WO2014027637A1 - 光格子時計、時計装置、およびレーザー光源 - Google Patents
光格子時計、時計装置、およびレーザー光源 Download PDFInfo
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
- H03L7/00—Automatic control of frequency or phase; Synchronisation
- H03L7/26—Automatic control of frequency or phase; Synchronisation using energy levels of molecules, atoms, or subatomic particles as a frequency reference
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- G—PHYSICS
- G04—HOROLOGY
- G04F—TIME-INTERVAL MEASURING
- G04F5/00—Apparatus for producing preselected time intervals for use as timing standards
- G04F5/14—Apparatus for producing preselected time intervals for use as timing standards using atomic clocks
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/067—Fibre lasers
- H01S3/06791—Fibre ring lasers
Definitions
- the present invention relates to an optical lattice clock, a clock device, and a laser light source. More particularly, the present invention relates to a high-precision optical lattice clock, a timepiece device, and a laser light source using the principle of an optical lattice clock.
- Atomic clocks have been adopted as the time reference.
- Atomic clocks are generally considered to represent highly accurate clocks, and are used, for example, as the primary standard for time.
- atomic clocks are also applied to applications requiring high accuracy, such as being mounted on each GPS satellite in GPS (Global Positioning System).
- a general atomic clock is a microwave region generated by a transition between electronic levels (hereinafter referred to as “clock transition”) in atoms (including ions, the same shall apply hereinafter) such as Cs (cesium) and Rb (rubidium).
- clock transition a transition between electronic levels (hereinafter referred to as “clock transition”) in atoms (including ions, the same shall apply hereinafter) such as Cs (cesium) and Rb (rubidium).
- the electromagnetic wave is used as a frequency reference.
- CSAC Chip Scale Atomic Clock
- optical atomic clock requires an ultra-high stability laser light source that requires a large vacuum device and a high-accuracy vibration isolator, and it is not easy to make the optical atomic clock portable.
- An optical lattice clock generally uses a periodic structure (hereinafter referred to as “optical lattice”) created by a standing wave of light (electromagnetic wave) in space, and the position of the antinode of the standing wave of the photoelectric field of the optical lattice (lattice) The clock transition in the atom trapped at point) is used.
- optical lattice clock may be described as an aspect of an atomic clock or an optical atomic clock.
- an atomic clock using an optical lattice is called an optical lattice clock, and neither an atomic clock nor an optical atomic clock includes an optical lattice clock.
- One of the factors that determine accuracy in atomic clocks and optical atomic clocks is interaction such as collision between atoms and the inner wall of a container or the like that accommodates the atoms.
- the miniaturization increases the relative influence of collision between atoms and the inner wall of the container, thereby deteriorating the accuracy of the atomic clock.
- atomic clock directed to miniaturization such as CSAC
- atoms are arranged in a narrow space, and the problem of interaction between atoms and walls becomes obvious.
- the interaction between the atom and the wall of the container or the like described above can be substantially avoided.
- the present invention provides a specific configuration that can increase the number of atoms in an optical lattice clock by taking advantage of the fact that atoms related to clock transition hardly interact with a wall such as a container. It contributes to the realization of optical lattice clocks, clock devices, and laser light sources.
- the inventors of the present application searched for a method capable of increasing the effective volume while realizing a highly accurate optical lattice clock operation.
- the effective volume is given as the product of the number of lattice points and the volume of each lattice point capable of trapping atoms.
- grating laser pairs that propagate space in the opposite directions, increase the number of grating points by enlarging the size of the spatial region of standing waves formed by interference
- the collision shift is a factor that limits uncertainty.
- the number of atoms per lattice is preferably about 1 to 10.
- the conventional optical grating generation method it is actually difficult to expand the spatial region of the standing wave to that size.
- This size restriction is related to the fact that the range of the spatial region suitable for high-accuracy spectroscopy of atoms is limited to a spatial region on the order of the Rayleigh length of the grating laser.
- the longer the Rayleigh length the closer the laser wavefront gets to the plane, so the Doppler effect associated with the in-plane motion of the atoms decreases, but the laser beam diameter increases and the atoms The effect of the trap will be weakened. This trade-off makes it difficult to expand the space suitable for high-precision atomic spectroscopy.
- the inventors of the present application searched for a technique that can avoid the limitations caused by the beam characteristics of the grating laser, that is, the Rayleigh length.
- the size of the spatial region suitable for high-precision atomic spectroscopy that is, the size of the spatial region suitable for functioning the optical lattice clock can be expanded.
- the present invention has been completed.
- an optical waveguide having a hollow path surrounded by a cylindrical wall as a waveguide path (optical) a two-level electronic state of atoms trapped in the optical lattice formed in the hollow passage is used for clock transition, and the optical lattice includes
- An optical lattice clock is provided which is an optical lattice having a magic wavelength, which is a wavelength that causes optical shifts corresponding to each of the two levels to coincide with each other.
- the hollow passage of the optical waveguide extends from the first end to the second end, and a pair of lattice lasers (a pair of lattice lasers) traveling in opposite directions to each other.
- a laser source that forms the optical grating in the hollow passage by cooling and cooling the atoms by the laser cooling unit to cool the atoms in the vicinity of the first end of the optical waveguide.
- a laser cooling section for supplying atoms), and the cooled atoms trapped in the optical lattice in the hollow passage cause the clock transition between the two-level electronic states.
- a hollow passage extending from a first end to a second end is surrounded by a cylindrical wall, an optical waveguide having the passage as a waveguide path, and an optical lattice forming the optical lattice in the passage.
- Laser light source for supplying a pair of lattice lasers traveling in opposite directions, and laser cooling for supplying a cooled atom of an atom having a two-level electronic state related to a clock transition to the vicinity of the first end of the optical waveguide
- the lattice laser pair is a pair of lasers having a magic wavelength, which is a wavelength that causes a light shift that coincides with each other of the two levels, and the cooling atom is
- An optical lattice clock is provided that causes the clock transition while being trapped by the optical lattice in the passage.
- the pair of lattice lasers of the laser light source is a moving light that moves from the first end to the second end in the hollow passage of the optical waveguide as the optical lattice.
- Moving photonic lattice, or moving the cooled atoms are trapped in the moving light lattice and introduced into the hollow passage from the first end, and the moving light lattice causes the second end to move from the first end to the second end.
- the clock transition is caused while being transported through the hollow passage toward.
- the optical waveguide in these aspects of the present invention surrounds a hollow passage with a cylindrical wall, for example, the passage extends from the first end to the second end. For this reason, the cooling atom can pass through the passage of the optical waveguide from the first end to the second end, for example, and pass through the passage.
- the cooled atom causes a clock transition in the path while being trapped by the optical lattice. Atoms are trapped and moved in the optical lattice of the hollow passage.
- Each aspect of the present invention employing an optical waveguide having a hollow passage provides several advantages over an optical lattice clock.
- the path Since the path is also a part of the optical path of the ring resonator, it becomes a position where a moving optical grating formed by a pair of grating lasers supplied to the ring resonator is formed.
- the cross-sectional shape of the passage surrounded by the cylindrical wall is not particularly limited, and can be selected from a circle, an ellipse, a polygon, a combination of these partial line segments, an indefinite shape, or the like.
- the optical waveguide is a hollow core photonic crystal fiber (HC-PCF), and the hollow base passage of the optical waveguide is the HC-
- HC-PCF hollow core photonic crystal fiber
- the hollow base passage of the optical waveguide is the HC-
- the passage formed by the hollow core of the PCF is advantageous.
- Hollow core photonic crystal fiber (hereinafter referred to as “HC-PCF”) is an example of a suitable optical waveguide.
- the HC-PCF is an optical waveguide having an elongated tube-like schematic structure as disclosed in Non-Patent Document 2, for example, and propagates light through a hollow portion (hollow core) extending in the central axis inside the optical waveguide. be able to.
- This hollow core is surrounded by a cladding made of photonic crystal.
- the moving optical grating for the optical lattice clock is formed in the hollow core by a laser having a wavelength that propagates through the hollow core and does not propagate into the cladding.
- a timepiece device that refers to light having a wavelength absorbed for the timepiece transition in any of the above optical lattice timepieces.
- the optical lattice timepiece according to any one of the above aspects further includes a ring resonator including the passage as a part of the optical path.
- the ring resonator of the optical lattice clock is provided as a laser resonator
- the cooling atom located in the hollow passage is used as a laser medium
- the cooling atom is
- a laser light source that outputs light stimulated by clock transition from the ring resonator.
- the optical lattice clock can be highly accurate and miniaturized.
- FIG. 1A is a configuration diagram showing a configuration of a conventional optical lattice clock
- FIG. 1B is a schematic diagram showing a state of a beam waist portion where atoms are trapped in the optical lattice
- C is a schematic diagram showing a potential state felt by atoms trapped in the optical lattice. It is explanatory drawing which shows the structure and operating principle of the optical lattice clock of embodiment of this invention. It is explanatory drawing for demonstrating the effect
- FIG. 3A is a configuration diagram showing an overall configuration for forming an optical grating in the optical waveguide and how atoms are trapped in the optical grating
- FIG. 3B is a cross-sectional view showing the structure of the optical waveguide. It is. It is a schematic diagram showing a configuration of HC-PCF which is one of specific examples of the optical waveguide of the embodiment of the present invention.
- 4A is a schematic cross-sectional view in a plane including the direction in which the passage of the HC-PCF 70 extends
- FIG. 4B is a schematic cross-sectional view in a direction of cutting the passage.
- FIG. 3 is a layout diagram illustrating a configuration example of an optical lattice clock employing HC-PCF in an optical waveguide according to an embodiment of the present invention.
- FIG. 6A is a plan view in which one surface of the substrate to which the ring resonator is attached is aligned with the paper surface
- FIG. 6B is a top view seen from above on the paper surface of FIG.
- the stability in determining the transition frequency means that there are few factors that hinder measurement for correctly determining the transition frequency if the transition frequency is sufficiently accurate.
- An index of stability in determining the transition frequency is an index called Alan's deviation.
- the expression of the Allan deviation is expressed as follows: f is the transition frequency when the electric field or magnetic field at the atomic position (hereinafter simply referred to as “external field”) is 0, and the Fourier limit of frequency measurement at the transition of the transition frequency is ⁇ f.
- N atoms are observed per time (for example, 1 second) over time t (for example, second), and the measurement is performed by taking an average of a total of Nt measurements, Given by.
- the stability is higher as the value of the Allan deviation is smaller.
- several approaches are taken to increase the stability in determining the transition frequency.
- the clock transition can be selected from the light wave region instead of the microwave.
- the optical atomic clock was developed from this viewpoint.
- the interaction time T in (b) can be set to about 1 second or more by using an atomic trap such as a Paul trap.
- a technique is employed in which the number of atoms involved in clock transition is increased or the measurement time t is increased to perform long-time measurement.
- the accuracy of the transition frequency means that the value of the transition frequency is close to the original value.
- the removal of the external field in (d) can be realized by adopting, for example, the trap in (b). This is because, for example, in the Paul trap, atoms are trapped at the center position in the electromagnetic quadrupole, that is, the position where the external field is theoretically zero.
- Lamb-Dicke constraint in which a clock transition is caused after the atoms are cooled by a technique such as laser cooling.
- Lamb-Dicke binding uses a phenomenon in which the Doppler effect due to atom motion is not observed when the trapped region of the trapped atom is sufficiently narrower than the wavelength that interacts with the atom, such as the transition frequency. .
- the stability in determining the transition frequency is determined by the Allan deviation of Equation (1) when the atoms are single atoms or the atoms belonging to the atomic group can be said to be uncorrelated with each other.
- N 1, and an allan deviation of 10 ⁇ 18 with high accuracy, atomic observation of at least about 10 days is required. In a watch that requires a great deal of time to determine an accurate time, the application is limited.
- an optical lattice clock forms a spatial pattern of electromagnetic field intensity by forming a standing wave by, for example, facing laser light in free space. Then, an electric dipole moment is induced in the atom by the influence of the photoelectric field. Since this electric dipole moment interacts with the photoelectric field, the atoms are attracted toward the region where the photoelectric field is strong, that is, toward the antinode of the standing wave of the photoelectric field that is the lattice point. If the optical field of the optical lattice is strong enough, the atoms are held at the lattice points of the optical lattice. Using this, in the optical lattice clock, the atoms involved in the clock transition are maintained in a floating state.
- Non-Patent Document 1 the electronic state of an atom is perturbed by an AC Stark shift due to a photoelectric field (hereinafter referred to as “light shift”).
- the energy due to this perturbation shows even-order dependence on the photoelectric field of the second (second order), fourth (fourth) order, etc. of the photoelectric field amplitude.
- the secondary component gives a substantial perturbation, and the perturbation energy U i of the level i is E L ( ⁇ ) as the photoelectric field and ⁇ i ( ⁇ ) as the perturbation energy of the level i.
- the proportional coefficient for It is expressed.
- the proportionality coefficient ⁇ i ( ⁇ ) is a value depending on the frequency ⁇ of the photoelectric field and the electron level i.
- the transition frequency determined as the difference between the upper level and the lower level of the clock transition generally also depends on the amplitude of the photoelectric field. That is, the transition frequency is affected by the frequency of the photoelectric field (wavelength of the optical grating). Furthermore, even if the intensity of the optical grating is sufficiently controlled, the transition frequency is affected by the intensity distribution within the wavelength of the photoelectric field depending on the relative position with respect to the grating point, and the clock transition inside the optical grating The transition frequency is spread.
- the frequency of the photoelectric field brings the same amount of light shift perturbation energy to both the upper and lower levels of the atomic clock transition. It can be a special frequency or wavelength. In the atoms in the optical lattice generated at that special frequency (wavelength), the dependence of the transition frequency on the intensity of the photoelectric field is canceled because the upper level energy and the lower level energy are the same dependence. Is done.
- the transition frequency of the clock transition does not depend on the photoelectric field, the necessity to control the amplitude of the optical grating for the purpose of increasing the accuracy of the transition frequency is reduced, and the intensity distribution within the range of each optical grating, that is, smaller than the wavelength of the photoelectric field
- the intensity distribution of the scale also does not cause the spread of the transition frequency.
- the inventors of the present application call this special wavelength of the optical grating a magic wavelength. This concept of cancellation of the photoelectric field dependence due to the magic wavelength gives an atomic physics proof that guarantees the potential ability of the optical lattice clock to suppress the perturbation of (d), which is a problem in the atomic clock. It can be said.
- This concept is an example of perturbation engineering because it intentionally controls the atomic properties of the perturbation that the atom receives from the external field.
- the factors affecting the accuracy in the operation of the optical lattice clock that uses the atoms trapped at the lattice points of the optical lattice of the magic wavelength will be described. If described in relation to the above-described explanation of the atomic clock, the estimation based on the Allan deviation of the above equation (1) and the specific approaches (a) to (e) are also effective for the optical lattice clock. . In particular, the characteristics of the optical lattice clock as compared with the atomic clock are explained. In the optical lattice clock, the clock transitions at all the atoms of the lattice points of the optical lattice are observed, so the stability in determining the transition frequency is improved.
- the atoms trapped at the lattice points of the optical lattice are confined in a region sufficiently narrower than the wavelength of the light involved in the clock transition.
- the above-mentioned conditions for removing the Doppler shift (Lamb-Dicke constraint) Is also satisfied.
- the perturbation (d) the substantial influence of the perturbation can be eliminated by the magic wavelength as described above.
- FIG. 1 is an explanatory diagram showing the configuration and operating principle of a conventional optical lattice clock.
- FIG. 1A is a configuration diagram showing a configuration of a conventional optical lattice clock 900
- FIG. 1B is a schematic diagram showing a state of a beam waist portion in which atoms are trapped in the optical lattice
- FIG. (c) is a schematic diagram showing a potential state felt by atoms trapped in the optical lattice trapped in the optical lattice.
- a conventional optical lattice clock 900 shown in FIG. 1A includes an optical path 920, laser light sources 930 and 932, and a laser cooling unit 940.
- the laser cooling unit 940 is a space part in which atoms are cooled by laser cooling in the vicinity of intersections on all extensions of white arrows from four directions in the drawing. In practice, however, the laser for cooling is further irradiated from two directions, that is, six directions in total. Between the lens 926 and the mirror 928 in the optical path 920, atomized cooled atoms 950 having two-level electronic states related to clock transition are supplied. The working space is maintained at a high vacuum so that the space in which the cooled atoms 950 fly is evacuated. Then, the cooling atom 950 in the optical path 20 becomes a reference atom, and light having a wavelength that causes absorption due to the clock transition is referred to for the optical lattice clock.
- the laser light source 930 supplies light propagating leftward on the paper through the polarizer 924 and the lens 926 to the atoms in the optical path 920, that is, the reference atoms.
- the polarizer 924 and the lens 926 When the light is reflected by the mirror 928, light propagating rightward on the paper surface is also supplied.
- These pairs of light become standing waves inside the optical path 920, and form a photoelectric field pattern in which positions where photoelectric fields oscillating at the frequency of light are formed and positions where photoelectric fields are not formed are arranged alternately.
- This pattern is an optical lattice, and a lattice point is a position where the amplitude of the photoelectric field is maximized.
- the laser light source 930 is a lattice laser having an action of supplying a lattice laser pair.
- An optical lattice clock that adopts the concept of magic wavelength is one in which the wavelength ⁇ L of the grating laser is the magic wavelength ⁇ m .
- the laser light source 932 supplies light to the same optical path as the laser light source 930, but has an action of exciting the clock transition of the cooled atom 950. For this reason, the laser light source 932 acts as a clock laser.
- the frequency of the laser light source 932 is stabilized using a reference cavity, and the frequency can be slightly changed by the acousto-optic element AOM.
- the absorption or emission of light due to the clock transition is detected by the detector 960, and is fed back to the laser light source 932 by the servo control unit 970.
- the clock laser from the laser light source 932 is irradiated to the cooling atom 950, and the excitation probability is observed depending on the intensity of light absorption or emission at each frequency.
- the clock laser frequency is feedback-controlled by the acousto-optic element AOM so that the excitation probability becomes 1/2.
- FIG. 1B shows an enlarged schematic view of the beam waist portion 910 where the intensity of the lattice laser pair from the laser light source 930 is strong.
- the intensity of the photoelectric field forming the standing wave has a spatial period of ⁇ L / 2 when the wavelength of the grating laser is ⁇ L.
- This spatial period ⁇ L / 2 becomes the lattice spacing of the optical lattice, and attracts the cooling atoms 950 to the antinode portion (lattice point) of the vibration of the photoelectric field.
- the intensity of the photoelectric field by the lattice laser shows a distribution corresponding to the laser mode in the radial direction.
- FIG. 1B shows the position dependency of the light intensity of the electric field forming the optical grating.
- the potential felt by the atoms in FIG. 1C has a shape obtained by inverting the intensity of the photoelectric field forming the standing wave of the optical lattice, and is proportional to the inverse sign of the square of the intensity of the photoelectric field.
- this potential is a value obtained by multiplying the value obtained by averaging the square of the absolute value of the photoelectric field over a longer time than the frequency of light by the negative proportionality coefficient.
- a position where the photoelectric field is strong forms a deeper potential well, and each lattice point has a minimum value, and a potential shape that vibrates spatially according to the intensity of the optical lattice is obtained.
- the actual potential also changes in the radial direction of the beam.
- the intensity of the photoelectric field by the grating laser is centered on the beam axis.
- the potential has a convex shape with the beam center being a minimum value.
- k p 2 ⁇ / ⁇ 0 (where ⁇ 0 is a laser light source 932 which is a clock laser or a laser (clock laser) light source (not shown) used for a probe having a frequency close thereto)
- ⁇ 0 is a laser light source 932 which is a clock laser or a laser (clock laser) light source (not shown) used for a probe having a frequency close thereto
- the above-mentioned Lamb-Dicke constraint is realized when k p ⁇ x ⁇ 1.
- the clock transition of the cooled atom 950 does not show the Doppler effect with respect to the wavelength of the clock transition light or the clock laser.
- the range in which traps actually occur is limited to a range of about 150 ⁇ m, which is sufficiently narrower than the Rayleigh length, as shown in FIG. At positions outside this range, even if a lattice laser pair exists, it is not suitable for the observation region of the optical lattice clock.
- FIG. 1C when the cooled atom 950 is a spin-polarized fermion, even if about 10 atoms are bound per potential well, atoms in the well In principle, this interaction does not occur, so that the interaction of individual atoms of the cooled atom 950 can be reduced.
- the optical lattice clock of this embodiment employs an optical waveguide having a hollow passage. This is expected to improve accuracy by increasing the number of atoms involved in clock transition, that is, N in the equation (1). In other words, in order to increase the number of atoms, increasing the effective volume of the spatial region in which atoms can be trapped in the optical lattice can ensure the uniformity of the lattice laser light over a long distance and increase the number of lattice points. Can do.
- FIG. 2 is an explanatory diagram showing a typical configuration and operation principle of the optical lattice clock of the present embodiment.
- FIG. 3 is an explanatory diagram for explaining the operation of the optical waveguide in the optical lattice clock of the present embodiment.
- FIG. 3A is a configuration diagram showing an overall configuration for forming an optical grating in the optical waveguide and how atoms are trapped in the optical grating
- FIG. 3B is a cross-sectional view showing the structure of the optical waveguide. It is.
- the optical lattice clock 100 includes an optical waveguide 10, a laser light source 30, and a laser cooling unit 40.
- a hollow passage 14 extending from the first end 16 to the second end 18 is surrounded by the cylindrical wall 12, and the passage 14 serves as a waveguide path.
- a part of the optical path 20 between the mirrors 26 and 28 passes through the passage 14.
- the laser light source 30 supplies a grating laser pair L 1 and L 2 that are opposite to each other to the optical path 20.
- the laser cooling unit 40 supplies a cooled atom 50 of an atom having a two-level electronic state related to a clock transition to the vicinity of the first end 16 of the optical waveguide 10.
- the operating space 8 is maintained at a high vacuum so that the space in which the cooled atoms 50 fly and are transported is evacuated.
- the optical path 20 is, for example, an optical path similar to a bow tie type resonator in which a semi-transmissive mirror 22 and a dichroic mirror 24 are combined in addition to the mirrors 26 and 28.
- the grating laser L1 is transmitted through the semi-transmissive mirror 22, reflected by the dichroic mirror 24, and irradiated by the mirror 28 to the opening of the passage 14 on the second end 18 side.
- the grating laser L2 passes through the semi-transmissive mirror 22 and is irradiated by the mirror 26 to the opening of the passage 14 on the first end 16 side.
- the lattice laser pairs L1 and L2 are a pair of lasers having a magic wavelength, which is a wavelength that causes optical shifts that coincide with each other for each of the two levels.
- the grating laser pairs L 1 and L 2 form a moving optical grating ML that moves from the first end 16 toward the second end 18 in the passage 14 of the optical waveguide 10.
- the cooled atom 50 is trapped in the moving light grating ML and is carried through the passage 14 from the first end 16 toward the second end 18, thereby causing a clock transition.
- the grating laser pair L1 and L2 have the same frequency when the grating laser L1 and the grating laser L2 have the same frequency (referred to as ⁇ 1 and ⁇ 2 respectively) when viewed from the moving frame at the velocity v at which the atoms move. In addition, it is similar to the frequency of the magic wavelength.
- the laser L from the laser light source 30 is branched into the grating laser L1 and the grating laser L2, and then a slight frequency shift is caused by the Doppler effect on at least one of the light beams by the acoustooptic device. Cause it to occur.
- ⁇ 1 and ⁇ 2 respectively
- the grating lasers L1 and L2 are passed through the acoustooptic elements AOM1 and AOM2, respectively.
- the nonpatent literature 3 it is reported to the nonpatent literature 3 by this inventor about the transport to the micro space of the cooling atom by a moving light grating.
- the optical grating of the moving optical grating ML has several planes that become antinodes of standing waves of the photoelectric field having a radial spread of the path 14. 14 are stacked in the extending direction.
- the cooling atoms 50 guided from the first end 16 into the cylindrical wall 12 pass through the passage 14 by the movement of the moving light grating ML at the speed v, and escape from the second end 18 through the passage 14. Therefore, the number determined by the supply amount of the cooling atom 50 is located on each plane that becomes the antinode of each standing wave of the moving optical grating ML from the first end 16 to the second end 18, that is, in the vicinity of each position where the photoelectric field is strong.
- each of the cooling atoms 50 causes a clock transition during a period when it is trapped in the moving light lattice ML and moves inside the passage 14. In one embodiment, this clock transition is detected by a clock laser incident from the outside. In another aspect, pumping to an excited state in advance and detecting the light caused by the clock transition as, for example, light L OUT is performed externally.
- the passage 14 is surrounded by the cylindrical wall 12 and extends from the first end 16 to the second end 18.
- the passage 14 communicates with the outside world at the first end 16 and the second end 18.
- optical waveguide 10 [2-2-1 Requirements for optical waveguides]
- the condition to be satisfied by the optical waveguide 10 is firstly to pass the cooling atom 50 together with the moving optical grating ML through the passage 14, and secondly, the moving optical grating ML by the lattice laser pair L1 and L2 is sufficient. It can be formed in the passage 14 with the intensity of the photoelectric field. For this reason, in the optical waveguide 10 of the present embodiment, the passage 14 communicates with the outside world at the first end 16 and the second end 18 as described above.
- the beam diameter and light intensity of the grating laser incident on the first end 16 (lattice laser L 1) and the second end 18 (lattice laser L 2) of the passage 14 are maintained in the passage 14 by the cylindrical wall 12.
- the cylindrical wall 12 reflects the lattice laser pair L1 and L2 and confines it in the passage 14.
- the optical waveguide 10 does not necessarily have a linear shape. Further, the pair of grating lasers L1 and L2 propagating through the optical waveguide 10 does not need to be a fundamental mode.
- FIG. 4 is a schematic diagram showing a configuration of an HC-PCF 70 that is one specific example of the optical waveguide 10.
- 4A is a schematic cross-sectional view in a plane including the direction in which the passage 74 of the HC-PCF 70 extends
- FIG. 4B is a schematic cross-sectional view in a direction in which the passage 74 is cut.
- the HC-PCF 70 is a specific example of the optical waveguide 10, and a passage 74 surrounded by the cylindrical wall 72 extends from the first end 76 to the second end 78.
- the cylindrical wall 72 is a photonic crystal under conditions that prohibit the propagation of light of the wavelength of the grating laser pair L1, L2 that is propagated in the passage 74.
- the passage 74 which is a hollow passage, the condition is not satisfied, and the propagation of light having the wavelengths of the grating laser pairs L1 and L2 is allowed.
- the grating laser pair L1, L2 propagates only in the path 74 from one end portion to the other end portion in the opposite directions to form an optical grating.
- the cylindrical wall 72 includes a PC clad 722 that functions as a photonic crystal and a sheath 724 that surrounds the PC clad 722.
- the PC clad 722 has a structure in which the refractive index medium is regularly arranged including voids, and the voids communicate with the axial direction of the HC-PCF 70. When the voids are disposed in a vacuum, the voids are also exhausted. It has such a structure.
- the PC clad 722 itself is a photonic crystal that prohibits the propagation of light of the wavelength of the grating laser pair L1, L2 by the arrangement of the refractive index medium and the gap.
- the total length (path) of the HC-PCF 70 illustrated in FIG. 4A is, for example, 30 mm or more.
- the inner diameter of the passage 74 is, for example, about 10 to 100 ⁇ m.
- the Rayleigh length is, for example, about 150 ⁇ m, but if it is changed to the optical waveguide 10 or HC-PCF 70 having a total length of about 30 mm (the path of the optical grating, that is, the length of the optical grating), the stability of the optical lattice clock is further improved. be able to.
- the size of the conventional optical atomic clock for example, even an optical lattice clock employing an optical waveguide with a size of about 30 mm can realize a highly accurate optical lattice clock that is sufficiently miniaturized. It can be said that.
- the length of the optical lattice is preferably about 30 mm in order to increase the number of atoms by expanding the spatial region.
- FIG. 5 is an explanatory diagram of an optical grating in the HC-PCF 70.
- the time average value in a certain time range of the intensity value Int obtained by squaring the photoelectric field of the optical grating formed by the fundamental mode laser is shown in the direction in which the passage 74 extends.
- each position x in the radial direction of the passage 74 is shown.
- the time average here is a time average over a time range that is longer than the period of vibration of the photoelectric field but not so long as to observe the movement of the moving light grating ML.
- the passage 74 is a columnar space extending in one direction.
- the moving light grating ML forms a lattice point at each position z, and at each lattice point, each position x
- the length of the path in the axial direction becomes the length of the entire optical grating, and at each position in the path, the intensity of the photoelectric field is repeated at a period ⁇ L / 2, which is half the wavelength ⁇ L of the grating laser. Yes.
- the maximum of the photoelectric field is located at the central axis portion of the passage 74, the photoelectric field decreases toward the inner surface of the cylindrical wall 72, and has a radial distribution similar to that of a Gaussian beam. For this reason, the cooling atom 50 trapped at the position of the stronger photoelectric field does not approach the position in contact with the inner surface of the cylindrical wall 72. As a result, long-range interactions such as van der Waals force, Casimir-Polder force, and Lifshits force between the cooled atom 50 and the inner surface of the cylindrical wall 72 are suppressed.
- the interaction between the atoms and the inner wall of the container as in the case of miniaturization in the atomic clock is avoided.
- This property is not limited to the case where the moving light grating ML is in the fundamental mode.
- the optical grating by the grating laser pair L1, L2 is in the vicinity of the inner surface of the cylindrical wall 72 and the photoelectric of the optical grating as it approaches the inner surface of the cylindrical wall 72. This is because the place weakens.
- the number N of atoms involved in observation per unit time (for example, 1 second) in equation (1) can be easily increased by using the optical waveguide 10 or the HC-PCF 70. If the length of the HC-PCF 70 is 30 mm or more, for example, the effective volume of the trapping range of atoms, which is limited to the Rayleigh length in FIG. For example, it can be increased to about 200 times (when the length of the HC-PCF 70 is 30 mm).
- the optical waveguide 10 and the HC-PCF 70 can realize a photoelectric field that is strong enough to maintain the trap of the cooled atom 50 over the entire length of the passage 14 and the passage 74. It is maintained even if the waveguide 10 and the HC-PCF 70 are lengthened. Therefore, according to the example of the 30 mm length described above, the number of atoms involved in observation can be sufficiently increased.
- the frequency ⁇ m of the optical grating is an arithmetic mean value of the frequencies of the grating laser L1 and the grating laser L2 ( ⁇ 1 and ⁇ 2 , respectively), and the moving pair moves at a speed v. It is also the frequency of the optical grating seen by the cooled atoms 50 in the frame.
- each lattice point is a spheroid collapsed in the thickness direction with a thickness of 30 nm and a diameter of 5 ⁇ m. It is close to the reality that it is about the body.
- the maximum value of atoms n that can trap this spheroid per lattice without degrading accuracy is about 10. This is because the atomic density at this time is about 1 ⁇ 10 13 cm ⁇ 3 , and if the atomic density is further increased, interatomic interaction occurs due to cryogenic (p-wave) collision, resulting in deterioration of accuracy. It is because it may invite. Fortunately, in this embodiment, since the optical waveguide 10 and the HC-PCF 70 are employed, the number of atoms involved in the clock transition can be increased without increasing the atomic density if the total length is increased. Is possible. For easy calculation, if the total length of the HC-PCF 70 in FIG.
- N is 106 or so.
- the number N of atoms can be varied if the total length of the HC-PCF 70 and the wavelength of the optical lattice are different. In particular, if the total length of the HC-PCF 70 is further extended, it is easy to increase the number N of atoms.
- the HC-PCF 70 more generally the optical waveguide 10, is employed, light attenuation does not occur in principle. Actually, the light attenuation is negligible when the total length of the HC-PCF 70 is about 40 mm or about 100 mm, for example.
- the optical waveguide 10 has an inner diameter of the passage 14 equal to or larger than a certain reference inner diameter in order to make the interaction of the cooling atom 50 with the cylindrical wall 12 surrounding the passage 14 smaller than a predetermined value. Is effective.
- the above-described long-range interaction can be considered as the effect of the inner wall of the container with respect to atoms. These interactions cause a shift in the frequency of the clock transition, degrading the clock accuracy. Therefore, the long-range interaction was estimated as an influence on the frequency shift ⁇ f / f.
- the inner diameter (diameter) of the cylindrical wall 72 is set to about 10 to 100 ⁇ m, for example, if the temperature of the fiber wall surface is set to about 70K or less, ⁇ f / f is suppressed to about 10 ⁇ 17 or less at the maximum. I was able to get an estimate.
- the optical waveguide 10 and the HC-PCF 70 it is possible to sufficiently increase the stability in which the expression (1) is an index in the optical lattice clock, that is, to reduce the value of the expression (1).
- long-range interaction between atoms and the inner wall of the container can be sufficiently suppressed.
- the optical waveguide 10 or the HC-PCF 70 is adopted, not only can the accuracy of the optical lattice clock be improved, but miniaturization of the optical lattice clock will become realistic.
- the cooled atoms 50 are polarized (spin polarized) and excited just prior to being introduced into the passage 14 or passage 74.
- the polarization is performed when the atom of the cooling atom 50 has a half integer spin in the excited state.
- This polarization or excitation is performed, for example, by irradiating a moving light grating ML including the cooling atom 50 with a unidirectional circularly polarized laser at a position immediately before the first end 16.
- the clock frequency f C of the clock transition in the optical lattice clock 100 is generally different from the frequency of the optical grating having the magic wavelength. Determining a clock frequency f C undergoing clock transition, for example, a dichroic mirror 24, if transmit light watch frequency f C is assumed to reflect light magic wavelength, cooling atoms through the dichroic mirror 24 This is possible by observing 50 excitation probabilities.
- This clock frequency f C is a sufficiently accurate frequency.
- the line width of the spectrum is determined as the reciprocal of the time passing through the passage 14 or the passage 74.
- the number of atoms is large and the stability is also increased.
- the cooled atom 50 in which the excitation probability is observed becomes a reference atom.
- the cooled atom 50 is first spin polarized. Then, at each of the entrance (first end 16) and the exit (second end 18) of the passage 14 or the passage 74, a ⁇ / 2 pulse of a clock transition is irradiated to cool the passage 14 or the passage 74 by the moving light grating ML. Ramsey spectroscopy is performed during the transport of atoms 50. This makes it possible to perform sensitive detection such that the spectral line width is the reciprocal of the time for passing through the passage 14 or the passage 74.
- the frequency of the clock laser beam is estimated from the absorption and phase change of the clock laser beam.
- the light for these clock transition probes is more generally shown as clock laser light L p , and a model is formed by introducing the light to be the clock transition probe into the optical waveguide 10 and monitoring its output. It is shown as a diagram.
- the light source (clock laser source, not shown) for the absorption distribution servo system by utilizing (not shown) by the clock laser beam L p feedback is performed on.
- This servo system is the same as that described using the detector 960 and the servo control unit 970 in FIG. Then, by dividing the frequency of the clock laser light source by a technique such as an optical frequency comb, the same frequency uncertainty can be obtained even at a microwave frequency that is easy to handle, and this is used as the clock device 1000. can do.
- Such a timepiece device 1000 enables time measurement with high accuracy.
- FIG. 6 is an arrangement diagram showing a configuration example of an optical lattice clock employing HC-PCF in the optical waveguide of the present embodiment.
- 6A is a plan view in which one surface of the substrate 60 defining the optical path 20 is aligned with the paper surface
- FIG. 6B is a top view seen from above on the paper surface of FIG. 6A.
- the timepiece device 1000 partially includes the optical lattice clock 100 shown in FIG.
- the optical waveguide 10, the optical path 20, and the laser cooling unit 40 are attached to a substrate 60 having a small coefficient of thermal expansion.
- the optical waveguide 10 and the optical path 20 are attached to the first surface 60A side of the substrate 60, and the laser cooling unit 40 is attached to the second surface 60B side.
- the substrate 60 is formed with a through hole 64 that connects the first surface 60A and the second surface 60B. Yes.
- the position of the through hole 64 is such that the opening on the first surface 60 ⁇ / b> A side is in the vicinity of the first end 16 of the optical waveguide 10.
- a mirror 62 is provided around the through hole 64 on the second surface 60B side.
- Laser cooling lasers 44, 44, and 44 are incident on the mirror 62 from different directions on the second surface 60B side. The lasers 44, 44 and 44 form a cooling photon field in the vicinity of the mirror 62 for cooling the atoms by the effect of the mirror 62.
- the atoms are supplied from the atom supply unit 42 to the space on the second surface 60B side. This atom interacts with the cooling photon field and is cooled, and then supplied to the first surface 60 ⁇ / b> A side through the through hole 64.
- an additional moving light grating ML 2 is formed in the through hole 64 so as to pass through the through hole 64.
- the cooling atoms 50 supplied to the first surface 60A side are moved from the first end 16 of the optical waveguide 10 to the passage 14 by the moving light grating ML formed in the space portion of the lattice laser pair L1 and L2. It is introduced inside and carried out from the second end 18.
- the lattice laser pair L1 and L2 spreads outside the optical waveguide 10 and propagates through the space. For this reason, as the distance from the optical waveguide 10 increases, the intensity of the photoelectric field of the moving optical lattice ML decreases, and the cooled atoms 50 cannot continue to be trapped. Accordingly, the cooled atom 50 after being largely separated from the second end 18 is not trapped by the moving light lattice ML by the lattice laser pair L1, L2, and is discharged by an exhaust system (not shown) for maintaining a high vacuum. go.
- the light due to the clock transition is output as the light L OUT or the clock transition is detected by a clock laser beam (not shown). Based on the highly accurate clock transition signal, the clock device 1000 generates a time reference signal. For this reason, the timepiece device 1000 can provide highly accurate time information. Further, by using the substrate 60, a relatively small configuration can be achieved.
- the substrate 60 is a thin plate (thickness of 5 to 100 mm) having a flat shape of about 50 mm ⁇ 100 mm. About 10 mm).
- the laser 44, the laser light source 30, the light source for excitation (polarization) operation, the light source for the moving light grating ML2 (all not shown), etc. can employ semiconductor lasers.
- the set size required to operate the timepiece device 1000 by supplying power only to the power source can be sufficiently accommodated in a 19-inch rack mount size.
- the timepiece device 1000 can be sufficiently downsized while being proud of high accuracy.
- the optical lattice clock 100 shown in FIG. 2 or 6 can also be operated as an active oscillator or a laser light source.
- the optical path 20 such as the optical path of the bow tie resonator serves as a laser resonator for laser emission by superradiation or stimulated emission, and the cooled atoms 50 in the passage 14 emit light. It becomes a medium to wake up and a laser medium.
- the cooling atom 50 when the cooling atom 50 is prepared in the upper state of the clock level, light is emitted by clock transition in the mode of the bow tie resonator, and laser oscillation is caused if stimulated emission occurs continuously.
- it when light is emitted, it operates as an active oscillator using that light as an output, that is, an oscillator that does not require a clock light for detection.
- It is easy to produce a timepiece device using the output of light from the oscillator as a frequency reference. It is also possible to use the frequency reference based on the behavior when light is absorbed between the clock levels of the cooled atom 50.
- the active oscillator can be operated under conditions that cause laser oscillation that continuously causes stimulated emission.
- optical pumping is performed to the upper level of the clock transition.
- the light or electromagnetic wave having a clock frequency f C output through the dichroic mirror 24 becomes a coherent laser output.
- an inversion distribution for laser oscillation is realized in this operation. This is because the cooling atoms 50 in the excited state are introduced into the passages 14 and 74 to cause a clock transition, and then the cooling atoms 50 that have reached the ground state are discharged from the passages 14 and 74.
- the stimulated emission is caused by the high density of light or electromagnetic waves with a clock frequency f C in a limited space in the passage 14. Is caused.
- the atoms in the transport process are optically pumped to the upper level via a clock transition via an auxiliary state. At this time, the laser behaves like a three-level laser. This optical pump can be performed on the same axis as the optical waveguide or from the wall surface of the cladding of the optical waveguide.
- the number of atoms per lattice is n in order to reduce collision shift.
- the moving speed of the moving optical grating ML v, the wavelength of light gratings was lambda L. This excited atom gives the laser oscillation gain.
- ⁇ p is about 10 6 / s.
- the condition of bad cavity limit is established to reduce the influence of the frequency pulling of the resonator. This reduces the limitation on the laser oscillation spectral width, which is limited by the thermal fluctuation of the laser resonator in the optical path 20.
- the HC-PCF 70 a rod shape which is a shape of an atomic cloud suitable for Dicke's super-radiation is realized. Therefore, the HC-PCF 70 can be said to be useful for realizing a laser light source using a super-radiation mechanism.
- the laser light source that oscillates using the principle of the optical lattice clock of the present embodiment outputs an ultra-high stability laser beam with a frequency uncertainty of about 10 ⁇ 17 and an output of about 0.1 pW. It becomes possible to make it.
- optical lattice clock, the clock device, and the laser light source of the present invention can be used for any device that uses time progress.
- DESCRIPTION OF SYMBOLS 1000 Clock apparatus 100 Optical lattice clock 8 Operation space 10 Optical waveguide 12 Cylindrical wall 14 Passage 16 1st end 18 2nd end 20 Optical path 22 Semi-transmission mirror 24 Dichroic mirror 26, 28 Mirror 30 Laser light source 40 Laser cooling part 42 Atom supply Portion 44 Laser 50 Cooling atom 60 Substrate 60A First surface 60B Second surface 62 Mirror 64 Through hole 72 Cylindrical wall 722 PC cladding 724 Sheath 74 Passage 76 First end 78 Second end L1, L2 Lattice laser ML, ML2 Moving light Lattice AOM1, AOM2 Acoustooptic device
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Abstract
Description
waveguide)を備え、該中空の通路に形成された光格子にトラップされている原子が有する2準位(two levels)の電子状態を時計遷移(clock transition)に利用し、前記光格子が、前記2準位の各準位に対して互いに一致した光シフトを生じさせる波長である魔法波長の光格子である光格子時計が提供される。
lattice)を形成するものであり、前記冷却原子は、該移動光格子にトラップされて前記第1端から前記中空の通路に導入され、該移動光格子によって、該第1端から前記第2端に向かって前記中空の通路を通して運ばれながら前記時計遷移を引き起こす。
本実施形態の光格子時計の動作原理を説明するために、まず従来の原子時計の動作と精度改善の手法を概説する(1-2)。その後、従来の光格子時計の動作を説明する(1-3)。
まず、原子の時計遷移の遷移周波数を利用する従来の原子時計(光原子時計を含む)の動作を、その精度を支配する要因と各要因に対するアプローチに着目し概説する。原子時計では、端的には、時計遷移の遷移周波数をいかに正しく測定できるか、および、遷移周波数自体が周囲から受ける影響をいかに排除できるか、により精度、すなわち、時計としての不確かさが決定される。このため、この不確かさの説明を、遷移周波数の値の決定における安定度(1-2-1)と、遷移周波数それ自体の正確さ(1-2-2)とに分けて進める。
遷移周波数の決定における安定度とは、仮に遷移周波数が十分に正確であるとした場合に、その遷移周波数を正しく決定するための測定を妨げる要因の少なさをいう。この遷移周波数の決定における安定度の指標がアラン偏差(Alan's deviation)とも呼ばれる指標である。アラン偏差の表現は、原子の位置における電場や磁場等(以下、単に「外場」という)が0のときの遷移周波数をf、遷移周波数の遷移における周波数測定のフーリエ限界をΔfとし、単位時間(例えば1秒)当りN個の原子観測を時間t(例えば秒)にわたり行い、合計Nt回の測定の平均を取る測定を行なうとする場合、
(a)遷移周波数fを増大させる、
(b)T=1/Δfという関係(フーリエリミット)によりΔfに影響を及ぼす相互作用時間Tを増大させる、
(c)測定結果の平均を取る延べ測定原子数Ntを増大させる、
という手法である。(a)の遷移周波数fを増大させるためには、例えば時計遷移をマイクロ波に代え光波領域のものから選択することができる。光原子時計はこの観点から開発されたものである。(b)の相互作用時間Tは、原子時計では、原子のトラップ、例えばPaulトラップ、を利用することにより1秒程度かそれ以上とすることができる。(c)のためには、時計遷移に関与する原子数を増大させたり、測定時間tを増大させて長時間測定を行なう手法が採用される。
その一方、遷移周波数の正確さとは、遷移周波数の値が本来の値に近いこと、である。この正確さを高めるためには、
(d)遷移周波数fに対する摂動(perturbation)の起源である外場を除去する、
(e)原子の運動に起因するドップラー効果によるシフト(「ドップラーシフト」)の影響を排除する、
といったアプローチが採用される。(d)の外場の除去は、例えば(b)の上記トラップを採用すれば実現される。例えばPaulトラップでは、電磁四重極における中心位置つまり外場が原理的に0となる位置に原子がトラップされるためである。また、(e)のドップラーシフトの影響を排除するためには、レーザー冷却などの手法により原子を冷却したうえで時計遷移を起こさせたり、ラム・ディッケ束縛とよばれる手法も採用される。ラム・ディッケ束縛は、トラップされている原子の閉じ込め領域を、遷移周波数などの原子と相互作用する波長よりも十分に狭くすると、原子の運動によるドップラー効果が観測されなくなる現象を利用するものである。
上述した種々のアプローチにより安定度および正確さが高められている原子時計においても、現実には、遷移周波数の決定における安定度の高さと、遷移周波数それ自体の正確さとを両立させがたい場合がある。例えば時計遷移を起こす原子が実際にはイオンである場合、(d)および(e)の点で遷移周波数それ自体の正確さは高めやすく(b)の点でも支障は少ないものの、(c)の原子数Nを増大させにくい。これは、イオンの互いのクーロン相互作用による斥力が生じるためである。このような場合、(c)の原子観測の測定時間tを増大させる対処が可能である。ただし、その測定時間は、時間の測定という時計の用途としては長くなり過ぎる。例えば、Δf/fを10-15程度、N=1としてアラン偏差を10-18という高精度な値とするには、少なくとも10日程度の原子観測が必要となる。正確な時間の決定のために多大な時間を要する時計では、用途が限定されてしまう。
次に、光格子時計の動作を説明する。光格子時計では一般に、例えば自由空間にレーザー光を対向させて定在波を形成することにより電磁場強度の空間的なパターンを形成する。すると原子には、光電場の影響により電気双極子モーメントが誘起される。この電気双極子モーメントは光電場と相互作用することから、原子は、光電場の強い領域に向かって、つまり、格子点である光電場の定在波の腹部分に向かって引きつけられる。もし、光格子の光電場が十分に強ければ、原子はその光格子の格子点に保持される。これを利用して、光格子時計では、時計遷移に関わる原子が宙に浮いた状態に維持される。
次に、本実施形態の光格子時計について説明する。本実施形態の光格子時計においては中空の通路を有する光導波路を採用する。これは、時計遷移に関わる原子数、すなわち式(1)におけるNを増大させることによる精度の向上を期待してのことである。つまり、上記原子数を増大させるべく、光格子において原子をトラップできる空間領域の実効的な体積を増加すると、長距離にわたって格子レーザー光の一様性を確保でき、格子点の数を増加させることができる。
図2は、本実施形態の光格子時計の典型的な構成および動作原理を示す説明図である。また、図3は、本実施形態の光格子時計における光導波路における作用を説明するための説明図である。図3(a)は、光導波路中に光格子を形成する全体構成と光格子において原子がトラップされる様子を示す構成図であり、図3(b)は、光導波路の構造を示す断面図である。
[2-2-1 光導波路に求められる条件]
次に光導波路10について説明する。光導波路10が満たすべき条件は、第1に、通路14に移動光格子MLとともに冷却原子50を通過させることであり、第2に、格子レーザー対L1、L2による移動光格子MLを、十分な光電場の強度で通路14に形成できることである。このために、本実施形態の光導波路10は、上述したように通路14が、第1端16と第2端18とにおいて外界と連通している。さらに光導波路10では、通路14の第1端16(格子レーザーL1)、第2端18(格子レーザーL2)に入射した格子レーザーのビーム径と光強度が筒状壁12により通路14において維持される。より具体的には、筒状壁12は格子レーザー対L1、L2を反射して通路14に閉じ込めるようになっている。なお、必ずしも、光導波路10は直線形状である必要はない。また、光導波路10を伝播する格子レーザー対L1、L2は、基本モードの伝播モードであることも必要ない。
図4は、光導波路10の具体例の一つであるHC-PCF70の構成を示す模式図である。図4(a)はHC-PCF70の通路74の延びる方向を含む面での概略断面図であり、図4(b)は通路74を切断する向きの概略断面図である。HC-PCF70は、光導波路10の具体例であり、筒状壁72に囲まれた通路74が第1端76から第2端78まで延びている。筒状壁72は、HC-PCF70では、通路74に伝播させる格子レーザー対L1、L2の波長の光の伝播が禁止されるような条件のフォトニッククリスタルをなしている。これに対し中空の通路となっている通路74ではその条件が成立せず格子レーザー対L1、L2の波長の光の伝播は許容される。その結果、格子レーザー対L1、L2は、通路74のみを一方の端部から他方の端部へ互いに逆向きに伝播してゆき、光格子を形成する。なお、筒状壁72は、フォトニッククリスタルとして機能するPCクラッド722と、それを取り巻くシース724により構成されている。PCクラッド722は、屈折率媒体が空隙を含んで規則的に配置された構造となり、その空隙がHC-PCF70の軸方向に連通していて、真空中に配置されると当該空隙内も排気されるような構造を有している。PCクラッド722は、それ自体が屈折率媒体と空隙の配置によって格子レーザー対L1、L2の波長の光の伝播を禁止するフォトニッククリスタルとなっている。図4(a)に例示したHC-PCF70の全長(道のり)は、例えば30mmまたはそれ以上のサイズにされている。通路74の内径は、例えば10~100μm程度である。
図5は、HC-PCF70における光格子の説明図であり、基本モードのレーザーにより形成した光格子の光電場を二乗した強度値Intの、ある時間範囲における時間平均値を、通路74の延びる向きに沿った各位置zと、通路74の径方向の各位置xについて示している。ここでの時間平均は、光電場の振動の周期よりは長いものの、移動光格子MLの移動が観察されるほどには長くない時間範囲にわたる時間平均である。通路74は、一方向に延びる柱状の空間になっている。この空間に移動光格子MLのために基本モードの格子レーザー対L1、L2を伝播させる場合には、移動光格子MLが各位置zにおいて格子点を形成し、各格子点においては、各位置xにおいて、筒状壁72の内側表面に近付くほど、振動の腹の部分の光格子の光電場が弱まる。また、通路の軸方向の長さが光格子全体の長さとなって、その通路の各位置においては、格子レーザーの波長λLの半分の周期λL/2で、光電場の強弱が繰り返している。例えば、通路74の中心軸部分に光電場の極大が位置し、筒状壁72の内側表面にむかって光電場が小さくなり、ガウシアンビームと同様の径方向の分布を持つ。このため、より強い光電場の位置にトラップされる冷却原子50は筒状壁72の内側表面に接する位置には近付かない。その結果、冷却原子50と筒状壁72の内側表面との間におけるvan der Waals力、Casimir-Polder力、そしてLifshits力といった長距離相互作用が抑制される。こうしてHC-PCF70に例示される光導波路10を採用する光格子時計では、原子時計においてミニチュア化する場合のような原子と容器内壁との相互作用は回避される。なお、この性質が得られるのは移動光格子MLが基本モードである場合だけには限定されない。高次の空間モードの移動光格子MLであっても、格子レーザー対L1、L2による光格子は筒状壁72の内側表面の近傍において、筒状壁72の内側表面に近付くにつれて光格子の光電場が弱まるためである。
[2-3-1 周波数決定における安定度]
次に、図4に関して説明したサイズを念頭に、本実施形態の光格子時計が達成する精度の概算的な見積りについて説明する。まず、式(1)における単位時間(例えば1秒)当りの観測に関与する原子の個数Nは、光導波路10やHC-PCF70を利用すれば、増大させることが容易である。これは、HC-PCF70の長さを例えば30mmまたはそれ以上とすれば、従来の光格子時計において図1(b)においてレイリー長に制限されていた原子のトラップの範囲の実効的な体積を、例えば200倍程度(HC-PCF70の長さが30mmの場合)に増大させることができるためである。光導波路10やHC-PCF70は、通路14や通路74の長さの全域において、冷却原子50のトラップを維持するだけの強さの光電場を実現することが可能であり、この性質は、光導波路10やHC-PCF70を長くしても維持される。そのため、上述した30mm長さの例に従えば、観測に関与する原子の個数を十分に増大させることができる。
v=λm(ω1-ω2)/2
=λmδω/2 式(3)
と表現される。すると、原子の寿命、すなわち、原子が通路14にて光格子と相互作用する時間τは、Lを通路14の全長として
τ=L/v 式(4)
により与えられる。このτの値は、例えば1秒程度とできる。
N=nl/(λL/2) 式(5)
となる。
そして、時計遷移の遷移周波数(時計周波数)に生じるシフトのうちドップラー効果によるシフトは正確に補償することができる。具体的には、原子から放射される時計周波数fCはドップラー効果の影響を受けた値、cを光速、fC0を原子がドップラー効果を受けない場合の周波数として、
fC=fC0(1+v/c) 式(6)
と表現される。式(3)をここに適用すれば、
fC=fC0(1+δω/2ωm) 式(7)
となる。このシフト量であるδω/2ωmは、正確に補償することができる。
典型的な動作において、冷却原子50は、通路14または通路74に導入される直前に、偏極(スピン偏極)され励起されている。なお、偏極は、冷却原子50の原子が励起状態において半整数スピンを有する場合に行なわれる。この偏極または励起は、例えば一方向の円偏光のレーザーを、冷却原子50を含む移動光格子MLに第1端16直前の位置において照射することにより行なわれる。
光格子時計100における時計遷移の時計周波数fCは、一般に、魔法波長の光格子の周波数とは別異のものである。時計遷移を起こしている時計周波数fCを決定することは、例えばダイクロイックミラー24を、時計周波数fCの光を透過し魔法波長の光を反射するものとしておけば、ダイクロイックミラー24を通じた冷却原子50の励起確率を観測することにより、可能である。この時計周波数fCは、十分に正確な周波数となる。具体的には、そのスペクトルの線幅は、通路14または通路74を通過する時間の逆数として決定される。しかも、通路14に含まれている原子のすべてが観測対象となっているため、原子数が多く安定度も高くなる。
次に構成例を挙げ、本発明の具体例をさらに詳細に説明する。以下の構成例に示す材料、使用量、割合、処理内容、処理手順、要素または部材の向きや具体的配置等は本発明の趣旨を逸脱しない限り適宜変更することかできる。したがって、本発明の範囲は以下の具体例に限定されるものではない。説明済の要素には、同様の符号を明示しその説明を省略する。
次に、上記実施形態の光格子時計100の変形例であるアクティブ型の発振器およびレーザー光源について説明する。図2または図6に示した光格子時計100は、アクティブ型の発振器やレーザー光源としても動作させることができる。具体的には、ボウタイ共振器の光路のような光路20が、超放射(superradiation)や誘導放出によるレーザー発振(lasing)のためのレーザー共振器となり、通路14中の冷却原子50がその発光を起こす媒質やレーザー媒質となる。つまり、冷却原子50を時計準位の上状態に準備するとき、ボウタイ共振器のモードで時計遷移により光を放出し、誘導放出が連続的に生じればレーザー発振を引き起こす。いずれにしても、光を放出する場合には、その光を出力としてアクティブ型の発振器、つまり、検出のための時計レーザー光を必要としない発振器として動作する。その発振器からの光の出力を周波数基準として時計装置を作製することも容易である。なお、冷却原子50の時計準位の間で光が吸収される際の挙動を利用して周波数基準とすることもできる。
Γp=nv/(λL/2) 式(8)
となる。ここで、移動光格子MLの移動速度をv、光格子の波長をλLとした。この励起された原子がレーザー発振の利得を与える。v=4cm/s、n=10とした場合、Γpは約106/sとなる。
hf0Γ/2 式(9)
程度となる。この出力Pの概算値は、式(8)に用いた値を利用すれば、約0.1pW程度となる。この出力Pは、周波数が安定化された一般的なレーザーを利用してPLL(Phase Lock Loop)を実現するために十分な値である。
100 光格子時計
8 動作空間
10 光導波路
12 筒状壁
14 通路
16 第1端
18 第2端
20 光路
22 半透過ミラー
24 ダイクロイックミラー
26、28 ミラー
30 レーザー光源
40 レーザー冷却部
42 原子供給部
44 レーザー
50 冷却原子
60 基板
60A 第1面
60B 第2面
62 ミラー
64 貫通孔
72 筒状壁
722 PCクラッド
724 シース
74 通路
76 第1端
78 第2端
L1、L2 格子レーザー
ML、ML2 移動光格子
AOM1、AOM2 音響光学素子
Claims (14)
- 筒状壁により囲まれた中空の通路を導波経路として有する光導波路
を備え、
該中空の通路に形成された光格子にトラップされている原子が有する2準位の電子状態を時計遷移に利用するものであり、
前記光格子が、前記2準位の各準位に対して互いに一致した光シフトを生じさせる波長である魔法波長の光格子である
光格子時計。 - 前記光導波路の前記中空の通路が第1端から第2端まで延びており、
互いに逆向きに進む格子レーザーの対を供給することにより前記中空の通路における前記光格子を形成するレーザー光源と、
前記レーザー冷却部により前記原子を冷却して前記光導波路の前記第1端の近傍に冷却原子を供給するレーザー冷却部と
をさらに備え、
前記中空の通路において前記光格子にトラップされている前記冷却原子が、前記2準位の電子状態の間において前記時計遷移を引き起こす
請求項1に記載の光格子時計。 - 前記レーザー光源の前記格子レーザーの対は、前記光格子として、前記光導波路の前記中空の通路において、前記第1端から前記第2端に向かって移動する移動光格子を形成するものであり、
前記冷却原子は、該移動光格子にトラップされて前記第1端から前記中空の通路に導入され、該移動光格子によって該第1端から前記第2端に向かって前記中空の通路を通して運ばれながら前記時計遷移を引き起こす
請求項2に記載の光格子時計。 - 前記光導波路が中空コアフォトニッククリスタルファイバー(HC-PCF)であり、
前記光導波路の前記中空の通路が、該HC-PCFの中空コアのなす通路である
請求項1~3のいずれか1項に記載の光格子時計。 - 前記冷却原子が、前記第1端に入射する前に偏極または励起されている
請求項2または3に記載の光格子時計。 - 前記移動光格子が前記光導波路の前記中空の通路の延びる向きに移動する1次元光格子であり、
前記原子はスピン偏極したフェルミ粒子となるものである
請求項1~3のいずれか1項に記載の光格子時計。 - 前記光導波路は、前記中空の通路の内径が、該中空の通路を囲む筒状壁に対する前記冷却原子の相互作用が所定の値より小さくなる基準内径以上にされている
請求項2または3に記載の光格子時計。 - 前記冷却原子の供給量は、前記光導波路の前記中空の通路のなす前記第1端から前記第2端までの道のりの範囲に同時に存在する前記冷却原子の総数が所定の数以上となるような供給量である
請求項2または3に記載の光格子時計。 - 前記光格子時計は、前記2準位の電子状態の間における前記時計遷移により前記中空の通路に位置する前記冷却原子により吸収される光を周波数基準とするものである
請求項1~3のいずれか1項に記載の光格子時計。 - 請求項1~3のいずれか1項に記載の光格子時計において前記時計遷移のために吸収される波長の光を参照する時計装置。
- 前記中空の通路を光路の一部に含むリング共振器
をさらに備える
請求項2または3に記載の光格子時計。 - 前記光格子時計は、前記2準位の電子状態の間における前記時計遷移により前記中空の通路に位置する前記冷却原子により放出される光を周波数基準とするものである
請求項11に記載の光格子時計。 - 請求項11に記載の光格子時計において前記時計遷移により放出される波長の光を参照する時計装置。
- 請求項11に記載の光格子時計の前記リング共振器をレーザー共振器として備え、
前記中空の通路に位置する前記冷却原子をレーザー媒質とし、
前記冷却原子の前記時計遷移により誘導放出される光を前記リング共振器から出力する
レーザー光源。
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EP2887466A4 (en) | 2016-06-01 |
US20150194972A1 (en) | 2015-07-09 |
JP6206973B2 (ja) | 2017-10-04 |
JPWO2014027637A1 (ja) | 2016-07-28 |
EP2887466A1 (en) | 2015-06-24 |
EP2887466B1 (en) | 2017-05-10 |
US9553597B2 (en) | 2017-01-24 |
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