Magnetic trap for cold atom ultrahigh / extremely-high vacuum measurement and design method

Abstract

The invention discloses a magnetic trap for cold atom ultrahigh / extremely-high vacuum measurement and a design method. Through three types of coils, the first type of coil generates an axial magnetic field with a bias magnetic field, the magnetic field generated by the second type of coil counteracts the bias magnetic field generated by the first type of coil, and the third type of coil generates a radial gradient magnetic field, so that the minimum value of the magnetic field intensity of the magnetic well is not zero; the uncertainty of vacuum measurement caused by Majorana loss is effectively inhibited, and more precise measurement of ultrahigh or extremely high vacuum is realized, so that a cold atom primary standard in an ultrahigh / extremely high vacuum range is constructed. Compared with an existing magnetic trap technology for restraining Majorana loss, the magnetic trap device has the remarkable advantages of being compact in structural design, sufficient in space of a laser beam transmission path, high in precision of magnetic field regulation and control and the like.

Description

Technical Field

[0001] The present invention relates to the technical fields of engineering electromagnetics, vacuum metrology and quantum mechanics, and in particular to a novel magnetic trap applied to a cold atom ultra-high / extremely high vacuum measuring device and a design method thereof. Background Art

[0002] Ultra-high / ultra-high vacuum measurement technology has a wide range of applications in aerospace science, nanoscience, semiconductor manufacturing, surface science, nuclear fusion, gravitational wave detection and other fields. In recent years, with the development of laser cooling and trapped atom technology, the technology of measuring vacuum degree by the loss of cold atoms is expected to develop into a new generation of vacuum metrology technology. After the cold atoms enter the magneto-optical trap, they collide with the background gas in the vacuum environment, causing the loss of cold atoms. The loss rate is determined by measuring the quasi-exponential change curve of the number N with time t, and the velocity-averaged collision loss rate coefficient is accurately calculated based on the collision theory, and then the vacuum degree of the background gas can be inverted. The pressure measurement range of this technology is 10 -5 Pa~10 -10 Pa, or even lower. Compared with the cold atom loss rate coefficient in the magneto-optical trap, which is easily affected by the trap depth, the magnetic trap has a shallower trap depth (<1mK), and the loss rate measurement depends only on the basic atomic properties (collision cross section), which can be used to construct the cold atom primary standard in the ultra-high / extremely high vacuum (UHV / XHV) range.

[0003] At present, the magnetic field required for the magnetic trap is usually generated by a powered coil, an electromagnet, a permanent magnet, etc. The simplest magnetic trap is a quadrupole magnetic trap composed of two coils of the same size and with opposite current directions. The quadrupole magnetic trap uses a linear gradient magnetic field to strongly bind cold atoms. However, there is a local minimum in the magnetic field strength in the geometric center area of ​​the magnetic trap, that is, the zero magnetic field area. When cold atoms approach this area, Majorana spin flip transitions are prone to occur, causing the atoms to change from a bound state to an unbound state, and may escape from the magnetic trap, resulting in atomic loss.

[0004] At present, some laser beams that have a repulsive effect on neutral atoms can be used to irradiate the center of the quadrupole magnetic field, so that the atoms trapped in the magnetic field cannot approach the central zero magnetic field area, thereby avoiding the Majorana spin flip, such as the quadrupole-plug beam magneto-optical hybrid trap. However, this method requires additional optical devices to produce the required blue detuned laser beam by adjusting the optical path. The focusing and alignment accuracy required in the adjustment process is high and the operation is difficult. In addition, the alternating magnetic field can be used to generate the maximum magnetic field intensity in space, and the atoms can be trapped in the strong field. The atoms are trapped in the lowest energy spin state, and the probability of spin flip collision is reduced, such as the AC / DC hybrid magnetic trap. However, the alternating magnetic field can easily magnetize the stainless steel cavity in the laboratory, causing interference to the cold atoms trapped in the cavity, resulting in an increase in the error in the atomic loss rate. Summary of the invention

[0005] In view of this, the present invention provides a magnetic trap for ultra-high / ultra-high vacuum measurement of cold atoms. By optimizing the coil structure and adjusting the coil current, the minimum magnetic field intensity of the magnetic well is not zero, thereby effectively avoiding Majorana loss and improving the measurement lower limit of ultra-high / ultra-high vacuum.

[0006] The magnetic trap for ultra-high / extremely high vacuum measurement of cold atoms of the present invention comprises three types of coils; the planes where all the coils are located are parallel to the radial plane where the center point of the vacuum glass chamber that traps the atomic cloud is located;

[0007] The first type of coil includes coil A and coil B; coil A and coil B are symmetrical along the radial plane where the center point is located, and the center of the coil is coaxial with the center point; the distance a between coil A and coil B is greater than or equal to the minimum size of the vacuum glass chamber; the inner diameters of coil A and coil B do not interfere with the laser beam passing along the axis where the center is located; currents of the same magnitude and direction are passed through coil A and coil B to generate an axial curvature field with a bias magnetic field;

[0008] The second type of coil includes coil C and coil D; coils C and D are coplanar with coils A and B respectively, and have the same center point as coils A and B respectively, and have diameters larger than coils A and B; currents of the same magnitude and direction are passed through coils C and D, and the direction of the current is opposite to that of the first type of coil to offset the bias magnetic field of the first type of coil, and the magnitude of the current should be set according to the magnitude of the reverse bias magnetic field;

[0009] The third type of coil includes eight coils; among them, two coils form a pair, and the four pairs of coils are respectively located on two planes symmetrical along the radial plane where the center point is located, and the interval between the two planes is greater than the interval between the planes where coils A and B are located; the four coils located on the same plane are evenly distributed on the plane with the laser beam as the center, and do not interfere with the laser beam; each pair of coils is passed through currents of equal magnitude and opposite directions, and the current directions between adjacent coils on the same plane are opposite, generating a radial gradient magnetic field, and the current magnitude should be set according to the magnitude of the gradient magnetic field.

[0010] Preferably, a≤10cm.

[0011] Preferably, the inner diameters of coils A and B are as small as possible without interfering with the laser beam passing along the axis at the center thereof.

[0012] Preferably, the number of layers and turns of coil A and coil B are both even numbers to ensure that a magnetic field symmetrical along the central plane can be generated; the number of layers of coil C and coil D is the same as that of the first type of coil. The number of turns should be kept at the maximum within the limited space, generally less than the number of turns of the first type of coil, and the specific value should be set according to the size of the reverse bias magnetic field generated.

[0013] Preferably, the outer diameters of coils C and D are as large as possible within the limited size range.

[0014] Preferably, in the third type of coils, the center points of the four coils located in the same plane are located on the same circle.

[0015] Preferably, the coil is wound with hollow square copper wire and the center pipe is filled with water.

[0016] The present invention also provides a design method for the magnetic trap, comprising:

[0017] Step 1, establish a rectangular coordinate system xyz, the geometric center of the vacuum glass chamber that traps the atomic cloud is located at the origin of the coordinate system, the planes where all coils are located are parallel to the radial plane where the origin of the coordinate system is located, and coils of various types are arranged symmetrically about the origin of the coordinate system;

[0018] Step 2, constructing the first type of coil, the second type of coil and the third type of coil respectively;

[0019] Step 3, setting the structural parameters of each type of coil wire by the magnetic field required to trap atoms, including wire type, number of turns, number of layers, outer diameter, and inner diameter;

[0020] Step 4, deriving the spatial distribution function of the magnetic field generated by a single coil according to the Biot-Savart law;

[0021] Step 5, adjusting the current in the first type of coil according to the spatial distribution function of the magnetic field so that it generates an axial magnetic field greater than the set initial curvature magnetic field. At this time, the minimum magnetic field strength is usually very large, which will weaken the radial curvature magnetic field.

[0022] Step 6, according to the spatial distribution function of the magnetic field, the current in the second type of coil is adjusted to generate a reverse bias magnetic field in the axial direction to offset the bias magnetic field generated by the first type of coil, ensuring that the final minimum magnetic field strength is not zero and is as small as possible to reduce the impact on the radial curvature magnetic field. At this time, part of the curvature magnetic field will also be offset accordingly.

[0023] Step 7, adjusting the current in the third type coil according to the spatial distribution function of the magnetic field to generate a gradient magnetic field in the radial direction. Since the radial curvature magnetic field is obtained by fitting the gradient magnetic field and the bias magnetic field, the gradient magnetic field to be set can be obtained according to the set radial curvature magnetic field.

[0024] Step 8, derive the relationship expression between the magnetic field strength and the number of trapped atoms based on the distribution of atoms trapped in the magnetic trap, and substitute the designed axial curvature magnetic field and the fitted radial curvature magnetic field into the expression to evaluate whether it meets the design requirements of trapping a sufficient number of atoms. If it does not meet the design requirements, return to step 5 and reset the size of the initial curvature magnetic field; if it meets the design requirements, the design of the magnetic trap is completed.

[0025] The order of steps 5 to 7 can be changed to adjust the current in the following way:

[0026] Step 5A, adjusting the current of the third type coil so as to generate a gradient magnetic field in the radial direction that meets the set value;

[0027] Step 6A, adjusting the current of the first type of coil so that the curvature of the axial magnetic field generated by the coil is greater than the set curvature value, and the first type of coil will generate a very high bias magnetic field;

[0028] Step 7A, adjusting the current of the second type of coil so as to offset the high bias magnetic field while avoiding the loss of axial magnetic field curvature as much as possible.

[0029] Beneficial effects:

[0030] The present invention uses three types of coils, wherein the first type of coil generates an axial magnetic field with a bias magnetic field, the magnetic field generated by the second type of coil offsets the bias magnetic field generated by the first type of coil, and the third type of coil generates a radial gradient magnetic field, thereby making the minimum magnetic field strength of the magnetic well non-zero, effectively suppressing the uncertainty caused by Majorana loss to vacuum measurement, and achieving more precise measurement of ultra-high or ultra-high vacuum, thereby constructing a cold atom primary standard within the ultra-high / ultra-high vacuum range. Compared with the existing magnetic trap technology designed to suppress Majorana loss, the present invention has significant advantages such as compactness of structural design, sufficient space for laser beam transmission path, and high precision of magnetic field control. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The center of the system is located at the origin 0 of the xyz coordinate system, and the magnetic trap is composed of three types of coils that are symmetrical about the origin.

[0032] Figure 2 It is a schematic diagram of the magnetic field intensity curve generated by the magnetic trap of the present invention within the axial range near the origin.

[0033] Figure 3 It is a schematic diagram of the magnetic field intensity curve generated by the magnetic trap of the present invention within the radial range near the origin.

[0034] Figure 4 It is a schematic diagram of the distribution of magnetic flux lines generated by the magnetic trap of the present invention in the axial direction, and the atomic groups are trapped near the origin.

[0035] Figure 5 It is a schematic diagram of the distribution of magnetic flux lines generated by the magnetic trap of the present invention in the radial direction, and the atomic groups are trapped near the origin. DETAILED DESCRIPTION

[0036] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0037] The present invention provides a magnetic trap for ultra-high / extremely high vacuum measurement of cold atoms, such as Figure 1 As shown, there are three types of coils, among which,

[0038] (1) The first type of coil has two coils: coil A and coil B; coil A and coil B are respectively located in two planes parallel to the radial plane where the center point of the vacuum glass chamber for trapping the atomic cloud is located, and coil A and coil B are symmetrical along the radial plane where the center point is located; the centers of coil A and coil B are coaxial with the center point; the spacing a between the planes where coil A and coil B are located should be within a certain range: if the spacing a is too large, the magnetic trap's ability to bind the atomic cloud will be weakened, and accordingly, a larger current may need to be introduced, which will produce higher requirements for the power supply system; if the spacing a is too small, the magnetic trap may not be able to be matched with the corresponding cavity; since the minimum size of the vacuum glass chamber for trapping the atomic cloud is currently greater than or equal to 2 cm, in this embodiment, 2 cm ≤ spacing a ≤ 10 cm;

[0039] The inner diameter of coil A and coil B should not interfere with the laser beam passing along the axis of their center, and should be as small as possible within the limited size range (≥5cm); under the same current and unchanged structure, a smaller inner diameter can produce a magnetic field with stronger binding force. The outer diameter of coil A and coil B is within the inner diameter range of the second type of coil; the number of layers and turns of coil A and coil B are both even numbers to ensure that a magnetic field symmetrical along the central plane can be generated, and currents of the same magnitude and direction can be passed to generate an axial curvature field with a bias magnetic field.

[0040] (2) The second type of coil also has two coils: coil C and coil D; coils C and D are coplanar with coils A and B respectively, and have the same center as coils A and B respectively;

[0041] The number of coil layers and turns of coil C and coil D are both even numbers, and the number of layers is consistent with that of the first type of coil. The number of turns should be kept at the maximum within the limited space, generally less than the number of turns of the first type of coil, and the specific value should be set according to the size of the reverse bias magnetic field. Coil C and coil D are connected to currents of the same magnitude and direction, and the direction of the current is opposite to that of the first type of coil, generating an axial curvature field with a bias magnetic field. The direction of the magnetic field is opposite to that of the first type of coil, which is used to offset the bias magnetic field of the first type of coil. The current size should be set according to the size of the reverse bias magnetic field;.

[0042] The inner diameter of coils C and D does not interfere with the outer diameter of the first type of coil; the outer diameter of coils C and D should be as large as possible within the limited size range (within the outer diameter range of the vacuum chamber and without interference with other parts of the instrument). In this way, while offsetting the bias magnetic field, the loss of the axial curvature field will be avoided as much as possible.

[0043] (3) The third type of coil has eight coils: coils E, F, G, H, I, J, K and L; among them, two coils form a pair (EF, GH, IJ, KL), and the four pairs of coils are located on two planes parallel to the radial plane where the center point of the magnetic trap is located (E, G, I, K plane and F, H, J, L plane); the four coils located on the same plane are evenly distributed on the plane, the centers of the four coils are located on the same circle, and each pair of coils is symmetrical along the radial plane where the origin of the magnetic trap is located; the distance between the two planes where the four pairs of coils are located is greater than the distance between the planes where coils A and B are located. Each pair of coils is passed through with equal and opposite currents, and the currents between adjacent coils on the same plane are in opposite directions, generating a radial gradient magnetic field. The current size should be set according to the size of the gradient magnetic field. The outer diameter of the third type of coil does not interfere with the laser beam, and is kept within the inner and outer diameters of the vacuum chamber, and the height does not interfere with the vacuum chamber and optical devices.

[0044] The coils are all wound with hollow square copper wires, and the wires are evenly cooled by injecting water into the central pipe.

[0045] The present invention also provides a design method for the magnetic trap, which is as follows:

[0046] Step 1: Establish a rectangular coordinate system xyz. The geometric center of the vacuum glass chamber that traps the atomic cloud, that is, the geometric center of the new magnetic trap, is located at the origin of the coordinate system. The planes where all coils are located are parallel to the radial plane where the origin of the coordinate system is located, and coils of various types are arranged symmetrically about the origin of the coordinate system.

[0047] Step 2: construct a first type of coil, arrange two coils of the first type of coil symmetrically along the axial direction of the magnetic trap and about the geometric center of the magnetic trap, and pass currents of the same direction and magnitude.

[0048] Step 3: Construct the second type of coil. The two coils of the second type of coil are arranged symmetrically along the axis of the magnetic trap and about the geometric center of the magnetic trap along the axial direction of the magnetic trap, and are passed with a current in the opposite direction to the current of the first type of coil. The second type of coil group is accurately positioned in the same radial plane as the first type of coil, ensuring that the two groups of coils are arranged concentrically and coaxially at the origin of the coordinate system.

[0049] Step 3: Construct the third type of coils. This group of coils consists of eight coils, divided into four pairs, and each pair is arranged symmetrically about the center plane of the magnetic trap along the axial direction of the magnetic trap. The two coils in each pair are passed through currents of equal magnitude but opposite directions, and the currents are opposite to those of the adjacent coils in the same plane. And the third type of coil group is positioned in an area higher than the plane where the first and second types of coil groups are located. After the construction is completed, please refer to the structural arrangement of the three types of coils Figure 1 .

[0050] Step 4: Set the structural parameters of each type of coil wire according to the magnetic field required to trap atoms, including wire type, number of turns, number of layers, outer diameter, and inner diameter. In the process of trapping the atomic cloud, the radial curvature field generated by the first and second types of coils will destructively interfere with the radial gradient field generated by the third type of coil. After a certain distance away from the origin in the axial direction, the radial magnetic field that binds the atomic cloud will disappear. While ensuring that the atomic cloud can be accurately observed, the size of the atomic cloud should not exceed the diameter of the laser beam. In order to prevent the radial gradient field within this range (within the diameter of the laser beam) from being destroyed, the magnetic field generated by the third type of coil should be much stronger than that generated by the first and second types of coils. This can be achieved during the design process by changing the structural layout, swapping the positions of the third type of coil and the first and second types of coil groups, that is, the third type of coil is located on the side close to the vacuum chamber, and the first and second types of coil groups are located outside the third type of coil, or increasing the current passed into the third type of coil.

[0051] Step 5: Based on the Biot-Savart law, derive the spatial distribution function of the magnetic field generated by a single coil; then, by superimposing the magnetic fields of a single coil, construct the magnetic field spatial distribution model of various coil combinations.

[0052] Step 6: Adjust the current in each type of coil so that a curvature field with a minimum magnetic field strength of non-zero is generated in the axial and radial directions within the range near the origin of the magnetic trap coordinate system. Since the axial magnetic field is relatively independent of the radial magnetic field, two methods can be used to adjust the current. The first method is to adjust the current of the first type of coil first, then the second type of coil, and finally the third type of coil. First, adjust the current of the first type of coil so that the curvature of the axial magnetic field generated by it is greater than the set curvature value. At this time, the first type of coil will generate a very high bias magnetic field. Secondly, adjust the current of the second type of coil so that it can offset the high bias magnetic field while avoiding the loss of axial magnetic field curvature as much as possible. Finally, adjust the current of the third type of coil so that it generates a gradient magnetic field that meets the set value in the radial direction. The second method is to adjust the current of the third type of coil first, then the first type of coil, and finally the second type of coil. First, adjust the current of the third type of coil so that it generates a gradient magnetic field that meets the set value in the radial direction. Secondly, adjust the current of the first type of coil so that the curvature of the axial magnetic field generated by it is greater than the set curvature value. Finally, adjust the current of the second type of coil so that it can offset the high bias magnetic field while minimizing the loss of axial magnetic field curvature. The first method is applicable to situations where the radial magnetic field has a wider adjustment range than the axial magnetic field. In this case, it is necessary to first ensure that the axial magnetic field meets the design requirements. Set the corresponding radial magnetic field based on the radial magnetic field generated by the first and second type coils at this time. The second method is applicable to situations where the axial magnetic field has a wider adjustment range than the radial magnetic field. In this case, it is necessary to first ensure that the radial magnetic field meets the design requirements. Set the corresponding axial magnetic field based on the ability of the radial magnetic field generated by the third type coil to offset the disturbances generated by the first and second type coils. For the adjusted axial curvature magnetic field and radial curvature magnetic field, please refer to Figure 2 and Figure 3 For the magnetic flux distribution of the new magnetic trap in the axial and radial range, please refer to Figure 4 and Figure 5 .

[0053] Step 7: By fitting the magnetic field strength curve to obtain the curvature value of the magnetic field, evaluate whether the generated magnetic field meets the design requirements of trapping a sufficient number of atoms. If the design effect does not meet the design goal, return to step 6; if the design effect meets the design goal, carry out the design of the cooling system of the energized coil.

[0054] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A magnetic trap for ultra-high / ultra-high vacuum measurement of cold atoms, characterized in that: include: Three types of coils; the planes where all coils are located are parallel to the radial plane where the center point of the vacuum glass chamber that traps the atomic cloud is located; The first type of coil includes coil A and coil B; coil A and coil B are symmetrical along the radial plane where the center point is located, and the center of the coil is coaxial with the center point; the distance a between coil A and coil B is greater than or equal to the minimum size of the vacuum glass chamber; the inner diameters of coil A and coil B do not interfere with the laser beam passing along the axis where the center is located; currents of the same magnitude and direction are passed through coil A and coil B to generate an axial curvature field with a bias magnetic field; The second type of coil includes coil C and coil D; coils C and D are coplanar with coils A and B respectively, and have the same center point as coils A and B respectively, and have diameters larger than coils A and B; currents of the same magnitude and direction are passed through coils C and D, and the direction of the current is opposite to that of the first type of coil, and the magnitude of the current is determined according to the bias magnetic field of the first type of coil to offset the bias magnetic field of the first type of coil; The third type of coil includes eight coils; among them, two coils form a pair, and the four pairs of coils are respectively located on two planes symmetrical along the radial plane where the center point is located, and the interval between the two planes is greater than the interval between the planes where coils A and B are located; the four coils located on the same plane are evenly distributed on the plane with the laser beam as the center, and do not interfere with the laser beam; each pair of coils is passed through currents of equal magnitude and opposite directions, and the current directions between adjacent coils on the same plane are opposite, and the current magnitude is determined according to the magnitude of the radial gradient magnetic field to be generated.

2. The magnetic trap according to claim 1, characterized in that Said a≤10cm.

3. The magnetic trap according to claim 1, characterized in that The inner diameters of coils A and B are as small as possible without interfering with the laser beam passing along the axis at their centers.

4. The magnetic trap according to claim 1, characterized in that The number of layers and the number of turns of coil A and coil B are both even numbers.

5. The magnetic trap according to claim 1, characterized in that The number of layers of coils C and D is the same as that of the first type of coils, the number of turns is less than that of the first type of coils, and is kept at the maximum within the limited space; the outer diameters of coils C and D are as large as possible within the limited size range.

6. The magnetic trap according to claim 1, characterized in that In the third type of coil, the center points of the four coils located in the same plane are located on the same circle.

7. The magnetic trap according to any one of claims 1 to 6, characterized in that: The coil is wound with hollow square copper wire and the center pipe is filled with water.

8. The method for designing a magnetic trap according to any one of claims 1 to 7, characterized in that: include: Step 1, establish a rectangular coordinate system xyz, the geometric center of the vacuum glass chamber that traps the atomic cloud is located at the origin of the coordinate system, the planes where all coils are located are parallel to the radial plane where the origin of the coordinate system is located, and coils of various types are arranged symmetrically about the origin of the coordinate system; Step 2, constructing the first type of coil, the second type of coil and the third type of coil respectively; Step 3, setting the structural parameters of each type of coil wire by the magnetic field required to trap atoms, including wire type, number of turns, number of layers, outer diameter and inner diameter; Step 4, deriving the spatial distribution function of the magnetic field generated by a single coil according to the Biot-Savart law; Step 5, adjusting the current in the first type of coil according to the spatial distribution function of the magnetic field so as to generate a magnetic field with an initial curvature greater than the set value in the axial direction; Step 6, adjusting the current in the second type of coil according to the spatial distribution function of the magnetic field, so that it generates a reverse bias magnetic field in the axial direction to offset the bias magnetic field generated by the first type of coil, ensuring that the final minimum value of the magnetic field intensity is not zero and is as small as possible; Step 7, adjusting the current in the third type coil according to the spatial distribution function of the magnetic field so as to generate a gradient magnetic field in the radial direction; Step 8, derive the relationship expression between the magnetic field strength and the number of trapped atoms based on the distribution of atoms trapped in the magnetic trap, and substitute the designed axial curvature magnetic field and the fitted radial curvature magnetic field into the expression to evaluate whether it meets the design requirements of trapping a sufficient number of atoms. If it does not meet the design requirements, return to step 5 and reset the size of the initial curvature magnetic field; if it meets the design requirements, the design of the magnetic trap is completed.

9. The design method according to claim 8, characterized in that: The steps 5 to 7 are replaced by steps 5A to 7A: Step 5A, adjusting the current in the third type coil according to the spatial distribution function of the magnetic field so as to generate a gradient magnetic field in the radial direction that meets the set value; Step 6A, adjusting the current in the first type of coil according to the spatial distribution function of the magnetic field so as to generate a magnetic field with an initial curvature greater than a set value in the axial direction; Step 7A, adjust the current in the second type of coil according to the spatial distribution function of the magnetic field, so that it generates an axial reverse bias magnetic field to offset the bias magnetic field generated by the first type of coil, ensuring that the final minimum magnetic field strength is not zero and is as small as possible.

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