JP4882491B2 - Quantum cryptographic communication device, communication terminal, and additional information transmission method - Google Patents

Description

  The present invention relates to a quantum cryptography communication device, a communication terminal, and an additional information transmission method that execute communication processing based on quantum cryptography.

  Specifically, the present invention sends a relatively strong reference light and a weak signal light with random phase modulation for each pulse from the sender side terminal to the receiver side terminal, and the receiver side terminal sends a signal. After random phase modulation is further applied to the light or reference light, homodyne detection is performed based on the reference light and signal light to obtain communication information. The transmitter side terminal increases the intensity of the signal light. In addition, the phase of the signal light is set to a phase that is not orthogonal to any of the phases constituting the plurality of bases related to the random phase modulation described above, or the transmitter terminal side increases the intensity of the signal light and The phase that forms the specific base among the plurality of bases related to the above-mentioned random phase modulation is added to the signal light, and the phase that forms the specific base in the signal light or reference light at the receiver side terminal And Quantum cryptography communication device that can transmit additional information from the sender-side terminal to the receiver-side terminal by adding phase modulation and obtaining additional information as the output of homodyne detection at the receiver-side terminal, etc. It is related to.

  The world is now entering an advanced information society. Information is delivered to people in the form of communications. Light is used as a carrier for information in communication because of its high speed and attenuation resistance. Optical communication is indispensable to the information society in this way, but it is constantly in danger of eavesdropping.

  Eavesdropping can be dealt with by encrypting information. DES, AES, etc. that encrypts after sharing the key in advance, and anyone can make ciphertext by releasing the encryption key, but it can be decrypted only by a legitimate recipient who knows the decryption key Public key cryptography such as RSA.

  The security of these keys is based on the assumption that an eavesdropper cannot know the decryption key only from a plaintext, a set of ciphertexts, or a public encryption key. The reason why the decryption key cannot be calculated from these is the reason why these ciphers are said to have computational security. However, this security is constantly threatened by the development and improvement of cryptanalysis and computing power, and there is a fear that there may be unpublished efficient cryptanalysis.

  An encryption method that uses quantum mechanical uncertainty to zero the amount of information obtained by an eavesdropper without seeking the basis of security for computational security is called quantum cryptography, and has been popular recently. Has come to be studied.

  Quantum cryptography is being researched and developed mainly in the field of optical communications. The signal such as the phase and polarization of weak signal light of about one photon is mapped on a plurality of non-orthogonal bases corresponding to 0 and 1, respectively, to obtain a signal. Since there are only one photon, wiretapping by branching is impossible. In addition, even if the signal is amplified due to non-orthogonality, it does not become the same as the original state (quantum non-replication theorem), and the eavesdropper's attack of amplification-branching can be nullified.

Using the quantum nature of these signals, even if an eavesdropper is used, the signal cannot reach or the nature of the signal changes. Therefore, an encryption system that can detect eavesdropping has been proposed.
However, there are serious problems in putting this into practical use. That is a synchronization problem at the transmitting and receiving ends. The signal originally used for quantum cryptography has an average number of photons of about one at the transmitting end, and the loss of the optical fiber is 0.2 dB / km or more. Therefore, when trying to transmit quantum cryptography information at a distance of 100 km, Becomes 1/100 or less.

  This represents that the signal reaches the receiving end only once every 100 times. This effectively prevents bit synchronization. Bit synchronization in this case is to identify the number of pulses sent in a certain time throttle. For this purpose, a synchronous clock is sent using an external line, or a clock pulse is sent with an optical signal having a sufficient amount of light at a different wavelength in the same optical fiber and strong against attenuation.

  However, in the external line, it is difficult to absorb the jitter in this signal line and it is difficult to identify the bit throttle. In another wavelength system in the same optical fiber, the Raman scattered light of the clock pulse interferes with the signal light, etc. The essential problem of not being able to achieve bit synchronization with the signal itself has emerged.

  The quantum cryptography communication has a field called continuous variable quantum cryptography that uses weak signal light with cipher and reference light having normal light intensity (see Patent Documents 1 and 2). That is, in this continuous variable quantum cryptography, the pulse of coherent light is separated into two, one is attenuated to the level of one photon, and the transmission information is loaded thereon as a phase change to obtain signal light. This phase consists of two types having different bases, and the base to be transmitted is randomly selected for each bit. The other optical pulse is transmitted as the reference light with the same phase and intensity. The receiving side takes out the phase information of the signal light by homodyne detection of the reference light and the signal light.

As is obvious from this quantum cryptography communication method, since the reference light having a large amount of light always reaches the receiving side in the bit synchronization, the clock information can be extracted therefrom. This is described in Patent Document 1. Yes.
JP 2000-101570 A JP 2005-286485 A

  In order to establish communication efficiently, in addition to bit synchronization, what is called frame synchronization for distinguishing a set of bits from other sets is required. Frame synchronization usually uses a special code to improve separation from other signals. For example, in a comma sequence in Gigabit Ethernet ([Ethernet] is a registered trademark), a pattern that does not appear in a normal 8B10B code is used to distinguish frames.

  An object of the present invention is to enable good transmission of additional information, for example, a frame synchronization signal, in continuous variable quantum cryptography communication.

The concept of this invention is
On the first communication terminal side, the pulse light generated from the light source is separated into signal light and reference light, the reference light is transmitted to the communication path, the signal light is attenuated to the first light amount, and for each pulse. Are added with random phase modulation having a predetermined phase selected from phases constituting a plurality of bases, and sent to the communication path, and on the second communication terminal side, the signal light and the reference light are transmitted from the communication path. Based on the signal light and the reference light after applying a random phase modulation having a predetermined phase selected from the phases constituting a plurality of bases for each pulse to the signal light or the reference light. An additional information transmission method in a quantum cryptography communication device that obtains communication information by performing homodyne detection,
On the first communication terminal side, the signal light is sent to the communication path as a second light quantity larger than the first light quantity and as a phase not orthogonal to any of the phases constituting the plurality of bases. In the method for transmitting additional information in a quantum cryptography communication device, additional information is obtained as an output of the homodyne detection on the second communication terminal side.

  In this invention, the quantum cryptography communication device includes a first communication terminal that is a sender-side terminal, a second communication terminal that is a receiver-side terminal, and an optical fiber that connects the first and second communication terminals, Alternatively, a communication path composed of free space is provided.

  The pulsed light generated from the light source of the first communication terminal (sender side terminal) is separated into signal light and reference light. The reference light is not attenuated, and the reference light transmitted to the communication path is relatively strong. The signal light is attenuated to the first light amount at the time of quantum cryptography communication, and the intensity of the signal light transmitted to the communication path is weak. However, the signal light is not attenuated at the time of transmitting additional information or the degree of attenuation is reduced to the first level. Is transmitted to the communication path with a second light amount larger than the first light amount.

  In addition, the signal light transmitted from the first communication terminal to the communication path has a predetermined phase selected from phases constituting a plurality of bases for each pulse corresponding to the communication information during quantum cryptography communication. Random phase modulation is added, but when additional information is transmitted, the phase is not orthogonal to any of the phases constituting the plurality of bases related to the random phase modulation described above.

  The above-mentioned reference light and signal light are sent to the second communication terminal (recipient side terminal) via the communication path, and these reference light and signal light are separated and extracted. After random phase modulation having a predetermined phase selected from phases constituting a plurality of bases for each pulse is applied to the signal light or the reference light, homodyne detection is performed based on the signal light and the reference light. During quantum cryptography communication, as described above, the intensity of the signal light transmitted from the first communication terminal to the communication path is reduced, and random phase modulation is applied for each pulse corresponding to the communication information. Communication information can be obtained at a small level as the output of homodyne detection.

  On the other hand, when transmitting additional information, the intensity of the signal light transmitted from the first communication terminal to the communication path is increased as described above, and the phase of the signal light forms a plurality of bases related to random phase modulation. Therefore, even if random phase modulation is performed on the second communication terminal side as an output of homodyne detection, additional information is always obtained at a large level.

  For example, when the unit of communication information transmitted in quantum cryptography communication is a frame, the additional information is transmitted in association with each frame, for example, at the head of each frame. This makes it possible to synchronize each frame.

  In addition, when the phase constituting the plurality of bases is on the first and second coordinate axes orthogonal to each other as the phase not orthogonal to any of the phases constituting the plurality of bases, the first and second coordinate axes You may make it use the phase which exists in the enclosed 1st quadrant and 3rd quadrant, or 2nd quadrant and 4th quadrant. In this case, it is possible to obtain information consisting of a bit string of 1, 0 from the output value of homodyne detection of the second communication terminal, and for example, information such as the type of communication information of each frame by quantum cryptography communication can be transmitted.

  The homodyne of the second communication terminal can also be obtained by amplitude-modulating the amount of signal light transmitted from the first communication terminal to the communication path when transmitting the additional information according to the content of the additional information, for example, bits 1 and 0. It is possible to obtain additional information including a bit string of 1 and 0 from the output value of detection.

  At the time of transmitting additional information, the first communication terminal side adds the phase modulation that is a phase constituting a specific base among the plurality of bases described above to the signal light, and transmits the signal light to the communication path. On the side, additional information may be obtained as an output of homodyne detection by adding phase modulation to the signal light or reference light, which is the phase constituting the specific base described above. In this case, on the second communication terminal side, when a random phase modulation is applied to the signal light or the reference light, when a large level signal starts to be output as a homodyne detection output at the time of additional information transmission, A state in which phase modulation that is a phase constituting the specific base is applied is added, and when transmission of the additional information is completed, a state in which random phase modulation is applied again is restored.

The concept of the present invention is
On the second communication terminal side, the pulsed light generated from the light source is separated into signal light and reference light, and the reference light is sent to the communication path through the second optical path in which no delay device is inserted. Is transmitted to the communication path through the first optical path in which the delay device is inserted, and the signal light and the reference light transmitted via the communication path are transmitted through the predetermined optical path on the first communication terminal side. When transmitting to the communication path, the light amount of the signal light is attenuated to a first light amount, and random phase modulation is applied to the signal light so as to have a predetermined phase selected from phases constituting a plurality of bases for each pulse. The second communication terminal side separates and extracts the signal light and the reference light transmitted via the communication path, and configures the plurality of bases for each pulse of the signal light or the reference light It becomes the predetermined phase selected from the phase to be After addition of random phase modulation or additional information transmission method in a quantum cryptography communication apparatus for obtaining communication information by performing a homodyne detection based signal light and the reference light,
On the first communication terminal side, the signal light is sent to the communication path as a second light quantity larger than the first light quantity and as a phase not orthogonal to any of the phases constituting the plurality of bases. In the method for transmitting additional information in a quantum cryptography communication device, additional information is obtained as an output of the homodyne detection on the second communication terminal side.

  In this invention, the quantum cryptography communication device includes: a second communication terminal that is a receiver side terminal; a first communication terminal that is a sender side terminal; and an optical fiber that connects the first and second communication terminals, Alternatively, a communication path composed of free space is provided.

  The pulsed light generated from the light source of the second communication terminal (receiver side terminal) is separated into signal light and reference light. Then, the signal light that has passed through the first optical path in which the delay device is inserted and the reference light that has passed through the second optical path in which the delay device is not inserted are transmitted to the communication path. In this case, the reference light is first transmitted to the communication path, and then the signal light is transmitted with a predetermined time difference.

  The above-described reference light and signal light are sent to the first communication terminal (sender side terminal) via the communication path. These reference light and signal light are sent again to the communication path via a predetermined optical path. The reference light is not attenuated, and the reference light transmitted to the communication path is relatively strong. The signal light is attenuated to the first light amount at the time of quantum cryptography communication, and the intensity of the signal light transmitted to the communication path is weak. However, the signal light is not attenuated at the time of transmitting additional information or the degree of attenuation is reduced to the first level. Is transmitted to the communication path with a second light amount larger than the first light amount.

  In addition, the signal light transmitted from the first communication terminal to the communication path is a random signal having a predetermined phase selected from phases constituting a plurality of bases for each pulse corresponding to communication information during quantum cryptography communication. However, when additional information is transmitted, the phase is set to a phase that is not orthogonal to any of the phases constituting the plurality of bases related to the random phase modulation described above.

  The above-mentioned reference light and signal light are sent to the second communication terminal (recipient side terminal) via the communication path, and these reference light and signal light are separated and extracted. As described above, when the reference light and the signal light are sent from the second communication terminal to the first communication terminal, the signal light passes through the first optical path, and the reference light passes through the second optical path. However, with respect to what is sent back from the first communication terminal to the second communication terminal, the reference light passes through the first optical path and the signal light passes through the second optical path. .

  After random phase modulation having a predetermined phase selected from phases constituting a plurality of bases for each pulse is applied to the signal light or the reference light, homodyne detection is performed based on the signal light and the reference light. During quantum cryptography communication, as described above, the intensity of the signal light transmitted from the first communication terminal to the communication path is reduced, and random phase modulation is applied for each pulse corresponding to the communication information. Communication information can be obtained at a small level as the output of homodyne detection.

  On the other hand, when transmitting additional information, the intensity of the signal light transmitted from the first communication terminal to the communication path is increased as described above, and the phase of the signal light forms a plurality of bases related to random phase modulation. Therefore, even if random phase modulation is performed on the second communication terminal side as an output of homodyne detection, additional information is always obtained at a large level.

  At the time of transmitting additional information, the first communication terminal side adds the phase modulation that is a phase constituting a specific base among the plurality of bases described above to the signal light, and transmits the signal light to the communication path. On the side, additional information may be obtained as an output of homodyne detection by adding phase modulation to the signal light or reference light, which is the phase constituting the specific base described above. In this case, on the second communication terminal side, when a random phase modulation is applied to the signal light or the reference light, when a large level signal starts to be output as a homodyne detection output at the time of additional information transmission, A state in which phase modulation that is a phase constituting the specific base is applied is added, and when transmission of the additional information is completed, a state in which random phase modulation is applied again is restored.

  According to the present invention, a relatively strong reference light and a weak signal light subjected to random phase modulation for each pulse are sent from the sender side terminal to the receiver side terminal, and the signal light is transmitted from the receiver side terminal. Alternatively, after random phase modulation is further applied to the reference light, homodyne detection is performed based on the reference light and signal light to obtain communication information, and the intensity of the signal light is increased at the transmitter side terminal. In addition, the phase of the signal light is set to a phase that is not orthogonal to any of the phases constituting the plurality of bases related to the above-described random phase modulation, or the signal light intensity is increased and the signal is increased on the transmitter terminal side. Phase modulation that is a phase that constitutes a specific base among a plurality of bases related to the random phase modulation described above is added to light, and a phase that constitutes the specific base described above in signal light or reference light at a receiver side terminal As much as Modulation was added, which a configuration to obtain additional information as the output of homodyne detection on the reception side terminal can be favorably transmit the additional information to the receiver terminal from the sender terminal.

  First, the outline of the present invention will be described. The present invention relates to quantum cryptography communication based on continuous variables using weak signal light carrying cryptographic information and reference light having normal light intensity.

  In continuous variable quantum cryptography communication, one coherent light is divided into two pulses of reference light and signal light attenuated to about one photon. In a continuous variable quantum cryptography in which information is placed on the phase state of signal light, when the signal light having an average photon number of 1 is represented on quadrature phase amplitude coordinates x and p, as shown in FIG. Expressed as four states at ± 1 position on the axis. In FIG. 1, the amplitude of the signal light is represented by a distance from the origin 0, and the phase is represented by a rotation angle around the origin 0.

  The transmission side randomly selects the basis of x or p, selects the phase state based on the predetermined relationship between each bit value of the communication information and the phase state, and determines the signal light state, that is, the phase. The receiving side selects a receiving base at random. The phase state of the signal light can be extracted by homodyne detection of the reference light and the signal light, and when the reception base matches the transmission base, a signal histogram as shown in FIG. 2 appears. This distribution is Gaussian around a certain center value due to the quantum nature.

  If the bases on the transmission side and the reception side are different, a signal histogram as shown in FIG. 3 appears. This is a distribution near the signal level 0. In continuous variable quantum cryptography, by combining the bases of the transmitting side and the receiving side in a classical channel, it is possible to extract only the same ground state and obtain 0 and 1 information under a certain error probability. it can.

  The frame synchronization signal as additional information must be unique so that it can be understood in any case. Here, the frame means a certain unit (unit) of communication information transmitted by quantum cryptography communication.

  By setting the level of the signal light (pulse light) to a level that does not disappear due to the attenuation of the communication path as a frame synchronization signal, the pulse will not disappear on the way, and the detection output after reaching the receiving side Is larger than a normal signal, so that the receiving side can reliably recognize the frame synchronization signal.

  FIG. 4 shows an example of the signal level and phase state of the frame synchronization signal sent from the transmission side, and shows a histogram of the signal obtained by homodyne detection on the reception side in that case of FIG.

  However, in a system in which the bases are randomly changed as a communication protocol, if the receiving side selects a base that is different from the frame synchronization base on the transmitting side, even if the amplitude of the signal light is large, FIG. As shown in FIG. 5, the output after homodyne detection is distributed around the 0 level. In this case, it becomes impossible to distinguish from a general signal, and the role of the signal as frame synchronization cannot be performed.

  The present invention relates to a proposal of a synchronization signal that can be stably distinguished from a normal signal and a device that can output the same even if the receiving side randomly selects a basis. FIG. 7 shows a representation of the frame synchronization signal according to the present invention in quadrature phase amplitude.

  As a normal signal, the average number of photons is 1 and the value placed on each base as a phase is taken, but the frame synchronization signal is not attenuated on the transmission side so that the amplitude level of light reaches the reception side reliably. Of course, the phase should not match the base of either signal. Then, whichever base is selected by the receiving side, the homodyne detection output is sufficiently separated from the normal signal light, as shown in FIG. By detecting this large signal level on the receiving side, the receiving side can surely know that the frame synchronization signal has been sent.

  Even if the frame synchronization signal alone is sufficient in terms of synchronization, for example, it can have information such as what kind of frame or the type of communication information transmitted in that frame. As shown in FIG. 9, when a frame synchronization signal is placed in a quadrant that is not adjacent to each other, for example, the first quadrant and the third quadrant, surrounded by the x axis and the p axis orthogonal to each other, the output is as shown in FIG. It is divided into positive and negative. If this is defined as 0 and 1, a frame synchronization signal composed of 0 and 1 bit strings can be transmitted. Note that the frame synchronization signal may be placed in the second quadrant and the fourth quadrant instead of the first quadrant and the third quadrant.

  By continuing the necessary number of pulses carrying the phase information, it is possible to send information that can identify what frame or the like from the transmission side to the reception side. When information is sent in adjacent quadrants, as shown in FIG. 11, depending on how the base on the receiving side is taken, there may be cases where 0 and 1 cannot be distinguished. It is necessary to send. In the case shown in FIG. 11, when the base on the receiving side is p, the outputs of homodyne detection are both positive, so 0 and 1 cannot be distinguished.

  Another way of putting information on the frame synchronization signal is to fix the homodyne detection phase to a predetermined base when the frame synchronization signal arrives. As a result, if a signal having a large amplitude is sent as a frame synchronization signal on the determined basis, information arrives. In this method, signal separation can be further increased with the same light amplitude as compared with the method in which the base is continuously changed at random. In this case, for example, when the frame synchronization signal is transmitted on the basis of p on the transmission side and the frame synchronization signal is detected on the reception side. The base on the receiving side is fixed to p.

  Of course, it is also possible to send the frame information by placing information on the amplitude of the continuous frame synchronization signal. In this case, the frame synchronization signal is amplitude-modulated.

  A first embodiment of the present invention will be described. FIG. 12 shows the configuration of the quantum cryptography communication device 100A as the first embodiment.

  The quantum cryptography communication device 100A connects the sender-side terminal 30 as the first communication terminal, the receiver-side terminal 31 as the first communication terminal, and the sender-side terminal 30 and the receiver-side terminal 31. For example, the communication path 32 comprised with an optical fiber is provided.

  This quantum cryptography communication device 100A is used as secret information (for example, in a common key cryptosystem) as communication information during quantum cryptography communication from the sender side terminal 30 to the receiver side terminal 31 via the communication path 32. A shared secret key) is transmitted, and a frame synchronization signal as additional information is transmitted when additional information is transmitted. This frame synchronization signal is transmitted at the head of each frame, for example, in association with each frame when a certain unit (unit) of communication information transmitted by quantum cryptography communication is used as a frame. Each frame can be synchronized by this frame synchronization signal.

  The sender-side terminal 30 includes a laser light source 33, a beam splitter 34, a reflecting mirror 35, a half-wave plate 36, a variable attenuator 37, a controller 38, a phase modulator 39, a reflecting mirror 40, and a polarizing beam splitter 41. .

  The receiver-side terminal 31 includes a polarization beam splitter 42, a beam splitter 43, a detector 44, a controller 45, a phase modulator 46, a half-wave plate 47, a reflecting mirror 48, a beam splitter 49, photodiodes 51 and 52, and subtraction. And a level discriminator 54.

  The quantum cryptography communication device 100A is a method for controlling reference light and signal light on different optical paths by using polarization of light in one-way communication from the sender-side terminal 30 to the receiver-side terminal 31. It is.

  Hereinafter, details of processing in each component will be described in accordance with an operation sequence of communication processing to which quantum cryptography is applied.

The pulsed light generated by the laser light source 33 of the sender terminal 30 is separated by the beam splitter 34 into pulsed light P1 that is signal light and pulsed light P2 that is reference light. Then, the pulsed light P1 (signal light) travels to the polarization beam splitter 41 through the first optical path. In the first optical path, a reflecting mirror 35, a half-wave plate 36, a variable attenuator 37, a phase modulator 39, and a reflecting mirror 40 are arranged in this order. The order of the variable attenuator 37 and the phase modulator 39 may be reversed. The half-wave plate 36 rotates the polarization plane of the pulsed light P1 by 90 degrees. The variable attenuator 37 can be an acousto-optic element or a LiNbO ₃ intensity modulator, and the phase modulator 39 can be a LiNbO ₃ phase modulator.

  The variable attenuator 37 adjusts the light amount of the pulsed light P1. The variable attenuator 37 attenuates the light amount of the pulsed light P1 during quantum cryptography communication under the control of the controller 38, and averages photons per pulse of the pulsed light P1 transmitted from the sender side terminal 30 to the communication path 32. The number should be about one. Further, the variable attenuator 37 does not attenuate the light amount of the pulsed light P <b> 1 or reduces the amount of attenuation when the additional information is transmitted under the control of the controller 38, and transmits it from the transmitter side terminal 30 to the communication path 32. The intensity of the pulsed light P1 is increased.

  Here, when the adjustment light quantity of the pulsed light P1 at the time of quantum cryptography communication is the first light quantity and the adjustment light quantity of the pulsed light P1 at the time of additional information transmission is the second light quantity, the second light quantity is the first light quantity. Become bigger. As a result, as will be described later, the homodyne detection output of the receiver-side terminal 31 at the time of transmitting additional information can be obtained at a level higher than that at the time of quantum cryptography communication, and a frame synchronization signal is transmitted from the receiver-side terminal 31. It will be possible to reliably recognize what has been done.

The average number of photons per pulse of the pulsed light P2 (reference light) transmitted from the sender-side terminal 30 to the communication path 32 is such that the S / N ratio in homodyne detection on the receiver-side terminal 31 side is optimized. Chosen. The typical intensity of the pulsed light P2 is about 10 ⁶ average photons per pulse.

  The phase modulator 39 adjusts the phase of the pulsed light P1. Under the control of the controller 38, the phase modulator 39 forms a plurality of bases for each pulse, and two bases in this embodiment (see x and p shown in FIG. 1) in the pulsed light P1 during quantum cryptography communication. Random phase modulation is applied to obtain a predetermined phase selected from the phases of 0 degrees (0 radians), 90 degrees (π / 2 radians), 180 degrees (π radians), and 270 degrees (3π / 2 radians). As described above, the quantum cryptography communication according to the present embodiment uses four quantum states.

  In addition, the phase modulator 39 controls the controller 38 so that the phase of the pulsed light P1 is any of the phases (0 degrees, 90 degrees, 180 degrees, and 270 degrees) constituting the above-described two bases when additional information is transmitted. Also, phase modulation is applied so that the phases are not orthogonal. For example, the pulsed light P1 has a phase of 45 degrees as shown by the frame region signal in FIG.

  As a result, as will be described later, the receiver side terminal 31 adds random phase modulation that becomes a predetermined phase selected from the phases constituting the two bases described above for each pulse to the pulsed light P1 or pulsed light P2. In this case, the output of homodyne detection at the time of transmitting additional information can be obtained at a large level regardless of the change in the basis.

  Here, the frame synchronization signal can have information such as the type of communication information of each frame. In this case, the phase modulator 39 changes the phase of the pulsed light P <b> 1 in accordance with the value of each bit of information composed of a bit string of 1 and 0 under the control of the controller 38. In this case, as shown in FIG. 9, the phases of the first quadrant and the third quadrant, which are not adjacent to each other and surrounded by the mutually orthogonal x-axis and p-axis, are used. The phase of the pulsed light P1 is set to the phase of the first quadrant, and the phase of the pulsed light P1 is set to the phase of the third quadrant corresponding to the bit 0.

  As a result, as shown in FIG. 10, the receiver-side terminal 31 can obtain a positive value “1” corresponding to the pulsed light P1 in the first quadrant as the output of the homodyne detection, A negative value “0” can be obtained corresponding to the pulsed light P1 having the phase of the third quadrant, and the information given to the frame synchronization signal can be obtained.

  Instead of using the phases of the first and third quadrants described above, the phases of the second and fourth quadrants that are not adjacent to each other may be used. Further, as described above, instead of changing the phase of the pulsed light P1 in accordance with the value of each bit of the information to be given to the frame synchronization signal, each information of the information to have the light quantity level of the pulsed light P1 to the frame synchronization signal. You may make it perform the amplitude modulation changed according to the value of a bit. In this case, the light amount level of the pulsed light P1 can be changed by the variable attenuator 37 described above. In this case, the receiver-side terminal 31 can obtain information given to the frame synchronization signal as a change in the output value of homodyne detection.

  The pulsed light P2 (reference light) separated by the beam splitter 34 travels to the polarization beam splitter 41 through the second optical path shorter than the first optical path described above. The polarization beam splitter 41 combines the pulsed light P1 (signal light) and the pulsed light P2 (reference light) and sends them to the communication path 32. The pulsed lights P1 and P2 have polarization planes orthogonal to each other and are separated in time. In this case, the polarization beam splitter 41 constitutes a light transmission unit.

  The pulsed light P1 (signal light) and the pulsed light P2 (reference light) transmitted from the sender side terminal 30 to the communication path 32 are incident on the receiver side terminal 31. These pulsed lights P1 and P2 are separated and extracted by the polarization beam splitter 42 into pulsed light P1 (signal light) and pulsed light P2 (reference light).

  The pulsed light P1 (signal light) travels to the beam splitter 49 through a fourth optical path having an optical path length corresponding to the second optical path of the sender-side terminal 30 described above. On the other hand, the pulsed light P2 (reference light) travels to the beam splitter 49 through a third optical path having an optical path length corresponding to the first optical path of the sender terminal 30 described above.

  In the third optical path, a beam splitter 43, a half-wave plate 47, and a reflecting mirror 48 are arranged in this order. A phase modulator 46 is disposed in the fourth optical path. The beam splitter 43 splits the pulsed light P <b> 2 (reference light) into pulsed light that travels to the half-wave plate 47 and pulsed light that travels to the detector 44. A typical branching ratio of the beam splitter 43 is 9 to 1, and is set so that most of the pulsed light travels toward the half-wave plate 47 side.

  The detector 44 supplies the detection output of the pulsed light branched by the beam splitter 43 to the controller 45. The controller 45 can know the arrival of the pulsed light P2 (reference light) from this detection output, and controls the phase modulation processing start timing for the pulsed light P1 (signal light) in the phase modulator 46. Thereby, in the phase modulator 46, the phase modulation process can be performed at the correct timing for each pulse constituting the pulsed light P1 (signal light).

The phase modulator 46 adjusts the phase of the pulsed light P1. As this phase modulator 46, a LiNbO ₃ phase modulator can be used similarly to the phase modulator 39 in the sender-side terminal 30 described above.

  The phase modulator 46 is selected from the 0 degree and 90 degree phases constituting the above-described two bases for each pulse in the pulse light P <b> 1 at the time of quantum cryptography communication and additional information transmission under the control of the controller 45. Random phase modulation with a predetermined phase is applied. The phase modulator 46 may be arranged in the third optical path where the half-wave plate 47 is arranged, and apply the same random phase modulation to the pulsed light P2 (reference light).

  The half-wave plate 47 rotates the polarization plane of the pulsed light P2 (reference light) by 90 degrees. As described above, in the transmitter terminal 30, the pulsed light P1 (signal light) passes through the long first optical path, the polarization plane is rotated by 90 degrees by the half-wave plate 36, and the pulsed light P2 (reference light) is In the receiver side terminal 31, the pulsed light P1 (signal light) passes through the short fourth optical path and the pulsed light P2 (reference light) passes through the long third optical path and is a half-wave plate. Since the polarization plane is rotated by 90 degrees in 47, the pulsed lights P1 and P2 incident on the beam splitter 49 coincide with each other in time and the polarization directions also coincide.

  The two outputs of the beam splitter 49 are input to the homodyne detector. That is, one output of the beam splitter 49 is input to the photodiode 51 constituting the homodyne detector, and the other output is input to the photodiode 52 constituting the homodyne detector.

  Then, the subtracter 53 obtains a difference signal between the outputs of the photodiodes 51 and 52, that is, a homodyne detection output. The homodyne detection output is supplied to the level discriminator 54. This level discriminator 54 discriminates the level of the homodyne detection output and discriminates and outputs the communication information Scm sent during the quantum cryptography communication and the frame synchronization signal FD delayed during the additional information transmission.

  During quantum cryptography communication, the intensity of the pulsed light P1 (signal light) transmitted from the sender-side terminal 30 to the communication path 32 is reduced as described above, and random phase modulation is performed for each pulse corresponding to the communication information. Therefore, communication information Scm is obtained at a small level as a homodyne detection output, and this is output from the level discriminator 54.

  On the other hand, when transmitting additional information, the intensity of the pulsed light P1 (signal light) transmitted from the sender-side terminal 30 to the communication path 32 is increased as described above, and the phase of the pulsed light P1 is randomly modulated. Since the phase is not orthogonal to any of the phases constituting the plurality of bases according to the above, random phase modulation is applied to the pulsed light P2 (reference light) as described above at the receiver-side terminal 31 as the homodyne detection output. Even if added, the frame synchronization signal FD is always obtained at a large level, and this is output from the level discriminator 54.

Here, an outline of a secret information sharing sequence by communication between the sender-side terminal 30 and the receiver-side terminal 31 in the quantum cryptography communication device 100A shown in FIG. 12 will be described with reference to FIGS. To do.
First, as shown in FIG. 13, pulsed light P <b> 1 and P <b> 2 are sent from the sender-side terminal 30 to the receiver-side terminal 31. Here, the transmitter-side terminal 30 applies the phase modulator 39 to the pulsed light P1 (signal light) to perform any one of phase modulation of {0, π / 2, π, 3π / 2}. This phase modulation sequence corresponds to the data transmission side phase modulation sequence in (b) of the lower table of FIG.

  The phase modulation sequence ((b) in the lower table of FIG. 13) executed by the transmitter terminal 30 on the pulsed light P1 may be a randomly selected sequence. Alternatively, after (a) a selection bit in the lower table of FIG. 13 is set in advance, modulation corresponding to the selection bit may be performed. For example, it is assumed that phase modulation light of 0 or π / 2 is associated with bit 0, and phase modulation of π or 3π / 2 is associated with bit 1.

The pulse light P1 subjected to such phase modulation is sent to the receiver side terminal 31 as signal light attenuated by the variable attenuator 37 (see FIG. 12). The pulsed light P2 (reference light) is sent to the receiver side terminal 31 without being attenuated. The pulsed light P1 sent from the sender-side terminal 30 to the receiver-side terminal 31 is weak signal light (the average number of photons per pulse is about one), and is sent from the sender-side terminal 30 to the receiver-side terminal 31. pulse light P2 to be is a relatively strong strong reference beam (typical mean photon number ¹⁰⁶ or so per pulse).

  The receiver-side terminal 31 that has received the pulsed light P1 and the light P2 uses the phase modulator 46 to randomly select any one of {0, π / 2}, for example, and the phase with respect to the pulsed light P1 (signal light). Modulation is performed and interference is measured in a homodyne detector.

  For example, when the phase modulation process of (c) shown in the lower table of FIG. 13 is executed in the phase modulator 46 of the receiver side terminal 31, the homodyne detector can detect the bit shown in (d). Become. (D) In the data shown in the confirmation bit based on interference, [0] and [1] are portions where bit identification due to interference can be performed, and [×] is a portion where bit identification cannot be performed. Whether or not bit identification is possible is determined by a combination of phase modulation processing executed in the sender-side terminal 1 and the receiver-side terminal 2 as described above.

  For example, the homodyne detector of the receiver-side terminal 31 is used only when the combination of phase modulation processing satisfies a predetermined condition as shown in the data shown in the confirmation bit based on (d) interference in the lower table of FIG. , Bits [0] or [1] will be detected. [×] is a portion where bit identification could not be executed.

  Thereafter, as shown in FIG. 14, the receiver-side terminal 31 notifies the transmitter-side terminal 30 of the modulation sequence information applied in the receiver-side terminal 31, that is, the information sequence in (c) of the lower table of FIG. 14. To do. It is {0, 0, π / 2, π / 2, 0.

  Based on the modulation sequence information received from the receiver-side terminal 31, the sender-side terminal 30 generates information indicating a portion where correct modulation suitable for bit detection is performed and a portion where incorrect modulation is performed. To the receiver side terminal 31. That is, the information column (e) in the lower table of the figure is notified to the receiver side terminal 2. {◯, ×, ○, ×, ○, ○ ··} shown in the figure. Note that the modulation sequence information {0, 0, π / 2, π / 2, 0... From the receiver side terminal 31 and the information {◯, ×, ○, × from the sender side terminal 30 shown in FIG. , ○, ○ ··} may apply public communication paths.

  Next, as illustrated in FIG. 15, the receiver-side terminal 31 notifies the detected bit information sequence to the transmitter-side terminal 30. {0, 0, 1, 0... On the other hand, the sender-side terminal 30 notifies the receiver-side terminal 31 of bit string information of only a portion subjected to phase modulation that can be detected on the receiver-side terminal 31 side. {0, 0, 1, 0... This is a bit series in the table in the lower part of FIG. 15 in which only (a) selected bits in which (e) is set in the modulation adaptation on the transmission / reception side are selected. These notification processes may also be executed via a public communication path.

  When communication data is not wiretapped on the communication path 32, all confirmation bits match in the bit mutual notification processing shown in FIG. However, when wiretapping of communication data is performed on the communication path 32, a shift in the notification bits occurs in the bit mutual notification processing. This is because the modulation state changes due to wiretapping of the communication path 32. When there is no eavesdropping on the communication path 32, there is no occurrence of misalignment between the notification bits.

  Such data communication makes it possible to share secret information such as a secret key in a common key cryptosystem. For example, when n bits of the secret key are shared, after confirming that the bits subjected to the mutual notification processing described with reference to FIG. 15 coincide with each other, the common bit selection processing that has been mutually notified in advance is performed. Processing such as selecting n bits from m bits (m> n) that can be shared by the above processing is executed.

  According to the quantum cryptography communication device 100A shown in FIG. 12, at the time of quantum cryptography communication, the intensity of the pulsed light P1 (signal light) transmitted from the sender side terminal 30 to the communication path 32 is reduced, and communication information is supported. Since random phase modulation is applied to each pulse, the communication information Scm is obtained at a small level as the homodyne detection output of the receiver side terminal 31 (output of the subtractor 53). Can be output separately.

  In addition, when transmitting additional information, the intensity of the pulsed light P1 (signal light) transmitted from the sender-side terminal 30 to the communication path 32 is increased, and the phase of the pulsed light P1 includes a plurality of phases related to random phase modulation. Since the phase is not orthogonal to any of the phases constituting the base (see FIGS. 7 and 9), the receiver side terminal 31 side is used as the homodyne detection output of the receiver side terminal 31 (output of the subtractor 53). Even if random phase modulation is performed by the phase modulator 46, the frame synchronization signal FD can always be obtained at a large level, and this can be distinguished and output by the level discriminator 54. Thereby, the frame synchronization signal FD can be surely recognized by the receiving side.

  Further, according to the quantum cryptography communication device 100A shown in FIG. 12, a frame synchronization signal is transmitted at the head of each frame, for example, in relation to a certain unit (unit) of communication information transmitted by quantum cryptography communication. Yes, each frame can be well synchronized.

  Further, according to the quantum cryptography communication device 100A shown in FIG. 12, at the time of transmitting additional information, the phase modulator 39 in the sender-side terminal 30 causes the phase of the pulsed light P1 (signal light) to be made up of information consisting of bit strings of 1 and 0. It can be changed according to each bit value, or the light amount level of the pulsed light P1 (signal light) is changed according to each bit value of information consisting of a bit string of 1, 0 by the variable attenuator 37 in the sender side terminal 30. The frame synchronization signal information can have information such as the type of communication information of each frame.

  Next explained is the second embodiment of the invention. FIG. 16 shows the configuration of the quantum cryptography communication device 100B as the second embodiment. The quantum cryptography communication device 100A connects a sender side terminal 1 as a first communication terminal, a receiver side terminal 2 as a second communication terminal, and the sender side terminal 1 and the receiver side terminal 2. And a communication path 3.

  Similarly to the above-described quantum cryptography communication device 100A, this quantum cryptography communication device 100B also sends a secret as communication information during quantum cryptography communication from the sender side terminal 1 to the receiver side terminal 2 via the communication path 3. Information (for example, a shared secret key used in a common key cryptosystem) is transmitted, and a frame synchronization signal as additional information is transmitted when additional information is transmitted. This frame synchronization signal is transmitted at the head of each frame, for example, in association with each frame when a certain unit (unit) of communication information transmitted by quantum cryptography communication is used as a frame. Each frame can be synchronized by this frame synchronization signal.

  The transmitter terminal 1 includes a beam splitter 19, a delay device 20, a phase modulator 21, a variable attenuator 22, a Faraday mirror 24, a detector 26, and a controller 28. The receiver side terminal 2 includes a laser light source 4, a circulator 5, a beam splitter 6 having a branching ratio of 1: 1, a phase modulator 7, a delay unit 8, a beam splitter 9, a polarization beam splitter 10, a detector 12, a homodyne. It has a detector 15, a level discriminator 16 and a controller 17.

  As the communication path 3, an optical fiber or a free space can be used. When the free space is the communication path 3, the influence of light diffraction can be reduced by using a telescope to increase the diameter of the light beam in the communication path 3.

  Hereinafter, details of processing in each component will be described in accordance with an operation sequence of communication processing to which quantum cryptography is applied.

  Pulse light generated by the laser light source 4 of the receiver side terminal 2 is transmitted to the transmitter side terminal 1 via the communication path 3, and the pulse light returns to the receiver side terminal 2 via the communication path 3 again. Since it operates in the order of coming, it explains according to the order.

  The circulator 5 of the receiver-side terminal 2 performs optical path control so that the light from the laser light source 4 is output to the beam splitter 6 and the light returned from the beam splitter 6 is output to the homodyne detector 15.

  When the pulsed light generated from the laser light source 4 of the receiver side terminal 2 is input to the beam splitter 6 through the circulator 5, the pulsed light P1 that is signal light and the pulsed light P2 that is reference light are transmitted to the beam splitter 6 by the beam splitter 6. Separated.

  The pulsed light traveling from the beam splitter 6 to the polarization beam splitter 10 through the phase modulator 7, the delay unit 8, and the beam splitter 9 is denoted by P1. Also, let P2 be the pulsed light that travels directly from the beam splitter 6 to the polarization beam splitter 10. In the figure, pulsed lights P1 and P2 heading from the receiver-side terminal 2 to the sender-side terminal 1 are indicated by solid line arrows, and pulsed lights P1 and P2 returning from the sender-side terminal 1 to the receiver-side terminal 2 are indicated by dotted-line arrows. ing.

  When the two paths from the beam splitter 6 to the polarizing beam splitter 10 are connected to each other by a polarization-maintaining fiber, the pulsed light P1 and the pulsed light P2 are merged by the polarizing beam splitter 10 and sent to the communication path 3. The pulsed light P1 and the pulsed light P2 are linearly polarized light orthogonal to each other.

  However, the pulsed light P1 is input to the communication path 3 later than the pulsed light P2 by the delay unit 8. The time difference between the pulsed light P1 and the pulsed light P2 must be sufficiently longer than the coherence time of the pulsed light of the laser light source 4, and the phase modulator 7 of the receiver side terminal 2 and the phase of the transmitter side terminal 1 The response time of the modulator 21 and the variable attenuator 22 is selected to be longer.

  The sender terminal 1 receives the pulsed lights P1 and P2 from the receiver terminal 2 via the communication path 3. In the sender-side terminal 1, the pulsed lights P <b> 1 and P <b> 2 from the communication path 3 are input to the beam splitter 19. The beam splitter 19 performs a branching process of the input light so that most of the light is output to the delay device 20 side and only a part of the light is output to the detector 26 side.

  The branching ratio of the beam splitter 19 is set so that as much light as possible travels to the delay device 20 within a range where the detector 26 can monitor the arrival of the pulsed light P2. For example, the branching ratio between the delay device 20 side and the detector 26 side is set to 9: 1.

  The detector 26 is used for monitoring the arrival of the pulsed light P2. As the detector 26, for example, a configuration in which a photodiode or an avalanche photodiode and an amplifier are combined with this can be applied. For the photodiode and the avalanche photodiode, Si can be used when the wavelength of the pulsed light is in the visible range or near infrared, and Ge or InGaAs can be used when the wavelength is 1.3 μm to 1.6 μm. Although not described in detail, the detector 12 of the receiver side terminal 2 is configured in the same manner as the detector 26.

  The detection output of the detector 26 is supplied to the controller 28. FIG. 16 shows an example in which the phase modulator 21 is installed on the communication path 3 side from the variable attenuator 22. However, the variable attenuator 22 is installed on the communication path 3 side from the phase modulator 21. May be.

  The controller 28 controls the phase modulator 21 and the variable attenuator 22. In this case, the transmittance of the variable attenuator 22 is increased with respect to the pulsed light P2 (reference light), and the phase modulator 21 is not operated.

  Moreover, the phase modulator 21 and the variable attenuator 22 are the same as the phase modulator 39 and the variable attenuator 37 in the transmitter side terminal 30 of the quantum cryptography communication apparatus 100A described above for the pulsed light P1 (signal light). The following operations are performed.

  That is, the phase modulator 21 adjusts the phase of the pulsed light P1. Under the control of the controller 28, the phase modulator 21 forms a plurality of bases for each pulse, and two bases in this embodiment (see x and p shown in FIG. 1) in the pulsed light P1 during quantum cryptography communication. Random phase modulation is applied to obtain a predetermined phase selected from the phases of 0 degrees (0 radians), 90 degrees (π / 2 radians), 180 degrees (π radians), and 270 degrees (3π / 2 radians). As described above, the quantum cryptography communication according to the present embodiment uses four quantum states.

  In addition, the phase modulator 21 controls the controller 28 so that the phase of the pulsed light P1 is any of the phases (0 degrees, 90 degrees, 180 degrees, and 270 degrees) constituting the above-described two bases when additional information is transmitted. Also, phase modulation is applied so that the phases are not orthogonal. For example, the pulsed light P1 has a phase of 45 degrees as shown by the frame region signal in FIG. As a result, as will be described later, the receiver-side terminal 2 adds a random phase modulation having a predetermined phase selected from the phases constituting the two bases described above for each pulse to the pulsed light P1 or the pulsed light P2. In this case, the output of homodyne detection at the time of transmitting additional information can be obtained at a large level regardless of the change in the basis.

  Although not described in detail, the frame synchronization signal can be provided with information such as the type of communication information of each frame in the same manner as the quantum cryptography communication device 100A described above.

  The controller 28 can know the arrival of the pulsed light P2 (reference light) based on the detection output of the detector 26, and controls the phase modulation processing start timing for the pulsed light P1 (signal light) in the phase modulator 21. To do. As a result, the phase modulator 21 can perform phase modulation processing at the correct timing for each pulse constituting the pulsed light P1 (signal light).

  The variable attenuator 22 adjusts the light amount of the pulsed light P1. The variable attenuator 22 attenuates the light amount of the pulsed light P1 during quantum cryptography communication under the control of the controller 28, and averages photons per pulse of the pulsed light P1 transmitted from the sender terminal 1 to the communication path 3. The number should be about one. Further, the variable attenuator 22 does not attenuate the light amount of the pulsed light P <b> 1 or reduces the amount of attenuation when the additional information is transmitted under the control of the controller 28, and sends it to the communication path 3 from the sender side terminal 1. The intensity of the pulsed light P1 is increased.

  Here, when the adjustment light quantity of the pulsed light P1 at the time of quantum cryptography communication is the first light quantity and the adjustment light quantity of the pulsed light P1 at the time of additional information transmission is the second light quantity, the second light quantity is the first light quantity. Become bigger. As a result, as will be described later, the homodyne detection output of the receiver-side terminal 2 at the time of transmitting additional information can be obtained at a level higher than that at the time of quantum cryptography communication. It will be possible to reliably recognize what has been done.

The average number of photons per pulse of the pulsed light P2 (reference light) transmitted from the sender-side terminal 1 to the communication path 3 is such that the S / N ratio in homodyne detection on the receiver-side terminal 2 side is optimized. Chosen. The typical intensity of the pulsed light P2 is about 10 ⁶ average photons per pulse. At this time, a typical ratio of the relative transmittances of the variable attenuator 22 on the transmitter side terminal 1 side to the pulsed light P1 and the pulsed light P2 is about 10 ⁻⁶ : 1.
The pulsed light P1 and the pulsed light P2 input from the receiver side terminal 2 to the transmitter side terminal 1 via the communication path 3 are reflected by the Faraday mirror 24 of the sender side terminal 1 and returned to the receiver side terminal 2. Therefore, the pulsed light P1 and the pulsed light P2 pass through the phase modulator 21 and the variable attenuator 22 of the sender terminal 1 twice in a reciprocating manner.

  The pulsed lights P1 and P2 input from the sender terminal 1 to the receiver terminal 2 via the communication path 3 are branched by the polarization beam splitter 10. In this case, the pulsed light P 1 is output to a short path that is directly input to the beam splitter 6, and the pulsed light P 2 is output to a long path that passes through the beam splitter 9, the delay unit 8, and the phase modulator 7. In the figure, the pulsed light P1 (signal light) and the pulsed light P2 (reference light) are indicated by dotted arrows.

  Since the pulsed light P1 and the pulsed light P2 are light reflected by the Faraday mirror 24 installed in the sender side terminal 1, the pulsed light P1 and the pulsed light returned to the polarization beam splitter 10 of the receiver side terminal 2 are used. P2 is linearly polarized light with the plane of polarization rotated by 90 degrees with respect to the pulsed light P1 and the pulsed light P2 output from the receiver side terminal 2, respectively.

  Due to this polarization, the pulsed light P1 input to the receiver side terminal 2 is output to the short path directly input to the beam splitter 6 by the polarization beam splitter 10, and the pulsed light P2 is output to the beam splitter 9, The signal is output to a long path passing through the delay unit 8 and the phase modulator 7. That is, the path between the polarization beam splitter 10 and the beam splitter 6 of the receiver side terminal 2 is the pulse light P1, P2 when traveling from the receiver side terminal 2 to the transmitter side terminal 1, and conversely, the sender side terminal. The pulse lights P1 and P2 returned from 1 to the receiver side terminal 2 are switched.

The pulsed light P <b> 2 (reference light) branched by the polarization beam splitter 10 is divided into a pulsed light traveling to the delay unit 8 and a pulsed light traveling to the detector 12 by the beam splitter 9. A typical branching ratio of the beam splitter 9 is 9 to 1, and is set so that most of the pulsed light travels toward the delay device 8.

  The configuration of the detector 12 of the receiver-side terminal 2 is the same as that of the detector 26 of the transmitter-side terminal 1, and as much light as possible is within the range where the arrival of the pulsed light P2 can be detected with respect to the branching ratio of the beam splitter 9 as well. The output is set to the delay unit 8 side.

  The output of the detector 12 is supplied to the controller 17. The controller 17 controls the operation of the phase modulator 7. The controller 17 can know the arrival of the pulsed light P2 (reference light) based on the detection output of the detector 12, and controls the phase modulation processing start timing for the pulsed light P2 (reference light) in the phase modulator 7. To do. Thereby, in the phase modulator 7, the phase modulation process is performed at the correct timing for each pulse constituting the pulsed light P2 (reference light).

  The phase modulator 7 is selected from the phases of 0 degree and 90 degrees constituting the two bases described above for each pulse for the pulsed light P2 under the control of the controller 17 during the quantum cryptography communication and the additional information transmission. Random phase modulation with a predetermined phase is applied. The phase modulator 7 may be disposed in the path of the pulsed light P1 (signal light), and the same random phase modulation may be applied to the pulsed light P1 instead of the pulsed light P2.

  When traveling from the receiver side terminal 2 to the transmitter side terminal 1, the pulsed light passes through a long path (beam splitter 6 → phase modulator 7 → delay unit 8 → beam splitter 9 → polarizing beam splitter 10). P1 passes through a short path (polarization beam splitter 10 → beam splitter 6) in the return path. On the other hand, the pulsed light P2 that has passed through a short path (beam splitter 6 → polarization beam splitter 10) in the forward path is a long path (polarization beam splitter 10 → beam splitter 9 → delay unit 8 → phase modulator 7 → beam splitter 6) in the return path. ) In this way, the pulsed lights P1 and P2 pass through a completely equidistant path in the round trip between the receiver side terminal 2 and the sender side terminal 1, and the pulsed light P1 (signal light) and the pulsed light are transmitted. P2 (reference light) arrives at the beam splitter 6 at the same time.

  The two outputs of the beam splitter 6 are input directly to the homodyne detector 15 through the circulator 5 on the other side and subjected to homodyne detection. The homodyne detection output is supplied to the level discriminator 16. This level discriminator 16 discriminates the level of the homodyne detection output and discriminates and outputs the communication information Scm transmitted at the time of quantum cryptography communication and the frame synchronization signal FD delayed at the time of additional information transmission.

  At the time of quantum cryptography communication, the intensity of the pulsed light P1 (signal light) transmitted from the sender terminal 1 to the communication path 3 is reduced as described above, and random phase modulation is performed for each pulse corresponding to the communication information. Therefore, communication information Scm is obtained at a small level as a homodyne detection output, and this is output from the level discriminator 16.

  On the other hand, when transmitting additional information, the intensity of the pulsed light P1 (signal light) transmitted from the sender-side terminal 1 to the communication path 3 is increased as described above, and the phase of the pulsed light P1 is random phase modulated. Since the phase is not orthogonal to any of the phases constituting the plurality of bases according to the above, random phase modulation is applied to the pulsed light P2 (reference light) as described above at the receiver side terminal 2 as the homodyne detection output. Even if added, the frame synchronization signal FD is always obtained at a large level, and this is output from the level discriminator 16.

  The secret information sharing sequence by communication between the sender-side terminal 1 and the receiver-side terminal 2 in the quantum cryptography communication device 100B shown in FIG. 16 is the same as that in the quantum cryptography communication device 100B shown in FIG. Since there is (see FIGS. 13 to 15), the description thereof is omitted.

  According to the quantum cryptography communication device 100B illustrated in FIG. 16, similarly to the quantum cryptography communication device 100A illustrated in FIG. 12 described above, when transmitting additional information, the pulse light P1 ( The intensity of the signal light) is increased, and the phase of the pulsed light P1 is not orthogonal to any of the phases constituting a plurality of bases related to random phase modulation (see FIGS. 7 and 9). ) As a homodyne detection output of the receiver side terminal 2, the frame synchronization signal FD can always be obtained at a large level even when random phase modulation is performed by the phase modulator 7 on the receiver side terminal 2 side. The receiving side can reliably recognize the synchronization signal FD.

  In the above-described embodiment, at the time of transmitting additional information, the phase of the pulsed light P1 (signal light) is not orthogonal to any of the phases constituting the plurality of bases described above on the sender-side terminals 30 and 1 side. The phase modulation which becomes the phase is added and transmitted to the communication path, and the receiver side terminals 31 and 2 side obtain the frame synchronization signal as the homodyne detection output, but the frame synchronization signal is transmitted as follows You may do it.

  That is, at the time of transmitting additional information, the transmitter side terminals 30 and 1 add pulse modulation P1 (signal light) to the communication path by adding phase modulation that is a phase constituting a specific base among the plurality of bases described above. At the receiver side terminals 31 and 2, homodyne detection output is performed by adding phase modulation to the pulse light P 1 (signal light) or pulse light P 2 (reference light) to the phase constituting the specific base described above. As a result, additional information is obtained.

  In this case, the receiver-side terminals 31 and 2 receive homodyne detection output when additional information is transmitted in a state where random phase modulation is applied to the pulsed light P1 (signal light) or the pulsed light P2 (reference light). When a large level signal starts to be output, the phase modulators 46 and 7 apply the phase modulation that becomes the phase constituting the specific base described above, and when the transmission of the additional information is completed, the random phase modulation is performed again. Return to the state of adding

  The present invention is capable of satisfactorily transmitting additional information such as a frame synchronization signal from a sender-side terminal to a receiver-side terminal when performing continuous variable quantum cryptography communication, such as a secret key in a secret key cryptosystem. The present invention can be applied to a quantum cryptography communication device that exchanges secret information.

It is the figure which represented the signal light of the average number of photons 1 in the continuous variable quantum cryptography which put information on the phase state of signal light on quadrature phase amplitude coordinates x and p. It is a figure which shows the histogram of a signal in case the receiving base corresponds with the transmission base, when the phase state of signal light is extracted by carrying out homodyne detection of reference light and signal light. It is a figure which shows the histogram of a signal when the bases of a transmission side and a receiving side differ when the phase state of signal light is extracted by carrying out homodyne detection of reference light and signal light. It is a figure which shows an example of the signal level and phase state of the frame synchronization signal sent from the transmission side. It is a figure which shows the histogram of the signal obtained by the homodyne detection of a receiving side in case the phase of a frame synchronizing signal exists on an x-axis. It is a figure which shows the histogram of the signal obtained by the homodyne detection of a receiving side in case the phase of a receiving side is p when the phase of a frame synchronizing signal exists on an x-axis. It is a figure which shows the state which does not make the phase of a frame synchronizing signal correspond to any base of x and P. It is a figure which shows the histogram of the signal obtained by the homodyne detection of a receiving side in the case where the level of the signal light of a frame synchronizing signal is enlarged and the phase is made not to coincide with any base of x and p. . It is a figure which shows the state which put the frame-synchronization signal in the quadrant which is not adjacent to each other, for example, the 1st quadrant and the 3rd quadrant surrounded by the mutually orthogonal x-axis and p-axis in order to put information on the frame synchronization signal. It is a figure which shows the histogram of the signal obtained by the homodyne detection of the receiving side in the state which put the frame-synchronization signal in the 1st quadrant and the 3rd quadrant. It is a figure which shows the state which put the frame-synchronization signal in the adjacent quadrant. It is a block diagram which shows the structure of the quantum cryptography communication apparatus as 1st Embodiment of this invention. It is a figure (1/3) for demonstrating an information communication process. It is a figure (2/3) for demonstrating an information communication process. It is a figure (3/3) for demonstrating an information communication process. It is a block diagram which shows the structure of the quantum cryptography communication apparatus as 2nd Embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1,30 ... Transmitter side terminal, 2,31 ... Receiver side terminal, 3,32 ... Communication path, 4,33 ... Laser light source, 5 ... Circulator, 6, 9, 19, 34, 43, 49 ... beam splitter, 7, 21, 39, 46 ... phase modulator, 8, 20 ... delay device, 10, 41, 42 ... polarization beam splitter, 12, 26, 44 ... detector, 15 ... homodyne detector, 16, 54 ... level discriminator, 17, 28, 38, 45 ... controller, 22, 37 ... variable attenuator, 24 ... Faraday mirror, 35, 40, 48 ... Reflector, 36, 47 ... Half-wave plate, 51, 52 ... Photodiode, 53 ... Subtractor, 100A, 100B ... Quantum Cryptographic communication device

Claims (18)

A quantum cryptography communication device that executes communication processing based on quantum cryptography,
A first communication terminal; a second communication terminal; a communication path connecting the first communication terminal and the second communication terminal;
The first communication terminal is
A light source that generates pulsed light;
A first light separation unit that separates pulsed light generated from the light source into signal light and reference light;
A variable attenuator for adjusting the amount of the signal light separated by the first light separation unit;
A first phase modulator that adjusts the phase of the signal light separated by the first light separation unit;
A first controller for controlling operations of the variable attenuator and the first phase modulator;
The reference light separated by the first light separation unit and the signal light separated by the first light separation unit and passed through the variable attenuator and the first phase modulator are used as the communication path. An optical transmission unit for transmitting,
The first controller is
At the time of quantum cryptography communication, the variable attenuator is controlled so that the light amount of the signal light becomes the first light amount, and the signal light has a predetermined phase selected from phases constituting a plurality of bases for each pulse. Controlling the first phase modulator to apply a random phase modulation such that
At the time of transmitting additional information, the variable attenuator is controlled so that the light amount of the signal light becomes a second light amount larger than the first light amount, and the phase of the signal light constitutes a plurality of bases. Controlling the first phase modulator so that the phase is not orthogonal to any of the above,
The second communication terminal is
A second light separation unit that separates and extracts the reference light transmitted from the first communication terminal via the communication path;
A homodyne detector for performing homodyne detection based on the signal light and the reference light separated by the second light separation unit;
A second phase modulator that adjusts the phase of the signal light or the reference light separated by the second light separation unit and supplied to the homodyne detector;
A second controller for controlling the operation of the second phase modulator;
A level discriminator that discriminates and outputs the communication information sent during the quantum cryptography communication and the additional information sent during the additional information transmission by determining the level of the output signal of the homodyne detector;
The second controller is
At the time of the quantum cryptography communication and the additional information transmission, the signal light or the reference light is subjected to random phase modulation having a predetermined phase selected from the phases constituting the plurality of bases for each pulse. A quantum cryptography communication device that controls the second phase modulator. The quantum cryptography communication device according to claim 1, wherein a unit of communication information transmitted in the quantum cryptography communication is a frame, and the additional information is transmitted in association with transmission of each frame. Surrounded by the first and second coordinate axes when the phases constituting the plurality of bases are on the first and second coordinate axes orthogonal to each other as a phase not orthogonal to any of the phases constituting the plurality of bases. 2. The quantum cryptography communication device according to claim 1, wherein phases existing in the first and third quadrants, or in the second and fourth quadrants are used. The quantum controller according to claim 1, wherein the first controller controls the variable attenuator so that the amount of the signal light is amplitude-modulated according to the additional information when the additional information is transmitted. Cryptographic communication device. A communication terminal used in a quantum cryptography communication device that executes communication processing based on quantum cryptography,
A light source that generates pulsed light;
A light separation unit that separates pulsed light generated from the light source into signal light and reference light;
A variable attenuator for adjusting the amount of the signal light separated by the light separation unit;
A phase modulator that adjusts the phase of the signal light separated by the light separation unit;
A controller for controlling the operation of the variable attenuator and the phase modulator;
A reference light separated by the light separation unit, and a light sending unit for sending the signal light separated by the light separation unit and passed through the variable attenuator and the first phase modulator to the communication path; Have
The above controller
At the time of quantum cryptography communication, the variable attenuator is controlled so that the light amount of the signal light becomes the first light amount, and the signal light has a predetermined phase selected from phases constituting a plurality of bases for each pulse. Control the phase modulator to add random phase modulation
At the time of transmitting additional information, the variable attenuator is controlled so that the light amount of the signal light becomes a second light amount larger than the first light amount, and the phase of the signal light is set to any of the plurality of phases. A communication terminal that controls the phase modulator so as to have a phase that is not orthogonal. A communication terminal used in a quantum cryptography communication device that executes communication processing based on quantum cryptography,
A light separation unit that separates and extracts signal light and reference light transmitted from another terminal device via a communication path;
A homodyne detector for performing homodyne detection based on the signal light and the reference light separated by the light separation unit;
A phase modulator that adjusts the phase of the signal light or the reference light separated by the light separation unit and supplied to the homodyne detector;
A controller for controlling the operation of the phase modulator;
A level discriminator that discriminates and outputs the communication information sent during the quantum cryptography communication and the additional information sent during the additional information transmission by determining the level of the output signal of the homodyne detector,
The above controller
During the quantum cryptography communication and during the transmission of the additional information, the signal light or the reference light is subjected to random phase modulation having a predetermined phase selected from phases constituting a plurality of bases for each pulse. A communication terminal characterized by controlling a modulator. On the first communication terminal side, the pulse light generated from the light source is separated into signal light and reference light, the reference light is transmitted to the communication path, the signal light is attenuated to the first light amount, and for each pulse. And applying random phase modulation having a predetermined phase selected from phases constituting a plurality of bases to the communication path, and then sending the signal light and the reference from the communication path on the second communication terminal side The signal light and the reference light are obtained by separating and extracting the light, and applying random phase modulation having a predetermined phase selected from the phases constituting the plurality of bases for each pulse to the signal light or the reference light. An additional information transmission method in a quantum cryptography communication device that obtains communication information by performing homodyne detection based on
On the first communication terminal side, the signal light is sent to the communication path as a second light quantity larger than the first light quantity and as a phase not orthogonal to any of the phases constituting the plurality of bases. The additional information transmission method in the quantum cryptography communication device is characterized in that additional information is obtained as an output of the homodyne detection on the second communication terminal side. A quantum cryptography communication device that executes communication processing based on quantum cryptography,
A first communication terminal; a second communication terminal; a communication path connecting the first communication terminal and the second communication terminal;
The first communication terminal is
A light source that generates pulsed light;
A first light separation unit that separates pulsed light generated from the light source into signal light and reference light;
A variable attenuator for adjusting the amount of the signal light separated by the first light separation unit;
A first phase modulator that adjusts the phase of the signal light separated by the first light separation unit;
A first controller for controlling operations of the variable attenuator and the first phase modulator;
The reference light separated by the first light separation unit and the signal light separated by the first light separation unit and passed through the variable attenuator and the first phase modulator are used as the communication path. An optical transmission unit for transmitting,
The first controller is
At the time of quantum cryptography communication, the variable attenuator is controlled so that the light amount of the signal light becomes the first light amount, and the signal light has a predetermined phase selected from phases constituting a plurality of bases for each pulse. Controlling the first phase modulator to apply a random phase modulation such that
At the time of transmitting additional information, the variable attenuator is controlled so that the light amount of the signal light is a second light amount larger than the first light amount, and a specific base is configured among the plurality of bases in the signal light Controlling the first phase modulator so as to apply phase modulation to be a phase to be
The second communication terminal is
A second light separation unit that separates and extracts the reference light transmitted from the first communication terminal via the communication path;
A homodyne detector for performing homodyne detection based on the signal light and the reference light separated by the second light separation unit;
A second phase modulator that adjusts the phase of the signal light or the reference light separated by the second light separation unit and supplied to the homodyne detector;
A second controller for controlling the operation of the second phase modulator;
A level discriminator that discriminates and outputs the communication information sent during the quantum cryptography communication and the additional information sent during the additional information transmission by determining the level of the output signal of the homodyne detector;
The second controller is
At the time of the quantum cryptography communication, the second phase modulator so as to apply, to the signal light or the reference light, random phase modulation having a predetermined phase selected from phases constituting the plurality of bases for each pulse. Control
A quantum cryptography communication device that controls the second phase modulator to add phase modulation to the signal light or the reference light to a phase that constitutes the specific base when transmitting the additional information . On the first communication terminal side, the pulse light generated from the light source is separated into signal light and reference light, the reference light is transmitted to the communication path, the signal light is attenuated to the first light amount, and for each pulse. Are added with random phase modulation having a predetermined phase selected from phases constituting a plurality of bases, and sent to the communication path, and on the second communication terminal side, the signal light and the reference light are transmitted from the communication path. The signal light and the reference light are subjected to random phase modulation having a predetermined phase selected from the phases constituting the plurality of bases for each pulse. An additional information transmission method in a quantum cryptography communication device that obtains communication information by performing homodyne detection based on:
On the first communication terminal side, the signal light is set to a second light amount larger than the first light amount, and the phase modulation that is a phase constituting a specific base among the plurality of bases is added and the communication is performed. The second communication terminal side adds phase modulation as a phase constituting the specific base to the signal light or the reference light, and obtains additional information as an output of the homodyne detection. A method for transmitting additional information in a quantum cryptography communication device. A quantum cryptography communication device that executes communication processing based on quantum cryptography,
A first communication terminal; a second communication terminal; a communication path connecting the first communication terminal and the second communication terminal;
The first communication terminal is
A first light sending section for sending the signal light and the reference light sent from the second communication terminal via the communication path to the communication path via a predetermined optical path;
A variable attenuator for adjusting the amount of the signal light passing through the predetermined optical path;
A first phase modulator for adjusting the phase of the signal light passing through the predetermined optical path;
A first controller for controlling the operation of the variable attenuator and the first phase modulator;
The first controller is
At the time of quantum cryptography communication, the variable attenuator is controlled so that the light amount of the signal light becomes the first light amount, and the signal light has a predetermined phase selected from phases constituting a plurality of bases for each pulse. Controlling the first phase modulator to apply a random phase modulation such that
At the time of transmitting additional information, the variable attenuator is controlled so that the light amount of the signal light becomes a second light amount larger than the first light amount, and the phase of the signal light constitutes a plurality of bases. Controlling the first phase modulator so that the phase is not orthogonal to any of the above,
The second communication terminal is
A light source that generates pulsed light;
A first light separation unit that separates pulsed light generated from the light source into signal light and reference light;
A first optical path in which a delay is inserted;
A second optical path in which no delay is inserted;
The signal light separated by the first light separation unit and passed through the first optical path is combined with the reference light separated by the first light separation unit and passed through the second optical path. A second light sending unit for sending to the communication path;
A second light separation unit that separates and extracts the signal light and the reference light transmitted from the first communication terminal via the communication path;
A homodyne detector for performing homodyne detection based on the signal light and the reference light separated by the second light separation unit;
A second phase modulator that adjusts the phase of the signal light or the reference light that is separated by the second light separation unit and supplied to the homodyne detector;
A second controller for controlling the operation of the second phase modulator;
A level discriminator that discriminates and outputs the communication information sent during the quantum cryptography communication and the additional information sent during the additional information transmission by determining the level of the output signal of the homodyne detector;
The second controller is
During the quantum cryptography communication and the additional information transmission, the signal light or the reference light is subjected to random phase modulation having a predetermined phase selected from phases constituting a plurality of bases for each pulse. 2. A quantum cryptography communication device characterized by controlling the phase modulator of 2. The quantum cryptography communication device according to claim 10, wherein a unit of information transmitted by the quantum cryptography communication is a frame, and the additional information is transmitted in association with transmission of each frame. Surrounded by the first and second coordinate axes when the phases constituting the plurality of bases are on the first and second coordinate axes orthogonal to each other as a phase not orthogonal to any of the phases constituting the plurality of bases. The quantum cryptography communication device according to claim 10, wherein phases existing in the first and third quadrants, or the second and fourth quadrants are used. 11. The quantum according to claim 10, wherein the first controller controls the variable attenuator so that the amount of the signal light is amplitude-modulated according to the additional information when the additional information is transmitted. Cryptographic communication device. A communication terminal used in a quantum cryptography communication device that executes communication processing based on quantum cryptography,
A light transmission unit for transmitting signal light and reference light transmitted from another communication terminal via a communication path to the communication path via a predetermined optical path;
A variable attenuator for adjusting the amount of the signal light passing through the predetermined optical path;
A phase modulator for adjusting the phase of the signal light passing through the predetermined optical path;
A controller for controlling the operation of the variable attenuator and the phase modulator,
The above controller
At the time of quantum cryptography communication, the variable attenuator is controlled so that the light amount of the signal light becomes the first light amount, and the signal light has a predetermined phase selected from phases constituting a plurality of bases for each pulse. Control the phase modulator to add random phase modulation
At the time of transmitting additional information, the variable attenuator is controlled so that the light amount of the signal light becomes a second light amount larger than the first light amount, and the phase of the signal light constitutes a plurality of bases. A communication terminal characterized by controlling the phase modulator so that the phase is not orthogonal to any of the above. A communication terminal used in a quantum cryptography communication device that executes communication processing based on quantum cryptography,
A light source that generates pulsed light;
A first light separation unit that separates pulsed light generated from the light source into signal light and reference light;
A first optical path in which a delay is inserted;
A second optical path in which no delay is inserted;
The signal light separated by the first light separation unit and passed through the first optical path is combined with the reference light separated by the first light separation unit and passed through the second optical path. , An optical transmission unit for transmitting to the communication path;
A second light separation unit that separates and extracts the signal light and the reference light transmitted from another communication terminal via the communication path;
A homodyne detector for performing homodyne detection based on the signal light and the reference light separated by the second light separation unit;
A phase modulator that adjusts the phase of the signal light or the reference light separated by the second light separation unit and supplied to the homodyne detector;
A controller for controlling the operation of the second phase modulator;
A level discriminator that discriminates and outputs the communication information sent during the quantum cryptography communication and the additional information sent during the additional information transmission by determining the level of the output signal of the homodyne detector;
The above controller
During the quantum cryptography communication and during the transmission of the additional information, the signal light or the reference light is subjected to random phase modulation having a predetermined phase selected from phases constituting a plurality of bases for each pulse. A communication terminal characterized by controlling a modulator. On the second communication terminal side, the pulsed light generated from the light source is separated into signal light and reference light, and the reference light is sent to the communication path through the second optical path in which no delay device is inserted. Is transmitted to the communication path through the first optical path in which the delay device is inserted, and the signal light and the reference light transmitted via the communication path are transmitted through the predetermined optical path on the first communication terminal side. When transmitting to the communication path, the light amount of the signal light is attenuated to a first light amount, and random phase modulation is applied to the signal light so as to have a predetermined phase selected from phases constituting a plurality of bases for each pulse. The second communication terminal side separates and extracts the signal light and the reference light transmitted via the communication path, and configures the plurality of bases for each pulse of the signal light or the reference light It becomes the predetermined phase selected from the phase to be After addition of random phase modulation or additional information transmission method in a quantum cryptography communication apparatus for obtaining communication information by performing a homodyne detection based signal light and the reference light,
On the first communication terminal side, the signal light is sent to the communication path as a second light quantity larger than the first light quantity and as a phase not orthogonal to any of the phases constituting the plurality of bases. The additional information transmission method in the quantum cryptography communication device is characterized in that additional information is obtained as an output of the homodyne detection on the second communication terminal side. A quantum cryptography communication device that executes communication processing based on quantum cryptography,
A first communication terminal; a second communication terminal; a communication path connecting the first communication terminal and the second communication terminal;
The first communication terminal is
A first light sending section for sending the signal light and the reference light sent from the second communication terminal via the communication path to the communication path via a predetermined optical path;
A variable attenuator for adjusting the amount of the signal light passing through the predetermined optical path;
A first phase modulator for adjusting the phase of the signal light passing through the predetermined optical path;
A first controller for controlling the operation of the variable attenuator and the first phase modulator;
The first controller is
At the time of quantum cryptography communication, the variable attenuator is controlled so that the light amount of the signal light becomes the first light amount, and the signal light has a predetermined phase selected from phases constituting a plurality of bases for each pulse. Controlling the first phase modulator to apply a random phase modulation such that
At the time of transmitting additional information, the variable attenuator is controlled so that the light amount of the signal light is a second light amount larger than the first light amount, and a specific base is configured among the plurality of bases in the signal light Controlling the first phase modulator so as to apply phase modulation to be a phase to be
The second communication terminal is
A light source that generates pulsed light;
A first light separation unit that separates pulsed light generated from the light source into signal light and reference light;
A first optical path in which a delay is inserted;
A second optical path in which no delay is inserted;
The signal light separated by the first light separation unit and passed through the first optical path is combined with the reference light separated by the first light separation unit and passed through the second optical path. A second light sending unit for sending to the communication path;
A second light separation unit that separates and extracts the signal light and the reference light transmitted from the first communication terminal via the communication path;
A homodyne detector for performing homodyne detection based on the signal light and the reference light separated by the second light separation unit;
A second phase modulator that adjusts the phase of the signal light or the reference light separated by the second light separation unit and supplied to the homodyne detector;
A second controller for controlling the operation of the second phase modulator;
A level discriminator that discriminates and outputs the communication information sent during the quantum cryptography communication and the additional information sent during the additional information transmission by determining the level of the output signal of the homodyne detector;
The second controller is
At the time of the quantum cryptography communication, the second phase modulator is added to the signal light or the reference light so as to apply random phase modulation having a predetermined phase selected from phases constituting a plurality of bases for each pulse. Control
A quantum cryptography communication device that controls the second phase modulator to add phase modulation to the signal light or the reference light to a phase that constitutes the specific base when transmitting the additional information . On the second communication terminal side, the pulsed light generated from the light source is separated into signal light and reference light, and the reference light is sent to the communication path through the second optical path in which no delay device is inserted. Is transmitted to the communication path through the first optical path in which the delay device is inserted, and the signal light and the reference light transmitted via the communication path are transmitted through the predetermined optical path on the first communication terminal side. When transmitting to the communication path, the light amount of the signal light is attenuated to a first light amount, and random phase modulation is applied to the signal light so as to have a predetermined phase selected from phases constituting a plurality of bases for each pulse. The second communication terminal side separates and extracts the signal light and the reference light transmitted via the communication path, and configures the plurality of bases for each pulse of the signal light or the reference light It becomes the predetermined phase selected from the phase to be After addition of random phase modulation or additional information transmission method in a quantum cryptography communication apparatus for obtaining communication information by performing a homodyne detection based signal light and the reference light,
On the first communication terminal side, the signal light is set to a second light amount larger than the first light amount, and the phase modulation that is a phase constituting a specific base among the plurality of bases is added and the communication is performed. And the second communication terminal side adds phase modulation to the signal light or the reference light to the phase constituting the specific base, and obtains additional information as an output of homodyne detection. A method for transmitting additional information in a quantum cryptography communication device.

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