JP2014197733A - Quantum interference device, atomic oscillator, magnetic sensor, and method for manufacturing quantum interference device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a quantum interference device in which the long-term stability can be enhanced more than conventional, an atomic oscillator, a magnetic sensor, and a method for manufacturing the quantum interference device.SOLUTION: An atomic oscillator 1 comprises: metal atoms; a semiconductor laser 10 for irradiating metal atoms with light containing a pair of resonance light generating electromagnetic induction transmission phenomenon; and a photodetector 14 for detecting the light transmitted through metal atoms. When the wavelength which is equivalent to the energy difference between a first ground level and a first excitation level of the metal atom, is determined as λ1, the wavelength which is equivalent to the energy difference between a second ground level and the first excitation level, is determined as λ2, the wavelength which is equivalent to the energy difference between the first ground level and the second excitation level, is determined as λ3, and the wavelength which is equivalent to the energy difference between the second ground level and the second excitation level, is determined as λ4, the center wavelength of the pair of resonance light is more than (λ3+λ4)/2 and less than (λ1+λ2)/2.

Description

  The present invention relates to a quantum interference device, an atomic oscillator, a magnetic sensor, and a method for manufacturing the quantum interference device.

As shown in FIG. 13, a cesium atom which is a kind of alkali metal atom is known to have a ground level of 6S _1/2 and two excited levels of 6P _1/2 and 6P _3/2. ing. Furthermore, each level of 6S _1/2 , 6P _1/2 , 6P _3/2 has a hyperfine structure divided into a plurality of energy levels. Specifically, 6S _1/2 has two ground levels of F = 3 and 4, 6P _1/2 has two excitation levels of F ′ = 3 and 4, and 6P _3/2 has F It has four excitation levels' = 2, 3, 4, and 5.

For example, a cesium atom in the ground level of F = 3 of 6S _1/2 transitions to the excited level of either F ′ = 2, 3 or 4 of 6P _3/2 by absorbing the D2 line. However, it is not possible to transition to the excitation level of F ′ = 5. The cesium atom in the ground level of F = 4 of 6S _1/2 must transition to the excited level of F ′ = 3, 4, 5 of 6P _3/2 by absorbing the D2 line. However, it is not possible to transition to the excitation level of F ′ = 2. These are based on the transition selection rule when electric dipole transition is assumed. Conversely, a cesium atom in the excited level of either F ′ = 3, 4 of 6P _3/2 emits a D2 line, and the ground level of F = 3 or F = 4 of 6S _1/2 ( Transition to either the original ground level or the other ground level. Here, three levels (two base levels and two ground levels of F = 3, 4 of 6S _1/2 and excitation levels of F ′ = 3, 4 of 6P _3/2 ) (Consisting of one excitation level) is called a Λ-type three-level because it can make a Λ-type transition by absorption and emission of the D2 line. Similarly, three levels consisting of two ground levels of F = 3, 4 of 6S _1/2 and an excited level of F ′ = 3, 4 of 6P _1/2 are the absorption level of the D1 line. Since a Λ-type transition by light emission is possible, a Λ-type three level is formed.

On the other hand, the cesium atom in the excitation level of F ′ = 2 of 6P _3/2 emits the D2 line, and the ground level of F = 3 of 6S _1/2 is always (original ground level). Similarly, the cesium atom in the excitation level of F ′ = 5 of 6P _3/2 emits the D2 line, and the ground level of F = 4 of 6S _1/2 (the original ground level) ). That is, three levels consisting of two ground levels of F = 3, 4 of 6S _1/2 and an excited level of F ′ = 2 or F ′ = 5 of 6P _3/2 are the absorption and emission of the D2 line. The Λ-type three-level is not formed because Λ-type transition by is impossible. It is known that alkali metal atoms other than cesium atoms also have two ground levels and excited levels that form Λ-type three levels.

By the way, the first ground level (in the case of a cesium atom, F = 3 ground level of 6S _1/2 ) and the excited level (cesium atom in a gaseous alkali metal atom). In this case, for example, resonant light (resonant light 1) having a frequency (frequency) corresponding to an energy difference from 6P _3/2 F ′ = 4 excitation level) and a second ground level ( In the case of a cesium atom, when simultaneously irradiating resonance light (resonance light 2) having a frequency (frequency) corresponding to an energy difference between 6S _1/2 F = 4 ground level and an excitation level. An electromagnetically induced transmission (EIT) phenomenon (CPT (Coherent Population Trapping) phenomenon) in which two ground levels are superposed, that is, a quantum coherence state (dark state), and excitation to the excited level stops. Sometimes called)) is known to happen The frequency difference between the resonance light pair (resonance light 1 and resonance light 2) that causes this EIT phenomenon exactly coincides with the frequency corresponding to the energy difference ΔE ₁₂ between the two ground levels of the alkali metal atom. For example, since the frequency corresponding to the energy difference between the two ground levels of cesium atoms is 9.192631770 GHz, two kinds of D1 line or D2 line laser light having a frequency difference of 9.192631770 GHz are simultaneously applied to the cesium atoms. When irradiated, the EIT phenomenon occurs.

Therefore, as shown in FIG. 14, when light having a frequency of ω ₁ and light having a frequency of ω ₂ are simultaneously irradiated onto a gaseous alkali metal atom, the two light waves become a resonance light pair, and the alkali metal atom becomes EIT. The intensity of the light transmitted through the alkali metal atom changes sharply depending on whether or not the phenomenon occurs. The signal indicating the intensity of the transmitted light that changes sharply is called an EIT signal (resonance signal), and when the frequency difference ω ₁ −ω _{2 of} the resonance light pair exactly matches the frequency ω ₁₂ corresponding to ΔE _12. The level of the EIT signal shows a peak value. Therefore, the atomic cell (gas cell) in which gaseous alkali metal atoms are sealed is irradiated with two light waves, and the peak top of the EIT signal is detected by the photodetector, that is, the frequency difference ω ₁ −ω ₂ between the two light waves. Is controlled so as to exactly match the frequency ω ₁₂ corresponding to ΔE ₁₂ , a highly accurate oscillator can be realized. A technique related to such an atomic oscillator is disclosed in Patent Document 1, for example.

US Pat. No. 6,320,472

Incidentally, the EIT phenomenon as absorption of light does not stop, two light waves of a certain frequency difference slightly offset from omega ₁₂ a (D1 line, which can be either of the D2 line), the cesium atom by sweeping the central wavelength When the transmittance of the light that is irradiated and transmitted through the alkali metal atom is observed, two absorption bands with lower transmittance can be observed as shown in FIG. In FIG. 15, the horizontal time axis represents the center wavelength of two light waves, and the vertical axis represents the transmittance. For example, if the two light waves are D2 lines, the cesium atom in the 6S _1/2 F = 3 ground level absorbs the D2 line with the smaller wavelength (higher frequency) and absorbs the F of 6P _3/2 At the same time as the transition to the excitation level of '= 4, the cesium atom in the 6S _1/2 F = 4 ground level absorbs the D2 line with the larger wavelength (smaller frequency) and absorbs 6P _3/2 A first absorption band is seen when transitioning to the excitation level of F ′ = 4. On the other hand, the cesium atom in the 6S _1/2 F = 3 ground level absorbs the D2 line having the smaller wavelength (higher frequency), and the excitation level of 6P _3/2 F ′ = 3. At the same time, the cesium atom in the ground level of F = 4 of 6S _1/2 absorbs the D2 line having the larger wavelength (smaller frequency) and excites F ′ = 3 of 6P _3/2 A second absorption band is observed when transitioning to a level. In FIG. 15, the first absorption band is the left absorption band, and when the center wavelength of the two light waves is λa, the first absorption band is the bottom, and the second absorption band is the right absorption band. The bottom of the second absorption band is when the center wavelength of the light wave is λb.

The energy difference between F '= 3 and F' = 4 in 6P _3/2 is about one order of magnitude smaller than the energy difference between F = 3 and F = 4 in 6P _1/2 , so it depends on the buffer gas enclosed in the gas cell. Including the effect (the effect of increasing the base of the absorption band), actually, the two absorption bands are observed as overlapped as shown in FIG. The intensity of absorption differs depending on the difference in transition intensity, and the second absorption band is buried in the first absorption band, making it difficult to understand. Conventionally, the EIT signal has been observed at the bottom of the first absorption band. In this case, a symmetrical EIT signal is obtained when only the first absorption band is considered, and an asymmetric EIT signal is obtained when only the second absorption band is considered. Eventually, by superimposing these two EIT signals, an asymmetric EIT signal is actually obtained when observed at the bottom of the first absorption band. Similarly, even when observed at the bottom of the second absorption band, an asymmetric EIT signal is obtained.

When the intensity of the light irradiated to the alkali metal atom fluctuates due to aged deterioration of the laser light source or gas cell, environmental change, etc., the peak position does not fluctuate if it is a symmetrical EIT signal. However, as shown in FIG. 17, the peak position may fluctuate in an asymmetric EIT signal. When the peak position of the EIT signal changes, the frequency of the light that resonates also changes, so the frequency of the atomic oscillator also changes. As a result, the conventional atomic oscillator has a problem that it is difficult to achieve high long-term stability.

  The present invention has been made in view of the above problems, and according to some aspects of the present invention, a quantum interference device, an atomic oscillator, A method of manufacturing a magnetic sensor and a quantum interference device can be provided.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following aspects or application examples.

[Application Example 1]
The quantum interference device according to this application example includes a metal atom, a light source that irradiates light including a resonance light pair that generates an electromagnetically induced transmission phenomenon in the metal atom, and light detection that detects the light transmitted through the metal atom. A wavelength corresponding to an energy difference between the first ground level and the first excited level of the metal atom is λ1, and the second base is higher than the first ground level. The wavelength corresponding to the energy difference between the level and the first excitation level is λ2, and the energy difference between the first ground level and the second excitation level higher than the first excitation level is When the corresponding wavelength is λ3 and the wavelength corresponding to the energy difference between the second ground level and the second excitation level is λ4, the center wavelength of the resonant light pair is (λ3 + λ4) / 2. And smaller than (λ1 + λ2) / 2.

The “center wavelength of the resonant light pair” is an intermediate wavelength of the resonant light pair. When the wavelength of one resonant light pair is λ _A and the wavelength of the other resonant light pair is λ _B , (λ _A + λ _B ) a wavelength corresponding to / 2. For example, when a semiconductor laser is used as a light source and two primary sideband lights included in the emitted light of the semiconductor laser are used as a resonant light pair, the center wavelength of the resonant light pair is determined by the emission wavelength of the semiconductor laser. It matches the center wavelength.

  In the quantum interference device according to this application example, the metal atoms are respectively when the center wavelengths of the resonant light pair are (λ3 + λ4) / 2 and (λ1 + λ2) / 2 (exactly, when slightly deviating from these wavelengths). It becomes the bottom of the absorption band that absorbs the resonant light pair. Therefore, by setting the center wavelength of the resonant light pair to be larger than (λ3 + λ4) / 2 and smaller than (λ1 + λ2) / 2, the EIT signal appearing at the output of the light detection unit due to the electromagnetically induced transmission (EIT) phenomenon is Considering only the respective absorption bands, they become asymmetrical shapes opposite to each other, but by overlapping these, the shape becomes more symmetrical in practice. Since the EIT signal becomes more symmetrical, even if the intensity of the resonant light pair decreases with time, the fluctuation of the peak position of the EIT signal becomes smaller, so the fluctuation of the frequency of the atomic oscillator becomes smaller and stable for a long time. The degree can be improved.

[Application Example 2]
The quantum interference device according to this application example includes a metal atom, a light source that irradiates light including a resonance light pair that generates an electromagnetically induced transmission phenomenon in the metal atom, and light detection that detects the light transmitted through the metal atom. And when the two light waves having different frequencies are irradiated to the metal atom, the resonance light pair is transmitted between two center wavelengths at which the transmittance of the two light waves through the metal atom is a minimum value. Of the central wavelength.

In the quantum interference device according to this application example, the metal atom absorbs the resonant light pair when the center wavelength of the resonant light pair is λa and λb (exactly, when slightly shifted from these wavelengths). When the center wavelength of the resonant light pair is set to a wavelength between λa and λb, the EIT signal that appears at the output of the light detection unit due to the electromagnetically induced transmission (EIT) phenomenon is only in the respective absorption band. Considering the above, it becomes an asymmetrical shape opposite to each other, but by overlapping these, in reality, the shape becomes more symmetrical. Since the EIT signal becomes more symmetrical, even if the intensity of the resonant light pair decreases with time, the fluctuation of the peak position of the EIT signal becomes smaller, so the fluctuation of the frequency of the atomic oscillator becomes smaller and the long-term stability. Can be improved.

[Application Example 3]
In the quantum interference device according to the application example described above, the graph in which the output signal and the frequency of the light detection unit are plotted has a frequency band in which the shape of the graph is axisymmetric about the frequency at the maximum value. May be.

  According to the quantum interference device according to this application example, since the EIT signal has a frequency band that is line-symmetric with respect to the peak, the peak position of the EIT signal is maintained even if the intensity of the resonant light pair decreases with time. Because it does not fluctuate easily, high long-term stability of the atomic oscillator can be realized.

[Application Example 4]
The atomic oscillator according to this application example includes any of the quantum interference devices described above.

[Application Example 5]
The magnetic sensor according to this application example includes any of the quantum interference devices described above.

[Application Example 6]
The quantum interference device manufacturing method according to this application example includes a metal atom, a light source that generates light including two light waves and irradiates the metal atom, a light detection unit that detects light transmitted through the metal atom, A physics package preparation step of preparing a physics package including: a wavelength corresponding to an energy difference between the first ground level and the first excited level of the metal atom is λ1, from the first ground level The wavelength corresponding to the energy difference between the second higher ground level and the first excited level is λ2, and the second excited level higher than the first ground level and the first excited level. When the wavelength corresponding to the energy difference from the position is λ3 and the wavelength corresponding to the energy difference between the second ground level and the second excitation level is λ4, the center wavelength of the two light waves is ( Waves larger than λ3 + λ4) / 2 and smaller than (λ1 + λ2) / 2 And a center wavelength setting step for setting the length.

[Application Example 7]
In the method for manufacturing a quantum interference device according to the application example, in the center wavelength setting step, the frequency difference between the two light waves is swept around a frequency at which the metal atom causes an electromagnetically induced transmission phenomenon, The output signal of the light detection unit is acquired while sweeping the center wavelength of the two light waves in a given range that is larger than (λ3 + λ4) / 2 and smaller than (λ1 + λ2) / 2, and the center wavelength of the two light waves May be set so that the output signal of the light detection unit is close to line symmetry with the frequency at the maximum value as the center.

[Application Example 8]
In the manufacturing method of the quantum interference device according to the application example, in the center wavelength setting step, the frequency spectrum of the acquired output signal of the light detection unit is observed, and the second order of the sweep frequency obtained by sweeping the frequency difference between the two light waves. The value at the time when the difference between the component spectrum and the primary component spectrum becomes the largest may be the center wavelength of the two light waves.

The figure which shows the specific structural example of the atomic oscillator of this embodiment. FIG. 2A is a diagram showing energy levels obtained by Zeeman splitting, and FIG. 2B is a diagram showing an example of split EIT signals. Schematic which shows an example of the frequency spectrum of the emitted light of a semiconductor laser. Explanatory drawing of the detection principle by a detection circuit. Explanatory drawing of the detection principle by a detection circuit. Explanatory drawing about the relationship between the light transmittance (EIT signal intensity) and a lock frequency. Explanatory drawing about the relationship between the light transmittance (EIT signal intensity) and a lock frequency. Explanatory drawing about the relationship between the setting of the conventional center wavelength, and the symmetry of an EIT signal. Explanatory drawing about the relationship between the setting of the center wavelength of this embodiment, and the symmetry of an EIT signal. The flowchart figure which shows an example of the manufacturing method of the atomic oscillator of this embodiment. The figure which shows the structural example of the magnetic sensor of this embodiment. Schematic which shows the frequency spectrum of the emitted light of the semiconductor laser in a modification. The figure which shows the energy level of a cesium atom typically. Schematic which shows an example of an EIT signal. Explanatory drawing of an absorption band. Explanatory drawing of the absorption band including the effect by buffer gas. Explanatory drawing of the peak shift of an EIT signal.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The embodiments described below do not unduly limit the contents of the present invention described in the claims. Also, not all of the configurations described below are essential constituent requirements of the present invention.

1. Atomic oscillator [Structure of atomic oscillator]
FIG. 1 is a diagram illustrating a configuration example of the atomic oscillator according to the present embodiment. As shown in FIG. 1, the atomic oscillator 1 of the present embodiment includes a semiconductor laser 10, a neutral density filter (ND filter) 11, a ¼ wavelength plate 12, a gas cell 13, a photodetector 14, a magnetic field generator 15, detection. Circuit 16, voltage controlled crystal oscillator (VCXO) 17, modulation circuit 18, low frequency oscillator 19, frequency conversion circuit 20, detection circuit 21, modulation circuit 22, low frequency oscillator 23, drive circuit 24, magnetic field setting circuit 25, bias setting A circuit 26, a memory 27, and a frequency conversion circuit 28 are included. Note that the atomic oscillator 1 of the present embodiment may have a configuration in which some of the components (parts) in FIG. 1 are omitted or changed as appropriate, or other components are added.

  The semiconductor laser 10 (an example of a light source) is, for example, a surface emitting laser such as a vertical cavity surface emitting laser (VCSEL) or an edge emitting laser, and the semiconductor laser 10 is generated. The light thus made is incident on the neutral density filter 11.

  The neutral density filter 11 transmits only part of the light emitted from the semiconductor laser 10, and the light transmitted through the neutral density filter 11 enters the quarter-wave plate 12.

  The quarter-wave plate 12 transmits incident light as σ + circularly polarized light, and the light transmitted through the quarter-wave plate 12 enters the gas cell 13.

  The gas cell 13 is a container made of a transparent member such as glass, and neon (Ne) or argon (Ar) together with gaseous alkali metal atoms (sodium (Na) atoms, rubidium (Rb) atoms, cesium (Cs) atoms, etc.). ) And other buffer gases are enclosed. A part of the light incident on the gas cell 13 passes through the gas cell 13 and enters the photodetector 14.

  The light detector 14 (an example of a light detection unit) detects light transmitted through the gas cell 13 and outputs a detection signal corresponding to the detected light intensity. The light detection unit 14 can be realized using, for example, a photodiode (PD) that outputs a detection signal corresponding to the intensity of received light. The output signal of the photodetector 14 is input to the detection circuit 16 and the detection circuit 21.

  The magnetic field generator 15 generates a stationary magnetic field (uniform magnetic field) inside the gas cell 13. The magnetic field generator 15 can be realized by, for example, a coil, and a desired magnetic field can be achieved by adjusting the position and shape of the coil (for example, the direction in which the coil is wound, the number of turns, the diameter, etc.), the current magnitude and direction, and the like. Can be generated.

When this stationary magnetic field is applied to the gas cell 13, each energy level of the alkali metal atom is split into 2F + 1 pieces (Zeeman splitting). For example, in the case of a cesium atom, as shown in FIG. 2A, the ground level of 6S _1/2 and F = 3 and the excited level of 6P _3/2 and F ′ = 3 are represented by the magnetic quantum number mF = It splits into seven levels corresponding to 0, ± 1, ± 2, ± 3, and the 6S _1/2 , F ′ = 4 ground level and 6P _3/2 , F = 4 excited level are magnetic The quantum number is divided into nine levels corresponding to mF = 0, ± 1, ± 2, ± 3, ± 4. When σ + circularly polarized light is incident on the cesium atom, excitation is performed according to the selection rule of ΔmF = + 1. For example, the ground level of 6S _1/2 , F = 3, 4 and the excitation level of 6P _3/2 , F ′ = 4 Seven Λ-type three levels are formed between the positions. Therefore, in this state, when the frequency difference between the two light waves is swept, seven EIT signals are observed as shown in FIG. In particular, the Λ-type 3 level formed between the ground level of mF = 0 of 6S _1/2 , F = 3, 4 and the excitation level of mF = + 1 of 6P _3/2 , F ′ = 4 Since the intensity of the corresponding EIT signal is the highest, it is effective to control the frequency difference of the resonant light pair so as to generate this EIT signal.

  The semiconductor laser 10, the neutral density filter 11, the quarter-wave plate 12, the gas cell 13, the photodetector 14, and the magnetic field generator 15 are housed in one housing and constitute a physical package 100.

  The detection circuit 16 detects the output signal of the photodetector 14 using the oscillation signal of the low frequency oscillator 19 that oscillates at a low frequency of about several Hz to several hundred Hz. Then, the oscillation frequency of the voltage controlled crystal oscillator (VCXO) 17 is finely adjusted according to the magnitude of the output signal of the detection circuit 16. The voltage controlled crystal oscillator (VCXO) 17 oscillates at about several MHz to several tens of MHz, for example.

  The modulation circuit 18 uses the oscillation signal of the low frequency oscillator 19 (the same signal as the oscillation signal supplied to the detection circuit 16) as a modulation signal in order to enable detection by the detection circuit 16, and a voltage controlled crystal oscillator (VCXO) 17 Modulate the output signal. The modulation circuit 18 can be realized by a frequency mixer (mixer), a frequency modulation (FM) circuit, an amplitude modulation (AM) circuit, or the like.

  The frequency conversion circuit 20 converts the frequency of the output signal of the modulation circuit 18 at a constant frequency conversion rate and outputs the converted signal to the drive circuit 24. The frequency conversion circuit 20 can be realized by, for example, a PLL (Phase Locked Loop) circuit.

  The detection circuit 21 detects the output signal of the photodetector 14 using the oscillation signal of the low frequency oscillator 23 that oscillates at a low frequency of about several Hz to several hundred Hz.

  The modulation circuit 22 modulates the output signal of the detection circuit 21 using the oscillation signal of the low frequency oscillator 23 (the same signal as the oscillation signal supplied to the detection circuit 21) as a modulation signal in order to enable detection by the detection circuit 21. And output to the drive circuit 24. The modulation circuit 22 can be realized by a frequency mixer (mixer), a frequency modulation (FM) circuit, an amplitude modulation (AM) circuit, or the like.

  The magnetic field setting circuit 25 performs processing for setting the strength of the magnetic field generated by the magnetic field generator 15 according to the setting information stored in the memory 27. For example, the magnetic field generation unit 15 may be realized by a coil, and the magnetic field setting circuit 25 may set the amount of current flowing through the coil according to setting information stored in the memory 27.

  The bias setting circuit 26 (an example of the center wavelength setting unit) sets a bias current in the semiconductor laser 10 according to the setting information stored in the memory 27 (generated by the semiconductor laser 10) via the drive circuit 24. Processing for setting the center wavelength of light).

  The memory 27 is a non-volatile memory, and stores setting information for the intensity of the magnetic field generated by the magnetic field generator 15 and setting information for the bias current of the semiconductor laser 10. The memory 27 can be realized by, for example, a flash memory such as a MONOS (Metal-Oxide-Nitride-Oxide-Silicon) memory, an EEPROM (Electrically Erasable Programmable Read-Only Memory), or the like.

The drive circuit 24 sets the bias current of the semiconductor laser 10 and finely adjusts the bias current according to the output signal of the modulation circuit 22 and supplies the bias current to the semiconductor laser 10. That is, by a feedback loop (first feedback loop) passing through the semiconductor laser 10, the neutral density filter 11, the quarter wavelength plate 12, the gas cell 13, the photodetector 14, the detection circuit 21, the modulation circuit 22, and the drive circuit 24, The center wavelength λ ₀ (center frequency f ₀ ) of the light generated by the semiconductor laser 10 is finely adjusted.

Further, the drive circuit 24 superimposes the current (modulation current) of the output frequency component (modulation frequency f _m ) of the frequency conversion circuit 20 on the bias current, and supplies it to the semiconductor laser 10. With this modulation current, the semiconductor laser 10 is frequency-modulated, and with the light having the center frequency f ₀ (center wavelength λ ₀ ), the frequencies f ₀ ± f _m and f ₀ ± 2f are shifted from each other by f _m on both sides. _m ,... light is generated. The semiconductor laser 10, the neutral density filter 11, the quarter wave plate 12, the gas cell 13, the photodetector 14, the detection circuit 16, the voltage controlled crystal oscillator (VCXO) 17, the modulation circuit 18, the frequency conversion circuit 20, and the drive circuit the feedback loop through 24 (second feedback loop), the frequency f ₀ + f _m of the light and the frequency f ₀ -f light resonant light pair for generating EIT phenomenon in the alkali metal atom enclosed in the gas cell 13 of the _m It is finely adjusted so that

For example, the second feedback loop, an alkali metal atom with a ground level of the magnetic quantum number mF = 0 is to undergo EIT phenomenon, the modulation frequency f _m is feedback controlled. Specifically, the second feedback loop, the frequency difference of the light of the light and the frequency f ₀ -f _m of frequency f ₀ + f _m is a first-order sideband (= 2f _m) is magnetic alkali metal atom Feedback control is applied so as to accurately match the frequency ω ₁₂ corresponding to the energy difference ΔE ₁₂ between the two ground levels of the quantum number mf = 0. For example, if the alkali metal atom is a cesium atom, ω ₁₂ is 9.192631770 GHz + ΔHz (Δ is a frequency expressed by a quadratic function of the magnetic field strength), so the modulation frequency f _m exactly matches 4.59631585 GHz + Δ / 2 Hz. . FIG. 3 shows an example of the frequency spectrum of the emitted light from the semiconductor laser 10. In FIG. 3, the horizontal axis represents the light frequency, and the vertical axis represents the light intensity.

  As described above, by using the EIT phenomenon of alkali metal atoms, the output signal of the frequency conversion circuit 20 and the output signal of the voltage controlled crystal oscillator (VCXO) 17 included in the second feedback loop are respectively set to a predetermined frequency. It stabilizes at.

The frequency conversion circuit 28 converts the output signal of the voltage controlled crystal oscillator (VCXO) 17 at a constant frequency conversion rate, and generates a clock signal having a desired frequency (for example, 10.00... MHz). This clock signal is output externally. The frequency conversion circuit 20 can be realized by, for example, DDS (Direct Digital Synthesizer).

  In FIG. 1, the components (circuits) excluding the physical package 100 can be realized by, for example, a one-chip integrated circuit (IC).

  In FIG. 1, the semiconductor laser 10, the neutral density filter (ND filter) 11, the quarter wavelength plate 12, the gas cell 13, the photodetector 14, the magnetic field generator 15, the detection circuit 16, and the voltage controlled crystal oscillator (VCXO) 17, a modulation circuit 18, a low-frequency oscillator 19, a frequency conversion circuit 20, a detection circuit 21, a modulation circuit 22, a low-frequency oscillator 23, a drive circuit 24, a magnetic field setting circuit 25, a bias setting circuit 26, and a memory 27. 200 is configured. However, the quantum interference device 200 of the present embodiment may be configured such that some of the components (each unit) in FIG. 1 are omitted or changed as appropriate, or other components are added.

[Detection principle]
Next, the principle of detection by the detection circuit 16 will be described. As described above, in the present embodiment, the modulation circuit 18 uses the sine wave of about several tens Hz to several hundreds Hz generated by the low frequency oscillator 19 as the modulation signal, and uses the oscillation signal of the voltage controlled crystal oscillator (VCXO) 17 as the frequency. Modulated and input to the frequency conversion circuit 20. As a result, the frequency difference between the two light waves generated by the semiconductor laser 10 is swept within a range of about several hundred Hz to several kHz determined by the amplitude of the sine wave, and the output signal of the photodetector 14 is detected by the sine wave by the detection circuit 16. By performing synchronous detection, the area where the left and right areas of the EIT signal appearing in the output of the photodetector 14 are equal is detected as a peak. When the EIT signal is symmetric (line symmetric about the frequency at the maximum value), as shown in FIG. 4A, a sine wave (sweep signal) having a frequency of f _s (period is 1 / f _s ). When the zero cross points a, c, e coincide with the peak top of the EIT signal, the output signal of the photodetector 14 has a constant amplitude with a DC component and a frequency of 2 f _s (period is ½ f _s ). Are included, but the low frequency component having a frequency of f _s (period is 1 / f _s ) is extremely small. Therefore, the frequency component of f _s is hardly detected by the detection circuit 16. On the other hand, as shown in FIG. 4B, when the zero cross points a, c, e of the sine wave having the frequency f _s (period is 1 / f _s ) are shifted in a direction lower than the peak top of the EIT signal. , the output signal of the photodetector 14, the frequency (the period 1 / 2f _s) is 2f _s amplitude changes every 1 / f _s period in. That is, the output signal of the photodetector 14, in addition to the frequency component of the DC component and 2f _s, the frequency component of f _s are also included. Therefore, the frequency component of f _s is detected by the detection circuit 16, and the voltage value of the output signal of the detection circuit 16 is higher than the voltage value (reference voltage value) in the case of FIG. Since the output signal of the detection circuit 16 is input to the voltage controlled crystal oscillator (VCXO) 17, the oscillation frequency of the voltage controlled crystal oscillator (VCXO) 17 is higher (the direction in which the frequency difference between the two light waves approaches ω ₁₂ ). Change. On the other hand, although not shown, in the state where the zero cross points a, c, e of the sine wave having the frequency f _s (period is 1 / f _s ) are shifted in a direction higher than the peak top of the EIT signal, the photodetector The output signal 14 is a signal that is 180 degrees out of phase with the signal in FIG. Therefore, the voltage value of the output signal of the detection circuit 16 becomes negative (voltage value lower than the reference voltage value), and the oscillation frequency of the voltage controlled crystal oscillator (VCXO) 17 is low (the frequency difference between the two light waves approaches ω ₁₂₎ . Direction).

Due to such a detection principle, when the EIT signal is symmetrical, the peak top of the EIT signal is locked. On the other hand, when the EIT signal is asymmetrical, as shown in FIG. 5A, zero cross points a, c, e of a sine wave (sweep signal) having a frequency of f _s (period is 1 / f _s ) are EIT. in the state to match the signal of the peak top, the output signal of the photodetector 14, in addition to the frequency component of the DC component and 2f _s, the frequency component of f _s are also included. Therefore, the frequency component of f _s is detected by the detection circuit 16, and the voltage value of the output signal of the detection circuit 16 becomes a voltage value lower than the voltage value (reference voltage value) in the case of FIG. Further, as shown in FIG. 5B, the zero cross points a, c, e of the sine wave having a frequency of f _s (period is 1 / f _s ) are shifted by a predetermined amount in a direction lower than the peak top of the EIT signal. In this state, the output signal of the photodetector 14 includes a DC component and a low-frequency component having a constant amplitude with a frequency of 2 f _s (cycle is 1/2 f _s ), but the frequency is f _s (cycle is 1 / cycle). The low frequency component of f _s ) is very small. Therefore, the frequency component of f _s is hardly detected by the detection circuit 16 and is stabilized in this state. That is, when the EIT signal is asymmetrical, the position is shifted from the peak top of the EIT signal.

[Improved long-term stability of frequency]
Since the detection circuit 16 has the property of locking the frequency by regarding the area where the left and right areas of the EIT signal are equal as a peak, when the EIT signal is asymmetrically distorted, the intensity of the EIT signal decreases due to changes in the light source and the external environment. The frequency that locks may also shift. FIGS. 6A and 6B are examples of graphs in which the EIT signal (output signal of the photodetector 14)) and the lock frequency (frequency difference between the resonant light pairs) are plotted. 6 (A) and 6 (B), the vertical axis represents the transmittance of light transmitted through the gas cell 13 (the output signal intensity of the detector 14), and the horizontal axis represents the frequency (frequency difference between the resonant light pair). is there. For example, as shown in FIG. 6A, when the EIT signal is tilted to the left side (low frequency side), the lock frequency decreases as f ₁ → f ₂ → f ₃ as the intensity of the EIT signal decreases. To go. Conversely, as shown in FIG. 6B, when the EIT signal is tilted to the right side (high frequency side), the lock frequency increases as f ₁ → f ₂ → f ₃ as the intensity of the EIT signal decreases. To go. Therefore, if the EIT signal is asymmetric, the frequency varies with time and it is difficult to achieve high long-term stability.

On the other hand, if the EIT signal is symmetrical, as shown in FIG. 7, even if the intensity of the EIT signal decreases, the lock frequency remains unchanged at the initial frequency f ₁ . Therefore, if the EIT signal is symmetrical, the lock frequency does not fluctuate even after a long time, and high long-term stability can be realized.

As described above, the center wavelength of two light waves having a frequency difference slightly different from ω ₁₂ (frequency corresponding to the energy difference ΔE ₁₂ between the two base levels) by a predetermined amount is swept within a predetermined range. However, when the alkali metal atom is irradiated, two absorption bands are observed when the transmittance is observed. In the conventional atomic oscillator, as shown in FIG. 8A, the center wavelength λ ₀ of the resonant light pair is set to the bottom of the first absorption band having a large absorption of the two overlapping absorption bands. In this case, as shown in FIG. 8B, the EIT signal when only the first absorption band is taken into consideration is symmetric, and as shown in FIG. The EIT signal when considering only the absorption band is asymmetrical. Therefore, as shown in FIG. 8D, the actually observed EIT signal is a superposition of these two EIT signals, and thus becomes asymmetrical.

Therefore, in the atomic oscillator 1 of this embodiment, as shown in FIG. 9A, the center wavelength λ ₀ of the resonant light pair has a wavelength λa corresponding to the bottom (first minimum value) of the first absorption band. And a desired wavelength (frequency) between the wavelength λb corresponding to the bottom of the second absorption band (second minimum value). Specifically, the bias setting circuit 26 sets the center wavelength λ _{0 of the} light generated by the semiconductor laser 10 to a desired wavelength between λa and λb in accordance with the bias current setting information stored in the memory 27. To do. In this case, as shown in FIG. 9B, the EIT signal when considering only the first absorption band is tilted to the right and is asymmetrical, and as shown in FIG. The EIT signal when considering only the absorption band is tilted to the left and is asymmetrical. Therefore, as shown in FIG. 9 (D), the actually observed EIT signal is a superposition of these two EIT signals, so that it is more symmetrical than before, and can be made substantially symmetrical. Is possible. Since the EIT signal approaches left-right symmetry, fluctuations in the lock frequency over time can be reduced, so that the long-term stability of the atomic oscillator can be improved as compared with the prior art.

  Note that the wavelength corresponding to the energy difference between the F = 3 ground level (first ground level) of the alkali metal atom and the F ′ = 3 excited level (first excited level) is λ1, F A wavelength corresponding to the energy difference between the ground level (second ground level) = 4 and the excitation level of F ′ = 3 is λ2, and the ground level (first ground level) is F = 3. The wavelength corresponding to the energy difference from the excitation level of F ′ = 4 (second excitation level) is λ3, and corresponds to the energy difference between the ground level of F = 4 and the excitation level of F ′ = 4. When the wavelength is λ4, λa = (λ3 + λ4) / 2 and λb = (λ1 + λ2) / 2.

[Method of manufacturing an atomic oscillator]
FIG. 10 is a flowchart showing an example of a method for manufacturing the atomic oscillator 1 of the present embodiment.

  First, the physical package 100 shown in FIG. 1 is prepared (S10). For example, an existing physical package 100 may be prepared, or the semiconductor package 10, the neutral density filter 11, the quarter wavelength plate 12, the gas cell 13, the photodetector 14, and the magnetic field generator 15 may be prepared. May be assembled.

Next, in a state where the modulation frequency f _m was swept at low frequencies around the omega _12/2 of the semiconductor laser 10 (a state of being swept around the peak top of the EIT signal), the central wavelength lambda ₀ is λa and λb The bias current of the semiconductor laser 10 is swept so as to be swept within a predetermined range between (S20). Low frequencies sweeping the modulation frequency f _m may be brought into coincidence with the frequency of the low frequency oscillator 19 in FIG. 1. Signal to sweep at low frequencies the modulation frequency f _m around the omega _12/2, for example, can be generated by a signal generator.

Next, the frequency spectrum of the output signal of the light detector 14 observes, when the difference between the spectra of the primary component of the secondary component of the sweep frequency was swept modulation frequency f _m is largest in step S20 A bias current value is acquired (S30). When the EIT signal is swept at the peak top of the EIT signal according to the detection principle described above, if the EIT signal is symmetric, the primary component of the sweep signal is included in the output signal of the photodetector 14 as shown in FIG. It is hardly included. On the other hand, if the EIT signal is asymmetric, the primary component of the sweep signal is included in the output signal of the photodetector 14 as shown in FIG. The closer the EIT signal is to symmetry, the smaller this primary component. Therefore, it can be said that the EIT signal is closest to the target when the difference between the spectrum of the secondary component and the spectrum of the primary component is the largest. For example, a spectrum analyzer can be used to observe the frequency spectrum of the output signal of the photodetector 14. If the sweep frequency was swept modulation frequency f _m, for example 100Hz, to create a graph of the difference between the spectra of the 200Hz component of 100Hz component included in the output signal of the photodetector 14, when the difference is largest Get the bias current value.

  Next, the bias current value acquired in step S30 is written in the memory 27 of FIG. 1 (S40). As a result, the center wavelength λ0 is fixed at a wavelength at which the symmetry of the EIT signal is best between the wavelength λa corresponding to the bottom of the first absorption band and the wavelength λb corresponding to the bottom of the second absorption band. .

  Finally, the physical package 100 and the circuit (IC) are connected to assemble the atomic oscillator 1 (S50).

  As described above, according to the atomic oscillator of this embodiment, the symmetry of the EIT signal is better than before, so that long-term stability can be improved.

2. Magnetic Sensor When the intensity of the magnetic field around the gas cell 13 shown in FIG. 1 changes, the Zeeman splitting level in the ground level and the excited level of the alkali metal atoms accommodated in the gas cell 13 changes. In accordance with the change in the Zeeman splitting level, the frequency difference ω ₁₂ of the resonant light pair that causes the EIT phenomenon in the alkali metal atom also changes. This ω ₁₂ is known to be proportional to the square of the magnetic field strength B. Since the frequency of the voltage controlled crystal oscillator (VCXO) 17 is proportional to ω ₁₂ , the magnetic field strength can be calculated from the frequency of the voltage controlled crystal oscillator (VCXO) 17. Therefore, a magnetic sensor can be realized by arranging a magnetic measurement object in the vicinity of the gas cell 13.

  FIG. 11 is a diagram illustrating a configuration example of the magnetic sensor of the present embodiment. In FIG. 11, the same components as those in FIG. As shown in FIG. 11, in the magnetic sensor 2 of the present embodiment, the frequency conversion circuit 28 of the atomic oscillator 1 shown in FIG. 1 is replaced with a magnetic field strength information generation circuit 29, and other configurations are shown in FIG. Similar to the atomic oscillator 1.

  The magnetic field strength information generation circuit 29 measures the oscillation frequency of the voltage controlled crystal oscillator (VCXO) 17 and generates magnetic field strength information indicating the strength of the external magnetic field based on the measurement result. Since the stationary magnetic field is applied to the gas cell 13 by the magnetic field generator 15, for example, information on the oscillation frequency of the voltage controlled crystal oscillator (VCXO) 17 when the external magnetic field is 0 (only the stationary magnetic field) is stored in the memory 27 in advance. The magnetic field strength information generation circuit 29 may store the measured frequency with the oscillation frequency stored in the memory 27 to calculate the strength of the external magnetic field. Alternatively, a correspondence table between the oscillation frequency of the voltage controlled crystal oscillator (VCXO) 17 and the intensity of the external magnetic field is stored in the memory 27, and the magnetic field strength information generation circuit 29 refers to the correspondence table and sets the measured frequency to the measured frequency. You may obtain | require the intensity | strength of a corresponding external magnetic field by complementary calculation etc.

  The other configuration of the magnetic sensor 2 shown in FIG. 11 is the same as that of the atomic oscillator 1 shown in FIG.

3. The present invention is not limited to this embodiment, and various modifications can be made within the scope of the present invention.

[Modification 1]
The atomic oscillator and the magnetic sensor of the present embodiment, the first feedback loop, the light (frequency f ₀ + f _m of the two first-order sideband included in the output light of the semiconductor laser 10 light and the frequency f ₀ -f _m like the light) becomes resonant light pair for generating EIT phenomenon in the alkali metal atom, ie control as the modulation frequency f _m is equal to omega _12/2 is applied, not limited to this. For example, FIG. 12 (A) and 12 (B), the light of one primary sideband (frequency f ₀ + f _m of the light or the frequency f ₀ -f light _m) and the central wavelength lambda ₀ ( Control may be performed so that light of frequency f ₀ ) forms a resonant light pair, that is, modulation frequency f _m matches ω ₁₂ . In this case, the bias setting circuit 26 may set the bias current of the semiconductor laser 10 so that the intermediate wavelength of the resonant light pair is a wavelength between λa and λb.

[Modification 2]
The atomic oscillator and the magnetic sensor of the present embodiment may be modified to a configuration using an electro-optic modulator (EOM). That is, the semiconductor laser 10 is not modulated by the output signal (modulation signal) of the frequency conversion circuit 20, and generates light having a single wavelength λ ₀ (frequency f ₀ ) corresponding to the set bias current. The light having the wavelength λ ₀ enters an electro-optic modulator (EOM) and is modulated by an output signal (modulation signal) of the frequency conversion circuit 20. As a result, light having a frequency spectrum similar to that in FIG. 3 can be generated. Then, the gas cell 13 is irradiated with light generated by the electro-optic modulator (EOM). In this atomic oscillator and magnetic sensor, a light source is constituted by the semiconductor laser 10 and an electro-optic modulator (EOM). Note that an acousto-optic modulator (AOM) may be used instead of the electro-optic modulator (EOM).

[Modification 3]
In the atomic oscillator and the magnetic sensor of the present embodiment, two primary sideband lights included in the light emitted from one semiconductor laser 10 are used as resonance light pairs. Wavelength light may be generated and used as a resonant light pair. Also in this case, the bias currents of the two semiconductor lasers may be set so that the intermediate wavelength of the resonant light pair is a wavelength between λa and λb.

  Note that the quantum interference device of the present embodiment can be applied to applications other than atomic oscillators and magnetic sensors. For example, since the quantum interference state (quantum coherence state) of alkali metal atoms can be created with the configuration of the quantum interference device of this embodiment, a quantum computer can be obtained by extracting the resonant light pair incident on the gas cell 13. It is also possible to realize a light source used for quantum information equipment such as a quantum memory and a quantum cryptography system.

  The above-described embodiments and modifications are merely examples, and the present invention is not limited to these. For example, it is possible to appropriately combine each embodiment and each modification.

  The present invention includes configurations that are substantially the same as the configurations described in the embodiments (for example, configurations that have the same functions, methods, and results, or configurations that have the same objects and effects). In addition, the invention includes a configuration in which a non-essential part of the configuration described in the embodiment is replaced. In addition, the present invention includes a configuration that exhibits the same operational effects as the configuration described in the embodiment or a configuration that can achieve the same object. Further, the invention includes a configuration in which a known technique is added to the configuration described in the embodiment.

DESCRIPTION OF SYMBOLS 1 Atomic oscillator, 2 Magnetic sensor, 10 Semiconductor laser, 11 Neutral filter, 12 1/4 wavelength plate, 13 Gas cell, 14 Photo detector, 15 Magnetic field generation part, 16 Detection circuit, 17 Voltage control crystal oscillator (VCXO), 18 modulation circuit, 19 low frequency oscillator, 20 frequency conversion circuit, 21 detection circuit, 22 modulation circuit, 23 low frequency oscillator, 24 drive circuit, 25 magnetic field setting circuit, 26 bias setting circuit, 27 memory, 28 frequency conversion circuit, 29 Magnetic field strength information generation circuit, 100 physical package, 200 quantum interference device

Claims (8)

Metal atoms,
A light source that emits light including a resonant light pair that generates an electromagnetically induced transmission phenomenon in the metal atom;
A light detection unit for detecting the light transmitted through the metal atom,
The wavelength corresponding to the energy difference between the first ground level and the first excited level of the metal atom is λ1, the second ground level higher than the first ground level, and the first ground level A wavelength corresponding to an energy difference from an excitation level is λ2, a wavelength corresponding to an energy difference between the first ground level and a second excitation level higher than the first excitation level is λ3, When the wavelength corresponding to the energy difference between the second ground level and the second excitation level is λ4, the center wavelength of the resonant light pair is larger than (λ3 + λ4) / 2 and (λ1 + λ2 ) / 2, a quantum interference device. Metal atoms,
A light source that emits light including a resonant light pair that generates an electromagnetically induced transmission phenomenon in the metal atom;
A light detection unit for detecting the light transmitted through the metal atom,
When the two light waves having different frequencies are irradiated to the metal atom, the center wavelength of the resonant light pair is between two center wavelengths at which the transmittance of the two light waves to pass through the metal atom is a minimum value. Quantum interference device.   3. The quantum interference device according to claim 1, wherein in the graph plotting the output signal and the frequency of the light detection unit, the quantum interference device has a frequency band in which the shape of the graph is line-symmetric about the frequency at the maximum value. .   An atomic oscillator including the quantum interference device according to claim 1.   The magnetic sensor containing the quantum interference apparatus as described in any one of Claims 1 thru | or 3. A physical package preparation step of preparing a physical package including a metal atom, a light source that generates light including two light waves and irradiates the metal atom, and a light detection unit that detects light transmitted through the metal atom;
The wavelength corresponding to the energy difference between the first ground level and the first excited level of the metal atom is λ1, the second ground level higher than the first ground level, and the first ground level A wavelength corresponding to an energy difference from an excitation level is λ2, a wavelength corresponding to an energy difference between the first ground level and a second excitation level higher than the first excitation level is λ3, When the wavelength corresponding to the energy difference between the second ground level and the second excitation level is λ4, the center wavelength of the two light waves is larger than (λ3 + λ4) / 2 and (λ1 + λ2) A center wavelength setting step for setting a wavelength smaller than / 2,
A method for manufacturing a quantum interference device. In the center wavelength setting step,
The center wavelength of the two light waves is larger than (λ3 + λ4) / 2 and (λ1 + λ2) in a state where the frequency difference between the two light waves is swept around the frequency at which the metal atom causes an electromagnetically induced transmission phenomenon. The output signal of the light detection unit is acquired while sweeping in a given range smaller than / 2, and the center wavelength of the two light waves is centered on the frequency when the output signal of the light detection unit is a maximum value. The method for manufacturing a quantum interference device according to claim 6, wherein the quantum interference device is set so as to approach line symmetry. In the center wavelength setting step,
The frequency spectrum of the acquired output signal of the light detection unit is observed, and the value when the difference between the spectrum of the secondary component of the sweep frequency obtained by sweeping the frequency difference of the two light waves and the spectrum of the primary component becomes the largest is obtained. The method of manufacturing a quantum interference device according to claim 7, wherein the center wavelength of the two light waves is used.

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