A method for electric field assisted tuning of the depth of a magnetic well for evaporative cooling

Abstract

The application discloses a method for adjusting the depth of a magnetic well in the process of evaporation cooling by means of an electric field, comprising the following steps: Step 1: preparation of a molecular state, wherein a molecule is prepared in an IBr molecular hyperfine level X 1 Σ state (J=0, G=2.5, F=4) |4, -4> state; Step 2: preparation of a magnetic well, wherein a set of anti-Helmholtz coils are used, equal current is passed through the coils in opposite directions, the magnetic field is equal to zero at the midpoint of the two coil axes, and an approximately linearly changing magnetic field is generated along the axis direction on both sides of the midpoint, at this time, the magnetic field distribution along the axis direction is similar to a magnetic well; Step 3: adjustment of the depth of the well, wherein two electrodes are placed on the sides of the two coils, direct current is passed through the electrodes, and the depth of the well is changed by adjusting the electric field intensity. In the process of molecular evaporation cooling, the method combines electromagnetism, quickly changes the depth of the magnetic well by adjusting the electric field intensity, achieves the purpose of evaporation cooling, and obtains cold molecules with a higher concentration.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for adjusting the depth of a magnetic well for evaporative cooling by means of an electric field, and belongs to the technical field of cold molecule research. BACKGROUND

[0002] In the past three decades, the research on cold molecules has become increasingly "hot". Compared with atoms, molecules have a more abundant energy level structure, with structural characteristics such as vibrational and rotational degrees of freedom that atoms do not have. Moreover, molecules have stronger electric and magnetic dipoles than atoms, and in a low-temperature environment, molecules can exhibit new physical or chemical phenomena, so cold molecules have a wider range of applications than cold atoms. For example, cold molecules can be applied in the fields of quantum simulation, quantum information, cold chemistry, and precision measurement, so cold molecules have become the research frontier in the fields of atoms, molecules, and optical physics. However, the abundant energy level structure not only broadens the application of molecules, but also brings many difficulties to the cooling of molecules. It makes it difficult to directly apply atomic cooling techniques, including laser cooling techniques, to molecular cooling. However, under the efforts of scientists around the world, a variety of efficient molecular cooling techniques have been developed, such as the cold atom association technique for generating cold molecules by associating cold atoms, the electrostatic Stark deceleration technique for obtaining polar cold molecules, the static magnetic Zeeman deceleration technique for obtaining paramagnetic cold molecules, and the direct laser cooling of molecules. These methods can all reduce the temperature of molecules to the order of mK. If further cooling of the molecules is required, deep cooling must be performed, and evaporative cooling is a method of deep cooling. By repeatedly evaporating molecules above the average energy from a trapped sample of molecules, the remaining molecules reach a new equilibrium through elastic collisions, and the average energy of the molecules is reduced, thereby achieving the purpose of deep cooling of the molecules.

[0003] Currently, in the research of cold molecules, a pair of Helmholtz coils with opposite currents is often used to trap cold molecules. The quadrupole magnetic field generated by the Helmholtz coils provides an additional position-dependent force, forming a trapping potential well, called a magnetic well. The device can adjust the spatial magnetic field distribution by changing the current in the anti-Helmholtz coil, i.e., the depth of the magnetic well can be adjusted. However, this method cannot achieve rapid adjustment of the well depth, especially during the process of molecular evaporative cooling, where the time for rethermalization is often on the order of milliseconds, so the speed of well depth adjustment is extremely important. Moreover, the process of changing the current intensity in the coil also causes the volume of the potential well to increase, thereby reducing the density of cold molecules, which is contrary to the actual demand. On the other hand, when the molecules are cooled to a certain extent, the hyperfine structure of the molecules cannot be ignored, so how to quickly adjust the depth of the magnetic well within a new thermal equilibrium time and ensure a high density of cold molecules is a problem that needs to be solved in the research of the hyperfine structure of cold molecules. SUMMARY

[0004] In view of the problems of the prior art, the application provides a method for adjusting an electric field assisted anti-Helmholtz coil magnetic trap for cold molecule evaporation cooling, so that in the process of molecule evaporation cooling, the depth of the magnetic trap is quickly changed by adjusting the electric field intensity in a combined manner of electric field and magnetic field, so that the purpose of evaporation cooling is achieved, and cold molecules with high concentration are obtained.

[0005] In order to achieve the above-mentioned purpose, the technical scheme adopted by the application is as follows: a method for adjusting the depth of a magnetic trap by an electric field for evaporation cooling, comprising the following steps:

[0006] Step one: preparation of a molecular state, first, molecules are prepared in a IBr molecular hyperfine level X 1 Σ state (J=0, G=2.5, F=4) |4, -4> state, wherein J represents the total angular momentum of the molecule except the nuclear spin, G represents the intermediate angular momentum after the total angular momentum is coupled with the I nucleus with larger nuclear spin in the IBr molecule, and F represents the total angular momentum after the intermediate angular momentum G is coupled with the Br nucleus;

[0007] Step two: preparation of a magnetic trap, a group of anti-Helmholtz coils are used, equal size and opposite direction currents are passed through the coils, the magnetic field is equal to zero at the midpoint of the two coil axes, and an approximately linearly changing magnetic field is generated along the axis direction on both sides of the midpoint, at this time, the magnetic field distribution along the axis direction is similar to a magnetic trap;

[0008] Step three: adjustment of the depth of the trap, two electrodes are placed on the sides of the two coils, direct current is passed through the electrodes, and the depth of the trap is changed by adjusting the electric field intensity.

[0009] Further, when the hyperfine level of the cold molecule in step one interacts with the electric field and the magnetic field at the same time, energy level crossing and energy level avoided crossing are generated due to perturbation interaction.

[0010] Further, the energy level avoided crossing point can be tuned on the magnetic axis by changing the electric field intensity, when the angle between the direct current electric field and the magnetic field direction remains unchanged, the depth of the potential well can be changed by adjusting the electric field intensity, when the electric field gradually decreases, the depth of the potential well gradually decreases, when evaporation cooling is achieved, the molecules with high energy (i.e. the molecules with high temperature) in the potential well can be removed by cutting the well wall, the electric field intensity can be quickly adjusted to match the thermal equilibrium time, when the molecules reach a new thermal equilibrium, the electric field size is continuously adjusted, the depth of the potential well is reduced again, and the system temperature is further reduced.

[0011] The application has the following beneficial effects:

[0012] (1) the application proposes a scheme that the hyperfine energy level produces perturbation interaction under the mixed field of electricity and magnetism, and the depth of the magnetic well is changed by adjusting the intensity of the electric field. Compared with the existing magnetic well for evaporation cooling, the traditional magnetic well not only utilizes the interaction of the electric field and the magnetic field to produce perturbation, but also needs to utilize the microwave field to adjust the depth of the magnetic well by changing the frequency of the microwave field, while the scheme can directly change the depth of the magnetic well by changing the intensity of the electric field, and the operation of the scheme is simpler.

[0013] (2) in the process of evaporation cooling, the time for the system to reach thermal equilibrium again is very short, generally in the order of milliseconds, so the adjustment time must be matched with the thermal equilibrium time to ensure that the temperature of the system is not increased, and the adjustment method of changing the intensity of the electric field in the scheme can quickly change the well depth, so as to match the thermal equilibrium time and achieve the purpose of evaporation cooling.

[0014] (3) in the scheme, changing the intensity of the electric field is like "a knife", in the case of not changing the geometry of the potential well, the depth of the potential well is changed by cutting the well wall, and the volume of the potential well is further reduced, so that the cold molecules with greater final concentration are obtained, which is convenient for further confinement or loading. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 Fig. 1 is a schematic diagram of the hyperfine energy level of IBr molecule in the embodiment of the application 1 Fig. 2 is a schematic diagram of the case that the state (J=0, G=2.5, F=4) is at an angle of 30° between the magnetic field and the electric field, wherein Fig. (a) is a schematic diagram of the change of the hyperfine energy level with the magnetic field when the electric field E=1100V / cm is kept unchanged, Fig. (b) is a detailed diagram of the case that the energy level avoids crossing due to the perturbation in Fig. (a), Fig. (c) is a schematic diagram of the change of the hyperfine energy level with the magnetic field when the electric field E=900V / cm is kept unchanged, and Fig. (d) is a detailed diagram of the case that the energy level avoids crossing due to the perturbation in Fig. (c);

[0016] Figure 2 Fig. 3 is a schematic diagram of the structure of the magnetic well device in the embodiment of the application;

[0017] Figure 3 Fig. 4 is a relationship between the depth of the magnetic well and the applied electric field and the equivalent temperature in the embodiment of the application.

[0018] In the figure: 1, coil, 2, electrode. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical scheme and advantages of the application more clear and obvious, the application will be further described in detail through the drawings and examples. However, it should be understood that the specific examples described here are only used to explain the application, and are not used to limit the scope of the application.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0021] The specific method in this embodiment is as follows:

[0022] (1) Preparation of molecular states. When molecules are cooled to a certain degree, their hyperfine structure can no longer be ignored. Furthermore, cold molecules have strong electric and magnetic dipole moments. When these interact with external electric and magnetic fields, the hyperfine energy levels exhibit Stark and Zeeman effects. Normally, a magnetic field couples sub-levels with the same parity together, while an electric field couples sub-levels with opposite parity together. However, when the hyperfine energy levels of cold molecules interact with both electric and magnetic fields simultaneously, perturbation interactions can lead to energy level crossings and avoidances of crossings, such as... Figure 1 As shown in (a) and (c), the molecules are first prepared at the hyperfine energy level X of the IBr molecule. 1 On the Σ state (J=0, G=2.5, F=4)|4,–4> state, such as Figure 1 (b) shows the solid energy levels, where J represents the total angular momentum of the molecule excluding nuclear spin, G represents the intermediate angular momentum after the total angular momentum excluding nuclear spin is first coupled with the I atom nucleus with the larger nuclear spin in the IBr molecule, and F represents the total angular momentum after the intermediate angular momentum G is coupled with the Br atom nucleus.

[0023] (2) Fabrication of the magnetic trap. For example... Figure 2 As shown, by using a set of anti-Helmholtz coils and passing equal but opposite currents through them, the magnetic field is zero at the midpoint of the central axis of the two coils. On both sides of the midpoint, an approximately linearly varying magnetic field is generated along the central axis. At this time, the magnetic field distribution along the central axis resembles a magnetic trap.

[0024] (3) Adjustment of the trap depth. Two electrodes are placed on the sides of the two coils, and a direct current is applied. The angle between the direction of the electric field and the direction of the magnetic field can be adjusted. Figure 1 It can be seen that the energy level avoidance point can be tuned along the magnetic axis by changing the electric field strength. When the angle between the DC electric and magnetic field directions remains constant at θ = 30°, and the applied electric field E = 1100 V / cm, the position of the avoidance point caused by the perturbation is around 660 G (e.g., ...). Figure 1 (b) At this point, the potential well depth is 1.917 MHz, and the temperature of the potential well can be calculated to be approximately 91 μK. If the magnitude of the electric field is changed, such as by applying an external electric field E = 900 V / cm, the location of the avoidance point due to the perturbation is around 435 G (e.g., Figure 1(d)), at which point the potential well depth is 1.270 MHz, corresponding to a potential well temperature of approximately 61 μK, such as Figure 3 As shown, adjusting the electric field strength changes the potential well depth. This means that as the electric field gradually decreases, the potential well depth gradually decreases. Therefore, the electric field acts like a "knife," cutting through the well wall and thus changing the well depth. When evaporative cooling is achieved, cutting through the well wall removes molecules with higher energy (i.e., higher temperatures) from the potential well. Generally, the thermal equilibrium time is about a few milliseconds, and the electric field strength can be quickly adjusted to match this time. Once the molecules reach a new thermal equilibrium, further adjusting the electric field strength can decrease the potential well depth again, thereby further reducing the system temperature. This achieves electric field-assisted adjustment of the anti-Helmholtz coil magnetic well depth, achieving the effect of evaporative cooling of molecules.

[0025] in Figure 1 IBr molecular hyperfine level X 1 For the Σ state (J=0, G=2.5, F=4) with an angle of 30° between the magnetic and electric fields, (a) shows the hyperfine level variation with the magnetic field while keeping the electric field E=1100V / cm constant; (b) is a detailed diagram of the energy level avoidance due to perturbation in Figure (a); (c) shows the hyperfine level variation with the magnetic field while keeping the electric field E=900V / cm constant; (d) is a detailed diagram of the energy level avoidance due to perturbation in Figure (c). Comparing Figures (a) and (c), it can be seen that the molecule is first prepared at the energy levels represented by the solid lines in both figures, i.e., J=0, G=2.5, F=4, M F At the -4 energy level, under different electric fields, the energy level avoidance point shifts to the left. This reduces the energy difference between the highest and lowest points of the level, meaning the corresponding well depth along the magnetic field axis decreases, as will be discussed later. Figure 3 As shown, when the external electric field is 1100V / cm and 900V / cm, the magnetic trap depth decreases, and the magnetic trap temperature corresponding to that energy also decreases accordingly. That is, adjusting the electric field strength is like using a knife to gradually cut the trap wall, thereby reducing the system temperature and achieving evaporative cooling.

[0026] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for adjusting the depth of a magnetic trap with electric field assistance for evaporative cooling, characterized in that, Includes the following steps: Step 1: Preparation of the molecular state. First, the molecule is prepared in the hyperfine energy level X of the IBr molecule. 1 In the Σ state (J=0,G=2.5,F=4)|4,–4> state, J represents the total angular momentum of the molecule excluding nuclear spin, G represents the intermediate angular momentum after the total angular momentum excluding nuclear spin is first coupled with the I atom nucleus with larger nuclear spin in the IBr molecule, and F represents the total angular momentum after the intermediate angular momentum G is coupled with the Br atom nucleus. Step 2: Preparation of magnetic trap. Using a set of anti-Helmholtz coils, currents of equal magnitude and opposite direction are passed through them. At the midpoint of the central axis of the two coils (1), the magnetic field is zero. On both sides of the midpoint, a linearly changing magnetic field is generated along the central axis. At this time, a magnetic trap is distributed along the central axis. Step 3: Adjusting the trap depth. Place two electrodes (2) on the sides of the two coils (1), apply DC current, and change the trap depth by adjusting the electric field strength.

2. The method for adjusting the depth of a magnetic trap with electric field assistance for evaporative cooling according to claim 1, characterized in that, In step one, when the hyperfine energy level of the cold molecule interacts with both the electric and magnetic fields, energy level crossing and energy level avoidance will occur due to the perturbation interaction.

3. The method for adjusting the magnetic trap depth with electric field assistance for evaporative cooling according to claim 2, characterized in that, The energy levels are prevented from crossing on the magnetic axis by adjusting the electric field strength. When the angle between the DC electric field and the magnetic field remains constant, adjusting the electric field strength changes the potential well depth. When the electric field gradually decreases, the potential well depth gradually decreases. When evaporative cooling is achieved, cutting the well wall removes molecules with higher energy from the potential well, and the electric field strength is rapidly adjusted to match the thermal equilibrium time. After the molecules reach a new thermal equilibrium, the electric field strength is further adjusted to reduce the potential well depth again, thereby further reducing the system temperature.

Images