 1 to 10 were evaluated. For gas cells with a bonding property rating of "A", the airtightness was further evaluated. The results of the bonding property and airtightness evaluations are shown in Table 4 below. Gas cells with a bonding property rating of "B" were not evaluated for airtightness. For those for which airtightness was not evaluated, "-" is indicated in the "Airtightness" column of Table 4 below. 【0095】 <Bonding Quality> Bonding quality was assessed by observing the appearance of the transparent substrate and the bonding area of ​​the gas cell. If there was no deformation or cracks in the transparent substrate and no peeling or other defects at the bonding area, the bond was considered "A". If any defects were observed when observing the appearance of the transparent substrate and the bonding area, the bond was considered "B". 【0096】<Airtightness> For airtightness, a heat cycle test was performed on gas cells that received an "A" rating for joint performance to evaluate their airtightness. The heat cycle test consisted of 100 cycles, with one cycle being 30 minutes at 180°C and 30 minutes at 0°C. An ESPEC Corporation thermal shock device, model TSA-72ES, was used for the heat cycle test. After the heat cycle test, a penetrant (manufactured by TASETO Corporation: penetrant CM-3P) was dripped along the joint interface (joint) between the transparent substrate and the main body of the gas cell and left to stand for 1 hour. After standing for 1 hour, the gas cell was observed using an optical microscope, and those in which the penetrant did not reach the glass substrate were classified as "A". If even one streak of penetrant reached the glass substrate, it was classified as "B". 【0097】 【0098】 【0099】 【0100】 As shown in Table 4, Examples 4, 8 and 9, which are embodiments, are ((α ｂ / α a ) + (α a / α ｃ )) = The value of Q was between 2.00 and 3.10, and in all cases the evaluation of jointability and airtightness was "A". Examples 1 and 2 are (α ｂ / α a ) + (α a / α ｃ The value of ) exceeded 3.10, and delamination occurred at the joint. In Examples 1 and 2, the joint performance evaluation was "B", so the airtightness evaluation was not performed. In Example 3, (α ｂ / α a ) + (α a / α ｃ The value of ) was above 3.10 but close to 3.18, so the jointability evaluation was "A". However, in Example 3, the airtightness evaluation was "B". In Example 5, (α ｂ / α a ) + (α a / α ｃ The value of ) exceeded 3.10, and delamination occurred at the joint. Furthermore, in Example 5, Tg a -Tsｂ The temperature was below -30°C, and deformation was observed in the transparent substrate of the gas cell. Note that in Example 5, the bonding performance evaluation was "B", so the airtightness evaluation was not performed. Examples 6, 7 and 10 are (α ｂ / α a ) + (α a / α ｃ The value of ) exceeded 3.10, resulting in delamination and cracking at the joint, as well as cracking in the transparent substrate. Note that in Examples 6, 7, and 10, the evaluation of the bonding performance was "B," so the airtightness evaluation was not performed. The entire contents of the specification, claims, drawings, and abstract of Japanese Patent Application No. 2024-217496, filed on December 12, 2024, are incorporated herein by reference as disclosure of the present invention. 【0101】 1, 2, 3, 4, 5, 6, 7, 8, 9: Gas cell 10, 100: Main body portion 11, 11a, 11b, 11c, 11d, 101: Opening portion 12: Base portion 13: Side wall portion 14, 14a, 14b, 14c, 14d: Fitting portion 15, 15a, 15b, 15c, 15d: Lid portion 16, 16a, 16b: Through hole 17, 17a, 17b: Sealing portion 20, 20a, 20b, 20c, 20d: Transparent substrate 21, 21a, 21b, 21c, 21d: First main surface 22, 22a, 22b, 22c, 22d: Second main surface 23, 23a, 23b, 23c, 23d: Side surface 30: Reflective member 40: Closed space 50: Joint portion 141, 141a, 141b, 141c, 141d: First fitting surface 142, 142a, 142b, 142c, 142d: Second fitting surface 143, 143a, 143b: Third fitting surface

Claims

1. A gas cell in which alkali metal atoms are sealed inside, comprising: a main body having at least one opening; a transparent substrate provided in the opening; and a joint between the opening of the main body and the transparent substrate, the joint being composed of a fused layer of glass frit and a filler having a smaller coefficient of thermal expansion than the glass frit, and the coefficient of thermal expansion of the transparent substrate being α ａ The thermal expansion coefficient of the joint is set to α ｂ The thermal expansion coefficient of the main body is set to α ｃ (α ｂ / α ａ ) + (α ａ / α ｃ A gas cell where Q is 2.00 ≤ Q ≤ 3.

10.

2. Let the glass transition temperature of the transparent substrate be Tg ａ (°C), and let the softening temperature of the joint be Ts ｂ (°C). When, -30°C ≤ Tg ａ - Ts ｂ holds, the gas cell according to claim 1.

3. The gas cell according to claim 1, wherein the main body is made of SiSiC.

4. The glass frit has the following glass composition: ２ O ３ 15-50 mol%, SiO ２ 0-30 mol%, Al ２ O ３ 0-25 mol%, Bi ２ O ３ 0-35 mol%, ZnO 0-65 mol%, TeO ２ 0-20 mol%, TiO ２ +ZrO ２ +Nb ２ O ５ 0-15 mol%, MgO + CaO + SrO + BaO 0-15 mol%, Li ２ O + Na ２ O+K ２ The gas cell according to claim 1, which contains 0 to 10 mol% of O and has a softening temperature Ts of less than 650°C.

5. The gas cell according to claim 1, wherein the transparent substrate has a first main surface that together with the main body constitutes a closed space for encapsulating the alkali metal atoms, a second main surface opposite to the first main surface, and a side surface that is incorporated into the outer edges of the first and second main surfaces, and the opening of the main body and the transparent substrate are joined to two or three surfaces of the first main surface, the second main surface and the side surface of the transparent substrate.

6. A quantum sensor comprising: a gas cell according to any one of claims 1 to 5; a light source that emits excitation light to excite electrons of alkali metal atoms contained in the gas cell; and a photodetector that detects light that has passed through the gas cell.

7. The quantum sensor according to claim 6, which is an optical pumping magnetic sensor.

8. The quantum sensor according to claim 6, which is an atomic oscillator.

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