Abstract:
A quantum interference device causing electromagnetically induced transparency in an alkali metal atom includes: a light source generating first and second resonant lights with frequency differences Δω; a magnetic field generator applying a magnetic field to the atom; a light detector detecting intensities of the first and second resonant lights passing through the atom; and a controller causing a frequency difference between specified first and second resonant lights to equal a frequency difference corresponding to an energy difference between two ground levels of the atom based on the detected light. The controller causes the frequency Δω or magnetic field intensity to satisfy 2×δ×n=Δω or Δω×n=2×δ. The frequency δ corresponds to an energy difference between two Zeeman split levels differentiated by one magnetic quantum number and generated in the two ground levels of the atom by energy splitting.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of U.S. Ser. No. 12/872,012 filed Aug. 31, 2010 which claims priority to Japanese Patent Application No. 2009-205643, filed Sep. 7, 2009 all of which are expressly incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a quantum interference device, an atomic oscillator and a magnetic sensor. 
     2. Related Art 
     An atomic oscillator based on an electromagnetically induced transparency (EIT) system (also called a coherent population trapping (CPT) system) is an oscillator using a phenomenon (EIT phenomenon) in which when two resonant lights different from each other in wavelength (frequency) are simultaneously irradiated to an alkali metal atom, the absorption of the two resonant lights is stopped. It is known that the interaction mechanism between the alkali metal atom and the two resonant lights can be explained in a Λ-type 3-level system model. The alkali metal atom has two ground levels, and when resonant light  1  having a wavelength (frequency) corresponding to an energy difference between the ground level  1  and the excited level or resonant light  2  having a wavelength (frequency) corresponding to an energy difference between the ground level  2  and the excited level are individually irradiated to the alkali metal atom, light absorption occurs as is well known. However, when the resonant light  1  and the resonant light  2  in which the frequency difference accurately coincides with the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  are simultaneously irradiated to the alkali metal atom, a superposition state of the two ground levels, that is, a quantum interference state occurs, the excitation to the excited level is stopped, and the transparency (EIT) phenomenon occurs in which the resonant light  1  and the resonant light  1  pass through the alkali metal atom. This phenomenon is used and an oscillator with high accuracy can be formed by detecting and controlling the abrupt change of light absorption behavior when the frequency difference between the resonant light  1  and the resonant light  2  shifts from the frequency corresponding to the energy difference between the ground level  1  and the ground level  2 . Besides, since the energy difference between the ground level  1  and the ground level  2  is sensitively changed by the intensity or fluctuation of external magnetic field, a highly sensitive magnetic sensor can also be formed by using the EIT phenomenon. 
     In the atomic oscillator or the magnetic sensor, in order to improve the signal to noise ratio (S/N ratio) of the output signal, the number of alkali metal atoms that cause the EIT phenomenon has only to be increased. For example, JP-A-2004-96410 (patent document 1) discloses a method in which in order to improve the S/N ratio of an output signal of an atomic oscillator, the thickness of a cell in which gaseous alkali metal atoms are confined is increased, or the beam diameter of a laser beam incident on the cell is increased. In either method, in order to widen an area where the alkali metal atoms are irradiated with the resonant light, the thickness or the height of the cell is increased. Besides, U.S. Pat. No. 6,359,916 (patent document 2) proposes an atomic oscillator in which D1 line is used as a light source, so that the intensity of an EIT signal (signal of light passing through the alkali metal atom by the EIT phenomenon) is improved theoretically as compared with the related art case of D2 line, and the sensitivity and frequency stability accuracy is improved by this. In the atomic oscillator disclosed in the patent document 1 or the patent document 2, only one pair of two kinds of laser lights satisfying the occurring condition of the EIT phenomenon is used. 
     When attention is paid to individual atoms constituting a group of gaseous alkali metal atoms in the cell, they have a certain velocity distribution corresponding to the respective motion states.  FIG. 15  is a schematic view of the velocity distribution of a gaseous alkali metal atom group confined in a container. The horizontal axis of  FIG. 15  indicates the velocity of a gaseous alkali metal atom, and the vertical axis indicates the ratio of the number of gaseous alkali metal atoms having the velocity. As shown in  FIG. 15 , the gaseous alkali metal atoms have the certain velocity distribution corresponding to temperature, the center of which is the velocity of 0. Here, the velocity means an atom velocity component parallel to the irradiation direction when a laser beam is irradiated to the gaseous alkali metal atom group, and the value of the velocity of the atom at rest relative to a light source is 0. As stated above, when the velocity of the gaseous alkali metal atoms has the distribution, by the light Doppler effect (Doppler shift), the apparent wavelength (frequency) of the resonant light, that is, the wavelength (frequency) of the resonant light when viewed from the gaseous alkali metal atom has a distribution. This means that in atoms different in velocity, the excited levels are seemingly different. As shown in  FIG. 16 , the excited level has the broadening of certain width (Doppler broadening). Accordingly, even if a pair of the resonant light  1  and the resonant light  2  are simultaneously irradiated, only a very small part of the atoms having a specific velocity component value (for example, 0) with respect to the incident direction of the laser can actually cause the EIT phenomenon. A considerable number of gaseous alkali metal atoms which do not cause the EIT phenomenon and remain exist in the group, and the ratio of atoms contributing to the EIT phenomenon is very low. In order to increase the intensity of the EIT signal in the state where the EIT occurrence efficiency is low as described above, the thickness or the height of the cell must be increased as disclosed in the patent document 1, and there is a problem that miniaturization is difficult. Besides, in the state where the EIT occurrence efficiency is low, since the use efficiency of laser power is low, when the intensity of the EIT signal is kept at a certain level or higher, it is difficult to reduce the laser power, and there is a disadvantage also in power saving. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a quantum interference device in which the occurrence efficiency of an EIT phenomenon in alkali metal atoms is raised and the use efficiency of laser power can be improved, and to provide a small atomic oscillator or a magnetic sensor by using this. 
     According to an aspect of the invention, there is provided a quantum interference device that causes an electromagnetically induced transparency phenomenon to occur in an alkali metal atom by a resonant light pair including a first resonant light and a second resonant light. The quantum interference device includes a light source to generate a plurality of the first resonant lights different from each other in frequency by Δω and a plurality of the second resonant lights different from each other in frequency by Δω, a magnetic field generation unit that applies a magnetic field to the alkali metal atom, a light detection unit that detects intensities of lights including the first resonant lights and the second resonant lights passing through the alkali metal atom, and a control unit that controls to cause a frequency difference between the specified first resonant light and the specified second resonant light to become equal to a frequency difference corresponding to an energy difference between two ground levels of the alkali metal atom based on a detection result of the light detection unit. The control unit controls at least one of the frequency Δω and intensity of the magnetic field generated by the magnetic field generation unit to satisfy at least one of 2×δ×n=Δω and Δω×n=2×δ (n is a positive integer) with respect to a frequency δ corresponding to an energy difference between two Zeeman split levels different from each other in magnetic quantum number by one among a plurality of Zeeman split levels generated in each of the two ground levels of the alkali metal atom by energy splitting due to the magnetic field. 
     According to the aspect of the invention, the plurality of first resonant lights  1  different from each other in frequency by Δω and the plurality of second resonant lights  2  different from each other in frequency by Δω are generated so that the frequency difference between the specified first resonant light and the specified second resonant light becomes equal to the frequency difference corresponding to the energy difference between the two ground levels of the alkali metal atom. Accordingly, when the frequency of the first resonant light is f 1 , the frequency of the second resonant light is f 2 , and the frequency corresponding to the energy difference between the two ground levels of the alkali metal atom is f E , with respect to the resonant light  1  of f 1 =ω 1  and the resonant light  2  of f 2 =ω 2 , when the relation of f 1 −f 2 =f E  is satisfied, the relation of f 1 −f 2 =f E  is satisfied also for resonant light  1  of f 1 =ω 1 +k×Δω and the resonant light  2  of f 2 =ω 2 +k×Δω (k is a positive integer), and the relation of f 1 −f 2 =f E  is satisfied also for the resonant light  1  of f 1 =ω 1 −k×Δω and the resonant light  2  of f 2 =ω 2 −k×Δω (k is a positive integer). Thus, an alkali metal atom whose velocity is not 0 can also cause the EIT phenomenon, and the ratio of the number of the alkali metal atoms that cause the EIT phenomenon can be increased as compared with the related art. 
     Further, according to the aspect of the invention, the control is performed so that at least one of 2×δ×n=Δω and Δω×n=2×δ (n is a positive integer) is satisfied. Thus, the alkali metal atom in which the magnetic quantum number is not 0 can also cause the EIT phenomenon, and the ratio of the number of alkali metal atoms that cause the EIT phenomenon can be greatly increased as compared with the related art. 
     Accordingly, according to this aspect of the invention, the quantum interference device can be provided in which the occurrence efficiency of the EIT phenomenon in the alkali metal atoms is raised, and the use efficiency of the laser power can be improved. 
     The quantum interference device of the aspect of the invention may be configured such that the control unit controls the frequency Δω based on the detection result of the light detection unit. 
     For example, the control unit may feedback control the frequency Δω so that the detection result of the light detection unit becomes maximum. 
     The quantum interference device of the aspect of the invention may be configured such that the control unit controls the frequency Δω based on the intensity of the magnetic field near the alkali metal atom. 
     For example, the control unit includes a magnetic field detection unit that detects the intensity of the magnetic field near the alkali metal atom, and may control the frequency Δω based on the detection result of the magnetic field detection unit. 
     Besides, for example, the control unit may include a magnetic field control unit that controls so that the magnetic field generation unit generates the magnetic field of a previously determined intensity, and may control the frequency Δω according to the magnetic field of the previously determined intensity. 
     The quantum interference device of the aspect of the invention may be configured such that the control unit includes a magnetic field control unit that controls the intensity of the magnetic field generated by the magnetic field generation unit according to the previously determined frequency Δω. 
     The quantum interference device of the aspect of the invention may be configured such that n=1 is established. That is, the control unit may control to satisfy a relation of Δω=2δ. 
     With this configuration, since the occurrence efficiency of the EIT phenomenon in the alkali metal atoms can be raised to the maximum, the use efficiency of the laser power can be extracted to the utmost. 
     According to another aspect of the invention, there is provided an atomic oscillator including the quantum interference device. 
     According to still another aspect of the invention, there is provided a magnetic sensor including the quantum interference device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of a quantum interference device of an embodiment. 
         FIG. 2  is a view showing a structure of an atomic oscillator of a first embodiment. 
         FIG. 3  is a view for explaining a relation between a Zeeman split ground level of a cesium atom and a resonant light. 
         FIGS. 4A and 4B  are views showing examples of a frequency spectrum of laser light emitted by a semiconductor laser in the embodiment. 
         FIG. 5  is a view for explaining an example of a combination of two resonant lights in which the cesium atom causes an EIT phenomenon. 
         FIG. 6  is a view for explaining an example of a combination of two resonant lights in which the cesium atom causes the EIT phenomenon. 
         FIG. 7  is a view for explaining another example of a combination of two resonant lights in which the cesium atom causes the EIT phenomenon. 
         FIG. 8  is a view for explaining another example of a combination of two resonant lights in which the cesium atom causes the EIT phenomenon. 
         FIG. 9  is a view showing a structure of an atomic oscillator of a second embodiment. 
         FIG. 10  is a view showing a structure of a modified example of the atomic oscillator of the second embodiment. 
         FIG. 11  is a view showing a structure of an atomic oscillator of a third embodiment. 
         FIG. 12  is a view showing a structure of a modified example of the atomic oscillator of the third embodiment. 
         FIG. 13  is a view showing a structure of a magnetic sensor of a first embodiment. 
         FIG. 14  is a view showing a structure of a magnetic sensor of a second embodiment. 
         FIG. 15  is a schematic view of a velocity distribution of a gaseous alkali metal atom group. 
         FIG. 16  is a view schematically showing an energy level in view of a Doppler effect by the motion of an alkali metal atom. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described in detail with reference to the drawings. Incidentally, the embodiments described below do not improperly limit the content of the invention recited in the claims. Besides, all components described below are not necessarily inevitable components of the invention. 
     1. Quantum Interference Device 
       FIG. 1  is a functional block diagram of a quantum interference device of an embodiment. 
     The quantum interference device  1  of the embodiment includes a light source  10 , an atomic cell  20 , a magnetic field generation unit  30 , a light detection unit  40  and a control unit  50 . 
     The light source  10  generates plural first resonant lights (resonant light  1 ) different from each other in frequency by Δω, and plural second resonant lights (resonant light  2 ) different from each other in frequency by Δω. 
     The atomic cell  20  contains gaseous alkali metal atoms, and the magnetic field generation unit  30  applies a magnetic field to the alkali metal atoms. 
     The light detection unit  40  detects intensities of lights passing through the alkali metal atom and including the resonant light  1  and the resonant light  2 . 
     Based on the detection result of the light detection unit  40 , the control unit  50  controls so that a frequency difference between a specified first resonant light and a specified second resonant light is equal to a frequency difference corresponding to an energy difference between two ground levels of the alkali metal atom. Further, the control unit  50  controls at least one of the frequency Δω and the intensity of the magnetic field generated by the magnetic field generation unit so that at least one of 2×δ×n=Δω and Δω×n=2×δ (n is a positive integer) is satisfied with respect to a frequency δ corresponding to an energy difference between two Zeeman split levels different from each other in magnetic quantum number by one among plural Zeeman split levels generated in each of the two ground levels of the alkali metal atom by energy splitting due to the magnetic field. In order to maximize the intensity of the EIT signal, it is preferable that the control unit  50  controls to satisfy the relation of n=1, that is, Δω=2δ. 
     Besides, the control unit  50  may include a magnetic field control unit  52  that controls the intensity of the magnetic field generated by the magnetic field generation unit  30  according to the previously determined frequency Δω. 
     Besides, the control unit  50  may control the frequency Δω based on the detection result of the light detection unit  40 . For example, the control unit  50  may feedback control the frequency Δω so that the detection result of the light detection unit  40  becomes maximum. 
     Besides, the control unit  50  may control the frequency Δω based on the intensity of the magnetic field near the alkali metal atom. For example, the control unit  50  includes a magnetic field detection unit  54  that detects the intensity of the magnetic field near the alkali metal atom, and may control the frequency Δω based on the detection result of the magnetic field detection unit  54 . Besides, for example, the control unit  50  controls so that the magnetic field generation unit  30  generates a magnetic field of a previously determined intensity by the magnetic field control unit  52 , and may control the frequency Δω according to the previously determined intensity of the magnetic field. 
     Hereinafter, a more specific structure will be described while an atomic oscillator and a magnetic sensor, which use the quantum interference device of the embodiment, are used as examples. 
     2. Atomic Oscillator 
     (1) First Embodiment 
       FIG. 2  is a view showing a structure of an atomic oscillator of a first embodiment using the quantum interference device of  FIG. 1 . 
     As shown in  FIG. 2 , the atomic oscillator  100  of the first embodiment includes a center wavelength control unit  110 , a semiconductor laser  120 , an atomic cell  130 , a magnetic field generation unit  140 , a light detector  150 , an amplifier  160 , a detection unit  170 , a modulation unit  180 , an oscillator  190 , a detection unit  200 , an oscillator  210 , a modulation unit  220 , an oscillator  230 , a frequency conversion unit  240 , a detection unit  250 , an oscillator  260 , a modulation unit  270 , an oscillator  280  and a modulation unit  290 . 
     The atomic cell  130  is such that gaseous alkali metal atoms (sodium (Na) atoms, rubidium (Rb) atoms, cesium (Cs) atom, etc.) are enclosed in a container. In this embodiment, a magnetic field is applied to the atomic cell  130  by the magnetic field generation unit  140  (for example, a coil). Thus, each of two ground levels of the alkali metal atom is divided into plural levels (Zeeman split levels) different from each other in magnetic quantum number m by energy splitting (Zeeman splitting) due to the magnetic field. Here, it is known that the number of Zeeman split levels varies according to the kind of the alkali metal atom. For example, in the case of the cesium atom, as shown in  FIG. 3 , the ground level  1  is divided into seven Zeeman split levels with magnetic quantum number m=−3, −2, −1, 0, +1, +2, +3 in descending order of energy. On the other hand, the ground level  2  is divided into nine Zeeman split levels with magnetic quantum number m=+4, +3, +2, +1, 0, −1, −2, −3, −4 in descending order of energy. Here, in both the ground level  1  and the ground level  2 , an energy difference E δ  between two Zeeman split levels different from each other in magnetic quantum number m by one is equal. The energy difference E δ  is changed according to the intensity of the magnetic field. That is, as the intensity of the magnetic field becomes large, the energy difference E δ  becomes large. Incidentally, in the following description, a frequency corresponding to the E δ  is δ. 
     When a resonant light pair (resonant light  1 , resonant light  2 ) having a frequency difference corresponding to an energy difference between two ground levels of the alkali metal atom are simultaneously irradiated to the atomic cell  130 , the alkali metal atom causes the EIT phenomenon. Incidentally, in the following description, the frequencies of the resonant light  1  and the resonant light  2  are respectively denoted by f 1  and f 2 . 
       FIG. 3  shows an example showing a case where the Λ-type 3-level system is established when the magnetic quantum number of the ground level  1  and that of the ground level  2  are equal to each other (common m). In the cesium atom with magnetic quantum number m=0, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz, when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz are simultaneously irradiated, the EIT phenomenon occurs. Besides, in the cesium atom with magnetic quantum number m=+1, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz+2δ, when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz+2δ are simultaneously irradiated, the EIT phenomenon occurs. Besides, in the cesium atom with magnetic quantum number m=−1, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz−2δ, when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz−2δ are simultaneously irradiated, the EIT phenomenon occurs. Similarly, in the respective cesium atoms with magnetic quantum numbers m=+2, +3, −2, −3, when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz+4δ, f 1 −f 2 =9.1926 GHz+6δ, f 1 −f 2 =9.1926 GHz−4δ, f 1 −f 2 =9.1926 GHz−6δ are respectively simultaneously irradiated, the EIT phenomenon occurs. Incidentally, the cesium atom with magnetic quantum number m=+4 or −4, since the ground level  1  does not exist, the EIT phenomenon does not occur. 
     In this embodiment, the semiconductor laser  120  emits the laser light including plural resonant light pairs and irradiates them to the alkali metal atoms so that the alkali metal atoms enclosed in the atomic cell  130  efficiently cause the EIT phenomenon. Specifically, with respect to the laser light emitted by the semiconductor laser  120 , its center wavelength λ 0  is controlled based on the output of the center wavelength control unit  110 , and modulation is applied based on the output of the modulation unit  290 . For example, when a laser driver to supply a drive current to the semiconductor laser  120  is used as the center wavelength control unit  110 , an AC current outputted by the modulation unit  290  is superimposed on the drive current, so that the laser light emitted by the semiconductor laser  120  can be modulated. In this embodiment, as described later, the output of the modulation unit  290  is feedback controlled so that the light corresponding to the modulation component becomes the resonant light  1  or the resonant light  2  to the alkali metal atom. 
     Incidentally, the semiconductor laser  120  may be an edge emitting laser, or a surface emitting laser such as a vertical cavity surface emitting laser (VCSEL). 
     The light detector  150  detects the light passing through the atomic cell  130 , and outputs the signal having the signal intensity corresponding to the amount of the detected light. The output signal of the light detector  150  is amplified by the amplifier  160 , and is inputted to the detection unit  170 , the detection unit  200  and the detection unit  250 . 
     The detection unit  170  synchronously detects the output signal of the amplifier  160  by the oscillation signal of the oscillator  190 . The modulation unit  180  modulates the output signal of the detection unit  170  by the oscillation signal of the oscillator  190 . The oscillator  190  is oscillated at a low frequency of, for example, several tens Hz to several hundreds Hz. The center wavelength control unit  110  controls the center wavelength λ 0  of the laser light emitted by the semiconductor laser  120  according to the output signal of the modulation unit  180 . The center wavelength λ 0  is stabilized by a feedback loop passing through the semiconductor laser  120 , the atomic cell  130 , the light detector  150 , the amplifier  160 , the detection unit  170 , the modulation unit  180  and the center wavelength control unit  110 . 
     The detection unit  200  synchronously detects the output signal of the amplifier  160  by the oscillation signal of the oscillator  230 . The oscillator  210  is the oscillator in which the oscillation frequency is changed according to the magnitude of the output signal of the detection unit  200 , and can be realized by, for example, a voltage controlled crystal oscillator (VCXO). Here, the oscillator  210  oscillates at, for example, about 10 MHz, and the oscillation signal becomes the output signal of the atomic oscillator  100 . The modulation unit  220  modulates the output signal of the oscillator  210  by the oscillation signal of the oscillator  230 . The oscillator  230  is oscillated at a low frequency of, for example, several tens Hz to several hundreds Hz. 
     The frequency conversion unit  240  converts the output signal of the modulation unit  220  into a signal of a frequency equal to ½ (in the case of the cesium atom, 9.1926 GHz/2=4.5963 GHz) of a frequency difference corresponding to an energy difference between the two ground levels of the alkali metal atom with magnetic quantum number m=0 enclosed in the atomic cell  130 . The frequency conversion unit  240  can be realized by, for example, a phase locked loop (PLL) circuit. Incidentally, as described later, the frequency conversion unit  240  may convert the output signal of the modulation unit  220  into a signal of a frequency equal to the frequency difference (9.1926 GHz in the case of the cesium atom) corresponding to the energy difference between the two ground levels of the alkali metal atom with magnetic quantum number m=0 enclosed in the atomic cell  130 . 
     The detection unit  250  synchronously detects the output signal of the amplifier  160  by the oscillation signal of the oscillator  280 . The oscillator  260  is the oscillator in which the oscillation frequency is changed according to the magnitude of the output signal of the detection unit  250 , and can be realized by, for example, a voltage controlled crystal oscillator (VCXO). Here, the oscillator  260  oscillates at a frequency Δω (for example, about 1 MHz to 10 MHz) sufficiently smaller than a frequency corresponding to the width of Doppler broadening of the excited level of the alkali metal atom enclosed in the atomic cell  130 . The modulation unit  270  modulates the output signal of the oscillator  260  by the oscillation signal of the oscillator  280 . The oscillator  280  is oscillated at a low frequency of, for example, several tens Hz to several hundreds Hz. 
     The modulation unit  290  modulates the output signal of the frequency modulation unit  240  by the output signal of the modulation unit  270  (the output signal of the modulation unit  270  may be modulated by the output signal of the frequency conversion unit  240 ). The modulation unit  290  can be realized by a frequency mixing unit (mixer), a frequency modulation (FM) circuit, an amplitude modulation (AM) circuit or the like. As described before, the laser light emitted by the semiconductor laser  120  is modulated based on the output of the modulation unit  290 , and the plural resonant lights  1  and the plural resonant lights  2  are generated. 
     In the atomic oscillator having the structure as described above, unless the frequency difference between the resonant light  1  and the resonant light  2  emitted by the semiconductor laser  120  accurately coincides with the frequency corresponding to the energy difference between the two ground levels of the alkali metal atom, the alkali metal atom does not cause the EIT phenomenon. Thus, the detection amount of the light detector  150  changes very sensitively in response to the frequency of the resonant light  1  and that of the resonant light  2 . Thus, by a feedback loop passing through the semiconductor laser  120 , the atomic cell  130 , the light detector  150 , the amplifier  160 , the detection unit  200 , the oscillator  210 , the modulation unit  220 , the frequency conversion unit  240  and the modulation unit  290 , the feedback control is performed so that the frequency of the output signal of the frequency conversion unit  240  very accurately coincides with the frequency equal to ½ of the frequency difference corresponding to the energy difference between the two ground levels of the alkali metal atom with magnetic quantum number m=0. As a result, the oscillator  210  existing in this feedback loop also oscillates at a very stable oscillation frequency, and the frequency accuracy of the output signal of the atomic oscillator  100  can be made very high. 
     However, even if the frequency of the output signal of the frequency conversion unit  240  very accurately coincides with the frequency equal to ½ of the frequency difference corresponding to the energy difference between the two ground levels of the alkali metal atom with magnetic quantum number m=0, as described later, unless at least the relation of 2×δ×n=Δω or Δω×n=2×δ (n=1 is desirable) is satisfied, the ratio of the alkali metal atoms that cause the EIT phenomenon becomes very low. As a result, the signal intensity of the EIT signal also becomes low. Then, in the atomic oscillator  100  of the embodiment, by the feedback loop passing through the semiconductor laser  120 , the atomic cell  130 , the light detector  150 , the amplifier  160 , the detection unit  250 , the oscillator  260 , the modulation unit  270  and the modulation unit  290 , the feedback control is performed so that the oscillation frequency Δω of the oscillator  260  satisfies the relation of 2×δ×n=Δω or Δω×n=2×δ (n=1 is desirable). 
     The semiconductor laser  120 , the atomic cell  130 , the magnetic field generation unit  140  and the light detector  150  correspond to the light source  10 , the atomic cell  20 , the magnetic field generation unit  30  and the light detection unit  40  of  FIG. 1 , respectively. Besides, a circuit including the center wavelength control unit  110 , the amplifier  160 , the detection unit  170 , the modulation unit  180 , the oscillator  190 , the detection unit  200 , the oscillator  210 , the modulation unit  220 , the oscillator  230 , the frequency conversion unit  240 , the detection unit  250 , the oscillator  260 , the modulation unit  270 , the oscillator  280  and the modulation unit  290  corresponds to the control unit  50  of  FIG. 1 . 
     Next, the frequency of the laser light emitted by the semiconductor laser  120  will be described in detail.  FIG. 4A  and  FIG. 4B  are views showing examples of a frequency spectrum of the laser light emitted by the semiconductor laser  120 . 
     As shown in  FIG. 4A  and  FIG. 4B , the laser light emitted by the semiconductor laser  120  is modulated, and has a center frequency band having a center frequency f 0  (=v/λ 0 : v is the velocity of light) and side bands on both sides of the center frequency band. Each of the center frequency band, the upper side band and the lower side band includes plural frequency components different from each other by Δω. Incidentally, in  FIG. 4A  and  FIG. 4B , for convenience, it is assumed that each of the center frequency band, the upper side band and the lower side band includes five frequency components (center frequency component, primary and secondary higher harmonic wave components on both sides thereof). However, the third or more higher harmonic wave components are actually included. 
     In the example of  FIG. 4A , both the frequency difference between the center of the upper side band and the center of the center frequency band and the frequency difference between the center of the center frequency band and the center of the lower side band are 4.5963 GHz. Thus, the frequency difference (ω 1 −ω 2 ) between the center (ω 1 ) of the upper side band and the center (ω 2 ) of the lower side band coincides with 9.1926 GHz as the frequency difference corresponding to the energy difference between the two ground levels of the cesium atom with magnetic quantum number m=0. Accordingly, when the center frequency f 0  (the center wavelength λ 0 ) of the laser light is set to a desired value, each of the frequency components included in the upper side band becomes the resonant light  1 , and each of the frequency components included in the lower side band becomes the resonant light  2 . 
     On the other hand, in the example of  FIG. 4B , the frequency difference (ω 1 −ω 2 ) between the center (ω 1 ) of the upper side band and the center (ω 2 =f 0 ) of the center frequency band coincides with 9.1926 GHz as the frequency difference corresponding to the energy difference between the two ground levels of the cesium atom with magnetic quantum number m=0. Accordingly, when the center frequency f 0  (the center wavelength λ 0 ) of the laser light is set to a desired value, each of the frequency components included in the upper side band becomes the resonant light  1 , and each of the frequency components included in the center frequency band becomes the resonant light  2 . Incidentally, since the frequency difference between the center of the center frequency band and the center of the lower side band is also 9.1926 GHz, the center frequency f 0  (center wavelength λ 0 ) may be set so that each of the frequency components included in the center frequency band becomes the resonant light  1 , and each of the frequency components included in the lower side band becomes the resonant light  2 . 
     Incidentally, the feedback control is performed so that the frequency of the output signal of the frequency conversion unit  240  becomes (ω 1 −ω 2 )/2=4.5963 GHz in the example of  FIG. 4A , and becomes (ω 1 −ω 2 )/2=9.1926 GHz in the example of  FIG. 4B . 
       FIG. 5  is a view for explaining a combination of the resonant light  1  and the resonant light  2  in which the cesium atom whose ground level is Zeeman split causes the EIT phenomenon. In  FIG. 5 , it is assumed that the relation of Δω=2δ is satisfied. Besides, each energy difference E Δω  of excited levels E 1 , E 2 , E 3 , E 4  and E 5  or excited levels E 1 ′, E 2 ′, E 3 ′, E 4 ′ and E 5 ′ represents the energy difference corresponding to Δω. 
     First, the cesium atom with magnetic quantum number m=0 will be considered. In the cesium atom with magnetic quantum number m=0, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz, the EIT phenomenon occurs when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz are simultaneously irradiated. Accordingly, in the cesium atom with apparent excited level E 1 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 +2Δω and the resonant light  2  of f 2 =ω 2 +2Δω are simultaneously irradiated. Besides, in the cesium atom with apparent excited level E 2 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 +Δω and the resonant light  2  of f 2 =ω 2 +Δω are simultaneously irradiated. Besides, the cesium atom with apparent excited level E 3 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1  and the resonant light  2  of f 2 =ω 2  are simultaneously irradiated. Besides, the cesium atom with apparent excited level E 4 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 −Δω and the resonant light  2  of f 2 =ω 2 −Δω are simultaneously irradiated. Besides, the cesium atom with apparent excited level E 5 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 −2Δω and the resonant light  2  of f 2 =ω 2 −2Δω are simultaneously irradiated. 
     Next, the cesium atom with magnetic quantum number m=+1 will be considered. In the cesium atom with magnetic quantum number m=+1, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz+2δ, the EIT phenomenon occurs when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz+2δ=9.1926 GHz+Δω are simultaneously irradiated. Accordingly, in the cesium atom with apparent excited level E 2 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 +2Δω and the resonant light  2  of f 2 =ω 2 +Δω are simultaneously irradiated. Besides, in the cesium atom with apparent excited level E 3 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 +Δω and the resonant light  2  of f 2 =ω 2  are simultaneously irradiated. Besides, the cesium atom with apparent excited level E 4 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1  and the resonant light  2  of f 2 =ω 2 −Δω are simultaneously irradiated. Besides, the cesium atom with apparent excited level E 5 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 −Δω and the resonant light  2  of f 2 =ω 2 −2Δω are simultaneously irradiated. 
     Next, the cesium atom with magnetic quantum number m=+2 will be considered. In the cesium atom with magnetic quantum number m=+2, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz+4δ, the EIT phenomenon occurs when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz+4δ=9.1926 GHz+2Δω are simultaneously irradiated. Accordingly, in the cesium atom with apparent excited level E 2 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 +2Δω and the resonant light  2  of f 2 =ω 2  are simultaneously irradiated. Besides, in the cesium atom with apparent excited level E 3 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 +Δω and the resonant light  2  of f 2 =ω 2 −Δω are simultaneously irradiated. Besides, the cesium atom with apparent excited level E 4 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1  and the resonant light  2  of f 2 =ω 2 −2Δω are simultaneously irradiated. 
     Next, the cesium atom with magnetic quantum number m=+3 will be considered. In the cesium atom with magnetic quantum number m=+3, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz+6δ, the EIT phenomenon occurs when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz+6δ=9.1926 GHz+3Δω are simultaneously irradiated. Accordingly, in the cesium atom with apparent excited level E 3 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 +2Δω and the resonant light  2  of f 2 =ω 2 −Δω are simultaneously irradiated. Besides, in the cesium atom with apparent excited level E 4 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 +Δω and the resonant light  2  of f 2 =ω 2 −2Δω are simultaneously irradiated. 
     Next, the cesium atom with magnetic quantum number m=−1 will be considered. In the cesium atom with magnetic quantum number m=−1, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz−2δ, the EIT phenomenon occurs when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz−2δ=9.1926 GHz−Δω are simultaneously irradiated. Accordingly, in the cesium atom with apparent excited level E 2 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 +Δω and the resonant light  2  of f 2 =ω 2 +2Δω are simultaneously irradiated. Besides, in the cesium atom with apparent excited level E 3 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1  and the resonant light  2  of f 2 =ω 2 +Δω are simultaneously irradiated. Besides, in the cesium atom with apparent excited level E 4 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 −Δω and the resonant light  2  of f 2 =ω 2  are simultaneously irradiated. Besides, the cesium atom with apparent excited level E 5 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 −2Δω and the resonant light  2  of f 2 =ω 2 −Δω are simultaneously irradiated. 
     Next, the cesium atom with magnetic quantum number m=−2 will be considered. In the cesium atom with magnetic quantum number m=−2, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz−4δ, the EIT phenomenon occurs when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz−4δ=9.1926 GHz−2Δω are simultaneously irradiated. Accordingly, in the cesium atom with apparent excited level E 2 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 2  and the resonant light  2  of f 2 =ω 2 +2Δω are simultaneously irradiated. Besides, in the cesium atom with apparent excited level E 3 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 −Δω and the resonant light  2  of f 2 =ω 2 +Δω are simultaneously irradiated. Besides, in the cesium atom with excited level E 4 , the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 −2Δω and the resonant light  2  of f 2 =ω 2  are simultaneously irradiated. 
     Next, the cesium atom with magnetic quantum number m=−3 will be considered. In the cesium atom with magnetic quantum number m=−3, since the frequency corresponding to the energy difference between the ground level  1  and the ground level  2  is 9.1926 GHz−6δ, the EIT phenomenon occurs when the resonant light  1  and the resonant light  2  satisfying the relation of f 1 −f 2 =9.1926 GHz−6δ=9.1926 GHz−3Δω are simultaneously irradiated. Accordingly, in the cesium atom with apparent excited level E 3 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 −Δω and the resonant light  2  of f 2 =ω 2 +2Δω are simultaneously irradiated. Besides, in the cesium atom with apparent excited level E 4 ′, the EIT phenomenon occurs when the resonant light  1  of f 1 =ω 1 −2Δω and the resonant light  2  of f 2 =ω 2 +Δω are simultaneously irradiated. 
     Incidentally, the cesium atom with magnetic quantum number m=+4 or −4 does not cause the EIT phenomenon since the ground level  1  does not exist. 
     The combination of the resonant light  1  and the resonant light  2  described in  FIG. 5  can be visually understood by using a graph shown in  FIG. 6 . In the graph of  FIG. 6 , the horizontal axis indicates the frequency f 1  of the resonant light  1 , and the vertical axis indicates the frequency f 2  of the resonant light  2 . It is indicated that the cesium atom in which the EIT phenomenon occurs exists in the combination of the frequency f 1  of the resonant light  1  and the frequency f 2  of the resonant light  2  in which the intersection point is indicated by a black dot, and the cesium atom in which the EIT phenomenon occurs does not exist in the combination of the frequency f 1  of the resonant light  1  and the frequency f 2  of the resonant light  2  in which the intersection point is indicated by a white dot. In more detail, the cesium atoms with magnetic quantum number m=−3, −2, −1, 0, +1, +2, +3 cause the EIT phenomenon in the combinations of the frequency f 1  of the resonant light  1  and the frequency f 2  of the resonant light  2  indicated by black dots on the respective straight lines of f 1 −f 2 =9.1926 GHz−3Δω, f 1 −f 2 =9.1926 GHz−2Δω, f 1 −f 2 =9.1926 GHz−Δω, f 1 −f 2 =9.1926 GHz, f 1 −f 2 =9.1926 GHz+Δω, f 1 −f 2 =9.1926 GHz+2Δω and f 1 −f 2 =9.1926 GHz+3Δω. 
     In the related art method, since only the resonant light  1  of f 1 =ω 1  and the resonant light  2  of f 2 =ω 2  are simultaneously irradiated, the cesium atom causes the EIT phenomenon only at the intersection point of f 1 =ω 1  and f 2 =ω 2  in  FIG. 6 . On the other hand, in this embodiment, the plural resonant lights  1  of f 1 =ω 1 −2Δω, ω 1 −Δω, ω 1 , ω 1 +Δω and ω 1 +2Δω and the plural resonant light  2  of f 2 =ω 2 −2Δω, ω 2 −Δω, ω 2 , ω 2 +Δω and ω 2 +2Δω are simultaneously irradiated, and at least the cesium atom with magnetic quantum number m=0 causes the EIT phenomenon (intersection points of f 1 −f 2 =9.1926 GHz). Further, when the feedback control is performed so as to satisfy Δω=2δ, the cesium atoms with magnetic quantum number m=−3, −2, −1, +1, +2, +3 also cause the EIT phenomenon. 
     As described above, according to the atomic oscillator  100  (the quantum interference device  1 ) of the embodiment, the control is performed so as to satisfy the relation of Δω=2δ, so that the ratio of alkali metal atoms that cause the EIT phenomenon can be greatly increased as compared with the related art, and therefore, the power efficiency of the semiconductor laser  120  can be greatly improved as compared with the related art. Thus, miniaturization is also easy. For example, when power assigned to one resonant light pair is set to be almost equal to power of the related art, the saturation limit of light absorption becomes high, and the total power increases. Thus, a high contrast EIT signal can be acquired. Besides, when the total light irradiation power is almost equal to that of the related art, since the power per one resonant light pair is decreased, the power broadening of the EIT signal (phenomenon in which when the light power is high, the line width of the EIT signal is increased) can be suppressed, and as compared with the related art, an excellent EIT signal with narrow half-width can be acquired. Accordingly, according to the embodiment, the frequency stability can be improved as compared with the related art. 
     Although the intensity of the magnetic field applied to the atomic cell  130  is changed by the influence of geomagnetic field and the influence of temperature change, according to this embodiment, the feedback control is performed in view of also the influence of geomagnetic field and the influence of temperature change. Accordingly, the influence of disturbance is cancelled, and the higher accuracy atomic oscillator (quantum interference device) can be provided. 
     Incidentally, in this embodiment, in order to maximize the number of alkali metal atoms that cause the EIT phenomenon, it is preferable to establish the relation of Δω=2δ. However, at least the relation of 2×δ×n=Δω or Δω×n=2×δ (n is a positive integer) has only to be satisfied. For example, in the case of Δω=4δ (in the case of n=2 in 2×δ×n=Δω), as shown in  FIG. 7 , the cesium atoms with magnetic quantum number m=−2, 0, +2 cause the EIT phenomenon at intersection points on f 1 −f 2 =9.1926 GHz−Δω, f 1 −f 2 =9.1926 GHz and f 1 −f 2 =9.1926 GHz+Δω. Besides, for example, in the case of Δω=δ (in the case of n=2 in Δω×n=2×δ), as shown in  FIG. 8 , the cesium atoms with magnetic quantum number m=−3, −2, −1, 0, +1, +2, +3 cause the EIT phenomenon at intersection points on f 1 −f 2 =9.1926 GHz−6Δω, f 1 −f 2 =9.1926 GHz−4Δω, f 1 −f 2 =9.1926 GHz−2Δω, f 1 −f 2 =9.1926 GHz, f 1 −f 2 =9.1926 GHz+2Δω, f 1 −f 2 =9.1926 GHz+4Δω, and f 1 −f 2 =9.1926 GHz+6Δω. 
     (2) Second Embodiment 
       FIG. 9  is a view showing a structure of an atomic oscillator of a second embodiment using the quantum interference device  1  of  FIG. 1 . As shown in  FIG. 9 , the atomic oscillator  100  of the second embodiment is different from the atomic oscillator  100  of the first embodiment shown in  FIG. 2  in that the detection unit  250 , the oscillator  260 , the modulation unit  270  and the oscillator  280  are deleted and a frequency conversion unit  300  and a magnetic field control unit  310  are newly added. In  FIG. 9 , the same component as that of  FIG. 2  is denoted by the same reference numeral and its description is omitted. 
     The frequency conversion unit  300  converts the oscillation signal of the oscillator  210  into a signal of a frequency Δω. The frequency conversion unit  300  may be realized by a divider circuit. 
     Here, in the atomic oscillator  100  of the second embodiment, the frequency Δω of the output signal of the frequency conversion unit  300  has a constant value previously determined irrespective of the intensity of a magnetic field. The magnetic field control unit  310  controls the intensity of the magnetic field generated by a magnetic field generation unit  140  so as to satisfy the relation of at least 2×δ×n=Δω or Δω×n=2×δ (n is a positive integer) (Δω=2δ is preferable). 
     Incidentally, a semiconductor laser  120 , an atomic cell  130 , the magnetic field generation unit  140 , a light detector  150  and the magnetic field control unit  310  correspond to the light source  10 , the atomic cell  20 , the magnetic field generation unit  30 , the light detection unit  40  and the magnetic field control unit  52  of  FIG. 1 , respectively. Besides, a circuit including a center wavelength control unit  110 , an amplifier  160 , a detection unit  170 , a modulation unit  180 , an oscillator  190 , a detection unit  200 , the oscillator  210 , a modulation unit  220 , an oscillator  230 , a frequency conversion unit  240 , a modulation unit  290 , the frequency conversion unit  300  and the magnetic field control unit  310  corresponds to the control unit  50  of  FIG. 1 . 
     According to the second embodiment, the magnetic field control unit  310  controls the intensity of the magnetic field generated by the magnetic field generation unit  140  in view of also the influence of geomagnetic field and the influence of temperature change. Accordingly, the influence of disturbance is cancelled, and the higher accuracy atomic oscillator (quantum interference device) can be provided. 
     Modified Example 
       FIG. 10  is a view showing a structure of a modified example of the atomic oscillator  100  of the second embodiment. In the atomic oscillator  100  shown in  FIG. 10 , an oscillator  320  is newly added to the atomic oscillator  100  shown in  FIG. 9 . A frequency conversion unit  300  converts the oscillation signal of the oscillator  320  into a signal of a frequency Δω. Here, although the frequency stability of the oscillator  320  is inferior to the oscillator  210  of  FIG. 9 , the accuracy of Δω has only to be secured to such a degree that alkali metal atoms enclosed in a atomic cell  130  can sufficiently cause the EIT phenomenon. The oscillator  320  can be realized by, for example, a crystal oscillator (XO). 
     Incidentally, in the atomic oscillator  100  shown in  FIG. 9  or  FIG. 10 , when Δω is made coincident with the oscillation frequency of the oscillator  210  or the oscillator  320 , the oscillator conversion unit  300  is unnecessary. In this case, the magnetic field control unit  310  controls the intensity of the magnetic field generated by the magnetic field generation unit  140  based on the oscillation signal of the oscillator  210  or the oscillator  320 . 
     (3) Third Embodiment 
       FIG. 11  is a view showing a structure of an atomic oscillator of a third embodiment using the quantum interference device  1  of  FIG. 1 . As shown in  FIG. 11 , the atomic oscillator  100  of the third embodiment is different from the atomic oscillator  100  of the first embodiment shown in  FIG. 2  in that the detection unit  250 , the oscillator  260 , the modulation unit  270  and the oscillator  280  are deleted, and a magnetic field detection unit  330 , a frequency setting unit  340 , a frequency conversion unit  350  and a magnetic field control unit  360  are newly added. In  FIG. 11 , the same component as that of  FIG. 2  is denoted by the same reference numeral and its description is omitted. 
     The magnetic field control unit  360  controls so that the magnetic field generation unit  140  generates a magnetic field of desired intensity. For example, when a magnetic field generation unit  140  is a coil, the magnetic field control unit  360  controls the magnitude of a current flowing though the coil. 
     The magnetic field detection unit  330  detects the intensity of a magnetic field applied to an atomic cell  130 . 
     The frequency setting unit  340  controls the frequency conversion ratio of the frequency conversion unit  350  based on the detection result of the magnetic field detection unit  330 . 
     The frequency conversion unit  350  converts an oscillation signal of an oscillator  210  into a signal of a frequency Δω based on an output signal (setting information of a frequency conversion ratio) of the frequency setting unit  340 . The frequency conversion unit  350  may be realized by a divider circuit. In this case, the frequency setting unit  340  sets the division ratio of the divider circuit. 
     Here, the frequency setting unit  340  controls the frequency conversion ratio (for example, the division ratio) of the frequency conversion unit  350  so as to satisfy the relation of at least 2×δ×n=Δω or Δω×n=2×δ (n is a positive integer) (Δω=2δ is preferable) based on the intensity of the magnetic field detected by the magnetic field detection unit  330 . 
     Incidentally, a semiconductor laser  120 , the atomic cell  130 , the magnetic field generation unit  140 , a light detector  150  and the magnetic field detection unit  330  correspond to the light source  10 , the atomic cell  20 , the magnetic field generation unit  30 , the light detection unit  40  and the magnetic field detection unit  54  of  FIG. 1 , respectively. Besides, a circuit including a center wavelength control unit  110 , an amplifier  160 , a detection unit  170 , a modulation unit  180 , an oscillator  190 , a detection unit  200 , the oscillator  210 , a modulation unit  220 , an oscillator  230 , a frequency conversion unit  240 , a modulation unit  290 , the magnetic field detection unit  330 , the frequency setting unit  340 , the frequency conversion unit  350  and the magnetic field control unit  360  corresponds to the control unit  50  of  FIG. 1 . 
     According to the third embodiment, the intensity of the magnetic field applied to the alkali metal atoms enclosed in the atomic cell  130  is detected by the magnetic field detection unit  330  in view of also the influence of geomagnetic field and the influence of temperature change. Then, the frequency conversion ratio of the frequency conversion unit  350  is controlled by the frequency setting unit  340  based on the intensity of the detected magnetic field. Accordingly, the influence of disturbance is cancelled, and the higher accuracy atomic oscillator (quantum interference device) can be provided. 
     Modified Example 
       FIG. 12  is a view showing a structure of a modified example of the atomic oscillator  100  of the third embodiment. In an atomic oscillator  100  shown in  FIG. 12 , the magnetic field detection unit  330  is deleted from the atomic oscillator  100  shown in  FIG. 11 . Then, a frequency setting unit  340  controls a frequency conversion ratio of a frequency conversion unit  350  based on control information of a magnetic field control unit  360  to a magnetic field generation unit  140 . That is, in the atomic oscillator  100  of  FIG. 12 , the intensity of a magnetic field is estimated based on the control information to the magnetic field generation unit  140 , and the conversion ratio of the frequency conversion unit  350  is controlled. Accordingly, although the frequency accuracy of the output signal of the frequency conversion unit  350  is inferior to the oscillator  210  of  FIG. 11 , the accuracy of Δω has only to be secured to such a degree that alkali metal atoms enclosed in an atomic cell  130  can sufficiently cause the EIT phenomenon. For example, when the environment is such that the influence of geomagnetic field or the influence of temperature change is neglected, there is no problem. 
     3. Magnetic Sensor 
     (1) First Embodiment 
       FIG. 13  is a view showing a structure of a magnetic sensor of a first embodiment using the quantum interference device  1  of  FIG. 1 . As shown in  FIG. 13 , the magnetic sensor  400  of the first embodiment has the same structure as that of the atomic oscillator  100  of the first embodiment shown in  FIG. 2  except that an oscillation signal of an oscillator  260  becomes an output signal. In  FIG. 13 , the same component as that of  FIG. 2  is denoted by the same reference numeral and its description is omitted. 
     As described before, when magnetic field is applied to an atomic cell  130 , the ground level  1  and the ground level  2  of an alkali metal atom are divided into plural Zeeman split levels different from each other in magnetic quantum number m. In both the ground level  1  and the ground level  2 , an energy difference E δ  between two Zeeman split levels different in magnetic quantum number m by one is proportional to the intensity of the magnetic field. According to the structure of  FIG. 13 , feedback control is performed so that the signal intensity of an output signal of a light detector  150  (output signal of an amplifier  160 ) becomes maximum. The signal intensity of the output signal of the light detector  150  (the output signal of the amplifier  160 ) becomes maximum when the relation of 2×δ×n=Δω or Δω×n=2×δ (n is a positive integer) (Δω=2δ is preferable) is satisfied with respect to an oscillation frequency Δω of the oscillator  260  and a frequency δ corresponding to the energy difference E δ  between the Zeeman split levels. That is, since the oscillation frequency Δω of the oscillator  260  is proportional to the intensity of the magnetic field, when the oscillation signal of the oscillator  260  is made the output signal, the function as the magnetic sensor can be exerted. In the structure of  FIG. 13 , although the magnetic field is always generated by a magnetic generation unit  140 , the intensity of an external magnetic field can be calculated by obtaining the relative frequency of the output signal based on the oscillation frequency of the oscillator  260  when the intensity of the external magnetic field is 0. 
     (2) Second Embodiment 
       FIG. 14  is a view showing a structure of a magnetic sensor of a second embodiment using the quantum interference device  1  of  FIG. 1 . As shown in  FIG. 14 , the magnetic sensor  400  of the second embodiment is different from the magnetic sensor  400  of the first embodiment shown in  FIG. 13  in that the detection unit  250 , the oscillator  260 , the modulation unit  270  and the oscillator  280  are deleted, and a signal processing unit  370  and an oscillator  380  are newly added. In  FIG. 14 , the same component as that of  FIG. 13  is denoted by the same reference numeral and its description is omitted. 
     The signal processing unit  370  sends a sweep signal  372  for changing an oscillation frequency stepwise to the oscillator  380 , and monitors an output signal of an amplifier  160  in each state of the sweep signal  372  and frequency information  382  (which may be the oscillation signal itself) of the oscillator  380 . In the magnetic sensor  400  of the second embodiment, the oscillation frequency of the oscillator  380  is Δω, and when the relation of 2×δ×n=Δω or Δω×n=2×δ (n is a positive integer) (Δω=2δ is preferable) is satisfied, the signal intensity of the output signal of the amplifier  160  abruptly increases. Since the frequency information  382  is proportional to the intensity of magnetic field, when information (information indicating the intensity of magnetic field) based on the frequency information  382  when the signal intensity of the output signal of the amplifier  160  abruptly increases is outputted, the function as the magnetic sensor can be exerted. Although magnetic field is always generated by a magnetic field generation unit  140  in the structure of  FIG. 14 , information indicating the intensity of the external magnetic field can be outputted by obtaining the relative frequency of the oscillator  380  based on the frequency information  382  when the intensity of the external magnetic field is 0. 
     Incidentally, when the signal processing unit  370  can estimate the oscillation frequency of the oscillator  380  from the state of the sweep signal  372 , the frequency information  382  may not be monitored. 
     According to the first embodiment and the second embodiment, the use efficiency of laser power is improved by raising the occurrence efficiency of the EIT phenomenon in the alkali metal atom, and the high sensitivity magnetic sensor can be provided. Thus, the miniaturization is also easy. 
     Incidentally, the invention is not limited to the embodiments, but can be variously modified within the scope of the gist of the invention. 
     For example, in the atomic oscillator  100  of the embodiment, although the AC current is superimposed on the drive current of the semiconductor laser  120 , and the semiconductor laser  120  emits the modulated light, an electro-optic modulator (EOM) is used, and frequency modulation may be applied to the light emitted from the semiconductor laser  120 . 
     The invention includes substantially the same structure as the structure described in the embodiments (for example, the structure having the same function, method and result, or the structure having the same object and effect). Besides, the invention includes a structure in which an unessential portion of the structure described in the embodiments is replaced. Besides, the invention includes a structure which can achieve the same operation and effect as the structure described in the embodiments or the structure which can achieve the same object. Besides, the invention includes a structure in which a well-known technique is added to the structure described in the embodiments.