Abstract:
The present invention provides an optical communication method and an optical communication system in which eavesdropping is more difficult than in conventional techniques. An optical communication system in one embodiment of the present invention comprises: a photon pair generator which generates a correlated photon pair; a polarizer which is provided on an optical path of one photon of the correlated photon pair and direction of which is changeable based on information to be transmitted; a shutter which is provided between the photon pair generator and the polarizer on the optical path of the one photon of the correlated photon pair and which is capable of blocking the one photon of the correlated photon pair; and a photon detector which is provided on an optical path of another photon of the correlated photon pair.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of International Application No. PCT/JP2015/003018, filed Jun. 17, 2015, which claims the benefit of Japanese Patent Application No. 2014-136718, filed Jul. 2, 2014. The contents of the aforementioned applications are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to an optical communication method and an optical communication system for transmitting and receiving information by using photons. 
       BACKGROUND ART 
       [0003]    In recent years, research and experiments have been made on quantum cryptography communication utilizing principles of quantum mechanics (Non Patent Documents 1 and 2). In the conventional quantum cryptography communication, studies have been made assuming that a photon in superposed states in the quantum mechanics exists in a communication path. When the photon in the superposed states is observed by an eavesdropper, the photon transitions from the superposed states to an eigenstate having definite information. Due to such an effect, the eavesdropper cannot perform eavesdropping without affecting the exchanged information, because the eavesdropper cannot reproduce the original superposed states. Hence, the fact that the proper sender and recipient can detect eavesdropping guarantees security. 
         [0004]    For example, Non Patent Document 3 discloses an optical communication method for a quantum cryptography communication. In the optical communication method described in Non Patent Document 3, a sender phase-modulates a photon according to information desired to be transmitted and transmits the photon to a recipient. If an eavesdropper exists in a transmission pass and measures the photon, the eavesdropper may fail to retransmit a photon modulated by the same phase modulation because the eavesdropper cannot know the phase used by the sender. As a result, mismatch (error) between the information transmitted by the sender and the information received by the recipient increases and the existence of the eavesdropper can be thereby detected. 
       CITATION LIST 
     Non Patent Document 
       [0000]    
       
         Non Patent Document 1: H. Takesue et al., “Differential phase shift quantum key distribution experiment over 105 km fibre”, New Journal of Physics, 2005, Vol. 7, 232 
         Non Patent Document 2: H. Goto, “Mechanism and development trends of quantum cryptography communication”, Kinyu Kenkyu, 2009, 28(3), pp. 107-150, Institute for Monetary and Economic Studies, Bank of Japan 
         Non Patent Document 3: K. Inoue, “Quantum Key Distribution Technologies”, IEEE Journal of Selected Topics in Quantum Electronics, 2006, Vol. 12 (4), pp. 888-896 
         Non Patent Document 4: M. Morimoto, “Resolution of Single Photon and Electron Interference Enigma”, http://vixra.org/abs/1312.0097, 2013 
       
     
       SUMMARY OF INVENTION 
       [0009]    The conventional optical communication method described above even enables detection of the existence of the eavesdropper after the eavesdropping, but still allows the eavesdropper to measure the photon exchanged in the communication path. When the existence of the eavesdropper is detected, the conventional optical communication method can take a countermeasure against the eavesdropping, such as destroying information (for example, encryption key) on a photon which may have been maliciously measured. However, it is undeniable that the conventional optical communication method allows the photon to give some information to the eavesdropper. Accordingly, it is more preferable to make it more difficult for the eavesdropper to measure the photon from the beginning. 
         [0010]    The present invention has been made in view of the problems described above, and an object thereof is to provide an optical communication method and an optical communication system in which eavesdropping is more difficult than in conventional techniques. 
         [0011]    A first aspect of the present invention is an optical communication system comprising: a photon pair generator which generates a correlated photon pair; a polarizer which is provided on an optical path of one photon of the correlated photon pair and direction of which is changeable based on information to be transmitted; a shutter which is provided between the photon pair generator and the polarizer on the optical path of the one photon of the correlated photon pair and which is capable of blocking the one photon of the correlated photon pair; and a photon detector which is provided on an optical path of another photon of the correlated photon pair. 
         [0012]    A second aspect of the present invention is an optical communication method comprising: setting a direction of a polarizer based on information to be transmitted; generating a correlated photon pair with a photon pair generator after the direction of the polarizer is set; blocking one photon of the correlated photon pair with a shutter after the correlated photon pair is generated; and detecting another photon of the correlated photon pair with a detector after the correlated photon pair is generated, wherein the shutter and the polarizer are arranged on an optical path of the one photon of the correlated photon pair, and the detector is arranged on an optical path of the another photon of the correlated photon pair. 
         [0013]    In the optical communication method and the optical communication system of the present invention, since the information is transmitted based on the direction of the polarizer and the one photon of the correlated photon pair is blocked by the shutter, it is difficult to perform eavesdropping by reading the photon. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a schematic configuration diagram of an optical communication system in one embodiment of the present invention. 
           [0015]      FIG. 2  is a view depicting a flowchart of an optical communication method in one embodiment of the present invention. 
           [0016]      FIG. 3A  is a view depicting a schematic plot of a result measured by the optical communication method in one embodiment of the present invention. 
           [0017]      FIG. 3B  is a view depicting a schematic plot of a result measured by the optical communication method in one embodiment of the present invention. 
           [0018]      FIG. 4  is a schematic configuration diagram of an optical communication system in one embodiment of the present invention. 
           [0019]      FIG. 5  is a view depicting a flowchart of an optical communication method in one embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0020]    Embodiments of the present invention are described below with reference to the drawings. However, the present invention is not limited to the embodiments. Note that, in the drawings described below, parts with the same function are denoted by the same reference numeral and overlapping description thereof is omitted in some cases. 
       First Embodiment 
       [0021]      FIG. 1  is a schematic configuration diagram of an optical communication system  100  in the embodiment. The optical communication system  100  includes a receiver  110  and a transmitter  120 , and the receiver  110  and the transmitter  120  are connected to each other by a communication path  130  which is a transmission path of information. Although the communication path  130  is a free space in the embodiment, the communication path  130  may at least partially be an optical waveguide such as an optical fiber or a PLC. Note that the names of the receiver  110  and the transmitter  120  are defined based on a direction in which the information is transmitted, and are opposite to a direction in which photons are transmitted as will be described later. Specifically, the transmission direction of the information is a direction from the transmitter  120  to the receiver  110 , but the transmission direction of the photons is a direction from the receiver  110  to the transmitter  120 . 
         [0022]    In the receiver  110 , there are provided a photon source  111  which outputs photons according to control of a reception controller  119  and a photon pair generator  112  which generates correlated photon pairs by receiving photons from the photon source  111 . One photon of each photon pair is referred to as photon p, and the other photon is referred to as photon s. The photon p and the photon s are correlated to each other to have polarizations orthogonal to each other. Specifically, when the photon p is a vertically-polarized photon, the photon s is a horizontally-polarized photon or vise versa. In the embodiment, a BBO crystal (β-BaB 2 O 4  crystal) which generates pairs of photons correlated to each other by means of parametric down-conversion is used as the photon pair generator  112 . However, any material or device which generates pairs of photons correlated to each other can be used. 
         [0023]    Two QWPs  113   a  and  113   b  (quarter wave plates) and a double slit plate  114  are provided corresponding to a direction in which the photons s are emitted by the photon pair generator  112 , that is, are provided on an optical path of the photons s. The double slit plate  114  has two slits parallel to each other. The first QWP  113   a  is arranged such that the photons s having passed the first QWP  113   a  enter one of the two slits, and the second QWP  113   b  is arranged such that the photons s having passed the second QWP  113   b  enter the other one of the two slits. 
         [0024]    When the linearly-polarized photons s enter the QWPs  113   a  and  113   b  at an angle of −45° or 45° with respect to the fast axes of the QWPs  113   a  and  113   b  (the polarization of the photons s in this case is assumed to be diagonal polarization), the QWPs  113   a  and  113   b  convert the photons s to a circularly-polarized photons and output the converted photons s. Meanwhile, when the linearly-polarized photons s enter the QWPs  113   a  and  113   b  at an angle of 0° or 90° with respect to the fast axes of the QWPs  113   a  and  113   b  (the polarization of the photons s in this case is assumed to be vertical polarization or horizontal polarization), the QWPs  113   a  and  113   b  output the photons s as they are as linearly-polarized photons. Moreover, the directions in which the respective QWPs  113   a  and  113   b  convert the photons s to circularly-polarized photons are opposite to each other. Specifically, the first QWP  113   a  converts the photons s to right circularly-polarized photons while the second QWP  113   b  converts the photons s to left circularly-polarized photons (or vise versa). In such a configuration, when the vertically-polarized or horizontally-polarized photons s enter the QWPs  113   a  and  113   b , interference occurs after the photons s pass through the double slit plate  114 . Meanwhile, when the diagonally-polarized photons s enter the QWPs  113   a  and  113   b , no interference occurs after the photons s pass through the double slit plate  114 . 
         [0025]    A slit plate  115  and a reception detector  116  are provided on a path of the photons s having passed the double slit plate  114 , that is, are provided on the optical path of the photons s. The slit plate  115  has a slit which allows the photons s to enter the reception detector  116  only in a predetermined direction. 
         [0026]    The reception detector  116  outputs a predetermined signal to the reception controller  119  upon detecting the photons s. Although an APD (avalanche photodiode) is used as the reception detector  116  in the embodiment, any device capable of detecting photons can be used. A driver  117  which moves the reception detector  116  in a direction perpendicular to the direction in which the photons s are emitted by the photon pair generator  112  (optical path of the photons s) is connected to the reception detector  116 . The driver  117  is any driver such as a motor or an actuator. By repeating the detection of the photons s while moving the reception detector  116  with the driver  117 , the number of photons s detected by the reception detector  116  at each position can be obtained. 
         [0027]    A shutter  118  is provided corresponding to a direction in which the photons p are emitted by the photon pair generator  112 , that is, is provided on an optical path of the photons p. The shutter  118  is switchable between an open state in which the photons p are allowed to pass and travel toward the communication path  130  and a closed state in which the photons p are blocked and prevented from traveling toward the communication path  130 , according to the control of the reception controller  119 . Any device which can be mechanically or electromagnetically switched between the open state and the closed state can be used as the shutter  118 . 
         [0028]    The reception controller  119  is connected to the photon source  111 , the reception detector  116 , and the shutter  118 . The reception controller  119  electrically controls the members, communicates with the transmitter  120  via a synchronization transmission path  140 , records measured data, and performs input and output for the user. The reception controller  119  includes any computer or electric circuit. 
         [0029]    In the transmitter  120 , a polarizer  121  (polarization plate) is provided corresponding to the direction in which the photons p from the receiver  110  enter, that is, is provided on the optical path of the photons p. The polarizer  121  is switchable between a first state in which the polarizer  121  allows photons having polarization in a predetermined direction to completely pass and a second state in which the direction to allow the photons to pass is rotated by +45° or −45° from that of the first state. In the embodiment, since the vertical polarization or the horizontal polarization is used as the polarization in the predetermined direction, the polarizer  121  in the first state allows the vertically-polarized or horizontally-polarized photons to pass while the polarizer  121  in the second state allows the diagonally-polarized photons to pass. The polarizer  121  is switchable between the first state and the second state according to the control by a transmission controller  122 . 
         [0030]    A slit plate  123  and a transmission detector  124  are provided on a path of the photons p having passed the polarizer  121 . The slit plate  123  has a slit which allows the photons p to enter the transmission detector  124  only in a predetermined direction. The transmission detector  124  outputs a predetermined signal to the transmission controller  122  upon detecting the photons p. Note that, in the embodiment, since the information is transmitted from the transmitter  120  to the receiver  110  based on the direction of the polarizer  121  as will be described below, the transmission detector  124  is not necessarily required for the information transmission. The transmission detector  124  is used to align the optical axis of the communication path  130  between the transmitter  120  and the receiver  110  or to measure the distance between the transmitter  120  and the receiver  110  by receiving photons from the receiver  110 . 
         [0031]    The transmission controller  122  is connected to the polarizer  121  and the transmission detector  124 . The transmission controller  122  electrically controls the members, communicates with the receiver  110  via the synchronization transmission path  140 , records measured data, and performs input and output for the user. The transmission controller  122  includes any computer or electric circuit. 
         [0032]    The receiver  110  and the transmitter  120  are connected to each other by the synchronization transmission path  140 . The synchronization transmission path  140  may be any communication path such as an optical fiber communication path, a radio communication path, and the like. The transmission controller  122  transmits a synchronization signal to the reception controller  119  via the synchronization transmission path  140  to perform transmission-reception timing synchronization. The transmission-reception timing synchronization is performed by means of any synchronization method by using a signal indicating a transmission time, a periodic signal, and the like. 
         [0033]    Principles of the present invention are described. It has been conventionally considered that: in a correlated photon pair (entangled photon pair) generated by a BBO crystal (photon pair generator), when one photon s has the vertical polarization |y&gt;, the other photon p has the horizontal polarization |x&gt; or vise versa; and these polarization directions are not determined until the photons are observed and the photons are in the superposed states. Specifically, it has been considered that the correlated photon pair is in a superposition of a state |y&gt;s|x&gt;p in which the photon s has the vertical polarization |y&gt; and the photon p has the horizontal polarization |x&gt; and a state |x&gt;s|y&gt;p in which the photon s has the horizontal polarization |x&gt; and the photon p has the vertical polarization |y&gt;. This state is expressed by formula (1). 
         [0000]    
       
         
           
             
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         [0034]    However, in this concept, a paradox in which time seems to be reversed occurs and a special situation such as a situation in which cause and effect are switched occurs. Meanwhile, explanation can be made without using the conventional concept by accepting existence of a non-localized potential as described below. 
         [0035]    Specifically, the horizontal polarization |x&gt; and the vertical polarization |y&gt; of the photons having passed a device, which allows only specific polarization to pass, such as the polarizer have been conventionally considered to be in a superposition of polarization rotated by +45° or −45° as expressed by formula (2). 
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         [0036]    In this formula, |+&gt; represents polarization of +45° with respect to the x axis, and |−&gt; represents polarization of −45° with respect to the x axis. Although this is correct in a classical electromagnetic field, applying this expression to a photon which cannot be divided such as a single photon leads to paradox. In the case of dealing with a very small number of photons as described above, such a situation can be explained well by using the non-localized potential which universally exists in a space, instead of considering such a situation as superposition. In this case, formula (3) in which a non-localized potential |ζ&gt; is added to formula (2) needs to be considered as a correct expression. 
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         [0037]    Therefore, the non-localized potential |ζ&gt; is expressed by formula (4). 
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         [0038]    In this formula, the letters attached to the non-localized potential |ζ&gt; each represent the direction of the polarizer (polarizer  121  in the optical communication system  100 ), |ζ 1 &gt; represents the horizontal direction, and |ζ 2 &gt; represents the vertical direction. As described above, the direction of the non-localized potential |ζ&gt; is determined by the existence of the polarizer. Specifically, the non-localized potential is |ζ 1 &gt; when the direction of the polarizer is horizontal and the non-localized potential is |ζ 2 &gt; when the direction of the polarizer is vertical. 
         [0039]    Since the non-localized potential |ζ&gt; follows the Maxwell&#39;s equations, the non-localized potential |ζ&gt; propagates from the polarizer at the speed of light. When the propagated non-localized potential |ζ&gt; reaches the BBO crystal (photon pair generator  112  in the optical communication system  100 ) which generates the photon pairs, the polarization of the photons which can be generated by the BBO crystal is determined by this non-localized potential |ζ&gt; whose direction is determined. 
         [0040]    Specifically, before the BBO crystal generates photons, the non-localized potential |ζ&gt; reaches the BBO crystal from the polarizer existing in the direction in which the photons are to be emitted from the BBO crystal. When the direction of the polarizer is horizontal, the non-localized potential |ζ 1 &gt; reaches the BBO crystal. Accordingly, the BBO crystal can generate only the photons p having the horizontal polarization |x&gt; in the direction toward the polarizer. In this case, the other photons s of the correlated photon pairs have the vertical polarization |y&gt;. Meanwhile, when the direction of the polarizer is vertical, the non-localized potential |ζ 2 &gt; reaches the BBO crystal. Accordingly, the BBO crystal can generate only the photons p having the vertical polarization |y&gt; in the direction toward the polarizer. In this case, the other photons s of the correlated photon pairs have the horizontal polarization |x&gt;. As described above, the polarization of the photons which can be generated by the BBO crystal is restricted by the direction of the polarizer existing in the direction in which the photons are to be emitted from the BBO crystal. Note that, although description is given of the vertical polarization and the horizontal polarization, the polarization of the photons which can be generated by the BBO crystal is restricted by the direction of the polarizer in a similar way also in the diagonal polarization rotated by +45° or −45° from the vertical polarization or the horizontal polarization. 
         [0041]    In other words, in the BBO crystal, the completely-correlated photon pair is generated from the beginning, and the correlated photon pair before being observed is not in the superposed states as in the conventional concept. However, since the non-localized potential itself cannot be observed and such determination of the direction cannot therefore be sensed, it has been conventionally considered that there is a strange correlation in the photon pairs. 
         [0042]    Since probability amplitudes of the non-localized potential are &lt;ζ 1 |ζ 1 &gt;=0 and &lt;ζ 2 |ζ 2 &gt;=0, it can be seen that the non-localized potential cannot be observed. This can be easily derived by multiplying formula (4) as it is and by using the relationship of formula (5). 
         [0000]      1/√{square root over (2)}=           x|+             =             +|x             =             y|+             =             +|y               
 
         [0000]      =           x|−             =             −|x             =−             y|−             =−             −|y                 (5)
 
         [0043]    In summary, the correlated photon pair generated by the BBO crystal has the complete correlation in which, when one photon has the vertical polarization |y&gt;, the other photon has the horizontal polarization |x&gt; (or vise versa). However, these directions of polarization are determined when the photon pair is generated, irrespective of whether the observation is performed or not, and the correlated photon pair is not in the superposed states as in the conventional concept. The non-localized potential |ζ&gt; whose direction is determined determines the directions of polarization upon reaching the BBO crystal before the generation of the photon pairs. 
         [0044]      FIG. 2  is a view depicting a flowchart of an optical communication method using the optical communication system  100 . First, in the state where the shutter  118  is open, the transmission controller  122  sets the direction of the polarizer  121  based on the information to be transmitted (step S 101 ). In the embodiment, in the case of transmitting “1” as first information, the polarizer  121  is set to the first state in which the vertically-polarized or horizontally-polarized photons are allowed to completely pass. Meanwhile, in the case of transmitting “0” as second information, the polarizer  121  is set to the second state in which the direction to allow the photons to pass is rotated by +45° or −45° from that of the first state. The direction of polarization allowed to pass in the first state may be set to a direction other than the horizontal polarization or the vertical polarization. Moreover, “0” and “1” which are the information to be transmitted may be opposite. In such a case, the receiver  110  may be configured such that definitions of relationships between the directions of polarization and the information to be transmitted are appropriately changed. 
         [0045]    Next, the reception controller  119  generates photons from the photon source  111  (step S 102 ). A timing at which the photons are generated from the photon source  111  is set such that the photons from the photon source  111  reaches the photon pair generator  112  after the non-localized potential from the polarizer  121  reaches the photon pair generator  112 . Specifically, assume that a time point at which the direction of the polarizer  121  is set is t 1 , a time required for the non-localized potential from the polarizer  121  to reach the photon pair generator  112  is a, and a time required for the photons from the photon source  111  to reach the photon pair generator  112  is b. In this case, a time point t 2  at which the photons are generated from the photon source  111  is expressed by formula (6). 
         [0000]      [Math 6] 
         [0000]        t   2   &gt;t   1   +a−b   (6)
 
         [0046]    Since the non-localized potential and the photons travel at the speed of light, the times a and b can be calculated from the distances between the members. The time point t 1  at which the direction of the polarizer  121  is set is determined based on the synchronization signal received from the transmission controller  122  via the synchronization transmission path  140 . 
         [0047]    Upon receiving the photons from the photon source  111 , the photon pair generator  112  generates the photons p and s which are the correlated photon pairs (step S 103 ). At this point, since the non-localized potential from the polarizer  121  has already reached the photon pair generator  112 , the polarization directions of the photons p and s are determined by the non-localized potential. Specifically, when the polarizer  121  is in the first state in which the vertically-polarized or horizontally-polarized photons are allowed to completely pass, the photons p traveling from the photon pair generator  112  toward the polarizer  121  are the vertically-polarized or horizontally-polarized photons, and the other photons s are also the vertically-polarized or horizontally-polarized photons. Meanwhile, when the polarizer  121  is in the second state in which the direction to allow the photons to pass is rotated by +45° or −45° from that of the first state and the diagonally-polarized photons are allowed to pass, the photons p traveling from the photon pair generator  112  toward the polarizer  121  are the diagonally-polarized photons and the other photons s are also the diagonally-polarized photons. 
         [0048]    Next, the reception controller  119  closes the shutter  118  (step S 104 ). A timing at which the shutter  118  is closed is a timing after the photons p and s are generated in the photon pair generator  112  and before the photons p reach the shutter  118 . Specifically, assume that a time required to generate the photons p and s in the photon pair generator  112  is c and a time required for the photons p from the photon pair generator  112  to reach the shutter  118  is d. In this case, a time point t 3  at which the shutter  118  is closed is expressed by formula (7). Since the photons travel at the speed of light, the times c and d can be calculated from the distances between the members. 
         [0000]      [Math 7] 
         [0000]        t   2   +b+c&lt;t   3   &lt;t   2   +b+c+d   (7)
 
         [0049]    In parallel with step S 104 , the reception detector  116  detects the photons s having passed through the QWPs  113   a  and  113   b  and the double slit plate  114  and records whether the photons s are detected or not in the reception controller  119  (step S 105 ). Thereafter, the reception controller  119  opens the shutter  118  (step S 106 ). 
         [0050]    In the embodiment, in order to determine the transmitted information (direction of the polarizer  121 ) from whether interference occurs or not in a measurement result, the detection needs to be performed at multiple measurement positions and performed multiple times at each measurement position. To achieve this, the driver  117  moves the reception detector  116  by a predetermined distance (step S 107 ) and steps S 102  to S 106  are repeated predetermined times at each measurement position (step S 108 ), without the direction of the polarizer  121  being changed. For example, steps S 102  to S 107  are performed 50 times at each of 20 positions (total of 1000 times). 
         [0051]    Lastly, the reception controller  119  determines the transmitted information by plotting a photon detection number measured at each measurement position (step S 109 ). Specifically, when the occurrence of interference is recognized in the plot, the photons s before entering the QWPs  113   a  and  113   b  are the vertically-polarized or horizontally-polarized photons, and the other photons p are therefore also the vertically-polarized or horizontally-polarized photons. From this, it is found that the polarizer  121  on the optical path of the photons p is in the first state in which the vertically-polarized or horizontally-polarized photons are allowed to completely pass. Hence, the reception controller  119  determines that the information transmitted from the transmitter  120  is “1” which is the first information. Meanwhile, when no occurrence of interference is recognized in the plot, the photons s before entering the QWPs  113   a  and  113   b  are the diagonally-polarized photons, and the other photons p are therefore also the diagonally-polarized photons. From this, it is found that the polarizer  121  on the optical path of the photons p is in the second state in which the direction to allow the photons to pass is rotated by +45° or −45° from that of the first state. Hence, the reception controller  119  determines that the information transmitted from the transmitter  120  is “0” which is the second information. 
         [0052]      FIGS. 3A and 3B  are views depicting schematic plots of results measured by the reception detector  116 . When the plot has a mountain shape with one peak as illustrated in  FIG. 3A , no interference is occurring. Accordingly, the photons s before entering the QWPs  113   a  and  113   b  are the diagonally-polarized photons. Meanwhile, when the plot has a wave shape with multiple peaks as illustrated in  FIG. 3B , interference is occurring. Accordingly, the photons s before entering the QWPs  113   a  and  113   b  are the vertically-polarized or horizontally-polarized photons. Such determination can be performed by the reception controller  119  or by the user. 
         [0053]    In the optical communication system  100  of the embodiment, the information is transmitted by utilizing the fact that the non-localized potential which cannot be observed is sent from the transmitter  120  to the receiver  110  and the polarization of the photons which can be generated in the photon pair generator  112  is restricted by the non-localized potential. Although the receiver  110  and the transmitter  120  of the optical communication system  100  are arranged at positions capable of transmitting and receiving the photons, the shutter  118  blocks the photons before the photons are actually emitted from the receiver  110  to the transmitter  120 . Accordingly, an eavesdropper cannot intercept the exchanged photons and read information. Moreover, since no observable photons travel through the transmission path of the information from the transmitter  120  to the receiver  110 , it is difficult for the eavesdropper to know the transmission path. 
       Second Embodiment 
       [0054]    In the optical communication system  100  of the first embodiment, the information is statistically determined by utilizing the existence or absence of interference. Accordingly, transmission needs to be performed multiple times for one piece of information (“1” or “0”). Meanwhile, in the embodiment, information can be determined by performing transmission once for one piece of information. 
         [0055]      FIG. 4  is a schematic configuration diagram of an optical communication system  200  of the embodiment. The optical communication system  200  includes a receiver  210  and a transmitter  220 , and the receiver  210  and the transmitter  220  are connected to each other by a communication path  230  which is a transmission path of information. Although the communication path  230  is a free space in the embodiment, the communication path  230  may at least partially be an optical waveguide such as an optical fiber or a PLC. 
         [0056]    In the receiver  210 , there are provided a photon source  211  which outputs photons according to control of a reception controller  219  and a photon pair generator  212  which generates correlated photon pairs by receiving photons from the photon source  211 . One photon of each photon pair is referred to as photon p, and the other photon is referred to as photon s. The photon p and the photon s are correlated to each other to have polarizations orthogonal to each other. Specifically, when the photon p is a vertically-polarized photon, the photon s is a horizontally-polarized photon or vise versa. In the embodiment, a BBO crystal (β-BaB 2 O 4  crystal) which generates pairs of photons correlated to each other by means of photometric down-conversion is used as the photon pair generator  212 . However, any material or device which generates pairs of photons correlated to each other can be used. 
         [0057]    A second shutter  213  and a second polarizer  214  (polarization plate) are provided corresponding to a direction in which the photons s are emitted by the photon pair generator  212 , that is, are provided on an optical path of the photons s. The second shutter  213  is switchable between an open state in which the photons s are allowed to pass and travel toward the second polarizer  214  and a closed state in which the photons s are blocked and prevented from traveling toward the second polarizer  214 , according to the control of the reception controller  219 . Any device which can be mechanically or electromagnetically switched between the open state and the closed state can be used as the second shutter  213 . 
         [0058]    The second polarizer  214  allows photons having polarization of a predetermined direction to pass and does not allow photons having polarization of directions other than the predetermined direction to pass. In the embodiment, since the horizontal polarization is used as the polarization of the predetermined direction, the second polarizer  214  allows the photons s to pass when the photons s are horizontally-polarized and does not allow photons s to pass when the photons s are polarized in other directions, that is, vertically-polarized or diagonally-polarized. 
         [0059]    A slit plate  215  and a reception detector  216  are provided on a path of the photons s having passed the second polarizer  214 . The slit plate  215  has a slit which allows the photons s enter the reception detector  216  only in a predetermined direction. 
         [0060]    The reception detector  216  outputs a predetermined signal to the reception controller  219  upon detecting the photons s. Although an APD (avalanche photodiode) is used as the reception detector  216  in the embodiment, any device capable of detecting photons can be used. 
         [0061]    A first shutter  218  is provided corresponding to a direction in which the photons p are emitted by the photon pair generator  212 , that is, is provided on an optical path of the photons p. The first shutter  218  is switchable between an open state in which the photons p are allowed to pass and travel toward the communication path  230  and a closed state in which the photons p are blocked and prevented from traveling toward the communication path  230 , according to the control of the reception controller  219 . Any device which can be mechanically or electromagnetically switched between the open state and the closed state can be used as the first shutter  218 . 
         [0062]    The reception controller  219  is connected to the photon source  211 , the reception detector  216 , the first shutter  218 , and the second shutter  213 . The reception controller  219  electrically controls the members, communicates with the transmitter  220  via a synchronization transmission path  240 , records measured data, and performs input and output for the user. The reception controller  219  includes any computer or electric circuit. 
         [0063]    In the transmitter  220 , a first polarizer  221  (polarization plate) is provided corresponding to the direction in which the photons p from the receiver  210  enter, that is, is provided on the optical path of the photons p. The first polarizer  221  is switchable between a first state in which the first polarizer  221  allows photons having polarization in a predetermined direction to completely pass and a second state in which the direction to allow the photons to pass is rotated by a predetermined angle (for example, +90° or −90°) from that of the first state. In the embodiment, since the vertical polarization is used as the polarization in the predetermined direction, the polarizer  221  in the first state allows the vertically-polarized photons to pass while the polarizer  221  in the second state allows the horizontally-polarized photons to pass. The first polarizer  221  is switchable between the first state and the second state according to the control by a transmission controller  222 . 
         [0064]    A slit plate  223  and a transmission detector  224  are provided on a path of the photons p having passed the first polarizer  221 . The slit plate  223  has a slit which allows the photons p to enter the transmission detector  224  only in a predetermined direction. The transmission detector  224  outputs a predetermined signal to the transmission controller  222  upon detecting the photons p. Note that, in the embodiment, since the information is transmitted from the transmitter  220  to the receiver  210  based on the direction of the first polarizer  221  as in the first embodiment, the transmission detector  224  is not necessarily required for the information transmission. The transmission detector  224  is used to align the optical axis of the communication path  230  between the transmitter  220  and the receiver  210  or to measure the distance between the transmitter  220  and the receiver  210  by receiving photons from the receiver  210 . 
         [0065]    The transmission controller  222  is connected to the first polarizer  221  and the transmission detector  224 . The transmission controller  222  electrically controls the members, communicates with the receiver  210  via the synchronization transmission path  240 , records measured data, and performs input and output for the user. The transmission controller  222  includes any computer or electric circuit. 
         [0066]    The receiver  210  and the transmitter  220  are connected to each other by the synchronization transmission path  240 . The synchronization transmission path  240  may be any communication path such as an optical fiber communication path, a radio communication path, and the like. The transmission controller  222  transmits a synchronization signal to the reception controller  219  via the synchronization transmission path  240  to perform transmission-reception timing synchronization. The transmission-reception timing synchronization is performed by means of any synchronization method by using a signal indicating a transmission time, a periodic signal, and the like. 
         [0067]      FIG. 5  is a view depicting a flowchart of an optical communication method using the optical communication system  200 . First, in the state where the second shutter  213  is closed and the first shutter  218  is open, the transmission controller  222  sets the direction of the first polarizer  221  based on the information to be transmitted (step S 201 ). In the embodiment, in the case of transmitting “1” as first information, the first polarizer  221  is set to the first state in which the vertically-polarized photons are allowed to completely pass. Meanwhile, in the case of transmitting “0” as second information, the first polarizer  221  is set to the second state in which the direction to allow the photons to pass is rotated by the predetermined angle (for example, +90° or −90°) from that of the first state. The direction of polarization allowed to pass in the first state may be set to a direction other than the vertical polarization. Moreover, “0” and “1” which are the information to be transmitted may be opposite. In such cases, the receiver  210  may be configured such that definitions of relationships between the directions of polarization and the information to be transmitted are appropriately changed. 
         [0068]    Next, the reception controller  219  generates photons from the photon source  211  (step S 202 ). A timing at which the photons are generated from the photon source  211  is set such that the photons from the photon source  211  reaches the photon pair generator  212  after the non-localized potential from the first polarizer  221  reaches the photon pair generator  212 . This timing can be calculated by using formula (6) as in the first embodiment. 
         [0069]    Upon receiving the photons from the photon source  211 , the photon pair generator  212  generates the photons p and s which are the correlated photon pairs (step S 203 ). At this point, since the non-localized potential from the first polarizer  221  has already reached the photon pair generator  212 , the polarization directions of the photons p and s are determined by the non-localized potential. Specifically, when the first polarizer  221  is in the first state in which the vertically-polarized photons are allowed to completely pass, the photons p traveling from the photon pair generator  212  toward the first polarizer  221  are the vertically-polarized photons, and the other photons s are the horizontally-polarized photons. Meanwhile, when the first polarizer  221  is in the second state in which the direction to allow the photons to pass is rotated by the predetermined angle (+90° or −90° in this case) from that of the first state and the horizontally-polarized photons are allowed to pass, the photons p traveling from the photon pair generator  212  toward the first polarizer  221  are the horizontally-polarized photons and the other photons s are the vertically-polarized photons. 
         [0070]    Next, the reception controller  219  closes the first shutter  218  (step S 204 ). A timing at which the first shutter  218  is closed is a timing after the photons p and s are generated in the photon pair generator  212  and before the photons p reach the first shutter  218 . This timing can be calculated by using formula (7) as in the first embodiment. 
         [0071]    In parallel with step S 204 , the reception controller  219  opens the second shutter  213  (step S 205 ). A timing at which the second shutter  213  is opened is a timing after the photons p and s are generated in the photon pair generator  212  and before the photons s reach the second shutter  213 . Specifically, assume that a time required for the photons s from the photon pair generator  212  to reach the second shutter  213  is e, in addition to t 2 , b, and c used in formulae (6) and (7). In this case, a time point t 4  at which the second shutter  213  is opened is expressed by formula (8). Since the photons travel at the speed of light, the time e can be calculated from the distances between the members. 
         [0000]        t   2   +b+c&lt;t   4   &lt;t   2   +b+c+e   (8)
 
         [0072]    The reception detector  216  detects the photons s having passed through the second polarizer  214  and records whether the photons s are detected or not in the reception controller  219  (step S 206 ). Thereafter, the reception controller  219  opens the first shutter  218  and closes the second shutter  213  (step S 207 ). 
         [0073]    Lastly, the reception controller  219  determines the transmitted information by using a result measured by the reception detector  216  (step S 208 ). Specifically, when the photons are detected by the reception detector  216 , the photons s before entering the second polarizer  214  are horizontally-polarized photons, and the other photons p are therefore vertically-polarized photons. From this, it is found that the first polarizer  221  on the optical path of the photons p is in the first state in which the vertically-polarized photons are allowed to completely pass. Hence, the reception controller  219  determines that the information transmitted from the transmitter  220  is “1” which is the first information. Meanwhile, when no photons are detected by the reception detector  216 , the photons s before entering the second polarizer  214  are not the horizontally-polarized photons, and the other photons p are therefore the horizontally-polarized photons. From this, it is found that the first polarizer  221  on the optical path of the photons p is in the second state in which the direction to allow the photons to pass is rotated by the predetermined angle from that of the first state. Hence, the reception controller  219  determines that the information transmitted from the transmitter  220  is “0” which is the second information. 
         [0074]    The optical communication system  200  in the embodiment has effects of the first embodiment and can also transmit one piece of information by performing transmission once. Accordingly, the optical communication system  200  can further increase the transmission speed. 
         [0075]    The present invention is not limited to the embodiments described above, and appropriate changes can be made within a scope not departing from the purport of the present invention. 
         [0076]    In this description, the present invention is described by using words such as vertical polarization, horizontal polarization, and diagonal polarization as the polarization of photons. However, the polarization in the present invention is not limited to polarization of specific directions. In the case of carrying out the present invention, polarization of other directions may be used based on the symmetry of polarization. In the case of using the polarization of other directions, the statements of the description may be replaced as appropriate.