Method for measuring second-order Zeeman frequency shift of atomic fountain

Abstract

The invention discloses a method for measuring the second-order Zeeman frequency shift of an atomic fountain, and the method comprises the steps: obtaining the variable quantity of an outer axial magnetic field through a measurement device disposed on a cesium atom fountain clock, and enabling the operation of the cesium atom fountain clock not to be interrupted when the variable quantity of the outer axial magnetic field does not exceed a change threshold value. According to the magnetic field change and the shielding factor, rough measurement of the center frequency of the current magnetic-sensitive Ramsey transition fringes is achieved, a corresponding second-order Zeeman frequency shift is obtained, when the outer axial magnetic field change quantity exceeds a change threshold value, operation of the cesium atom fountain clock is interrupted, accurate measurement of the center frequency of the current magnetic-sensitive Ramsey transition fringes is achieved through a magnetic-sensitive Ramsey transition method, and the precision of the center frequency of the current magnetic-sensitive Ramsey transition fringes is improved. And the corresponding second-order Zeeman frequency shift is obtained. According to the method, the second-order Zeeman frequency shift can be rapidly and accurately measured in real time, and great help is provided for reducing the deviation between the cesium atom fountain clock and international atomic time, improving the monthly operation rate of the cesium atom fountain clock and improving the status of the cesium atom fountain clock in international atomic time calibration.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of atomic fountain clocks, and particularly relates to a method for measuring second-order Zeeman frequency shift of an atomic fountain. BACKGROUND

[0002] International atomic time is an integral time scale with the time unit of international system of units "second", and is adjusted by "leap second" to form international standard time-coordinated universal time. The time unit "second" is defined as "second", and the symbol is s, which is the unit of time in the international system of units. Its definition is to take the frequency of the transition between the hyperfine energy levels of cesium-133 atoms in the absence of interference, that is, the cesium frequency , which is a fixed value of 9192631770 Hz, and its unit Hz is equal to s -1 . The cesium atomic fountain clock is a time-frequency reference device that directly reproduces this definition, has the highest accuracy performance at present, and is used to calibrate other types of time-keeping atomic clocks. In the process of calibrating the free atomic time to generate international atomic time, the frequency deviation, frequency stability, frequency uncertainty and monthly running rate of the cesium atomic fountain clock and the reference hydrogen clock are the main performance indicators. In order to accurately calculate the frequency deviation of the cesium atomic fountain clock and the reference hydrogen clock, it is necessary to deduct various frequency shifts of the cesium atoms due to the influence of environmental physical effects and the motion state of the cesium atoms, mainly including cold atom collision frequency shift, second-order Zeeman frequency shift, blackbody radiation frequency shift, gravitational redshift and the like.

[0003] Among them, the cold atom collision frequency shift and the blackbody radiation frequency shift are obtained by real-time measurement during the operation of the cesium atomic fountain clock, the gravitational redshift is a fixed value related to the altitude of the cesium atomic fountain clock, and the second-order Zeeman frequency shift will change due to the influence of the geomagnetic field. Generally, the second-order Zeeman frequency shift needs to be evaluated by using the traditional measurement method at the end of each operation cycle (one month) of the cesium atomic fountain clock. The traditional method mainly includes: magnetic sensitive Ramsey transition method, low frequency transition method and Rabi transition method. The atomic |3, 1>→|4, 1> Ramsey transition fringe center frequency is obtained by gradually increasing the atomic upward height from the upper end of the excitation cavity to the conventional operation height, so as to calculate the second-order Zeeman frequency shift.

[0004] The measurement process of the above traditional method is relatively complicated, and a complicated experimental device needs to be built, and too many system parameters need to be adjusted. The measurement is time-consuming, the average monthly running rate of the cesium atomic fountain clock is about 90%, and the measurement of the second-order Zeeman frequency shift occupies 1.7%, which will occupy about half a day of operation time of the cesium atomic fountain clock, and will affect the monthly running rate of the cesium atomic fountain clock. In addition, the second-order Zeeman frequency shift cannot be deducted in real time in the measurement of the frequency deviation of the cesium atomic fountain clock and the reference hydrogen clock, which will bring certain error to the subsequent measurement. SUMMARY

[0005] In order to solve the above problems existing in the prior art, the application provides a method for measuring second-order Zeeman frequency shift of an atomic fountain. The application provides a method for measuring second-order Zeeman frequency shift of an atomic fountain, comprising the following steps: A measuring device is installed on a cesium atomic fountain clock, and the measuring device is used to measure the external axial magnetic field data of a magnetic shielding device of the cesium atomic fountain clock. During the operation of the cesium atomic fountain clock, at a preset sampling time, the external axial magnetic field variation of the magnetic shielding device is determined according to the obtained external axial magnetic field data, and it is judged whether the external axial magnetic field variation exceeds a variation threshold. If the external axial magnetic field variation does not exceed the variation threshold, the center frequency shift of a magnetic-sensitive Ramsey transition fringe and the current center frequency of the magnetic-sensitive Ramsey transition fringe are obtained according to the external axial magnetic field variation, and the corresponding second-order Zeeman frequency shift is calculated according to the current center frequency of the magnetic-sensitive Ramsey transition fringe. If the external axial magnetic field variation exceeds the variation threshold, the operation of the cesium atomic fountain clock is interrupted, the current magnetic-sensitive Ramsey transition fringe is measured by using a magnetic-sensitive Ramsey transition method, the current magnetic-sensitive Ramsey transition fringe is compared with the previous magnetic-sensitive Ramsey transition fringe, the center frequency of the current magnetic-sensitive Ramsey transition fringe is determined, and the corresponding second-order Zeeman frequency shift is calculated according to the center frequency of the current magnetic-sensitive Ramsey transition fringe.

[0006] In an embodiment of the application, the cesium atomic fountain clock comprises a magneto-optical trap, a state selection cavity, a detector and a fluorescence collection system, the magnetic shielding device and an excitation cavity. The magneto-optical trap is used to vertically upwardly project a cold atom group according to a set initial speed and time interval. The state selection cavity, the detector and the fluorescence collection system, the magnetic shielding device and the excitation cavity are sequentially arranged from bottom to top along the running path of the cold atom group.

[0007] In an embodiment of the application, the measuring device comprises a single-axis magnetometer probe and a single-axis magnetometer connected therewith, the single-axis magnetometer probe is installed at the outer top of the magnetic shielding device and close to the central axis of the magnetic shielding device, and the installation direction of the single-axis magnetometer probe is parallel to the central axis of the magnetic shielding device.

[0008] In an embodiment of the application, the sampling time is consistent with the operation period of the cesium atomic fountain clock.

[0009] In an embodiment of the application, the variation threshold is 1400nT.

[0010] In one embodiment of the present application, the center frequency shift of the magnetic sensitive Ramsey transition fringe and the current center frequency of the magnetic sensitive Ramsey transition fringe are obtained according to the variation of the external axial magnetic field, comprising: The variation of the internal axial magnetic field of the magnetic shielding device is calculated according to the variation of the external axial magnetic field, and the calculation formula is as follows: ; In the formula, is the variation of the internal axial magnetic field, is the variation of the external axial magnetic field, is the axial shielding factor of the magnetic shielding device; The center frequency shift of the magnetic sensitive Ramsey transition fringe is calculated according to the variation of the internal axial magnetic field, and the calculation formula is as follows: ; In the formula, is the center frequency shift of the magnetic sensitive Ramsey transition fringe; The current center frequency of the magnetic sensitive Ramsey transition fringe is obtained according to the center frequency shift and the center frequency of the previous magnetic sensitive Ramsey transition fringe.

[0011] In one embodiment of the present application, the current magnetic sensitive Ramsey transition fringe is measured by the magnetic sensitive Ramsey transition method, comprising: The magneto-optical trap is adjusted to have an initial velocity of The cold atom group is vertically upward projected every 2 seconds; The microwave of the state selection cavity is adjusted to have a pulse of , so that the cold atom group is in a magnetic sensitive state after passing through the state selection cavity ; The excitation microwave frequency of the excitation cavity is scanned according to a preset scanning condition; The flight time signal intensity of the falling cold atom group under each excitation microwave frequency is measured and recorded by the detector and the fluorescence collection system, and the current magnetic sensitive Ramsey transition fringe is obtained.

[0012] In one embodiment of the present application, the scanning condition comprises: the scanning range is , is the center frequency of the previous magnetic sensitive Ramsey transition fringe, and the frequency scanning step is 0.2 Hz.

[0013] In one embodiment of the present application, the calculation formula of the second-order Zeeman frequency shift is as follows: ​ ; wherein, represents the relative value of the second-order Zeeman frequency shift, represents the unperturbed clock transition frequency, represents the absolute value of the second-order Zeeman frequency shift, represents the center frequency of the magnetic Ramsey transition fringe.

[0014] Compared with the prior art, the present application has the beneficial effects that: The method for measuring the second-order Zeeman frequency shift of the atomic fountain of the present application acquires the variation amount of the external axial magnetic field through the measuring device installed on the cesium atomic fountain clock. When the variation amount of the external axial magnetic field does not exceed the variation threshold, the cesium atomic fountain clock does not need to be interrupted, and the coarse measurement of the center frequency of the current magnetic Ramsey transition fringe is realized according to the magnetic field variation and the shielding factor, so that the corresponding second-order Zeeman frequency shift is obtained. When the variation amount of the external axial magnetic field exceeds the variation threshold, the cesium atomic fountain clock is interrupted, and the current magnetic Ramsey transition fringe is measured by using the magnetic Ramsey transition method. The current magnetic Ramsey transition fringe is compared with the previous magnetic Ramsey transition fringe, the accurate measurement of the center frequency of the current magnetic Ramsey transition fringe is realized, and the corresponding second-order Zeeman frequency shift is obtained. Compared with the traditional measurement method, the present application has the advantages that the measuring device is simple, the operation process is simple, the measurement time is short, the second-order Zeeman frequency shift can be measured in real time, quickly and accurately, and the present application is very helpful for reducing the deviation of the cesium atomic fountain clock from the international atomic time, improving the monthly operation rate of the cesium atomic fountain clock, and improving the position of the cesium atomic fountain clock in the international atomic time calibration.

[0015] The above description is only a summary of the technical scheme of the present application. In order to more clearly understand the technical means of the present application, the content of the specification can be implemented, and in order to make the above and other purposes, characteristics and advantages of the present application more obvious and easy to understand, the following preferred embodiments are described in detail below, and the accompanying drawings are described as follows. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a flowchart of a method for measuring the second-order Zeeman frequency shift of an atomic fountain provided by an embodiment of the present application; Figure 2 is a structural schematic diagram of a cesium atomic fountain clock provided by an embodiment of the present application, which is installed with a measuring device; Figure 3 is a schematic diagram of the external axial magnetic field data of the magnetic shielding device within the sampling time provided by an embodiment of the present application; Figure 4 is a schematic diagram of the magnetic Ramsey transition fringe corresponding to the variation amount of the external axial magnetic field which does not exceed the variation threshold provided by an embodiment of the present application; Figure 5This is a schematic diagram of magnetic Ramsey transition fringes obtained using the magnetic Ramsey transition method according to an embodiment of the present invention.

[0017] Icons: 1-Magneto-optical trap; 2-Cold atom cluster; 3-Selective cavity; 4-Detector and fluorescence collection system; 5-Magnetic shielding device; 6-Excitation cavity; 7-Single-axis magnetometer probe; 8-Single-axis magnetometer; 9-Computer. Detailed Implementation

[0018] To further illustrate the technical means and effects adopted by the present invention to achieve the intended purpose, the following describes in detail a method for measuring the second-order Zeeman shift of an atomic fountain according to the present invention, in conjunction with the accompanying drawings and specific embodiments.

[0019] The foregoing and other technical contents, features, and effects of the present invention will be clearly presented in the following detailed description of specific embodiments in conjunction with the accompanying drawings. Through the description of the specific embodiments, a more in-depth and concrete understanding can be gained of the technical means and effects adopted by the present invention to achieve its intended purpose. However, the accompanying drawings are for reference and illustration only and are not intended to limit the technical solutions of the present invention.

[0020] This invention provides a method for measuring the second-order Zeeman shift of an atomic fountain. Please refer to [link to relevant documentation]. Figure 1 , Figure 1 This is a flowchart illustrating a method for measuring the second-order Zeeman shift of an atomic fountain according to an embodiment of the present invention, as shown below. Figure 1 As shown, the method for measuring the second-order Zeeman shift of an atomic fountain in this embodiment may include the following steps: Step 1: Install a measuring device on the cesium atomic fountain clock. The measuring device is used to measure the external axial magnetic field data of the magnetic shielding device of the cesium atomic fountain clock.

[0021] Please see Figure 2 , Figure 2 This is a schematic diagram of the cesium atomic fountain clock with the installation and measuring device provided in an embodiment of the present invention. Figure 2 As shown, in this embodiment, the cesium atom fountain clock includes: a magneto-optical trap 1, a state selection cavity 3, a detector and fluorescence collection system 4, a magnetic shielding device 5, and an excitation cavity 6; wherein, the magneto-optical trap 1 is used to vertically and upwardly project cold atom clusters 2 according to a set initial velocity and time interval; the state selection cavity 3, the detector and fluorescence collection system 4, the magnetic shielding device 5, and the excitation cavity 6 are arranged sequentially from bottom to top along the running path of the cold atom clusters 2.

[0022] During normal operation, the cesium atomic fountain clock generates excitation atoms within the selected cavity 3. Leap Microwave, the excitation cavity 6 is for excitation of atomic generation. Leap microwave.

[0023] In this embodiment, the measuring device includes a connected single-axis magnetometer probe 7 and a single-axis magnetometer 8. The single-axis magnetometer probe 7 is installed on the top outer side of the magnetic shielding device 5 near the central axis of the magnetic shielding device 5, and the installation direction of the single-axis magnetometer probe 7 is parallel to the central axis of the magnetic shielding device 5.

[0024] Understandably, the external axial magnetic field data measured by the single-axis magnetometer 8 can be transmitted and recorded in real time by the computer 9 connected to it.

[0025] Step 2: During the operation of the cesium atomic fountain clock, at the preset sampling time, the change in the external axial magnetic field of the magnetic shielding device is determined based on the acquired external axial magnetic field data, and it is determined whether the change in the external axial magnetic field exceeds the change threshold.

[0026] In this embodiment, the sampling time is consistent with the operating cycle of the cesium atomic fountain clock, which is 4 seconds. A schematic diagram of the external axial magnetic field data of the magnetic shielding device during the sampling time is shown below. Figure 3 As shown.

[0027] The full width at half maximum (FWHM) of the Ramsey transition spectral line at the normal operating height of the cesium atomic fountain clock is 1 Hz. When the center frequency of the magnetically sensitive Ramsey transition fringe shifts by more than 1 Hz, it becomes difficult to discern the direction of the shift.

[0028] The relationship between the change in center frequency and the change in the inner axial magnetic field of the magnetic shielding device is shown in the following formula: , In the formula, The shift in the center frequency of the magnetically sensitive Ramsey transition fringes. This represents the change in the axial magnetic field.

[0029] Therefore, based on the relationship between the change in center frequency and the change in the inner axial magnetic field of the magnetic shielding device, it can be obtained that when the center frequency shift is 1Hz, the change in the inner axial magnetic field is 0.14nT.

[0030] Axial shielding factor of magnetic shielding device Approximately 10 4 Therefore, according to the relationship between the axial shielding factor of the magnetic shielding device and the inner and outer axial magnetic fields... , This represents the change in the inner axial magnetic field. The change in the outer axial magnetic field is given by the value of 1 Hz. It can be calculated that when the center frequency shift is 1 Hz, the change in the outer axial magnetic field is 1400 nT.

[0031] Therefore, in this embodiment, the change threshold is 1400nT.

[0032] It can be understood that since the radial shielding factor of the magnetic shielding device is at least one order of magnitude higher than the axial shielding factor, the change of the radial magnetic field is not considered here.

[0033] Step 3: If the change of the outer axial magnetic field does not exceed the change threshold, the change of the center frequency of the magnetic Ramsey transition stripe and the current center frequency of the magnetic Ramsey transition stripe are obtained according to the change of the outer axial magnetic field, and the corresponding second-order Zeeman frequency shift is calculated according to the current center frequency of the magnetic Ramsey transition stripe.

[0034] When the change of the outer axial magnetic field is less than 1400nT, the magnetic Ramsey transition stripe is as shown in Figure 4 As can be seen from the figure, when the change of the center frequency of the magnetic Ramsey transition stripe is less than 1Hz, the moving direction and size of the center frequency can be easily determined. Therefore, in this embodiment, without interrupting the operation of the cesium fountain clock, the center frequency of the magnetic Ramsey transition stripe is coarsely measured according to the change of the magnetic field and the shielding factor, so as to obtain the second-order Zeeman frequency shift.

[0035] Specifically, the change of the center frequency of the magnetic Ramsey transition stripe and the current center frequency of the magnetic Ramsey transition stripe are obtained according to the change of the outer axial magnetic field, which can include the following steps: Step 3.1: The change of the inner axial magnetic field of the magnetic shielding device is calculated according to the change of the outer axial magnetic field, and the calculation formula is as follows: ; In the formula, is the change of the inner axial magnetic field, is the change of the outer axial magnetic field, is the axial shielding factor of the magnetic shielding device; Step 3.2: The change of the center frequency of the magnetic Ramsey transition stripe is calculated according to the change of the inner axial magnetic field, and the calculation formula is as follows: ; In the formula, is the change of the center frequency of the magnetic Ramsey transition stripe; Step 3.3: The current center frequency of the magnetic Ramsey transition stripe is obtained according to the change of the center frequency and the center frequency of the previous magnetic Ramsey transition stripe; In this embodiment, after obtaining the current center frequency of the magnetic Ramsey transition stripe, the corresponding second-order Zeeman frequency shift can be calculated by using the calculation formula of the second-order Zeeman frequency shift, wherein The calculation formula of the second-order Zeeman frequency shift is as follows: ; wherein, represents the relative value of the second order Zeeman shift, represents the frequency of the unperturbed clock transition , represents the absolute value of the second order Zeeman shift, represents the center frequency of the magnetic sensitive Ramsey transition fringe.

[0036] Step 4: If the variation of the external axial magnetic field exceeds the variation threshold, the operation of the cesium atomic fountain clock is interrupted, the current magnetic sensitive Ramsey transition fringe is measured by using the magnetic sensitive Ramsey transition method, the center frequency of the current magnetic sensitive Ramsey transition fringe is determined by comparing the current magnetic sensitive Ramsey transition fringe with the previous magnetic sensitive Ramsey transition fringe, and the corresponding second order Zeeman shift is calculated according to the center frequency of the current magnetic sensitive Ramsey transition fringe.

[0037] When the second order Zeeman shift is measured by using the magnetic sensitive Ramsey transition method, the microwave parameters in the state selection cavity 3 are adjusted to be the pulses that excite the atoms to occur transition. The microwave parameters in the excitation cavity 6 are adjusted to be the pulses that excite the atoms to occur transition.

[0038] Specifically, the current magnetic sensitive Ramsey transition fringe is measured by using the magnetic sensitive Ramsey transition method, including the following steps. S1: The magneto-optical trap vertically throws the cold atom group upward at an initial speed of every 2 seconds; S2: The microwave of the state selection cavity is adjusted to be the pulses that excite the atoms to occur transition, so that the cold atom group is in the magnetic sensitive state after passing through the state selection cavity. S3: The excitation microwave frequency of the excitation cavity is scanned according to the preset scanning condition. In this embodiment, the scanning condition includes that the scanning range is , and generally the center frequency variation range of the magnetic sensitive Ramsey transition fringe is less than ±2 Hz, is the center frequency of the previous magnetic sensitive Ramsey transition fringe, and the frequency scanning step is 0.2 Hz. With the change of the excitation microwave frequency, the magnetic sensitive Ramsey transition probability of the cold atom group changes.

[0039] S4: The falling time signal intensity of the cold atom group under each excitation microwave frequency is measured and recorded by using the detector and the fluorescence collection system, and the current magnetic sensitive Ramsey transition fringe is obtained.

[0040] A schematic diagram of magnetic Ramsey transition fringes obtained using the magnetic Ramsey transition method is shown below. Figure 5 As shown. In this embodiment, the shift in the center frequency of the magnetic Ramsey transition fringe can be obtained by comparing the current magnetic Ramsey transition fringe with the previous one. Then, the center frequency of the current magnetic Ramsey transition fringe can be obtained based on the center frequency of the previous fringe. Finally, the corresponding second-order Zeeman frequency shift can be calculated using the formula for calculating the second-order Zeeman frequency shift. The formula for calculating the second-order Zeeman frequency shift is as follows: ; In the formula, This represents the relative value of the second-order Zeeman frequency shift. This indicates that there is no interference with Zhong Yueqian. frequency, Represents the absolute value of the second-order Zeeman shift. This represents the center frequency of the magnetically sensitive Ramsey transition fringe.

[0041] In this embodiment, the measurement time for the second-order Zeeman frequency shift using the magnetic Ramsey transition method is 40 seconds, which is negligible in the monthly measurement cycle. After the magnetic Ramsey transition method measurement is completed, the cesium atomic fountain clock resumes normal operation, and the measuring device continues to monitor changes in the magnetic field.

[0042] It should be noted that the second-order Zeeman frequency shift can be measured in real time using the method of this invention. Therefore, when calculating the frequency deviation between the cesium atomic fountain clock and the reference hydrogen clock for the current month, the second-order Zeeman frequency shift for the corresponding time period can be deducted in real time.

[0043] The method for measuring the second-order Zeeman frequency shift of the cesium atomic fountain clock of the present invention is simpler in terms of measuring device, operation process, and measurement time compared with traditional measurement methods. It can measure the second-order Zeeman frequency shift in real time, quickly and accurately, which is of great help to reduce the deviation between the cesium atomic fountain clock and international atomic time, improve the monthly operating rate of the cesium atomic fountain clock, and enhance the status of the cesium atomic fountain clock in international atomic time calibration.

[0044] It is to be understood that the terminology "first", "second", and the like used herein is merely intended to differentiate one element from another element, and does not imply or suggest any actual relationship or sequence between the elements. Also, the terms "comprise", "comprising", or any other variant thereof are intended to cover non-exclusive inclusions, such that an item or apparatus that comprises a list of elements does not exclude other elements not expressly listed. An element defined by the phrase "comprising a..." does not exclude the presence of additional identical elements in the item or apparatus comprising the element. The terms "connected" or "coupled" or similar terms are not limited to a direct physical or mechanical connection, but can also include an electrical connection, whether direct or indirect. The terms "upper", "lower", "left", "right", and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are merely intended to facilitate the description of the present application and simplify the description, and thus cannot be construed as indicating or implying that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus cannot be construed as limiting the present application.

[0045] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific feature or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present application. The illustrative expressions of the above terms in the present specification do not necessarily refer to the same embodiment or example. Also, the specific feature or characteristic described can be combined in any suitable manner in one or more embodiments or examples. In addition, a person skilled in the art can combine and integrate different embodiments or examples described in the present specification.

[0046] The above is a further detailed description of the present application in conjunction with specific preferred embodiments, and the specific implementation of the present application cannot be limited to these descriptions. For those of ordinary skill in the art to which the present application belongs, a number of simple deductions or replacements can be made without departing from the concept of the present application, and all should be considered to fall within the protection scope of the present application.

Claims

1. A method of measuring the second order Zeeman shift of an atomic fountain, characterized in that, The application relates to a method for measuring the second-order Zeeman frequency shift of a cesium atomic fountain clock. The method comprises the following steps: installing a measuring device on a cesium atomic fountain clock, wherein the measuring device is used for measuring the outer axial magnetic field data of a magnetic shielding device of the cesium atomic fountain clock; during the operation of the cesium atomic fountain clock, determining the outer axial magnetic field variation of the magnetic shielding device according to the obtained outer axial magnetic field data at a preset sampling time; judging whether the outer axial magnetic field variation exceeds a variation threshold; if the outer axial magnetic field variation does not exceed the variation threshold, obtaining the center frequency movement of a magnetic sensitive Ramsey transition fringe and the current center frequency of the magnetic sensitive Ramsey transition fringe according to the outer axial magnetic field variation; and calculating the corresponding second-order Zeeman frequency shift according to the current center frequency of the magnetic sensitive Ramsey transition fringe; if the outer axial magnetic field variation exceeds the variation threshold, interrupting the operation of the cesium atomic fountain clock, measuring the current magnetic sensitive Ramsey transition fringe by using a magnetic sensitive Ramsey transition method, comparing the current magnetic sensitive Ramsey transition fringe with a previous magnetic sensitive Ramsey transition fringe, determining the center frequency of the current magnetic sensitive Ramsey transition fringe, and calculating the corresponding second-order Zeeman frequency shift according to the center frequency of the current magnetic sensitive Ramsey transition fringe. The cesium atomic fountain clock comprises a magneto-optical trap, a state selection cavity, a detector and a fluorescence collection system, the magnetic shielding device and an excitation cavity. The magneto-optical trap is used for vertically upwardly projecting a cold atom group according to a set initial speed and time interval. The state selection cavity, the detector and the fluorescence collection system, the magnetic shielding device and the excitation cavity are sequentially arranged along the running path of the cold atom group from bottom to top.

2. The method of measuring the second order Zeeman shift of an atomic fountain according to claim 1, wherein, The measuring device comprises a single-axis magnetometer probe and a single-axis magnetometer connected in series, the single-axis magnetometer probe is installed on the outer top of the magnetic shielding device and is close to the central axis of the magnetic shielding device, and the installation direction of the single-axis magnetometer probe is parallel to the central axis of the magnetic shielding device. The sampling time is consistent with the running period of the cesium atomic fountain clock. The variation threshold is 1400 nT.

3. The method of measuring the second order Zeeman shift of an atomic fountain according to claim 1, wherein, The method for obtaining the center frequency movement of the magnetic sensitive Ramsey transition fringe and the current center frequency of the magnetic sensitive Ramsey transition fringe according to the outer axial magnetic field variation comprises the following steps:

4. The method of claim 1, wherein, The inner axial magnetic field variation of the magnetic shielding device is calculated according to the outer axial magnetic field variation, and the calculation formula is as follows:

5. The method of measuring the second order Zeeman shift of an atomic fountain according to claim 1, wherein, The center frequency movement of the magnetic sensitive Ramsey transition fringe is calculated according to the inner axial magnetic field variation, and the calculation formula is as follows:

6. The method of measuring the second order Zeeman shift of an atomic fountain of claim 1, wherein, The current center frequency of the magnetic sensitive Ramsey transition fringe is obtained according to the center frequency movement and the center frequency of the previous magnetic sensitive Ramsey transition fringe. The method for measuring the current magnetic sensitive Ramsey transition fringe by using the magnetic sensitive Ramsey transition method comprises the following steps: ； In the formula, is the inner axial magnetic field variation, is the outer axial magnetic field variation, is the axial shielding factor of the magnetic shielding device; The excitation microwave frequency of the excitation cavity is scanned according to a preset scanning condition; ； In the formula, is the amount of center frequency shift of the magnetic sensitive Ramsey transition fringe; The time-of-flight signal intensity of the cold atom group falling under each excitation microwave frequency is measured and recorded by using the detector and the fluorescence collection system, and the current magnetic sensitive Ramsey transition fringe is obtained.

7. The method of measuring the second order Zeeman shift of an atomic fountain according to claim 2, wherein, ​ The magneto-optical trap is according to an initial velocity of The cold atom groups are vertically upward projected every 2 seconds; adjusting a microwave of the selected cavity to excite atoms to undergo a transition to a magnetic sensitive state ; ​ ​ 8. The method of measuring the second order Zeeman shift of an atomic fountain according to claim 7, wherein, The scanning condition includes that the scanning range is , is the center frequency of the previous magnetic sensitive Ramsey transition fringe, and the frequency scanning step is 0.2 Hz.

9. The method of measuring the second order Zeeman shift of an atomic fountain of claim 1, wherein, The formula for calculating the second-order Zeeman shift is as follows: ； wherein represents the relative value of the second order Zeeman shift, , represents the unperturbed clock transition frequency, represents the absolute value of the second order Zeeman shift, represents the center frequency of the magnetic sensitive Ramsey transition fringe.

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