A magnetic trap and its design method for ultra-high / ultra-high vacuum measurements of cold atoms

Abstract

This invention discloses a magnetic trap and its design method for cold atom ultra-high / ultra-high vacuum measurements. The invention utilizes three types of coils: a first type generates an axial magnetic field with a bias magnetic field; a second type generates a magnetic field that cancels out the bias magnetic field generated by the first type; and a third type generates a radial gradient magnetic field. This ensures that the minimum magnetic field strength of the magnetic well is not zero, effectively suppressing the uncertainty caused by Majorana loss in vacuum measurements and enabling more precise measurements of ultra-high or ultra-high vacuum. This allows for the construction of a primary cold atom standard within the ultra-high / ultra-high vacuum range. Compared to existing magnetic trap technologies aimed at suppressing Majorana loss, this invention offers significant advantages such as compact structural design, ample space for the laser beam transmission path, and high precision in magnetic field control.

Description

Technical Field

[0001] This invention relates to the fields of engineering electromagnetics, vacuum metrology, and quantum mechanics, specifically to a novel magnetic trap and its design method for use in ultra-high / extremely high vacuum measurement devices for cold atoms. Background Technology

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

[0003] Currently, the magnetic field required for magnetic traps is typically generated by current-carrying coils, electromagnets, permanent magnets, etc. The simplest magnetic trap is a quadrupole magnetic trap consisting of two coils of the same size with opposite current directions. Quadrupole magnetic traps utilize linear gradient magnetic fields to strongly bind cold atoms. However, there is a local minimum of magnetic field strength, i.e., a zero magnetic field region, in the geometric center region of the magnetic trap. When cold atoms approach this region, they are prone to Majorana spin-flip transitions, causing the atoms to change from a bound state to an unbound state and potentially escape from the magnetic trap, resulting in atomic loss.

[0004] Currently, it's possible to irradiate the center of a quadrupole magnetic field with a laser beam that repels neutral atoms, preventing trapped atoms from approaching the central zero-magnetic-field region and thus avoiding Majorana spin flips, as seen in quadrupole-plug beam magneto-optical hybrid traps. However, this method requires additional optical equipment; the blue-detuned laser beam is generated by adjusting the optical path, and the focusing and alignment precision required during this adjustment process is high, making it difficult to operate. Alternatively, alternating magnetic fields can be used to generate the maximum magnetic field strength in space, trapping atoms in their lowest-energy spin states, reducing the probability of spin-flip collisions, as in AC / DC hybrid magnetic traps. However, alternating magnetic fields can easily magnetize the stainless steel cavity in the laboratory, interfering with the cold atoms trapped within and increasing the error in the atom 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 strength of the magnetic trap is not zero, thereby effectively avoiding Majorana loss and improving the measurement lower limit for ultra-high / ultra-high vacuum.

[0006] The magnetic trap for ultra-high / ultra-high vacuum measurement of cold atoms of the present invention includes three types of coils; the plane in which all coils are located is parallel to the radial plane in which the center point of the vacuum glass chamber trapping the atomic cloud is located.

[0007] The first type of coil includes coil A and coil B; coil A and coil B are symmetrical about 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 through along the axis where their centers are 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 share the same center point with coils A and B respectively, and their diameters are larger than those of coils A and B; coils C and D are supplied with current of the same magnitude and direction, and the direction of the current is opposite to that of the first type of coil, in order to counteract 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; two coils form a pair, and the four pairs of coils are located on two planes symmetrical about the radial plane where the center point is located, and the distance between the two planes is greater than the distance 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 supplied with a current of equal magnitude and opposite direction, and the current directions between adjacent coils on the same plane are opposite, generating a radial gradient magnetic field, and the magnitude of the current should be set according to the magnitude of the gradient magnetic field.

[0010] Preferably, a ≤ 10 cm.

[0011] Ideally, the inner diameters of coils A and B should be as small as possible without interfering with the laser beam passing through along their central axis.

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

[0013] Ideally, the outer diameters of coils C and D should be as large as possible within the specified size range.

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

[0015] A preferred design uses hollow square copper wire wound around the coil, with water injected into the central pipe.

[0016] The present invention also provides a design method for the above-mentioned 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 plane containing all the coils is parallel to the radial plane containing the origin of the coordinate system. The coils of each type are arranged symmetrically about the origin of the coordinate system.

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

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

[0020] Step 4: Based on the Biot-Savart law, derive the spatial distribution function of the magnetic field generated by a single coil;

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

[0022] Step 6: Adjust the current in the second type of coil according to the spatial distribution function of the magnetic field to generate a reverse bias magnetic field in the axial direction to counteract the bias magnetic field generated by the first type of coil. This ensures that the final minimum magnetic field strength is not zero and is as small as possible to reduce the influence on the radial curvature magnetic field. At this time, the curvature magnetic field will also be correspondingly canceled.

[0023] Step 7: Adjust the current in the third type of 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: Based on the distribution of atoms trapped in the magnetic trap, derive the expression relating the magnetic field strength to the number of trapped atoms. Substitute the designed axial curvature magnetic field and the fitted radial curvature magnetic field into the expression to evaluate whether they meet the design requirement of trapping a sufficient number of atoms. If they do not meet the design requirement, return to step 5 and reset the initial curvature magnetic field. If they meet the design requirement, the design of the magnetic trap is complete.

[0025] The order of steps 5 through 7 can be reversed, and the current can be adjusted in the following way:

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

[0027] Step 6A: Adjust the current of the first type of coil so that the curvature of the axial magnetic field it generates is greater than the set curvature value. At this time, the first type of coil will generate a very high bias magnetic field.

[0028] Step 7A: Adjust the current of the second type of coil so that it can counteract the high bias magnetic field while minimizing the loss of axial magnetic field curvature.

[0029] Beneficial effects:

[0030] This invention utilizes three types of coils: a first type generates an axial magnetic field with a bias magnetic field; a second type generates a magnetic field that cancels out the bias magnetic field generated by the first type; and a third type generates a radial gradient magnetic field. This ensures that the minimum magnetic field strength of the magnetic well is not zero, effectively suppressing the uncertainty caused by Majorana loss in vacuum measurements. This enables more precise measurements of ultra-high or extremely high vacuum, thus establishing a cold atom primary standard within the ultra-high / extreme high vacuum range. Compared to existing magnetic trap technologies aimed at suppressing Majorana loss, this invention offers significant advantages such as compact structural design, ample space for the laser beam transmission path, and high precision in magnetic field control. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the magnetic trap coil structure of the present invention. The system center is located at the origin O of the xyz coordinate system, and the magnetic trap consists of three types of coils that are symmetrical about the origin.

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

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

[0034] Figure 4 This is a schematic diagram of the magnetic field lines generated by the magnetic trap of the present invention along the axial direction, with atomic clusters trapped near the origin.

[0035] Figure 5 This is a schematic diagram of the distribution of magnetic field lines generated radially by the magnetic trap of the present invention, with atomic clusters trapped near the origin. Detailed Implementation

[0036] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0037] This invention provides a magnetic trap for ultra-high / extremely high vacuum measurements of cold atoms, such as... Figure 1 As shown, it includes 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 located in two planes parallel to the radial plane of the center point of the vacuum glass chamber where the atomic cloud is trapped, 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 distance a between the planes where coil A and coil B are located should be within a certain range: if the distance a is too large, the magnetic trap will weaken its ability to confine the atomic cloud, and a larger current may be required, which will place higher demands on the power supply system; if the distance 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 where the atomic cloud is trapped is greater than or equal to 2cm, in this embodiment, 2cm≤distance a≤10cm;

[0039] The inner diameters of coils A and B must not interfere with the laser beam passing along their central axis, and should be as small as possible within a limited size range (≥5cm). With the same current and unchanged structure, a smaller inner diameter can generate a stronger magnetic field. The outer diameters of coils A and B are within the range of the inner diameters of second-type coils. The number of layers and turns of coils A and B are both even to ensure the generation of a symmetrical magnetic field along the central plane. When a current of the same magnitude and direction is passed through, an axial curvature field with a bias magnetic field is generated.

[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 are concentric with coils A and B respectively;

[0041] Both coils C and D have an even number of coil layers and turns, with the number of layers consistent with that of the first-type coil. The number of turns should be kept to a maximum within the confined space, generally less than that of the first-type coil; the specific value should be set according to the magnitude of the generated reverse bias magnetic field. Coils C and D are supplied with current of the same magnitude and direction, but in the opposite direction to that of the first-type coil, generating an axial curvature field with a bias magnetic field. This magnetic field, in the opposite direction to that of the first-type coil, is used to counteract the bias magnetic field of the first-type coil. The magnitude of the current should be set according to the magnitude of the reverse bias magnetic field.

[0042] The inner diameters of coils C and D should not interfere with the outer diameter of the first type of coil; the outer diameters 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). This will minimize the loss of the axial curvature field while counteracting the bias magnetic field.

[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 containing the center point of the magnetic trap (planes E, G, I, K and F, H, J, L); the four coils located on the same plane are evenly distributed on the plane, and the centers of the four coils are located on the same circle. Each pair of coils is symmetrical along the radial plane containing the origin of the magnetic trap; the distance between the two planes containing the four pairs of coils is greater than the distance between the planes containing coils A and B. Each pair of coils is supplied with a current of equal magnitude and opposite direction, and the current directions of adjacent coils on the same plane are opposite, generating a radial gradient magnetic field. The magnitude of the current should be set according to the magnitude 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 range of the inner and outer diameters of the vacuum chamber, and its height does not interfere with the vacuum chamber or optical devices.

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

[0045] The present invention also provides a design method for the above-mentioned magnetic trap, as detailed below:

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

[0047] Step 2: Construct the first type of coil. Arrange the two coils of the first type of coil symmetrically about the geometric center of the magnetic trap along the axis of the magnetic trap and pass currents of the same direction and magnitude through them.

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

[0049] Step 3: Construct the third type of coil. This group consists of eight coils arranged in four pairs, each pair symmetrically arranged about the central plane of the magnetic trap along its axial direction. Two coils in each pair are supplied with equal but opposite currents, opposite to the currents of adjacent coils on the same plane. The third type of coil group is positioned in a region higher than the plane containing the first and second type coil groups. For the structural arrangement of the three types of coils after construction, please refer to [link / reference needed]. Figure 1 .

[0050] Step 4: Set the structural parameters of each type of coil wire according to the magnetic field required to trap the atoms, including wire type, number of turns, number of layers, outer diameter, and inner diameter. During the process of trapping the atomic cloud, the radial curvature fields generated by the first and second type coils will destructively interfere with the radial gradient field generated by the third type coil. After moving a certain distance away from the origin along the axial direction, the radial magnetic field binding the atomic cloud will disappear. To ensure 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 coil should be much stronger than the magnetic fields generated by the first and second type coils. In the design process, this can be achieved by changing the structural layout, such as swapping the positions of the third type coil and the first and second type coil groups, i.e., placing the third type coil closer to the vacuum chamber side and the first and second type coil groups outside the third type coil, or by increasing the current flowing into the third type 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 a spatial distribution model of the magnetic field of various coil combinations.

[0052] Step Six: Adjust the current in each type of coil to generate a curvature field with a minimum magnetic field strength that is not zero in the axial and radial directions near the origin of the magnetic trap coordinate system. Since the axial and radial magnetic fields are relatively independent, two methods can be used to adjust the current. The first method adjusts the current in the order of first type of coil, then second type, and finally third type. First, adjust the current of the first type of coil so that the curvature of its axial magnetic field is greater than the set curvature value; at this point, the first type of coil will generate a very high bias magnetic field. Second, adjust the current of the second type of coil to counteract the high bias magnetic field while minimizing the loss of axial magnetic field curvature. Finally, adjust the current of the third type of coil to generate a gradient magnetic field in the radial direction that meets the set value. The second method adjusts the current in the order of third type of coil, then first type, and finally second type. First, adjust the current of the third type of coil to generate a gradient magnetic field in the radial direction that meets the set value. Second, adjust the current of the first type of coil so that the curvature of its axial magnetic field is greater than the set curvature value. Finally, adjust the current of the second type of coil to counteract the high bias magnetic field while minimizing the loss of axial magnetic field curvature. The first method is suitable when the radial magnetic field has a wider adjustment range than the axial magnetic field. In this case, it's necessary to first ensure the axial magnetic field meets the design requirements, and then set the corresponding radial magnetic field based on the radial magnetic fields generated by the first and second type of coils. The second method is also suitable when the axial magnetic field has a wider adjustment range than the radial magnetic field. In this case, it's necessary to first ensure the radial magnetic field meets the design requirements, and then set the corresponding axial magnetic field based on the ability of the radial magnetic field generated by the third type of coil to counteract the disturbances generated by the first and second type of coils. Please refer to [link to adjusted axial and radial curvature magnetic fields]. Figure 2 and Figure 3 For the magnetic field line distribution of the novel magnetic trap in the axial and radial ranges, please refer to [link / reference]. Figure 4 and Figure 5 .

[0053] Step 7: Obtain the curvature value of the magnetic field by fitting the magnetic field intensity curve to evaluate whether the generated magnetic field meets the design requirement 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, proceed with the design of the cooling system for the energized coil.

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

Claims

1. A magnetic trap for ultra-high / extremely high vacuum measurements of cold atoms, characterized in that, include: Three types of coils; the plane containing all the coils is parallel to the radial plane containing the center point of the vacuum glass chamber where the atomic cloud is trapped; The first type of coil includes coil A and coil B; coil A and coil B are symmetrical about 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 through along the axis where their centers are 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; coil C and D are coplanar with coil A and B respectively, and share the same center point with coil A and B respectively, and their diameters are larger than those of coil A and B; coil C and coil D are supplied with current of the same magnitude and direction, and the direction of the current is opposite to that of the first type of coil. The magnitude of the current is determined according to the bias magnetic field of the first type of coil, so as to counteract the bias magnetic field of the first type of coil. The third type of coil includes eight coils; two coils form a pair, and the four pairs of coils are located on two planes symmetrical about the radial plane where the center point is located, and the distance between the two planes is greater than the distance 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 supplied with a current of equal magnitude and opposite direction, and the current directions between adjacent coils on the same plane are opposite, and the magnitude of the current is determined according to the magnitude of the radial gradient magnetic field to be generated.

2. The magnetic trap as claimed in claim 1, characterized in that, The value of a is less than or equal to 10 cm.

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

4. The magnetic trap as claimed in claim 1, characterized in that, Both coil A and coil B have an even number of layers and turns.

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

6. The magnetic trap as claimed in 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 made of hollow square copper wire, and water is injected into the central pipe.

8. The design method of the magnetic trap as described in 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 plane containing all the coils is parallel to the radial plane containing the origin of the coordinate system. The coils of each type are arranged symmetrically about the origin of the coordinate system. Step 2: Construct the first type of coil, the second type of coil, and the third type of coil respectively; Step 3: Set the structural parameters of each type of coil wire by using the magnetic field required to trap the atoms, including wire type, number of turns, number of layers, outer diameter and inner diameter; Step 4: Based on the Biot-Savart law, derive the spatial distribution function of the magnetic field generated by a single coil; Step 5: Adjust the current in the first type of coil according to the spatial distribution function of the magnetic field so that it generates a magnetic field with a curvature greater than the set initial value in the axial direction. Step 6: Adjust the current in the second type of coil according to the spatial distribution function of the magnetic field to generate a reverse bias magnetic field in the axial direction to counteract 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. Step 7: Adjust the current in the third type of coil according to the spatial distribution function of the magnetic field to generate a gradient magnetic field in the radial direction; Step 8: Based on the distribution of atoms trapped in the magnetic trap, derive the expression relating the magnetic field strength to the number of trapped atoms. Substitute the designed axial curvature magnetic field and the fitted radial curvature magnetic field into the expression to evaluate whether they meet the design requirement of trapping a sufficient number of atoms. If they do not meet the design requirement, return to step 5 and reset the initial curvature magnetic field. If they meet the design requirement, the design of the magnetic trap is complete.

9. The design method as described in claim 8, characterized in that, Steps 5 to 7 are replaced with steps 5A to 7A: Step 5A: Adjust the current in the third type of coil according to the spatial distribution function of the magnetic field so that it generates a gradient magnetic field in the radial direction that satisfies the set value. Step 6A: Adjust the current in the first type of coil according to the spatial distribution function of the magnetic field so that it generates a magnetic field with a curvature greater than the set initial 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 a reverse bias magnetic field in the axial direction to counteract 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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