An atomic interferometric gravity gradiometer based on prism beam-splitting magneto-optical trap and a gravity gradient measurement method

Abstract

This invention discloses an atomic interferometric gravity gradiometer and gravity gradient measurement method based on a prism-splitting magneto-optical trap, comprising a beam expander / collimator, a first prism-splitting magneto-optical trap assembly, a second prism-splitting magneto-optical trap assembly, a Raman laser collimator / expander, a Raman quarter-wave plate, a Raman mirror, and a vacuum cavity. The first and second prism-splitting magneto-optical trap assemblies are mounted on the vacuum cavity. The cooled beam emitted by the beam expander / collimator is split into two beams, which enter the first and second prism-splitting magneto-optical trap assemblies respectively, generating a first cold atom cluster and a second cold atom cluster. The power of the three sets of opposing beams in each magneto-optical trap is strictly equal, and the intensity and noise are common-mode, which can effectively improve the measurement accuracy of gravity and gravity gradient, reduce the size and weight of the instrument, and has strong robustness, meeting the requirements of engineering applications and dynamic measurements.

Description

Technical Field

[0001] This invention relates to the field of cold atom interferometry precision measuring instruments, specifically to an atomic interferometer gravity gradiometer based on a prism beam splitter magneto-optical trap and a gravity gradient measurement method. Background Technology

[0002] Gravity gradient measurement is of great significance to earth science, autonomous navigation, and space science. It is also an effective means of resource exploration, with important applications in basic geological surveys and research, and the exploration of oil and gas deposits. Gravity gradient measurement can measure not only the first derivative and its components of gravity potential, but also the second derivative.

[0003] Compared with traditional gravity measurement techniques, cold atom interferometry has significant advantages in high-precision gravity measurement and gravity gradient measurement. The cold atom interferometric gravity gradiometer used for gravity gradient measurement can simultaneously measure absolute gravity value and gravity gradient value. It is a high-precision gravity measurement device with self-calibration capability. However, the existing cold atom interferometric gravity gradiometer still has many shortcomings, and the practical performance and application range of the instrument are greatly limited. The main shortcomings are: (1) The intensity fluctuation of the beam of the opposing beam that forms the magneto-optical trap in the cold atom interferometric gravity gradiometer is inconsistent, which leads to the destruction of the intensity balance between the opposing beams, causing the generated cold atom parameters to change randomly, making it difficult to maintain the measurement accuracy and resulting in poor instrument stability; (2) The random differences in the number of atoms, temperature and other parameters of the cold atom clusters that serve as the gravity detection mass in the cold atom interferometric gravity gradiometer are large, which makes the gravity measurement accuracy of the cold atom interferometric loop inconsistent, resulting in large systematic errors and low sensitivity in gravity gradient measurement; (3) The defects of large instrument size and weight.

[0004] In 2016, the University of Birmingham in the UK developed a cold atom interferometric gravity gradiometer (A. Hinton, “Development of a transportable cold atom gradiometer,” PhD thesis, University of Birmingham, 2016.). This instrument uses a traditional three-dimensional magneto-optical trap (3D-MOT) to generate cold atom clusters as the detection medium for gravity gradient measurements. Because the 3D-MOT uses multiple polarization-maintaining fibers to guide the laser beam used for atom cooling, it is susceptible to random noise in the cooling light intensity caused by changes in ambient temperature. This noise disrupts the intensity balance between the cooling beams in the MOT, severely weakening the atom cooling effect. It leads to significant random fluctuations in the number and temperature of cold atoms, and also causes random changes in the initial position, projection velocity, and trajectory of the cold atom clusters during upward / downward movement, reducing the stability of the instrument. Furthermore, the intensity noise also causes significant differences in the parameters of the two cold atom clusters, increasing the instrument's systematic error.

[0005] US Patent (US9134450B2) discloses a cold atom interferometric gravity gradiometer. It utilizes a hollow pyramidal reflector embedded in a vacuum cavity and a single-beam laser incident from the bottom of the cavity to form two pyramidal magneto-optical traps. Single and double reflections from the four reflective surfaces of the pyramidal reflector create a six-beam magneto-optical trap that generates cold atoms. Because the intensity and noise between beam pairs are common-mode during operation, the beam intensity between the back-propagating beam pairs remains balanced, resulting in relatively stable parameters for the two cold atom clusters. However, the laser power distribution between the two magneto-optical traps cannot be freely adjusted, leading to significant differences in the cold atom cluster parameters between the two cold atom interferometric loops. This results in inconsistent gravity measurement accuracy between the two loops, causing large systematic errors and low sensitivity in gravity gradient measurement. Furthermore, the hollow pyramidal reflector must be embedded in an ultra-high vacuum cavity. Due to the size limitations of the glass hollow pyramidal reflector, it is difficult to increase the number of atoms in the cold atom clusters, limiting the number of cold atoms participating in the interferometric measurement. This results in a low signal-to-noise ratio and makes it difficult to further improve the resolution and precision of the instrument. Summary of the Invention

[0006] The purpose of this invention is to solve the technical problems of poor measurement accuracy and stability of existing gravity gradiometers, and to provide an atomic interferometric gravity gradiometer and gravity gradient measurement method based on a prism-splitting magneto-optical trap. The power of the beams in each magneto-optical trap is strictly equal and the light intensity and noise are common mode, which effectively improves the measurement accuracy of gravity and gravity gradient.

[0007] To achieve the above objectives, the technical solution provided by this invention is as follows:

[0008] An atomic interferometer based on a prism-beam-splitting magneto-optical trap is unique in that:

[0009] It includes a beam expander and collimator, a beam splitter, a first prism beam splitter magneto-optical trap assembly, a second prism beam splitter magneto-optical trap assembly, a Raman laser collimator and beam expander, a Raman quarter-wave plate, a Raman reflector, and a vacuum cavity;

[0010] The vacuum chamber is used for introducing... 87 Rb atom release agent; the first prism beam splitter magneto-optical trap assembly and the second prism beam splitter magneto-optical trap assembly are disposed in the vacuum cavity; the first prism beam splitter magneto-optical trap assembly, the second prism beam splitter magneto-optical trap assembly, the Raman beam quarter-wave plate, and the Raman beam reflector are sequentially disposed on the output light path of the Raman laser beam emitted by the Raman laser collimator and beam expander.

[0011] The beam expander and collimator is used to guide the cooling laser through a single-mode polarization-maintaining fiber. The cooling beam emitted by the beam expander and collimator is split into two beams by the beam splitter and enters the first prism beam splitting magneto-optical trap assembly and the second prism beam splitting magneto-optical trap assembly respectively to generate the first cold atom cluster and the second cold atom cluster. The Raman laser beam emitted by the Raman laser collimator and beam expander passes through the first prism beam splitting magneto-optical trap assembly and the second prism beam splitting magneto-optical trap assembly in sequence. After being reflected upward by the Raman light quarter-wave plate and the Raman light reflector, it re-enters the second prism beam splitting magneto-optical trap assembly and the first prism beam splitting magneto-optical trap assembly to achieve interference between the first cold atom cluster and the second cold atom cluster.

[0012] Furthermore, the first prism beam splitting magneto-optical trap assembly and the second prism beam splitting magneto-optical trap assembly have the same structure, both including a stereo beam splitter for splitting the incident cooling beam into one transmitted beam and four reflected beams, a prism assembly for reflecting the reflected light from the stereo beam splitter, a plane mirror for receiving the transmitted light from the stereo beam splitter, a first quarter-wave plate and a second quarter-wave plate sequentially disposed between the stereo beam splitter and the plane mirror, and a first anti-Helmholtz coil disposed at the incident end of the stereo beam splitter, and a second anti-Helmholtz coil disposed between the first quarter-wave plate and the second quarter-wave plate.

[0013] The incident cooling beam is transmitted through a stereo beam splitter, then passes through a first quarter-wave plate and a second quarter-wave plate before being incident on a plane mirror. After being reflected by the plane mirror, it passes through the second quarter-wave plate again and forms the first set of opposing atomic cooling beams collinear with the incident cooling beam.

[0014] The prism assembly receives four reflected beams from the stereo beam splitter and forms a second set of through-beam atomic cooling beams and a third set of through-beam atomic cooling beams. The second set of through-beam cooling beams and the third set of through-beam cooling beams are orthogonal and in the same plane, and are perpendicular to the first set of through-beam atomic cooling beams. Their intersection point forms an atomic cooling region.

[0015] The first and second anti-Helmholtz coils are electrically connected to external devices to generate magnetic fields. The central axes of the first and second anti-Helmholtz coils are collinear with the optical axis of the first set of opposing atomic cooling lights, and the distances from both to the center of the atomic cooling region are equal.

[0016] Furthermore, the stereo beam splitter is a square truncated prism; the prism assembly includes a first trapezoidal prism, a second trapezoidal prism, a third trapezoidal prism, and a fourth trapezoidal prism, used to reflect the four reflected beams of the stereo beam splitter respectively; the second trapezoidal prism and the third trapezoidal prism are respectively located on two opposite sides of the stereo beam splitter, and the first trapezoidal prism and the fourth trapezoidal prism are respectively located on the other two sides, and the four reflected beams are respectively reflected to form a second set of through-beam atomic cooling light and a third set of through-beam atomic cooling light; the first trapezoidal prism, the second trapezoidal prism, the third trapezoidal prism, and the fourth trapezoidal prism are isosceles trapezoidal prisms, with the angle between their two inclined surfaces and the lower base surface being 45 degrees, the two inclined surfaces being coated with a total reflection film, and the lower base surface being coated with an anti-reflection film.

[0017] Furthermore, quarter-wave plates are provided on the exit surfaces of the first, second, third, and fourth trapezoidal prisms.

[0018] Furthermore, the stereo beam splitter is a square truncated prism; the angle between its four sides and the bottom surface is 45 degrees, the four sides are coated with a total reflection film, and the top and bottom surfaces are coated with an anti-reflection film.

[0019] Furthermore, the stereo beam splitter consists of four right-angle prisms and a glass cube. The inclined surfaces of the right-angle prisms are coated with a total reflection film, and the upper and lower surfaces of the glass cube are light-transmitting surfaces coated with an anti-reflection film. The four sides of the glass cube are respectively overlapped with one right-angle face of the four right-angle prisms and bonded together to form a whole.

[0020] Furthermore, the stereo beam splitter is composed of four right-angle prisms, the inclined surfaces of which are coated with a total reflection film. The square area enclosed by one right-angle facet of the four right-angle prisms is used to transmit the central part of the cooling beam, and the four 45° inclined surfaces are used to reflect the cooling beam.

[0021] Furthermore, the vacuum cavity is a hollow cavity made of non-magnetic metal, and along the central axis of the cavity in the vertical direction, it sequentially includes a first magneto-optical trap region, a first interference region, a first detection region, a vacuum pump connection region, a second magneto-optical trap region, a second interference region, and a second detection region; and the hollow parts inside each region are connected as one.

[0022] The first cold atom cluster is generated in the first magneto-optical trap region, and after interference in the first interference region, it enters the first detection region;

[0023] The first cold atom cluster is generated in the second magneto-optical trap region, and after interference in the second interference region, it enters the second detection region;

[0024] A vacuum flange is installed on the vacuum pump connection area to connect the getter pump and the ion pump.

[0025] Furthermore, the first magneto-optical trap region and the second magneto-optical trap region have the same structure, which is a three-dimensional octagonal structure, with glass windows on four oblique sides; circular windows on the front and rear sides and the top end face; the first interference region and the second interference region are prisms with square cross-sections; the first detection region and the second detection region are prisms with square cross-sections, and multiple circular glass windows are provided on the four sides for detection light transmission and cold atom interference fluorescence detection; a circular window is provided on the bottom end face of the second detection region for passing a Raman laser beam.

[0026] A method for measuring gravity gradient, characterized by the following steps:

[0027] Step 1: Introduce an atomic release agent into the vacuum cavity. The beam expander and collimator emits a cooling beam, which is split into a first beam and a second beam that are parallel to each other. The first beam enters the first prism beam splitting magneto-optical trap assembly, and the second beam enters the second prism beam splitting magneto-optical trap assembly. Adjust the magnetic fields of the first prism beam splitting magneto-optical trap assembly and the second prism beam splitting magneto-optical trap assembly respectively to meet the working conditions of the magneto-optical trap, thereby generating the first cold atom cluster and the second cold atom cluster.

[0028] Step 2: Turn off the magnetic fields in the first prism beam splitting magneto-optical trap assembly and the second prism beam splitting magneto-optical trap assembly, and the first cold atom cluster and the second cold atom cluster fall freely under the action of gravity; at the same time, Raman laser beams are emitted through the Raman laser collimator and beam expander to apply three light pulses with a time interval of T to the first cold atom cluster and the second cold atom cluster to interfere with the cold atom clusters;

[0029] Step 3: After interference, the first and second cold atom clusters continue to fall freely. Excited by the probe beam, they release fluorescence signals containing atomic population probabilities. These signals are converted by a photodetector to obtain atomic interference signals. Data inversion yields gravity measurements, and subsequently, the gravity gradient value is obtained. for:

[0030]

[0031] in, and , respectively, represent the gravity measurements of the first cold atom cluster and the second cold atom cluster, and z represents the geometric center height difference z of the first prism beam splitting magneto-optical trap assembly and the second prism beam splitting magneto-optical trap assembly.

[0032] Compared with the prior art, the present invention has the following beneficial technical effects:

[0033] 1. In the atomic interferometric gravity gradiometer based on prism beam splitting magneto-optical traps of this invention, the cooling laser is introduced by a single-mode polarization-maintaining fiber and split into two beams of equal power using a free-space polarization beam splitter or fiber beam splitter. The beams then enter two three-dimensional magneto-optical traps (3D-MOTs) respectively. The cooling light power distribution between the two three-dimensional magneto-optical traps is adjustable. This method can maximize the consistency of key parameters such as the number of atoms, temperature, and loading speed of the two cold atomic clusters, reduce system errors, and ensure that the instrument obtains higher gravity gradient measurement resolution.

[0034] 2. In the atomic interferometric gravity gradiometer based on a prism-splitting magneto-optical trap of this invention, the three-dimensional magneto-optical trap (3D-MOT) uses a stereoscopic beam-splitting prism to split the incident single-beam cooled laser into five beams. Then, four trapezoidal prisms and one mirror are used to refract and reflect the split beams, forming three sets of opposing magneto-optical trap beams with equal power and orthogonal polarization. This magneto-optical trap optical path exhibits less fluctuation in the common-mode noise of the six cooled beams, resulting in smaller fluctuations in parameters such as the spatial position of the cold atom clusters, thus improving the robustness of the instrument.

[0035] 3. The present invention is based on an atomic interferometric gravity gradiometer using a prism beam splitting magneto-optical trap. Its vacuum cavity is made of a single piece of material in an integrated process, which has the characteristics of small size, light weight and robust structure. The structural strength meets the dynamic measurement requirements on a moving carrier. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the magneto-optical trap with an optical field distribution of (0, 1, 1) in an embodiment of the atomic interferometer gravity gradiometer based on a prism beam splitter magneto-optical trap of the present invention.

[0037] Figure 2 This is a schematic diagram of the structure of the first prism beam-splitting magneto-optical trap assembly in an embodiment of the atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap of the present invention;

[0038] Figure 3 This is a schematic diagram of the structure of the stereo beam splitter in the embodiment of the atomic interference gravity gradiometer based on the prism beam splitter magneto-optical trap of the present invention; wherein (a) is a schematic diagram of one structure and (b) is a schematic diagram of another structure;

[0039] Figure 4 This is a schematic diagram of the optical path structure of an embodiment of the atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap of the present invention;

[0040] Figure 5 This is a schematic diagram of the vacuum cavity in an embodiment of the atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap of the present invention;

[0041] Figure 6 This is a schematic diagram of the vacuum cavity optical path structure of an embodiment of the atomic interferometer gravity gradiometer based on a prism beam splitter magneto-optical trap according to the present invention;

[0042] The annotations in the attached figures are explained as follows:

[0043] 100 - Cooling beam, 101 - First beam, 102 - Second beam, 105 - Beam expander and collimator, 103 - Beam splitter, 200 - First prism beam-splitting magneto-optical trap assembly, 300 - Second prism beam-splitting magneto-optical trap assembly, 201 - Stereo beam splitter, 202 - First quarter-wave plate, 203 - Second quarter-wave plate, 204 - Plane mirror, 205 - First trapezoidal prism, 206 - Second trapezoidal prism, 207 - Third trapezoidal prism, 208 - Fourth trapezoidal prism, 209 - Third quarter-wave plate, 210 - Fourth quarter-wave plate, 211 - Fifth quarter-wave plate, 212 - Sixth quarter-wave plate, 213a - First anti-Helmholtz coil, 213b - Second anti-Helmholtz coil Mönchs coil, 220-right-angle prism, 221-glass cube, 400-vacuum cavity, 410a-first magneto-optical trap region, 420a-first interference region, 430a-first detection region, 440-vacuum pump connection region, 410b-second magneto-optical trap, 420b-second interference region, 430b-second detection region; 412a-glass window of the first magneto-optical trap region, 413-circular window of the first magneto-optical trap region, 411-top window, 422a-glass window of the second magneto-optical trap region, 423a-circular window of the second magneto-optical trap region, 431-bottom window, 500-Raman laser collimator and expander, 501-Raman laser beam, 502-Raman beam quarter-wave plate, 503-Raman beam reflector. Detailed Implementation

[0044] To make the objectives, advantages, and features of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Those skilled in the art should understand that these embodiments are merely used to explain the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0045] This embodiment provides an atomic interferometric gravity gradiometer based on a prism-beam-splitting magneto-optical trap. The principle is that the gravitational force between any two masses is inversely proportional to the square of the distance between them. The local gravity gradient is defined as the change in the local gravitational field as a function of the distance along the measurement axis of the gradiometer. The local gravitational field in the z-direction generated by the Earth exhibits a natural gradient along the z-axis. To determine the local gravity gradient along the z-direction, two local gravity measurements must be performed at a distance z apart. and The result of two such measurements can be used to determine the one-dimensional gravity gradient in the z-direction by subtracting the two measurements and dividing by the distance between them.

[0046] like Figure 1As shown in the diagram, the Z-direction coincides with the gravitational g-direction. Four of the six cooling beams lie in the same plane (ZY plane), distributed at 90° angles to each other. These four beams form two sets of opposing beams, with the optical axes of both sets forming an angle of 45° with the Z-direction. The remaining two cooling beams also form a set of collinear opposing beams, with their optical axes perpendicular to the ZY plane and passing through the center of the two anti-Helmholtz coils. The convergence point of the six cooling beams will generate a cold atom cluster.

[0047] Figure 4 As shown, the atomic interferometer gravity gradiometer based on the prism beam splitter magneto-optical trap of the present invention includes a beam expander collimator 105, a beam splitter 103, a first prism beam splitter magneto-optical trap assembly 200, a second prism beam splitter magneto-optical trap assembly 300, a Raman laser collimator beam expander 500, a Raman beam quarter-wave plate 502, a Raman beam reflector 503, and a vacuum cavity 400. The cooled beam 100 emitted by the beam expander collimator 105 is split into two beams by the beam splitter 103. These beams enter the first prism beam splitting magneto-optical trap assembly 200 and the second prism beam splitting magneto-optical trap assembly 300, respectively, generating a first cold atom cluster and a second cold atom cluster. The Raman laser beam 501 emitted by the Raman laser collimator beam expander 500 is incident downwards and enters the first prism beam splitting magneto-optical trap assembly 200 and the second prism beam splitting magneto-optical trap assembly 300 in sequence. The Raman laser beam 501 passes through the Raman quarter-wave plate 502, and its polarization state becomes circularly polarized. The circularly polarized light is reflected by the Raman light reflector 503 and passes through the Raman quarter-wave plate 502 again, becoming linearly polarized light perpendicular to the incident light. It returns upwards and meets the downward incident light in the first prism beam splitting magneto-optical trap assembly 200 and the second prism beam splitting magneto-optical trap assembly 300, thus interfering with the first and second cold atom clusters.

[0048] The beam splitter 103 employs a free-space polarization beam splitter or an optical fiber beam splitter to split the cooling beam 100 into two beams of equal power.

[0049] The first prism beam-splitting magneto-optical trap assembly 200 and the second prism beam-splitting magneto-optical trap assembly 300 have the same structure, such as Figure 2 As shown, the first prism beam splitting magneto-optical trap assembly 200 is spatially distributed in a (0, 1, 1) pattern and includes: a stereo beam splitter 201, a first quarter-wave plate 202, a second quarter-wave plate 203, a plane mirror 204, a first trapezoidal prism 205, a second trapezoidal prism 206, a third trapezoidal prism 207, a fourth trapezoidal prism 208, a third quarter-wave plate 209, a fourth quarter-wave plate 210, a fifth quarter-wave plate 211, a sixth quarter-wave plate 212, a first anti-Helmholtz coil 213a, and a second anti-Helmholtz coil 213b.

[0050] The stereo beam splitter 201 is used to split the vertically incident circularly polarized cooling beam 101 into five beams, including one transmitted beam 101a and four reflected beams. The center of the cross-section of the circularly polarized cooling beam is collinear with the geometric center of the light-transmitting surface on the stereo beam splitter 201, so that the power of the four reflected beams is equal.

[0051] After the transmitted beam 101a passes through the first quarter-wave plate 202 and the second quarter-wave plate 203 in succession, it is reflected by the plane mirror 204 and passes through the second quarter-wave plate 203 again to form the reflected beam 101A. The reflected beam 101A and the incident beam 101a are collinear to form a set of atomic cooling light in the X direction.

[0052] The reflected beam 101b is reflected by the two 45° reflecting surfaces of the trapezoidal prism 205 and becomes circularly polarized light after passing through the third quarter-wave plate 209; the reflected beam 101d is reflected by the two 45° reflecting surfaces of the trapezoidal prism 208 and becomes circularly polarized light after passing through the third quarter-wave plate 212; the reflected beams 101b and 101d form a set of opposing atomic cooling beams in the ZY plane, and the angle between this set of opposing beams and the vertical direction (Z direction) is -45°.

[0053] The reflected beam 101c is reflected by the two 45° reflecting surfaces of the trapezoidal prism 206 and becomes circularly polarized light after passing through the third quarter-wave plate 210; the reflected beam 101e is reflected by the two 45° reflecting surfaces of the trapezoidal prism 207 and becomes circularly polarized light after passing through the third quarter-wave plate 211; the reflected beam 101c and the third reflected beam 101e form another set of opposing atomic cooling beams in the ZY plane, and the angle between this set of opposing beams and the vertical direction (Z direction) is 45°.

[0054] The first cold atom cluster is generated at the intersection of the above three sets of cooling light.

[0055] The central axes of the first anti-Helmholtz coil 213a and the second anti-Helmholtz coil 213b coincide with the optical axis of the transmitted beam 101a, and the distances between the two anti-Helmholtz coils and the intersection centers of the three sets of cooling lights are equal.

[0056] The stereo beam splitter 201 in this embodiment of the invention, as shown... Figure 3 As shown in (a), the stereo beam splitter 201 consists of four right-angle prisms 220 and a glass cube 221. The inclined surfaces of the right-angle prisms 220 are coated with a total reflection film. The upper and lower surfaces of the glass cube 221 are light-transmitting surfaces and are coated with an anti-reflection film. The four sides of the glass cube 221 are respectively overlapped with a right-angle surface of the right-angle prisms 220 and bonded together with glue to form a whole.

[0057] Other embodiments may also employ, such as Figure 3(b) shows a stereo beam splitter 201, which consists of four right-angle prisms 220. The inclined surfaces of the right-angle prisms 220 are coated with a total reflection film. The four right-angle prisms 220 are distributed at 90° on the horizontal plane with a right-angled surface. The square aperture formed by the four right-angle prisms 220 is used to transmit the central part of the cooling beam, and the four 45° inclined surfaces are used to reflect the cooling beam.

[0058] like Figure 4 As shown, the cooling beam 100 is emitted by the collimator beam expander 105. After being split by the spatial light beam splitter, it becomes two beams with equal power. The laser beam generated by the laser collimator beam expander 105 is split into a first beam 101 and a second beam 102 by the beam splitter 103. The first beam 101 and the second beam 102 enter the first prism beam splitting magneto-optical trap assembly 200 and the second prism beam splitting magneto-optical trap assembly 300, respectively, to generate the light field required for the preparation of the first cold atom cluster and the second cold atom cluster.

[0059] The Raman laser beam 501 emitted by the Raman laser collimator 500 enters the vacuum cavity 400 vertically along the direction of gravity to perform interference operations on the first and second cold atom clusters.

[0060] like Figure 5 As shown, the vacuum chamber 400 is made entirely of titanium metal. The light-transmitting window of the chamber is made of quartz glass, and the glass window is coated with anti-reflection coating on both sides to ensure that the laser transmittance is >99.9%. The various functional areas of the vacuum chamber (400) are internally connected to ensure the balance and consistency of the gas pressure in different functional areas.

[0061] The vacuum cavity 400 includes, from top to bottom, a first magneto-optical trap region 410a, a first interference region 420a, a first detection region 430a, a vacuum pump connection region 440, a second magneto-optical trap region 410b, a second interference region 420b, and a second detection region 430b along the vertical cavity axis; the hollow parts inside each of these regions are connected as one unit. The first magneto-optical trap region 410a is used to generate the first cold atom cluster, the first interference region 420a is used to generate interference from the free fall of the first cold atom cluster, and the first detection region 430a is used to detect the atomic population probability to obtain the interference signal of the first cold atom cluster; the vacuum pump connection region 440 has vacuum flanges on its four sides for connecting the getter pump and the ion pump; the second magneto-optical trap region 410b is used to generate the second cold atom cluster, the second interference region 420b is used to generate interference from the free fall of the second cold atom cluster, and the second detection region 430b is used to detect the atomic population probability to obtain the interference signal of the second cold atom cluster; the first magneto-optical trap region 410a and the second magneto-optical trap region 410b have the same structure and geometry, the first interference region 420a and the second interference region 420b have the same structure and geometry, and the first detection region 430a and the second detection region 430b have the same structure and geometry.

[0062] The ultra-high vacuum maintaining system of vacuum chamber 400 includes a getter pump and an ion pump. These three vacuum pumps are connected to vacuum pump connection area 440 via vacuum conduits and connecting angle valves. The vacuum level of vacuum chamber 400 is below 1.0 × 10⁻⁷ Pa. Both surfaces of all light-transmitting windows in vacuum chamber 400 are coated with an anti-reflection coating using 780 nm laser light.

[0063] Working atom selection in this embodiment 87 The cooling and trapping laser wavelength for Rb is approximately 780.24 nm. The frequency of the cooling laser beam is red-detuned by 10–20 MHz relative to the transition from |52S1 / 2, F=3> to |52P3 / 2, F′=3>, with a total power greater than 160 mW. The repump laser frequency resonates with the transition from |52S1 / 2, F=1> to |52P3 / 2, F′=2>, with a power greater than 6 mW. The laser used for cooling and trapping atoms consists of a superposition of cooling and repump light, coupled into a single-mode polarization-maintaining fiber.

[0064] The first magneto-optical trap region 410a has an octagonal front side, four oblique sides with glass windows 412a, a circular window 413 on the front, and a top window 411 on the top end face. The first interference region 420a is a prism with a square cross-section. The first detection region 430a is a prism with a square cross-section and multiple circular glass windows on its four sides for detecting light transmission and cold atom interference fluorescence detection.

[0065] The second magneto-optical trap region 420b has an octagonal front side, four oblique sides with glass windows 422a, and two circular windows 423a on the front side. The second interference region 420b is a prism with a square cross-section. The second detection region 430b is a prism with a square cross-section and multiple circular glass windows on its four sides for detection light transmission and cold atom interference fluorescence detection. The bottom end face of the second detection region 430b has a bottom window 431 for passing through the Raman laser beam 501.

[0066] like Figure 6 As shown, the laser formed by the superposition of cooling light and repump light is output to the laser collimator and expander 105 through a single-mode polarization-maintaining fiber. The laser beam generated by the laser collimator and expander 105 is split into a first beam 101 and a second beam 102 by an optical fiber beam splitter. The beams enter the first prism beam splitting magneto-optical trap assembly 200 and the second prism beam splitting magneto-optical trap assembly 300, respectively, to generate two cold atom clusters.

[0067] The laser beam generated by the superposition of cooling light and repump light is output to the laser collimator and expander 105 through a single-mode polarization-maintaining fiber. The laser beam 100 generated by the laser collimator and expander 105 is split into a first beam 101 and a second beam 102 by a spatial beam splitter. The first beam 101 is perpendicularly incident on the stereo beam splitter 201 and split into five beams. Four horizontal beams are generated by reflection, and one vertical beam is generated by transmission. The four reflected beams are in the same plane and propagate in four directions within the plane, with an angle of 90 degrees between adjacent beams. The four reflected beams enter four identical trapezoidal beams around the stereo beam splitter 201. The trapezoidal prism, after being reflected twice by the two inclined surfaces, enters the vacuum cavity 400 through the four side windows of the first magneto-optical trap region 410a, forming two pairs of opposing laser beams with orthogonal polarization directions in a plane. The vertically transmitted beam passes through the front window of the first magneto-optical trap region 410a, and is reflected by the quarter-wave plate 203 and the total reflection mirror 204 to form another pair of opposing laser beams with orthogonal polarization directions. Together with the two pairs of beams mentioned above, they form three pairs of MOT beams with a spatial orientation of (0, 1, 1) for the generation of cold atoms, which are used to generate the first cold atom cluster.

[0068] Similarly, the second beam 102 generated by the laser collimator 105 enters the magneto-optical trap assembly 200b to generate a second MOT beam with three pairs of spatial orientations in a (0, 1, 1) structure, which is used to generate a second cold atom cluster.

[0069] This invention is based on an atomic interferometer gravity gradiometer using a prism-beam-splitting magneto-optical trap. The specific working process is as follows:

[0070] Step 1, Preparation of dual-cold atomic clusters: Installing in the first magneto-optical trap region 410a and the second magneto-optical trap region 410b on the vacuum cavity 400 87 Rb atom releasing agent, through 87 A current is applied to the electrode of the Rb atom-releasing agent to release atoms, which then participate in interference. 87 Rb atomic vapor; after the cooling light and repump light are superimposed, they are transmitted to the beam expander collimator 105 through a single-mode polarization-maintaining fiber and converted into a parallel beam 100. After spatial beam splitting, the parallel beam 100 is divided into two beams with equal power and opposite directions: a first beam 101 and a second beam 102.

[0071] The first beam 101 enters the first prism beam-splitting magneto-optical trap assembly 200, forming six orthogonally opposed laser beams with a spatial structure of (0, 1, 1). These six orthogonal laser beams pass through the corresponding glass windows of the first magneto-optical trap region 410a and enter the vacuum cavity 400. The beams converge at the geometric center of the vacuum region of the first magneto-optical trap region 410a. Adjusting the cooling light and the parameters of the anti-Helmholtz coil satisfies the operating conditions of the magneto-optical trap, generating the first cold atom cluster.

[0072] Similarly, the second beam 102 enters the second prism beam-splitting magneto-optical trap assembly 300, forming six orthogonally opposed laser beams with a spatial structure of (0, 1, 1). These six orthogonal laser beams pass through the corresponding glass windows of the first magneto-optical trap region 410b and enter the vacuum cavity 400. The beam converges at the geometric center of the vacuum region of the first magneto-optical trap region 410b. Adjusting the cooling light and the parameters of the anti-Helmholtz coils satisfies the operating conditions of the magneto-optical trap, generating the first cold atom cluster.

[0073] The height difference between the geometric centers of the first and second magneto-optical traps is z, therefore the vertical distance between the first and second cold atom clusters is z;

[0074] The second step is the interference of the two cold atomic clusters: After the two cold atomic clusters are prepared, the magnetic fields generated by the anti-Helmholtz magnetic field coils in the two magneto-optical traps are first turned off, and then the trapped laser beams in the two magneto-optical traps are turned off. The cold atomic clusters in the first magneto-optical trap region 410a and the second magneto-optical trap region 410b fall freely under the action of gravity. A Raman laser beam 501 with a frequency difference of 6.8 GHz enters the first interference region 420a and the second interference region 420b of the vacuum cavity 400 vertically through the top window 411 of the vacuum cavity 400. Three light pulses with a time interval of T, π / 2, π, and π / 2, are simultaneously applied to the two cold atomic clusters, which are in a magnetically insensitive state after the preparation. This is used to manipulate the cold atomic clusters through interference, realizing the splitting, reflection, and recombining of the atomic clusters, and constructing two falling cold atom interferometer gravimeters in the vertical direction.

[0075] The third step is the acquisition of the gravitational gradient: the two cold atom clusters, having completed their interferometric manipulation, continue to fall freely into the first detection region 430a and the second detection region 430b, respectively. After being excited by the detection beam, they release fluorescent signals containing the atomic population probabilities. These signals are collected by lens groups installed in the side glass windows of the first detection region 430a and the second detection region 430b, and then converted into electrical signals by photodetectors to obtain atomic interference signals. Finally, interference information is obtained through data inversion. The measurements from the two gravimeters are as follows: and Then the gravitational gradient value in the vertical (Z) direction is:

[0076] ,

[0077] This completes one measurement cycle of the gravity gradient.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the present invention.

Claims

1. An atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap, characterized in that: It includes a beam expander collimator (105), a beam splitter (103), a first prism beam splitter magneto-optical trap assembly (200), a second prism beam splitter magneto-optical trap assembly (300), a Raman laser collimator beam expander (500), a Raman beam quarter-wave plate (502), a Raman beam mirror (503), and a vacuum cavity (400). The vacuum chamber (400) is used for introducing... 87 Rb atom release agent; the first prism beam splitter magneto-optical trap assembly (200) and the second prism beam splitter magneto-optical trap assembly (300) are disposed on the vacuum cavity (400); the first prism beam splitter magneto-optical trap assembly (200), the second prism beam splitter magneto-optical trap assembly (300), the Raman beam quarter-wave plate (502), and the Raman beam reflector (503) are sequentially disposed on the output light path of the Raman laser beam (501) emitted by the Raman laser collimator beam expander (500); The beam expander collimator (105) is used to guide the cooling laser through a single-mode polarization-maintaining fiber. The cooling beam (100) emitted by the beam expander collimator (105) is split into two beams by the beam splitter (103) and enters the first prism beam splitting magneto-optical trap assembly (200) and the second prism beam splitting magneto-optical trap assembly (300) respectively to generate the first cold atom cluster and the second cold atom cluster. The Raman laser beam (501) emitted by the Raman laser collimator beam expander (500) passes through the first prism beam splitting magneto-optical trap assembly (200) and the second prism beam splitting magneto-optical trap assembly (300) in sequence, and is reflected upward by the Raman 1 / 4 wave plate (502) and the Raman reflector (503) and enters the second prism beam splitting magneto-optical trap assembly (300) and the first prism beam splitting magneto-optical trap assembly (200) again to achieve interference between the first cold atom cluster and the second cold atom cluster.

2. The atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap according to claim 1, characterized in that: The first prism beam splitting magneto-optical trap assembly (200) and the second prism beam splitting magneto-optical trap assembly (300) have the same structure. They both include a stereo beam splitter (201) for splitting the incident cooling beam into a transmitted beam and four reflected beams, a prism assembly for reflecting the reflected light from the stereo beam splitter (201), a plane mirror (204) for receiving the transmitted light from the stereo beam splitter (201), a first quarter-wave plate (202) and a second quarter-wave plate (203) arranged sequentially between the stereo beam splitter (201) and the plane mirror (204), and a first anti-Helmholtz coil (213a) arranged at the incident end of the stereo beam splitter (201) and a second anti-Helmholtz coil (213b) arranged between the first quarter-wave plate (202) and the second quarter-wave plate (203). The incident cooling beam is transmitted through a stereo beam splitter (201), then through a first quarter-wave plate (202) and a second quarter-wave plate (203) before being incident on a plane mirror (204). After being reflected by the plane mirror (204), it passes through the second quarter-wave plate (203) again and forms the first set of opposing atomic cooling beams collinear with the incident cooling beam. The prism assembly receives four reflected beams from the stereo beam splitter (201) and forms a second set of through-beam atomic cooling beams and a third set of through-beam atomic cooling beams. The second group of through-beam cooling beams and the third group of through-beam cooling beams are orthogonal and in the same plane, and are perpendicular to the first group of through-beam atomic cooling beams. Their intersection point forms an atomic cooling region. The first anti-Helmholtz coil (213a) and the second anti-Helmholtz coil (213b) are electrically connected to external devices to generate magnetic fields. The central axes of the first anti-Helmholtz coil (213a) and the second anti-Helmholtz coil (213b) are collinear with the optical axis of the first set of opposing atomic cooling lights, and the distances from both to the center of the atomic cooling region are equal.

3. The atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap according to claim 2, characterized in that: The stereo beam splitter (201) is a regular square truncated prism; the prism assembly includes a first trapezoidal prism (205), a second trapezoidal prism (206), a third trapezoidal prism (207), and a fourth trapezoidal prism (208), used to reflect the four reflected beams of the stereo beam splitter (201) respectively; the second trapezoidal prism (206) and the third trapezoidal prism (207) are respectively located on two opposite sides of the stereo beam splitter (201), and the first trapezoidal prism... The mirror (205) and the fourth trapezoidal prism (208) are located on the other two sides respectively. The four reflected beams are reflected to form the second set of through-beam atomic cooling light and the third set of through-beam atomic cooling light. The first trapezoidal prism (205), the second trapezoidal prism (206), the third trapezoidal prism (207) and the fourth trapezoidal prism (208) are isosceles trapezoidal prisms. The angle between the two inclined surfaces and the bottom surface is 45 degrees. The two inclined surfaces are coated with a total reflection film and the bottom surface is coated with an anti-reflection film.

4. The atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap according to claim 3, characterized in that: A quarter-wave plate is provided on the exit surface of the first trapezoidal prism (205), the second trapezoidal prism (206), the third trapezoidal prism (207), and the fourth trapezoidal prism (208).

5. The atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap according to claim 2, characterized in that: The stereo beam splitter (201) is a square truncated prism; the angle between its four sides and the bottom surface is 45 degrees, the four sides are coated with a total reflection film, and the top and bottom surfaces are coated with an anti-reflection film.

6. The atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap according to claim 2, characterized in that: The stereo beam splitter (201) consists of four right-angle prisms (221) and a glass cube (220). The inclined surfaces of the right-angle prisms (221) are coated with a total reflection film, and the upper and lower surfaces of the glass cube (220) are light-transmitting surfaces coated with an anti-reflection film. The four sides of the glass cube (220) are respectively overlapped with one right-angle face of the four right-angle prisms (221) and bonded together to form a whole.

7. The atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap according to claim 2, characterized in that: The stereo beam splitter (201) is composed of four right-angle prisms (221). The inclined surfaces of the right-angle prisms (221) are coated with a total reflection film. The square area enclosed by one right-angle surface of the four right-angle prisms (221) is used to transmit the central part of the cooling beam, and the four 45° inclined surfaces are used to reflect the cooling beam.

8. The atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap according to claim 1, characterized in that: The vacuum cavity (400) is a hollow cavity made of non-magnetic metal. Along the vertical central axis of the vacuum cavity (400), it sequentially includes a first magneto-optical trap region (410a), a first interference region (420a), a first detection region (430a), a vacuum pump connection region (440), a second magneto-optical trap region (410b), a second interference region (420b), and a second detection region (430b); and the hollow parts inside each region are connected as one. The first cold atom cluster is generated in the first magneto-optical trap region (410a), and after interference in the first interference region (420a), it enters the first detection region (430a). The first cold atom cluster is generated in the second magneto-optical trap region (410b), and after interference in the second interference region (420b), it enters the second detection region (430b). A vacuum flange is provided on the vacuum pump connection area (440) for connecting the getter pump and the ion pump.

9. The atomic interferometer gravity gradiometer based on a prism beam-splitting magneto-optical trap according to claim 8, characterized in that: The first magneto-optical trap region (410a) and the second magneto-optical trap region (410b) have the same structure, which is a three-dimensional octagonal structure, with glass windows on four oblique sides; circular windows on the front and back sides and the top end face; the first interference region (420a) and the second interference region (420b) are prisms with square cross-sections; the first detection region (430a) and the second detection region (430b) are prisms with square cross-sections, and multiple circular glass windows are provided on the four sides for detection light transmission and cold atom interference fluorescence detection; a circular window (431) is provided on the bottom end face of the second detection region (430b) for passing through the Raman laser beam (501).

10. A method for measuring gravity gradient, based on the atomic interferometric gravity gradiometer based on a prism-beam-splitting magneto-optical trap as described in any one of claims 1-9, characterized in that, Includes the following steps: Step 1: Atom release agent is introduced into the vacuum cavity (400), and the beam expander collimator (105) emits a cooling beam (100), which is split into a parallel first beam (101) and a second beam (102); the first beam (101) enters the first prism beam splitting magneto-optical trap assembly (200), and the second beam (102) enters the second prism beam splitting magneto-optical trap assembly (300); the magnetic fields of the first prism beam splitting magneto-optical trap assembly (200) and the second prism beam splitting magneto-optical trap assembly (300) are adjusted respectively to meet the working conditions of the magneto-optical trap, thereby generating the first cold atom cluster and the second cold atom cluster; Step 2: The magnetic fields in the first prism beam splitting magneto-optical trap assembly (200) and the second prism beam splitting magneto-optical trap assembly (300) are turned off, and the first and second cold atom clusters fall freely under the action of gravity; at the same time, a Raman laser beam (501) is emitted through the Raman laser collimator (500) to apply three light pulses with a time interval of T to the first and second cold atom clusters to interfere with the cold atom clusters; Step 3: After interference, the first and second cold atom clusters continue to fall freely. Excited by the probe beam, they release fluorescence signals containing atomic population probabilities. These signals are converted by a photodetector to obtain atomic interference signals. Data inversion yields gravity measurements, and subsequently, the gravity gradient value is obtained. for: ； in, and , respectively, represent the gravity measurements of the first cold atom cluster and the second cold atom cluster, and z represents the geometric center height difference z of the first prism beam splitting magneto-optical trap assembly and the second prism beam splitting magneto-optical trap assembly.

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