Diffuse Reflection Device for Pre-Cooling of Magneto-Optical Trap and Laser Cooling Method

Abstract

A diffuse reflection device and a laser cooling method for pre-cooling of a magneto-optical trap. The device uses a quartz vacuum chamber with a special configuration. The surface of the vacuum chamber has a coating with a high diffuse reflectivity, and corresponding small holes are opened on the coating. The laser enters the vacuum chamber through the small holes by means of an optical fiber, and forms an isotropic cooling light field in the chamber through diffuse reflection by the coating. There is a pair of anti-Helmholtz coils outside the vacuum chamber, which form a magneto-optical trap with the cooling laser, and realize diffuse reflection cooling as the pre-stage cooling of the magneto-optical trap through timing control. The present invention can increase the optical thickness of the atoms obtained by diffuse reflection cooling, improve the loading rate of the magneto-optical trap for trapping and cooling atoms, and at the same time provide ideas for studying new cooling methods.

Description

Technical Field

[0001] The present invention relates to laser cooling of atoms, and in particular to a diffuse reflection device and a laser cooling method for pre-cooling of a magneto-optical trap. Background Art

[0002] Technologies such as Zeeman decelerators and magneto-optical traps developed since the 1980s have enabled the rapid development of the field of laser cooling of atoms. Conventional pre-cooling devices for magneto-optical traps, such as Zeeman decelerators or two-dimensional magneto-optical trap devices, require a magnetic field and have strict polarization requirements for the cooling light required. Diffuse reflection cooling has been widely used due to its many advantages. For example, cooling atoms using diffuse reflection light does not require fine adjustment of the spatial position of the laser, has no requirement for the polarization of the laser, and does not require an additional magnetic field device. Therefore, diffuse reflection cooling technology is mostly applied to integrated, stable, and miniaturized devices.

[0003] However, only using diffuse reflection cooling cannot trap atoms, so the optical thickness of the obtained cold atoms is relatively low; on the other hand, the loading rate of cold atoms obtained only through magneto-optical trap technology is not high. Pre-cooling the magneto-optical trap by diffuse reflection cooling greatly simplifies the complexity of the experimental system compared with traditional pre-cooling schemes. At the same time, the use of the magneto-optical trap will greatly increase the optical thickness of atoms in diffuse reflection cooling. Summary of the Invention

[0004] The purpose of the present invention is to provide a diffuse reflection device and a laser cooling method for pre-cooling of a magneto-optical trap. First, use diffuse reflection cooling technology to cool atoms in a diffuse reflection quartz vacuum chamber. Subsequently, turn on the magnetic field and optical field required for the magneto-optical trap to cool and trap the atoms in the magneto-optical trap, so as to realize using diffuse reflection cooling as the pre-cooling of the magneto-optical trap.

[0005] The present invention is realized through the following technical solutions:

[0006] A diffuse reflection device for pre-cooling of a magneto-optical trap, characterized in that it mainly includes a quartz vacuum chamber with a special configuration, an external device of the vacuum chamber, and a cold atom signal detection device.

[0007] The quartz vacuum chamber with a special configuration includes three mutually perpendicular quartz tubes, a horizontal quartz tube, a vertically oriented quartz tube, and a metal flange. A window plate is pasted on the end face of each of the three mutually perpendicular quartz tubes. The outer surface of the vacuum chamber is evenly coated with a surface coating having an extremely high diffuse reflectivity. Two laser incident small holes are respectively opened on the surface coatings of the three mutually perpendicular quartz tubes for connecting multimode optical fibers. The upper end of the vertically oriented quartz tube is connected to the vacuum chamber formed by the three mutually perpendicular quartz tubes, and the lower end of the vertically oriented quartz tube is connected to the knife-edge flange. The horizontal quartz tube is horizontally connected to the vacuum chamber;

[0008] The external device of the vacuum chamber includes six multimode optical fibers, a pair of anti-Helmholtz coils, a quarter-wave plate, and a mirror. Six lasers for diffuse reflection cooling enter the vacuum chamber through the six multimode optical fibers and the laser incident small holes. After the lasers are diffusely reflected by the surface coating of the cavity, an isotropic light field is formed in the cavity. The wave plate and the mirror are respectively arranged on the outer axes at one end of each of the three mutually perpendicular quartz tubes. Three lasers respectively pass through the centers of the three mutually perpendicular quartz tubes and are reflected by the quarter-wave plate and the mirror, and the reflected light completely coincides with the incident light. Three pairs of light beams form a magneto-optical trap light field. The pair of anti-Helmholtz coils is arranged outside the two ends of the horizontally forward quartz tube. The energized anti-Helmholtz coils form a gradient magnetic field in the vacuum chamber, and this gradient magnetic field and the magneto-optical trap light field constitute a magneto-optical trap;

[0009] The cold atom signal detection device includes a CCD camera and a photodetector. The CCD camera is placed in front of the horizontal quartz tube for imaging the atomic fluorescence in the vacuum chamber. A beam of detection light passes through the center of the vacuum chamber cavity and is incident on the photodetector for cold atom signal detection.

[0010] A method for laser cooling using the above diffuse reflection device for pre-cooling of magneto-optical trap includes the following steps:

[0011] 1) Turn on the lasers for diffuse reflection cooling input via the multimode optical fibers, and pre-cool the atoms in the vacuum chamber through the three mutually perpendicular quartz tubes;

[0012] 2) Subsequently, turn off the diffuse reflection lasers, and the pair of anti-Helmholtz coils is energized to form a gradient magnetic field in the vacuum chamber;

[0013] 3) After the gradient magnetic field reaches stability, turn on the cooling light and the repumping light of the magneto-optical trap. The cooling light and the repumping light required by the magneto-optical trap are combined, shaped, and then incident equally power along the central axes of three quartz pipes. The polarization of the light beam is circular polarization. After passing through the center of the vacuum chamber, the light beam passes through a quarter-wave plate and is reflected back along the original path by a mirror. The optical paths of the incident light and the reflected light completely coincide. Three pairs of light beams and the magnetic field jointly form a magneto-optical trap to cool and trap atoms;

[0014] 4) In the detection stage, turn off the magneto-optical trap and turn on the detection light. A beam of detection light is incident on the photodetector along one of the quartz pipes at a small angle with the magnetic field axis. The photodetector receives the signal of the detection light to detect the temperature and quantity of atoms; Place the CCD camera along the axis of the horizontal quartz pipe to monitor the cold atom cloud in the magneto-optical trap.

[0015] Connect to the vacuum pump and the atomic source through a four-way or six-way joint, and maintain the entire vacuum chamber at a high vacuum degree through a vacuum pump group.

[0016] Special window pieces are glued to the end faces of the quartz pipes of the vacuum chamber using optical glue. The window pieces are used to improve the transmittance of the laser incident into the vacuum chamber.

[0017] A beam of detection light is incident along one of the quartz pipes at a small angle with the magnetic field axis. The signal of the detection light is received by the photodetector to detect the temperature and quantity of atoms; Place a CCD camera along the axis of the horizontal quartz pipe to monitor the cold atom cloud in the magneto-optical trap.

[0018] Compared with the pre-cooling scheme of the existing magneto-optical trap or the separate diffuse reflection cooling scheme, the present invention has the following advantages:

[0019] 1. The pre-cooling has the advantages of diffuse reflection cooling, that is, the pre-cooling has no requirement for the polarization of the light field and does not require precise adjustment of the spatial position of the laser.

[0020] 2. Diffuse reflection cooling does not require a magnetic field. Therefore, using it as pre-cooling will not affect the magnetic field distribution of the magneto-optical trap, and thus there will be no additional magnetic field affecting the atoms therein.

[0021] 3. Compared with using diffuse reflection cooling alone, the atoms can be trapped by the magneto-optical trap, increasing the optical thickness of the atoms.

[0022] Experiments show that the present invention can increase the optical thickness of the atoms obtained by diffuse reflection cooling, improve the loading rate of the magneto-optical trap for trapping and cooling atoms, and at the same time provide ideas for studying new cooling methods. Description of the Drawings

[0023] Figure 1Schematic diagram of a diffuse reflection quartz vacuum chamber (including coating and openings)

[0024] Figure 2 Schematic diagram of using an optical fiber to incident diffuse reflection light

[0025] Figure 3 Schematic diagram of the optical field and magnetic field structures in a magneto-optical trap

[0026] Figure 4 Schematic diagram of atomic signal detection

[0027] Figure 5 Timing control diagram

[0028] Figure 6 Rb atomic transition energy levels corresponding to laser frequencies

[0029] In the figure: 1-1 is a CF35 stainless steel metal flange, 1-2 is a vertical quartz tube, 1-3 is an opening for the diffuse reflection coating, 1-4 is the diffuse reflection coating on the surface of the quartz vacuum chamber, 1-5 is a window plate (only one is labeled), 1-6, 1-7, and 1-9 are three mutually perpendicular quartz tubes, 1-8 is a horizontal quartz tube, 2-1 is a multimode optical fiber for incident diffuse reflection light, 3-1 is a pair of magnetic field coils of the magneto-optical trap, 3-2 is the cooling light beam of the magneto-optical trap (including cooling light and repump light, only one is labeled), 3-3 is a quarter-wave plate (only one is labeled), 3-4 is a 0-degree mirror (only one is labeled), 4-1 is a photodetector, 4-2 is the detection light, 4-3 is atomic fluorescence, and 4-4 is a CCD camera. Specific implementation mode

[0030] The present invention will be clearly and detailedly described below in combination with the embodiments of the present invention and the drawings, but the scope of the present invention should not be limited thereby.

[0031] First, please refer to Figure 1 、 Figure 2 、 Figure 3 、 Figure 4 As can be seen from the figures, the diffuse reflection device for pre-cooling in a magneto-optical trap according to the present invention mainly includes a vacuum chamber with a special configuration, devices outside the vacuum chamber, and a cold atom signal detection device. The vacuum chamber includes three mutually perpendicular quartz tubes 1-6, 1-7, 1-9, a horizontal quartz tube 1-8, a vertical quartz tube 1-2, and a metal flange 1-1. Window plates 1-5 are pasted on the end faces of the three mutually perpendicular quartz tubes 1-6, 1-7, 1-9. The surface of the vacuum chamber is evenly coated with a coating 1-4 with extremely high diffuse reflectivity, and corresponding laser incident holes 1-3 are opened on the surface coating 1-4 for connection with the multimode optical fiber 2-1. The vertical quartz tube 1-2 is connected to the knife-edge flange 1-1.

[0032] The external device of the vacuum chamber includes six multi-mode optical fibers 2-1, a pair of anti-Helmholtz coils 3-1, a quarter-wave plate 3-3, and a mirror 3-4. Six beams of laser for diffuse reflection cooling enter the vacuum chamber through the small holes 1-3 via the multi-mode optical fibers 2-1. After the laser is diffusely reflected by the surface coating of the chamber, an isotropic light field is formed inside the chamber. Three beams of laser for magneto-optical trap pass through the centers of three mutually perpendicular quartz tubes 1-6, 1-7, and 1-9 and are reflected by the quarter-wave plate 3-3 and the mirror 3-4. The optical path of the reflected light coincides completely with that of the incident light, and three pairs of light beams form a magneto-optical trap light field. The pair of energized anti-Helmholtz coils 3-1 forms a gradient magnetic field inside the vacuum chamber, and this gradient magnetic field and the magneto-optical trap light field constitute a magneto-optical trap.

[0033] The cold atom signal detection device includes a CCD camera 4-4 and a photodetector 4-1. The CCD camera 4-4 is placed in front of the horizontal quartz tube 1-8 for imaging the atomic fluorescence 4-3 inside the vacuum chamber. A beam of detection light 4-2 passes through the center of the vacuum chamber and is incident on the photodetector 4-1 for cold atom signal detection.

[0034] A method for laser cooling using the above diffuse reflection device for pre-cooling of magneto-optical trap includes the following steps:

[0035] 1) First, turn on the laser for diffuse reflection cooling input via the multi-mode optical fibers 2-1, and form an isotropic light field inside the chamber through diffuse reflection on the surface of the chamber via the three mutually perpendicular quartz tubes 1-6, 1-7, and 1-9 to pre-cool the atoms inside the chamber.

[0036] 2) Subsequently, turn off the diffuse reflection light, energize the pair of anti-Helmholtz coils 3-1, and form a gradient magnetic field inside the vacuum chamber.

[0037] 3) After the gradient magnetic field reaches stability, turn on the cooling light and the repump light of the magneto-optical trap. The gradient magnetic field and three pairs of magneto-optical trap lasers incident through the three mutually perpendicular quartz tubes 1-6, 1-7, and 1-9 and reflected form a magneto-optical trap. The magneto-optical trap cools and traps the atoms.

[0038] 4) In the detection stage, turn off the magneto-optical trap, and incident a beam of detection light 4-2 along one of the quartz tubes at a small angle with the magnetic field axis. The signal of the detection light is received by the photodetector 4-1 to detect the temperature and quantity of atoms. Place the CCD camera 4-4 along the axis of the horizontal quartz tube 1-8 to monitor the cold atom cloud in the magneto-optical trap.

[0039] Embodiment

[0040] The vacuum chamber composed of the multiple quartz tubes is connected to a vacuum pump and a rubidium atom source through a four-way or six-way joint, and the entire vacuum chamber can be maintained at a relatively high vacuum degree by the vacuum pump.

[0041] A diffuse reflection coating 1-4 with a thickness of about 2 mm is evenly applied on the surface of the vacuum chamber wall. The diffuse reflectivity of the coating to 780 nm laser reaches 98%. Six small holes 1-3 with a diameter of about 3 mm are symmetrically opened on the coatings on the surfaces of three mutually perpendicular quartz tubes 1-6, 1-7, and 1-9. The small holes are used to directly inject the diffuse reflection cooling light output by the multimode optical fiber 2-1 into the vacuum chamber. The diffuse reflection light includes cooling light and repumping light. See Figure 6 , the frequency of the cooling light corresponds to the 5S 1 / 2 , F = 2 to 5P 3 / 2 , F’ = 3 energy level transition and has a red detuning of about 20 MHz. The frequency of the repumping light corresponds to the 5S of rubidium atoms 1 / 2 , F = 1 to 5P 3 / 2 , F’ = 2 energy level transition.

[0042] A pair of anti-Helmholtz coils 3-1 are placed outside the vacuum chamber. After the coils are energized, a gradient magnetic field is generated, and the zero magnetic field coincides with the center of the vacuum chamber.

[0043] The cooling light and repumping light required for the magneto-optical trap are combined, shaped, and then incident equally power along the axis centers of the three quartz tubes. The diameter of the three incident light beams is about 6 mm, and the polarization of the light beams is circular polarization. The light beam 3-2 passes through the center of the vacuum chamber, then passes through a quarter-wave plate 3-3 and is reflected back along the original path by a mirror 3-4. The optical paths of the incident light and the reflected light are completely coincident. The three pairs of light beams and the magnetic field jointly form a magneto-optical trap. The frequency of the cooling light corresponds to the 5S of rubidium atoms 1 / 2 , F = 2 to 5P 3 / 2 , F’ = 3 energy level transition and has a red detuning of about 20 MHz. The frequency of the repumping light corresponds to the 5S of rubidium atoms 1 / 2 , F = 1 to 5P 3 / 2 , F’ = 2 energy level transition.

[0044] As Figure 4 shown, a probe light 4-2 is incident along the direction with an angle of 15 degrees to the quartz tube 1-7. The signal of the probe light is received by a photodetector 4-1 and is used to detect the temperature and quantity of atoms. The frequency of the probe light corresponds to the 5S of rubidium atoms 1 / 2 , F = 2 to 5P 3 / 2 , F’ = 3 energy level transition; A CCD camera 4-4 is placed along the axis of the horizontal quartz tube 1-8, and the signal of the atomic cluster in the magneto-optical trap is monitored through the fluorescence 4-3 emitted by the atoms in the cavity.

[0045] The diffused reflection light, detection light, optical field and magnetic field required for the magneto-optical trap are all controlled by timing. The cooling light of diffused reflection and the repumping light are turned on simultaneously, and after cooling the atomic temperature to microkelvin, the diffused reflection light is turned off and the current of the magnetic field coil is turned on simultaneously. It takes a certain amount of time for the magnetic field to turn on. After the magnetic field is stable, the cooling light and repumping light of the magneto-optical trap are turned on. During the detection stage, the magneto-optical trap is turned off and the detection light is turned on.

[0046] The cooling system built in the present invention can increase the optical thickness of the atoms obtained by diffused reflection cooling, improve the loading rate of the magneto-optical trap for trapping and cooling atoms, and at the same time provide ideas for researching new cooling methods.

Claims

1. A diffuse reflection device for pre-cooling of a magneto-optical trap, characterized in that: It mainly includes a quartz vacuum chamber with a special configuration, an external device outside the vacuum chamber, and a cold atom signal detection device. The quartz vacuum chamber with the special configuration includes three mutually perpendicular quartz tubes (1-6, 1-7, 1-9), a horizontal quartz tube (1-8), a vertically oriented quartz tube (1-2), and a metal flange (1-1). A window piece (1-5) is pasted on the end face of each of the three mutually perpendicular quartz tubes (1-6, 1-7, 1-9). The outer surface of the vacuum chamber is evenly coated with a surface coating (1-4) with extremely high diffuse reflectivity. A number of laser incident small holes (1-3) are respectively opened on the surface coatings (1-4) of the three mutually perpendicular quartz tubes (1-6, 1-7, 1-9). The laser incident small holes (1-3) are used to connect multimode optical fibers (2-1). The upper end of the vertically oriented quartz tube (1-2) is connected to the vacuum chamber formed by the three mutually perpendicular quartz tubes (1-6, 1-7, 1-9), and the lower end of the vertically oriented quartz tube (1-2) is connected to the metal flange (1-1). The horizontal quartz tube (1-8) is horizontally connected to the vacuum chamber. The external device outside the vacuum chamber includes six multimode optical fibers (2-1), a pair of anti-Helmholtz coils (3-1), a quarter-wave plate (3-3), and a mirror (3-4). Six beams of laser for diffuse reflection cooling enter the vacuum chamber through the laser incident small holes (1-3) via the six multimode optical fibers (2-1). After the laser is diffusely reflected by the surface coating on the chamber wall, an isotropic light field is formed inside the chamber. The horizontally forward quartz tube (1-7) is one of the three mutually perpendicular quartz tubes (1-6, 1-7, 1-9). The pair of anti-Helmholtz coils (3-1) is arranged outside the two ends of the horizontally forward quartz tube (1-7). The quarter-wave plate (3-3) and the mirror (3-4) are respectively arranged on the outer axis of one end of the three mutually perpendicular quartz tubes (1-6, 1-7, 1-9). Three beams of laser for magneto-optical trap pass through the centers of the three mutually perpendicular quartz tubes (1-6, 1-7, 1-9) respectively and are reflected by the quarter-wave plate (3-3) and the mirror (3-4). The reflected light completely coincides with the incident light, and three pairs of light beams form a magneto-optical trap light field. The pair of energized anti-Helmholtz coils (3-1) forms a gradient magnetic field inside the vacuum chamber, and this gradient magnetic field and the magneto-optical trap light field constitute a magneto-optical trap. The cold atom signal detection device includes a CCD camera (4-4) and a photodetector (4-1). The CCD camera (4-4) is placed in front of the horizontal quartz tube (1-8) and is used for imaging the atomic fluorescence (4-3) inside the vacuum chamber. A beam of detection light (4-2) passes through the center of the vacuum chamber cavity and is incident on the photodetector (4-1) for cold atom signal detection.

2. A method for laser cooling using the diffuse reflection device for pre-cooling of a magneto-optical trap according to claim 1, characterized in that This method includes the following steps: 1) Turn on the laser input via the multimode optical fiber (2-1). After diffuse reflection from the surface coatings of the three mutually perpendicular quartz pipes (1-6, 1-7, 1-9), an isotropic optical field is formed to pre-cool the atoms in the cavity. 2) Subsequently, turn off the diffuse reflection laser, and energize the pair of anti-Helmholtz coils (3-1) to form a gradient magnetic field in the vacuum cavity. 3) After the gradient magnetic field reaches stability, turn on the cooling light and the repump light of the magneto-optical trap to form the optical field of the magneto-optical trap. The gradient magnetic field and the laser incident from the three mutually perpendicular quartz pipes (1-6, 1-7, 1-9) constitute a magneto-optical trap to cool and trap the atoms. 4) In the detection stage, turn off the magneto-optical trap and turn on the probe light. A beam of probe light (4-2) is incident on the photodetector (4-1) along one of the quartz pipes at a small angle with respect to the magnetic field axis. The photodetector (4-1) receives the signal of the probe light to detect the temperature and quantity of the atoms. Place the CCD camera (4-4) along the axis of the horizontal quartz pipe (1-8) to monitor the cold atom cloud in the magneto-optical trap.

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