JP4766420B2 - Magneto-optical trap device for neutral atoms - Google Patents

Description

  This invention uses a trap with a laser beam and a magnetic field to capture neutral atoms such as alkali metal atoms in an ultra-high vacuum and cool the captured atoms to near the Doppler limit temperature. The present invention relates to an optical trap device.

  FIG. 1 shows a schematic view around the trap portion of a conventional magneto-optical trap (MOT). The trap is performed using an anti-helmholtz coil (quadrupole coil) and a laser beam for irradiation in a vacuum vessel. In the example of FIG. 1, three pairs of laser beams traveling opposite to each other are used. Further, a convex lens for collimating the beam and a quarter wave plate for converting linearly polarized light into circularly polarized light are provided at the tip of the fiber.

  In an actual apparatus, in order to make the apparatus compact while effectively using the power of the laser beam, as shown in FIG. [lambda] / 4) A structure that substitutes a reflecting mirror that combines a plate and a total reflecting mirror is often employed. An example of this is shown in the schematic diagram of FIG. Here, the quarter-wave plate is used to convert the direction of rotation of the circularly polarized light into a suitable one for the operation of the MOT. In other words, when there is no quarter wave plate, the direction of rotation of the reflected circularly polarized light is rightward and the MOT cannot be operated, but by passing the quarter wave plate twice, It is well known to convert right-handed circularly polarized light to the left, and the MOT can be operated.

  As is well known, in these MOTs, the frequency of the laser beam to be irradiated is set slightly lower than the frequency of the absorbed light of the trapped atoms. Cooling is also performed by changing the wavelength of the laser beam to be irradiated.

  However, in the configuration using a reflecting mirror that combines a quarter-wave plate and a total reflecting mirror, the irradiated laser beam passes through the captured atomic cloud once and the beam whose intensity is attenuated is reflected again after the reflection. Since the laser beam passes through the cloud and the cooling capacity of the laser beam decreases accordingly, the laser beam power is not used effectively, and the operation of the MOT is impaired.

  Although the apparatus configuration is simplified, a magneto-optical trap apparatus is realized which does not have the disadvantage that the capture capability is reduced and the operation of the MOT is impaired.

  According to the present invention, it is possible to effectively use the limited laser beam power without damaging the operation of the MOT and to simplify the MOT apparatus configuration. Further, when a roof mirror is used, there is an effect that the adjustment of the incident angle of the laser beam is facilitated.

The present invention is intended to improve the performance of the MOT, and uses a plurality of laser beams including a magnetic field generated by an anti-helmholtz coil and at least one pair of circularly polarized laser beams traveling directly or through a mirror, In a magneto-optical trap device that traps multiple atoms in a reduced-pressure atmosphere,
A pair of laser beams traveling in opposition is composed of an introduced laser beam and a laser beam obtained by reflecting the introduced laser beam using a reflector such as a roof mirror, a right-angle prism, or a knife-edge right-angle prism. The apex angle portion of the roof mirror, right angle prism, or knife edge right angle prism is placed at a position where the introduced laser beam is irradiated but the shadow of the trap portion laser beam does not reappear on the trap portion. It is.

  In addition, since the reflected laser beam is a laser beam having a reduced intensity at the central portion, a beam shaping means is provided between the reflector and the trap portion in order to improve the intensity at the central portion. This beam shaping means converts a beam pattern of a laser beam having a dark part at the center into a beam pattern having no dark part at the center.

In addition, the present invention uses a plurality of laser beams including a magnetic field generated by an anti-helmholtz coil and at least one pair of circularly polarized laser beams traveling directly or through a mirror, to form a plurality of atoms in a reduced-pressure atmosphere. In a magneto-optical trap device for trapping , a pair of laser beams traveling in opposition to each other is introduced using a laser beam introduced, and a reflector composed of a quarter-wave plate and a total reflection mirror for the introduced laser beam. It consists of reflected laser light. Further, the beam shaping means is provided between the reflector and the trap unit or between the quarter-wave plate of the reflector and the total reflection mirror. This beam shaping means converts a beam pattern of a laser beam having a dark part at the center into a beam pattern having no dark part at the center.

  In addition, the circularly-polarized fiber is used to further reduce the size of the apparatus. The above-mentioned circularly-polarized laser light is obtained by converting the laser light transmitted through the polarization-maintaining optical fiber to a quarter of the wavelength. A circularly-polarized fiber adapted to convert linearly polarized light into circularly polarized light, having a length obtained by adding an integral multiple, and continuous with the above-described polarization-maintaining optical fiber with an inclination of 45 degrees around the optical axis It introduces using.

  Further, by disposing a part or all of the reflector in a reduced-pressure atmosphere provided with a trap portion, it is possible to suppress dissipation due to light attenuation or scattering caused by the window material of the container forming the reduced-pressure atmosphere.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  3B and 3C are schematic views of the present invention. For comparison, FIG. 3A shows a conventional configuration. Although FIG. 3 shows the case where left-handed circularly polarized light (LHC) is incident, if the direction of the magnetic field of MOT is reversed, it is necessary to reverse all the directions of circularly polarized light. The same applies to the case of using (RHC).

  In the configuration of FIG. 3A, the laser beam whose intensity has been attenuated by passing once through the captured atomic cloud 4 passes through the atomic cloud 4 again after reflection. In this case, the intensity of the laser beam 20 irradiated from the left direction in the figure and the intensity of the laser beam irradiated from the right direction are remarkably different, and the cooling ability of the laser beam is different in the left and right directions. The result is a decrease in performance.

  FIG. 3B is a case of the present invention and shows a method for overcoming the above-mentioned drawbacks. A feature is that a roof mirror 7 is used instead of the quarter-wave plate 2 and the total reflection mirror 6. Since the light is reflected twice inside the roof mirror 7, the direction of rotation of the circularly polarized light becomes suitable for MOT by repeating conversion from left to right to left, so that a wave plate is unnecessary. A right-angle prism may be used as the roof mirror 7. The roof mirror is intentionally shifted in the direction perpendicular to the optical axis so as not to enter the “shadow” region formed by the atomic cloud 4. By displacing in this way, the shadow area of the beam that has once passed through the atomic cloud will not be applied to the atomic cloud again, and the intensity of the laser light irradiated from the left side and the laser light irradiated from the right side will be almost the same. Can be equal. For this reason, the performance of MOT is not impaired.

  FIG. 3C shows a configuration in which a beam shaping unit 9 is arranged at an intermediate portion between the quarter-wave plate 2 and the total reflection mirror 6 of FIG. The beam shaping unit 9 is used for the purpose of “backfilling” the shadow generated in the central part after passing through the atomic cloud 4 to reproduce a uniform beam pattern, and is configured without loss of light intensity by a Fourier transform optical system or the like. It is desirable that The Fourier transform optical system here is most simply constituted by a single concave mirror placed at a focal distance away from the center of the trap. In this case, the laser beam travels once through the beam shaping section. In addition, a beam in which the amplitude distribution of the cross section of the beam is Fourier transformed is output. By doing so, the central portion of the output beam is always “brighter”, and the shadow generated in the central portion of the beam can be “backfilled”.

  In the above description, it goes without saying that the quarter-wave plate can be arranged inside the beam shaping unit, and can also be arranged between the beam shaping unit and the total reflection mirror. Moreover, the wavelength plate and the total reflection mirror of FIG. 3C can be omitted by providing the function of the wavelength plate and the total reflection mirror inside the beam shaping unit.

  Further, in the conventional MOT apparatus shown in FIG. 2, the operation characteristics are improved by replacing the reflecting mirror portion composed of the quarter-wave plate and the total reflection mirrors 6B, 6C, and 6F with the roof mirrors 7B, 7C, and 7F. FIG. 4 shows an embodiment for achieving the above.

  Although not shown in the drawing, it is desirable to form the reflection portion in the decompression portion where the reduction trap portion is disposed in order to prevent light attenuation. This is because when a window that transmits light is provided on the wall of the decompression section and laser light is introduced, the laser light is attenuated by absorption or scattering by the window material.

  As described above, in the example of the MOT in FIG. 1, a pair of quadrupole coils 3C and 3D and three pairs of laser beams 20A, 20B, 20C, 20D, 20E, and 20F are used.

  In addition, by placing one total reflection mirror near the center of the MOT and effectively using the region where the incident laser beam and the reflected laser beam overlap, 1 of the three pairs of laser beams A method of omitting the pair is known (this is called “mirror MOT”).

  Furthermore, it is possible to replace the reflecting mirror portion constituted by the quarter wavelength plate of the mirror MOT and the total reflecting mirror with the roof mirrors 7B and 7F. Further, in the case of FIG. 1, a convex lens for collimating the beam and a quarter wave plate for converting linearly polarized light into circularly polarized light are provided at the tip of the fiber. A compatible fiber can be used and is shown in FIG. The circular polarization compatible optical fiber referred to here is as follows.

  First, since the quarter-wavelength optical fibers are the same as ordinary polarization-maintaining optical fibers, they can be fused and connected to each other. A quarter-wavelength optical fiber is connected to a normal polarization maintaining optical fiber, and can be made to function as a polarization state converter by fusion splicing with the optical axes inclined at 45 degrees. In the above, the optical fiber module assembled by fusion-bonding with the optical axes inclined at 45 degrees is referred to as “circularly polarized optical fiber”.

  The magneto-optical trap (MOT) device is one of the fundamental technologies in atomic optics experiments. Bose-Einstein condensate (BEC) generation, atomic laser, atomic wave holography, atomic wave interferometer, quantum information processing device, quantum computer In any device in which neutral atoms are used, such as atomic clocks, it is used as a means for stably supplying a large number of cooled neutral atoms.

It is a schematic diagram which shows the conventional MOT. It is a figure which shows the structure which used the total reflection mirror and the quarter wavelength plate as a means to supply the beam which opposes by MOT of this invention. It is a figure which shows the structure (A) of the conventional beam reflector part, and the structure (B, C) of the reflection means used by this invention. It is a figure which shows the MOT apparatus comprised using the roof mirror for the reflection means. It is a figure which shows the mirror MOT apparatus comprised using the circular polarization compatible fiber and the roof mirror.

Explanation of symbols

1A, 1B, 1C, 1D, 1E, 1F Convex lens 2A, 2B, 2C, 2D, 2E, 2F Quarter wave plate 3C, 3D Coil 4 Atomic cloud 5 Depressurization vessel 6, 6B, 6C Total reflection mirror 7C, 7B , 7F Roof mirror 8 Total reflection mirror 9 Beam shaping part 10A, 10B, 10C, 10D, 10E, 10F Polarization-maintaining optical fiber 20, 20A, 20B, 20C, 20D, 20E, 20F Circularly polarized light

Claims (5)

A plurality of laser beams including a magnetic field generated by an anti-helmholtz coil and at least one pair of circularly polarized laser beams traveling directly or through a mirror are introduced into the trap portion to trap a plurality of atoms in a reduced-pressure atmosphere. In the magneto-optical trap device ,
A pair of laser beams traveling in opposition is composed of an introduced laser beam and a laser beam obtained by reflecting the introduced laser beam using a reflector such as a roof mirror, a right-angle prism, or a knife-edge right-angle prism. And
Neutrality characterized in that the apex portion of the roof mirror, right-angle prism, or knife-edge right-angle prism is arranged at a position where the introduced laser beam is irradiated but the shadow of the trap portion is not affected by the laser beam. Atomic magneto-optical trap device. Between the reflector and the trap part, there is a beam shaping means,
2. The neutral atom magnetism according to claim 1, wherein the beam shaping unit converts a beam pattern of a laser beam having a dark portion in the center into a beam pattern having no dark portion in the center. 3. Optical trap device. A plurality of laser beams including a magnetic field generated by an anti-helmholtz coil and at least one pair of circularly polarized laser beams traveling directly or through a mirror are introduced into the trap portion to trap a plurality of atoms in a reduced-pressure atmosphere. In the magneto-optical trap device ,
A pair of laser beams traveling in opposition is composed of an introduced laser beam and a laser beam obtained by reflecting the introduced laser beam using a quarter-wave plate and a reflector made of a total reflection mirror. ,
Furthermore, a beam shaping means is provided between the reflector and the trap part, or between the quarter wave plate of the reflector and the total reflection mirror,
The neutral beam magneto-optical trap apparatus, wherein the beam shaping means converts a beam pattern of a laser beam having a dark portion in the center into a beam pattern having no dark portion in the center.   The above circularly polarized laser beam has a length obtained by adding an integral multiple of a quarter of the wavelength of the laser beam transmitted through the polarization maintaining optical fiber. 4. A polarization maintaining optical fiber that is continuous at a rotation angle of 45 degrees and is introduced by using a circular polarization compatible fiber that converts linearly polarized light into circularly polarized light. The neutral atom magneto-optical trap device described.   5. The neutral atom magneto-optical trap apparatus according to claim 1, wherein a part or all of the reflector is disposed in a reduced-pressure atmosphere in which a trap portion is provided.

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