JP2015122675A - Modulation device and modulation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a modulation device capable of sufficiently suppressing an output intensity variation when applying output modulation to an optical pulse.SOLUTION: The modulation device includes intensity modulation means 101, detection means 102 and control means 103. The intensity modulation means 101 applies intensity modulation to an inputted optical pulse and outputs a result as an output pulse. The detection means 102 detects an intensity of the output pulse. The control means 103 corrects a modulation amount on the basis of the intensity of the output pulse and controls the modulation amount of the intensity modulation means 101 so as to provide an output intensity selected from among a plurality of stages of preset intensities other than maximum output.

Description

  The present invention relates to a quantum cryptography communication system, and more particularly to a technique for transmitting a quantum cryptography key using a decoy method.

  With the development of the information and communication society, various information is transmitted on the communication network, and it is expected that highly confidential information can be transmitted on the communication network. Therefore, in the process of transmitting information, highly confidential information leaks to a third party, that is, encryption communication technology for actively sharing information with a communication partner to prevent wiretapping is being actively developed. ing.

  As an encryption communication system with extremely high security, for example, there is a quantum encryption communication system using a quantum key distribution (QKD) system. In the quantum encryption key method, it is ideal to perform transmission by adding information to a single photon. Even if a third party attempts to eavesdrop on the transmission line, once observed photons cannot be completely restored to their original quantum state. Therefore, in the case of a single photon, it is possible to detect that an eavesdropping action has been performed from a change in received data. However, it is difficult to obtain a light source that stably generates a single photon, and a method of obtaining a pseudo single photon is often used by weakening the output light from the laser light source to an extremely low intensity.

  When a pseudo single photon from a laser light source is used, it is inevitable that one pulse includes two or more photons with a certain probability. When two or more photons are included in one pulse, even if a third party intercepts photon information on the transmission path, it becomes impossible to detect the wiretapping action. As an eavesdropping method for a system in which two or more photons are included in one pulse, for example, there is an eavesdropping method called a photon number splitting attack (PNS attack). If a PNS attack is possible, the security of the quantum key distribution type quantum cryptography communication system is greatly reduced. For example, a decoy method may be used as a means for responding to the PNS attack.

  In the decoy method, decoy light, that is, a light pulse that becomes a trap is included in the light pulse used as a signal and transmitted, and the information on the transmitting side is compared with the detection result on the receiving side to check for eavesdropping. This is an estimation method. In the decoy method, light pulses having three or more types of intensity are used. In the case of the decoy method using three kinds of light pulses, one of the three kinds of intensity settings is assigned to the light pulse of the signal light, and the other two are used as the light pulse of the decoy light. Assigned. The signal light is an optical pulse used for transmitting information of the quantum encryption key. In the decoy method, for example, 90% of the transmitted optical pulses are used as signal light, and the remaining 10% of optical pulses are used as decoy light. The ratio of the two decoy lights is set as 6% and 4%.

  In the decoy method, an intensity modulator for generating light pulses of three types of intensity is required. Further, in optical communication, it is necessary to perform signal processing at high speed, and an intensity modulator is also required to modulate light intensity at high speed. For example, an LN (Lithium Niobate) intensity modulator that combines an interference effect by a Mach-Zehnder interferometer and phase modulation is suitable as an intensity modulator when intensity modulation is performed on an optical pulse in the decoy method. The LN intensity modulator branches an input optical pulse into two waveguides. The optical pulse branched into the two waveguides is subjected to phase modulation by a phase modulator provided in each waveguide. The optical pulses that have undergone phase modulation in each waveguide are output as one optical pulse due to the interference effect. The LN intensity modulator controls the amount of phase modulation applied to two pulses, and performs intensity modulation by causing optical pulses having different phases to interfere with each other. Therefore, the LN intensity modulator controls the intensity modulation amount by the difference between the phase modulation amounts applied to the two pulses, that is, the phase difference.

  In a quantum cryptosystem quantum cryptography communication system, the average number of photons contained in one pulse is extremely small. Therefore, when generating optical pulses with different intensities used in the decoy scheme, high-precision intensity modulation is required. However, the optical path length of the waveguide of the intensity modulator varies due to a change in temperature or the like. Therefore, even if the amount of phase modulation applied to each optical pulse is the same, interference may occur with a phase difference different from the intended phase difference due to fluctuations in the optical path length. As a result, the intensity of the optical pulse output due to the difference in phase difference can be in a state where it deviates from the intended intensity. In addition, a change in the voltage value applied to the electrode for applying an electric field to the optical pulse during phase modulation may cause a change in the phase modulation amount, resulting in a change in output intensity from the intensity modulator. In the decoy method, increasing the accuracy of the intensity of signal light and decoy light is one of important techniques. For this reason, a technique for suppressing such fluctuations in intensity generated in the intensity modulator has been studied. In the decoy method, as a technique for suppressing fluctuations in output intensity generated in the intensity modulator, for example, there is a technique disclosed in Patent Document 1.

  Patent Document 1 discloses a technique in which an LN intensity modulator provided in a quantum cryptography transmission device for generating optical pulses having different intensities for the decoy method suppresses fluctuations in output intensity by correcting the modulation amount. It is shown. The quantum cryptography transmission device disclosed in Patent Document 1 includes a control circuit having a function of correcting the phase modulation amount of the LN intensity modulator in order to suppress fluctuations in output intensity from the LN intensity modulator. The control circuit determines the correction amount of the phase modulation amount based on the information on the number of photons detected by the quantum cryptography reception device and transmitted via the communication network. In Patent Document 1, since the signal light is set to have the maximum output intensity of the intensity modulator, the number of detected photons almost indicates the number of photons of the signal light. In Patent Document 1, the presence or absence of intensity fluctuation is determined based on the information on the number of photons sent from the receiving side, and the voltage applied to the electrode during phase modulation is controlled, thereby providing a signal from the intensity modulator. The fluctuation of output intensity is suppressed. Patent Document 1 also discloses a technique in which a light detector is provided after the intensity modulator, and correction is performed based on the intensity detection result in the quantum cryptography transmission device. Patent Document 1 states that, by using such a method, a stable intensity modulation operation can be performed for a long time even when an optical pulse with a small intensity is used.

International Publication No. 2009/011255

  However, the technique of Patent Document 1 is not sufficient in the following points. In Patent Document 1, the intensity at which the output intensity of the intensity modulator is maximized is used for the optical pulse of the signal light. In the intensity modulator using phase modulation, the intensity change with respect to the variation in phase difference is the smallest in the vicinity of the maximum output intensity. That is, in the vicinity of the maximum output intensity, fluctuations in the intensity caused by changes in the optical path length, changes in the voltage applied to the electrodes, and the like are small, so fluctuations in the phase difference may not be detected accurately from fluctuations in intensity. As a result, there is a possibility that the communication may be continued in a state where the intensity of the decoy light having a large fluctuation in intensity with respect to the fluctuation in the phase modulation amount is largely changed without sufficiently detecting the fluctuation in the phase modulation amount. Therefore, the technique of Patent Document 1 is not sufficient as a technique used in a quantum cryptography communication system using a decoy method that needs to suppress fluctuations in intensity in order to obtain a stable intensity of output light. An object of the present invention is to obtain a modulation device that can sufficiently suppress fluctuations in output intensity when intensity modulation is performed on an optical pulse.

  In order to solve the above problems, the modulation device of the present invention includes intensity modulation means, detection means, and control means. The intensity modulation means performs intensity modulation on the input optical pulse and outputs it as an output pulse. The detection means detects the intensity of the output pulse. The control unit corrects the modulation amount based on the intensity of the output pulse, and controls the modulation amount of the intensity modulation unit so that the output intensity is selected from the intensity set in a plurality of stages other than the maximum output.

  According to the modulation method of the present invention, intensity modulation is performed on an input optical pulse and output as an output pulse to detect the intensity of the output pulse. In the modulation method of the present invention, the modulation amount is corrected based on the intensity of the optical pulse, and the modulation amount is controlled so that the output intensity is selected from the intensities set in a plurality of stages other than the maximum output. .

  According to the present invention, it is possible to sufficiently suppress fluctuations in output intensity when intensity modulation is performed on an optical pulse.

It is a figure which shows the outline | summary of a structure of the 1st Embodiment of this invention. It is a figure which shows the outline | summary of a structure of the 2nd Embodiment of this invention. It is the figure which showed the intensity | strength and frequency of each signal in the 2nd Embodiment of this invention. It is a figure which shows the structure of a part of apparatus in the 2nd Embodiment of this invention. It is the figure which showed the graph of the characteristic of the output intensity in the 2nd Embodiment of this invention. It is the figure which showed the relationship between the modulation amount in the 2nd Embodiment of this invention, output intensity, and the kind of signal. It is the figure which showed the graph of the characteristic of the output intensity in the example contrasted with this invention. It is the figure which showed the relationship of the modulation amount, output intensity, and the kind of signal in the example contrasted with this invention.

  A first embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows an outline of the configuration of the modulation apparatus of this embodiment. The modulation apparatus according to the present embodiment includes intensity modulation means 101, detection means 102, and control means 103. The intensity modulation means 101 performs intensity modulation on the input optical pulse and outputs it as an output pulse. The detection means 102 detects the intensity of the output pulse. The control unit 103 corrects the modulation amount based on the intensity of the output pulse, and controls the modulation amount of the intensity modulation unit 101 so that the output intensity is selected from the intensity set in a plurality of stages other than the maximum output. To do.

  In the modulation device according to the present embodiment, the control unit corrects the modulation amount of the intensity modulation and controls the intensity to be an intensity other than the maximum intensity, and the intensity modulation unit performs intensity modulation on the optical pulse. Intensities other than the maximum intensity have a greater variation in intensity relative to the variation in modulation amount than the maximum intensity. For this reason, a change in the modulation amount is easily detected at an intensity other than the maximum intensity as compared with the maximum intensity. Therefore, the accuracy of correction of the modulation amount is improved by performing the intensity modulation with an intensity other than the maximum intensity and detecting the modulation amount fluctuation. By improving the accuracy of correction of the modulation amount, it is possible to suppress the fluctuation of the intensity of the optical pulse output from the intensity modulation means from the set intensity. As a result, in the modulation device according to the present embodiment, fluctuations in output intensity when intensity modulation is performed on an optical pulse can be sufficiently suppressed.

  A second embodiment of the present invention will be described in detail with reference to the drawings. FIG. 2 shows an outline of the configuration of the quantum cryptography communication system of the present embodiment.

  The quantum cryptography communication system of this embodiment includes a quantum cryptography transmission device 10 and a quantum cryptography reception device 20. In addition, the quantum cryptography communication system according to the present embodiment includes an optical network 31 that sends a quantum cryptography key from the quantum cryptography transmission device 10 to the quantum cryptography reception device 20. The quantum cryptography transmission device 10 and the quantum cryptography reception device 20 are connected via a communication network 32.

  The quantum cryptography transmission device 10 includes a light source unit 11, an encoding unit 12, a decoy modulation unit 13, a light attenuation unit 14, a light detection unit 15, and a key distillation processing unit 16. The light source unit 11 has a function of outputting a light pulse. In the present embodiment, the light source unit 11 includes a semiconductor laser and outputs light pulses having a predetermined wavelength and intensity.

  Based on the information from the key distillation processing unit 16, the encoding unit 12 has a function of performing modulation for adding encryption key information to the input optical pulse and outputting the modulated light pulse. In the present embodiment, information encoded according to the BB84 protocol is added to the optical pulse by the phase modulation method. The BB84 protocol is a quantum key distribution protocol proposed in 1984 by Bennett and Brassard. The encoding unit 12 of this embodiment includes a Mach-Zehnder interferometer and a phase modulator. The optical pulse input to the encoding unit 12 is separated into double pulses by a Mach-Zehnder interferometer having waveguides with different optical path differences. The phase modulator of the encoding unit 12 performs phase modulation on the optical pulse so that the phase difference between the double pulses becomes a phase difference corresponding to the information input from the key distillation processing unit 16. In the present embodiment, phase modulation is performed so that the phase difference becomes one of 0, π, π / 2, and 3π / 2 corresponding to the four types of codes input from the key distillation processing unit 16. The encoding unit 12 of the present embodiment includes two phase modulators corresponding to binarization or one unit corresponding to binarization in order to perform phase modulation corresponding to four types of codes on an optical pulse. A phase modulator is provided.

  The decoy modulation unit 13 has a function of performing intensity modulation on an optical pulse in order to generate optical pulses having different intensities corresponding to the decoy method. In the quantum cryptography communication system of the present embodiment, three intensity light pulses corresponding to the decoy method are set. FIG. 3 is a table showing the assignment of light pulses of three intensities to signal light and decoy light according to this embodiment. In FIG. 3, the signal light is shown as signal light (S), and the decoy light is shown as 囮 1 (D) and 囮 2 (Z). The intensity in FIG. 3 indicates the average number of photons that the signal light and the decoy light each contain per pulse. The intensity shown in FIG. 3 is the average number of photons contained in one pulse after the attenuation process is performed in the light attenuating unit 14. The intensity ratio in FIG. 3 indicates the relative intensity of the decoy light when the intensity of the signal light is 1. The mixing ratio in FIG. 3 indicates the ratio of the number of optical pulses respectively selected as signal light or decoy light with respect to all the number of optical pulses. As shown in FIG. 3, the maximum intensity among the three intensities is assigned to the signal light. The signal light is selected at a rate of 90%. The remaining two intensities are assigned to the two decoy lights, respectively. When the intensity of the signal light is 1, the intensity of the two decoy lights is assigned 0.4 and 0, respectively. The decoy light having an intensity of 0.4 is selected at a ratio of 6%, and the decoy light having an intensity of 0 is selected at a ratio of 4%. In the quantum cryptography communication system of the present embodiment, 90% of the optical pulses to be transmitted are used as signal light, and the remaining 10% of the optical pulses are used as decoy light.

  An LN (Lithium Niobate) intensity modulator can be used for the decoy modulator 13 of the present embodiment. The LN intensity modulator is an intensity modulator that combines a light interference effect by a Mach-Zehnder interferometer and a phase modulation method. The LN intensity modulator branches an input optical pulse into two waveguides, phase modulates the optical pulse passing through each of the waveguides, causes interference, and outputs the result to one waveguide. In the LN intensity modulator, by controlling the phase modulation amount of the optical pulse that passes through each of the two waveguides, the intensity of light according to the difference in phase modulation amount, that is, the phase difference of the optical pulse that passes through the two waveguides. Pulses can be generated.

  FIG. 4 shows an outline of the configuration of the decoy modulation unit 13 of the present embodiment. As shown in FIG. 4, the decoy modulation unit 13 includes a control unit 41, a correction amount control unit 42, a first phase modulation unit 43, and a second phase modulation unit 44. The optical pulse input to the decoy modulation unit 13 is branched into two waveguides, modulated by a phase modulation unit provided on each waveguide, and then sent to one waveguide. Output from the decoy modulator 13.

  Based on the information on the intensity of the optical pulse sent from the key distillation processing unit 16, the control unit 41 determines the phase modulation amount to be applied to the optical pulse by the first phase modulation unit 43 and the second phase modulation unit 44, respectively. calculate. The control unit 41 stores in advance the relationship between the intensity of the output optical pulse and the amount of phase modulation applied to the optical pulse by the first phase modulation unit 43 and the second phase modulation unit 44. The control unit 41 converts the calculated phase modulation amount into a signal indicating a voltage value applied to the electrodes by the first phase modulation unit 43 and the second phase modulation unit 44 to convert the first phase modulation unit 43 and the second phase modulation unit 43 into second signals. To the phase modulation unit 44. The control unit 41 stores the relationship between the phase modulation amount and the voltage value in advance. In addition, when calculating the phase modulation amount, the control unit 41 corrects the phase modulation amount using the correction amount information transmitted from the correction amount control unit 42.

  The correction amount control unit 42 has a function of calculating the correction amount of the phase modulation amount based on the intensity information sent from the light detection unit 15. The correction amount control unit 42 stores in advance the relationship between the detected intensity and the correction amount of the phase modulation amount. The correction amount control unit 42 sends information on the correction amount of the phase modulation amount to the control unit 41.

The first phase modulation unit 43 and the second phase modulation unit 44 perform phase modulation on the input optical pulse. The first phase modulation unit 43 and the second phase modulation unit 44 apply intensity modulation to an optical pulse passing through the waveguide by applying an electric field to the waveguide formed of LiNbO ₃ (lithium niobate) crystal. The LN intensity modulator of the present embodiment is a two-electrode modulator having two phase modulation units each provided with an electrode for applying an electric field to a waveguide formed of LiNbO ₃ crystal. The first phase modulation unit 43 and the second phase modulation unit 44 generate an electric field by applying a voltage based on the voltage signal from the control unit 41 to the electrodes, and perform phase modulation on the input optical pulse. Apply. The amount of modulation when the optical pulse is subjected to phase modulation by the first phase modulation unit 43 and the second phase modulation unit 44 is controlled by the voltage value applied to the electrodes.

FIG. 5 shows the relationship between the intensity of the optical pulse output from the decoy modulation unit 13 of the present embodiment and the difference between the phase modulation amounts applied by the first phase modulation unit 43 and the second phase modulation unit 44. It is a thing. It is assumed that the modulation amount of the phase modulation applied to the optical pulse by the first phase modulation unit 23 is represented by φ1, and the modulation amount of the phase modulation applied to the optical pulse by the second phase modulation unit 24 is represented by φ2. The horizontal axis of FIG. 5 represents the difference in the amount of phase modulation applied to the optical pulse by the first phase modulation unit 43 and the second phase modulation unit 44, that is, after the phase modulation of the optical pulse passing through the two waveguides. The phase difference φ1-φ2 is shown. The vertical axis in FIG. 5 indicates the intensity of the optical pulse output from the decoy modulator 13. It is assumed that the intensity of the optical pulse input to the decoy modulation unit 13 is represented as Iin, and the intensity of the optical pulse output from the decoy modulation unit 13 is represented as Iout. At that time, the intensity Iout of the output optical pulse can be expressed as Iout = Iin × cos ² ((φ1−φ2) / 2). Therefore, the intensity of the output light from the decoy modulation unit 13 exhibits characteristics as shown in FIG. In the present embodiment, the decoy modulator 13 is controlled so as to output light pulses having intensities shown as S, D and Z in FIG. The intensity s indicated by S in FIG. 5 is assigned to the signal light. Also, the intensities indicated by D and Z in FIG. 5 are assigned to two decoy lights, respectively. In FIG. 5, the intensity of D used as one of the decoy lights is set so that the intensity d of D satisfies d = 0.4 × s. The intensity of Z used as another decoy light is set to 0 as the intensity of the light pulse.

  FIG. 6 shows the relationship between the combination of the phase modulation amounts φ1 and φ2 in the first phase modulation unit 43 and the second phase modulation unit 44 and the intensity of the optical pulse output from the decoy modulation unit 13 in the present embodiment. Is shown. The output light intensity in FIG. 6 is shown with 1 indicating the maximum output intensity. The output light intensity becomes the maximum output when the phase modulation amount is φ1 = φ2, that is, when the phase difference is 0 °. In the quantum cryptography transmission device 10 of the present embodiment, the intensity is assigned. Not done. In this embodiment, the phase modulation amount of the signal having the intensity indicated by S in FIG. 5 used as signal light is set to φ1 = 0 ° and φ2 = 90 ° so that the output intensity is 50% of the maximum output intensity. . Further, the combination in which the phase modulation amounts are φ1 = θ ′ and φ2 = 180 ° is assigned to D in FIG. 5 which is one of the two decoy lights. In the present embodiment, θ ′ is set such that the intensity of the optical pulse assigned to D is 40% of the intensity of the optical pulse assigned to S. At this time, θ ′ is about 53 °. A combination in which the phase modulation amounts are φ1 = 0 ° and φ2 = 180 ° is assigned to Z in FIG. 5 which is the other of the two decoy lights. A combination in which the phase modulation amounts are φ1 = θ ′ and φ2 = 90 ° is not used in the present embodiment.

  For comparison with the present embodiment, FIG. 7 shows the relationship between the intensity of the output light and the difference between the phase modulation amounts in the two phase modulation sections when the maximum output intensity is assigned to the signal light S. Show. FIG. 7 shows a case where the intensity that is the maximum output is assigned to S, which is signal light, when an LN intensity modulator having the same configuration as the decoy modulator 13 of the present embodiment is used. The horizontal axis in FIG. 7 indicates the difference in phase modulation amount between the two phase modulation units as the phase difference, as in FIG. The vertical axis in FIG. 7 indicates the intensity of the output light from the LN intensity modulator as in FIG. In the example of FIG. 7, the signal light S is assigned to the maximum intensity s, and in FIG. 7, one of the two decoy lights is assigned to D, which is 40% of the intensity s. The other decoy light is assigned to Z having an intensity of zero. When S is assigned to the maximum output, 40% of the intensity D is an output with a phase difference of 102 °. FIG. 8 shows the correspondence between the combination of the phase modulation amounts and the intensity of the output light when the maximum output intensity is assigned to S, which is the signal light, as in FIG. When setting so as to have the characteristics of output light as shown in FIG. 7, a combination of phase modulation amounts whose phase modulation amounts are both 0 is assigned to the signal light as the maximum output light as shown in FIG. ing.

  As described above, in the present embodiment, the intensity that is the maximum output of the decoy modulator 13 is not assigned to the signal light and the decoy light used in the decoy method. Here, the influence of the fluctuation of the phase modulation amount on the intensity of the output light between the case where the intensity is assigned to each signal as in the present embodiment and the case where the intensity corresponding to the maximum output is assigned to the signal light. A comparison is made with reference to FIGS. The position S in FIG. 5 assigned to the signal light in the present embodiment is a position where the variation in the intensity of the output light is maximized when the phase difference varies by a certain amount. That is, when the variation of the phase modulation amount is to be detected by measuring the intensity of the output light, the position S in FIG. 5 can detect the variation of the phase difference most sensitively by measuring the intensity. Further, since the signal light is selected very frequently, fluctuations in the intensity detected by the light detection unit 15 subsequent to the decoy modulation unit 13 are represented by fluctuations in the phase difference of the signal light, that is, fluctuations in the phase modulation amount. Can be considered. Therefore, it is possible to detect the fluctuation of the phase modulation amount of the signal light by using the average value of the intensity of the optical pulse output from the decoy modulator 13.

  On the other hand, in the case of FIG. 7 in which S is set at the position of the maximum output intensity, the intensity fluctuation when the phase difference fluctuates by a certain amount is the smallest in the vicinity of S. Therefore, in the case of FIG. 7, the intensity measurement may not sufficiently detect the phase difference variation. In the case of FIG. 7, it can be considered that the intensity measurement is performed at the position D, but the frequency at which the intensity at the position D allocated to the decoy light is selected is small. Therefore, it is difficult to separately detect the intensity of only the optical pulse whose intensity at the position D is selected. Therefore, even in the case of FIG. 7 in which the signal light is assigned to the maximum output intensity, it is necessary to measure the intensity at the position S, so that the detection accuracy of the variation in the phase modulation amount can be greatly inferior to the present embodiment. There is sex. The influence of the signal light is small because the intensity fluctuation is small, but if the fluctuation of the phase modulation amount cannot be detected, it is determined that there is no abnormality and the operation can be continued. In that case, there is a possibility that the operation may be continued while the intensity of the decoy light set in a region where the intensity variation with respect to the variation of the phase modulation amount is large varies greatly. In this embodiment, by assigning an intensity other than the maximum output intensity to the intensity of the signal light, it is possible to sensitively detect a variation in the phase modulation amount based on the intensity measurement result. Therefore, the fluctuation in the intensity of the signal light can be detected to correct the phase modulation amount, and the fluctuation in the output intensity of not only the signal light but also the decoy light can be suppressed. In addition, by setting the intensity of the signal light to an intensity at which the intensity fluctuation with respect to the change in phase difference is maximized, the effect of improving the detection accuracy of the phase fluctuation amount becomes the highest.

  The light attenuating unit 14 has a function of attenuating the intensity of the input optical pulse so that the average number of photons included in the optical pulse becomes a predetermined number. In the present embodiment, a variable optical attenuator is used for the optical attenuator 14. In the present embodiment, the intensity of the optical pulse output from the optical attenuator 14 is set according to the design of the quantum cryptography communication system. In addition, the amount of attenuation corresponding to the setting of the intensity of the optical pulse output from the optical attenuator 14 is set in advance when the quantum cryptography transmitter 10 is installed or when the quantum cryptography communication system is maintained or adjusted. ing. Instead of such a configuration, the amount of attenuation of the light attenuating unit 14 may be controlled according to the intensity detected by the light detecting unit 15.

  The light detection unit 15 has a function of measuring the intensity of an optical pulse branched between the decoy modulation unit 13 and the light attenuation unit 14. The light detection unit 15 includes a photodiode, and converts the intensity of the input light pulse into an electric signal. The intensity of the light pulse detected by the light detection unit 15 is an average value of the intensity of the light pulse input to the light detection unit 15 at a predetermined time. The light detection unit 15 outputs the intensity measurement result to the correction amount control unit 42 of the decoy modulation unit 13. All three kinds of intensity light pulses output from the decoy modulator 13 are input to the light detector 15. However, since the generation rate of signal light and the intensity of the light pulse are higher than that of decoy light, the intensity of the detected light pulse is almost due to the signal light, and the intensity of the decoy light hardly appears in the measurement result. Therefore, the intensity of the light pulse measured by the light detection unit 15 can be regarded as the intensity of the signal light.

  The key distillation processing unit 16 has a function of performing general control relating to generation of a quantum encryption key in the quantum cryptography transmission device 10. The key distillation processing unit 16 sends information on the code to be added to the optical pulse to the encoding unit 12. The key distillation processing unit 16 sends information on the intensity of the optical pulse corresponding to the decoy method to the decoy modulation unit 13. The key distillation processing unit 16 has two random number generation functions of a first random number generation unit and a second random number generation unit. The key distillation processing unit 16 selects one of the four types of codes corresponding to the BB84 protocol based on the random number generated by the first random number generation unit, and transmits information on the selected code to the encoding unit 12. And send. The four types of codes are set to be selected randomly based on random numbers. The key distillation processing unit 16 stores information on the selected code and information on the time when the code is output. The key distillation processing unit 16 selects one of three different intensities based on the random number generated by the second random number generation unit, and sends information on the selected intensity to the decoy modulation unit 13. The three different intensities correspond to one signal light and two decoy lights. Further, the key distillation processing unit 16 stores the selected strength information and the time when the strength information is output. The frequency with which each intensity is selected is set in advance as shown in FIG. 3, and is stored in advance in the key distillation processing unit 16 as a condition for assigning a random number.

  The key distillation processing unit 16 transmits / receives data necessary for generating a quantum encryption key and data for determining the presence / absence of wiretapping via the key distillation processing unit 23 of the quantum cryptography receiving device 20 and the communication network 32. The key distillation processing unit 16 collates the information of the code added to the optical pulse by the encoding unit 12 with the data sent from the key distillation processing unit 23 of the quantum cryptography reception device 20, and generates a quantum cryptography key. . Further, the key distillation processing unit 16 is based on the detection result of the decoy light pulse sent from the key distillation processing unit 23 of the quantum cryptography receiver 20 and the intensity information output to the decoy modulation unit 13. It has a function of determining whether or not there is an eavesdropping action on the optical network 31 by a predetermined method.

  The quantum cryptography receiving device 20 includes a decryption unit 21, a photon detection unit 22, and a key distillation processing unit 23. The decoding unit 21 has a function of performing phase modulation on the input optical pulse based on the phase modulation amount information sent from the key distillation processing unit 23. The amount of modulation when the decoding unit 21 performs phase modulation on the optical pulse is set to correspond to the amount of phase modulation performed by the encoding unit 12 of the quantum cryptography transmission device 10. The phase modulation amount applied to the optical pulse by the decoding unit 21 of the present embodiment is set as 0 or π / 2. The decoding unit 21 of this embodiment includes a Mach-Zehnder interferometer and a phase modulator. The optical pulse input to the decoding unit 21 is branched into two waveguides, and the optical pulse passing through one of the waveguides is input to the phase modulator. The optical path length difference between the two waveguides is designed to match the optical path length difference of the encoding unit 12 of the quantum cryptography transmission device 10. The optical pulse that has passed through the two waveguides is subjected to phase modulation on the optical pulse in one of the waveguides, and then sent to one waveguide using the interference effect and input to the photon detector 22.

  The photon detection unit 22 has a function of detecting the number of photons of the input optical pulse and the phase difference between the double pulses. The photon detection unit 22 has a function of sending information about the number of detected photons and the phase difference to the key distillation processing unit 23.

  The key distillation processing unit 23 has a function of performing general control related to generation of an encryption key in the quantum cryptography reception device 20. The key distillation processing unit 23 includes a random number generation unit that generates a random number used when two types of modulation amounts set as phase modulation amounts in the decoding unit 21 are randomly selected. The key distillation processing unit 23 selects one of the two types of phase modulation amounts based on the random number generated by the random number generation unit 23. The key distillation processing unit 23 sends information on the selected phase modulation amount to the decoding unit 21. The key distillation processing unit 23 stores the selected phase modulation amount information and the time when the phase modulation amount information is output to the decoding unit 21. When the key distillation processing unit 23 receives the information on the number of photons and the phase difference detected from the photon detection unit 22, the information on the amount of phase modulation when the phase modulation is performed on the optical pulse including the photon in which the received information is detected. And save it in association with the time information.

  The key distillation processing unit 23 transmits / receives data necessary for generating a quantum encryption key and data for determining the presence / absence of wiretapping via the key distillation processing unit 16 of the quantum cryptography transmission device 10 and the communication network 32. The key distillation processing unit 23 receives the phase modulation amount applied to the optical pulse by the decoding unit 21 and the phase information detected by the photon detection unit 22 and the data sent from the key distillation processing unit 16 of the quantum cryptography transmission device 10. The quantum encryption key is generated by collating. The key distillation processing unit 23 also uses the optical network based on the information on the intensity of the decoy light pulse sent from the key distillation processing unit 16 of the quantum cryptography transmission device 10 and the detection result of the photon detection unit 22. 31 has a function of determining whether or not there is an eavesdropping action on a predetermined method.

  The optical network 31 includes an optical fiber and a relay device, and transmits the optical pulse transmitted from the quantum cryptography transmission device 10 to the quantum cryptography reception device 20. The communication network 32 is configured by an optical communication network or the like. The quantum cryptography transmission device 10 and the quantum cryptography reception device 20 transmit and receive data bidirectionally via the communication network 32. In addition, the communication network 32 can share a communication network used for data transmission such as information devices other than the quantum cryptography transmission device 10 and the quantum cryptography reception device 20.

  In the quantum cryptography communication system of this embodiment, an operation when a quantum cryptography key is transmitted from the quantum cryptography transmission device 10 to the quantum cryptography reception device 20 via the optical network 31 will be described. The light source unit 11 generates an optical pulse with a predetermined wavelength and intensity and outputs the optical pulse to the encoding unit 12. When an optical pulse is input, the encoding unit 12 adds encryption key information to the input optical pulse based on information from the key distillation processing unit 16. In the present embodiment, information encoded according to the BB84 protocol in the key distillation processing unit 16 is added to the optical pulse by the phase modulation method. The key distillation processing unit 16 randomly selects one code from four types of codes based on a random number generated inside. The key distillation processing unit 16 stores the code information output to the encoding unit 12 and the time when the code information is output.

  The encoding unit 12 splits the input optical pulse into two waveguides having different optical path lengths to generate a double pulse. The encoding unit 12 performs phase modulation on the light pulse that has passed through one of the duplex pulses based on information from the key distillation processing unit 16. The phase modulation is performed so that the phase difference of the double pulse becomes one of the four phase differences assigned to each code. The relationship between the code and the phase difference is stored in the encoding unit 12 in advance. In the present embodiment, each code is assigned to four phase difference states of 0, π, π / 2, and 3π / 2. The optical pulse subjected to phase modulation by the encoding unit 12 is input to the decoy modulation unit 13.

  When an optical pulse is input to the decoy modulation unit 13, the input optical pulse is branched into two waveguides. The key distillation processing unit 16 sends intensity information for generating optical pulses of each intensity corresponding to the decoy method to the decoy modulation unit 13. The key distillation processing unit 16 selects one of the three set intensities, and sends information on the selected intensity to the decoy modulation unit 13. The key distillation processing unit 16 selects the strengths so that the three set strengths have frequencies as shown in FIG. The distillation processing unit 16 selects the strength so that any of the three set strengths is selected at random while satisfying the frequency condition as shown in FIG. The key distillation processing unit 16 associates and stores the selected intensity information and the time when the selected intensity information is output to the decoy modulation unit 13.

  The control unit 41 of the decoy modulation unit 13 uses the phase modulation amount in the phase modulation unit based on the intensity information input from the key distillation processing unit 16 and the correction amount information sent from the correction amount control unit 42. Is converted into a voltage signal. The control unit 41 sends the converted voltage signal to the first phase modulation unit 43 and the second phase modulation unit 44, respectively. The phase modulation section provided in each waveguide applies an electric field to the waveguide through which the optical pulse passes based on the voltage signal sent from the control section 41, and performs phase modulation on the optical pulse. The optical pulses subjected to the phase modulation by the first phase modulation unit 43 and the second phase modulation unit 44 are sent to one waveguide so as to become one optical pulse due to the interference effect. The optical pulse sent to one waveguide is output from the decoy modulator 13. The optical pulse that is phase-modulated and output by the decoy modulation unit 13 is an optical pulse having one of the three intensities of the signal light or the two types of decoy light. The optical pulse output from the decoy modulator 13 is sent to the optical attenuator 14. A part of the optical pulse sent from the decoy modulator 13 to the optical attenuator 14 is branched and input to the optical detector 15.

  When a light pulse is input to the light detection unit 15, the light detection unit 15 measures the intensity of the input light pulse and sends information on the measured light pulse intensity to the correction amount control unit 42. The intensity of the light pulse measured by the light detection unit 15 is an average value of the intensity per predetermined time. When the correction amount control unit 42 receives the intensity information of the optical pulse, the correction amount control unit 42 calculates the correction amount of the phase modulation amount based on the intensity information. The correction amount control unit 42 stores in advance the relationship between the intensity and the correction amount of the phase modulation amount. When the correction amount control unit 42 calculates the correction amount of the phase modulation amount, the correction amount control unit 42 sends information on the correction amount to the control unit 41. Upon receiving the correction amount information, the control unit 41 calculates a phase modulation amount based on the received correction amount information for a new phase modulation amount that is calculated after the correction amount information is received.

  When an optical pulse is input from the decoy modulation unit 13 to the optical attenuation unit 14, the optical attenuation unit 14 attenuates the optical pulse by a predetermined attenuation amount. The optical pulse attenuated by the optical attenuator 14 is output from the quantum cryptography transmitter 10 to the optical network 31.

  It is assumed that an optical pulse from the quantum cryptography receiver 10 is input to the quantum cryptography receiver 20 via the optical network 31. The optical pulse input to the quantum cryptography reception device 20 is sent to the decryption unit 21. When an optical pulse is input to the decoding unit 21, it is branched into two waveguides. The decoding unit 21 performs phase modulation on the optical pulse input to one of the two waveguides. The decryption unit 21 performs phase modulation on the pulse that has not been phase-modulated by the encoding unit 10 of the quantum cryptography transmission device 10 among the two consecutive pulses. The decoding unit 21 performs phase modulation on the optical pulse based on the phase modulation amount information sent from the key distillation processing unit 23. In the present embodiment, the phase modulation amount is set as 0 or π / 2. The phase modulation amount is randomly selected based on the random number generated by the random number generation unit of the key distillation processing unit 23. Further, when sending the phase modulation amount information to the decoding unit 21, the key distillation processing unit 23 associates and stores the phase modulation amount information and the time information information. After the optical pulse of one waveguide is subjected to phase modulation, the optical pulses of the two waveguides are output after being interfered with each other by a Mach-Zehnder interferometer. The difference between the optical path lengths of the two waveguides of the decryption unit 21 is designed to be the same as the difference of the optical path lengths of the two waveguides of the encoding unit 12 of the quantum cryptography transmission device 10.

  The optical pulse output from the decoding unit 21 is sent to the photon detection unit 22. When an optical pulse is input to the photon detection unit 22, the photon detection unit 22 measures the number of photons of the optical pulse with a phase difference of 0 and an optical pulse with a phase difference of π among the input optical pulses. Then, the phase difference information and the photon number information are sent to the key distillation processing unit 23. Upon receiving information on the phase difference and the number of photons, the key distillation processing unit 23 stores the phase modulation amount information and time information output to the decoding unit 23 in association with each other.

  Next, the operation when the quantum cryptography transmission device 10 and the quantum cryptography reception device 20 each generate a quantum cryptography key will be described. The key distillation processing unit 23 of the quantum cryptography receiving device 20 uses the communication network 31 together with information on the phase modulation amount applied to the optical pulse by the decryption unit 21 and information on the phase difference detected by the photon detection unit 22 together with time information. To the quantum cryptography receiving device 10.

  When the quantum cryptography transmission device 10 receives information on the amount of phase modulation applied to the optical pulse from the quantum cryptography reception device 20, the quantum cryptography transmission device 10 generates a bit string based on the received information. The bit string can be generated as follows, for example. The quantum cryptography transmission device 10 detects an optical pulse having a phase modulation amount of 0 and a phase difference of 0 in the decryption unit 21 with respect to an optical pulse having a phase difference of 0 when the encoding unit 12 performs phase modulation. When the information that it has been received is received, “0” is assigned. The quantum cryptography apparatus 10 detects an optical pulse having a phase modulation amount of 0 and a phase difference of π in the decryption unit 21 with respect to an optical pulse having a phase difference of π when the encoding unit 12 performs phase modulation. Is received, “1” is assigned. The quantum cryptography apparatus 10 uses an optical pulse having a phase difference of π / 2 when the encoding unit 12 performs phase modulation, and an optical signal having a phase modulation amount of π / 2 and a phase difference of 0 in the decryption unit 21. When information indicating that a pulse has been detected is received, “0” is assigned. The quantum cryptography transmission device 10 has a phase modulation amount of π / 2 and a phase difference of π with respect to an optical pulse with a phase difference of 3π / 2 when the encoder 12 performs phase modulation. When information indicating that an optical pulse has been detected is received, “1” is assigned. If the combination of the modulation amounts and the like applied by each device is another combination, the correctness of the combination cannot be determined, and the key distillation processing unit 16 does not use it as information for generating a bit string. That is, the key distillation processing unit 16 generates a bit string by allocating bits only when it can be determined that the combination is correct from the information on the phase modulation amount and the detected phase difference applied to the optical pulses by the devices.

  The key distillation processing unit 16 transmits the time information of the optical pulse used for generating the bit string and the information of the phase modulation amount applied to the optical pulse by the encoding unit 12 to the quantum cryptography receiving device 20 via the communication network 31. . When the quantum cryptography receiving device 20 receives time information and phase modulation amount information from the quantum cryptography transmission device 10 via the communication network 31, the quantum cryptography receiving device 20 generates a bit string based on the received information and information stored in the own device. . The key distillation processing unit 23 of the quantum cryptography receiving device 20 generates a bit string as follows. The key distillation processing unit 23 collates the correspondence between the optical pulse included in the received information based on the time information and the optical pulse included in the information stored in the own device. When the key distillation processing unit 23 receives information that the phase modulation amount in the encoding unit 12 is 0 with respect to the optical pulse detected with the modulation amount in the decoding unit 21 being 0 and the phase difference being 0, Assign “0”. When the key distillation processing unit 23 receives the information that the phase modulation amount in the encoding unit 12 is π with respect to the optical pulse detected with the modulation amount in the decoding unit 21 being 0 and the phase difference being π, Assign “1”. The key distillation processing unit 23 receives information that the phase modulation amount in the encoding unit 12 is 3π / 2 with respect to the optical pulse detected as the modulation amount in the decoding unit 21 is π / 2 and the phase difference is π. If it is, “1” is assigned. The key distillation processing unit 23 receives information that the phase modulation amount in the encoding unit 12 is π / 2 with respect to the optical pulse detected with the modulation amount in the decoding unit 21 being π / 2 and the phase difference being 0. If “0” is assigned, “0” is assigned. When the combination of the modulation amounts and the like applied by each device is another combination, the correctness of the combination cannot be determined, and the key distillation processing unit 23 does not use it as information for generating a bit string.

  As described above, the quantum cryptography transmission device 10 and the quantum cryptography reception device 20 each generate a bit string based on information that can be determined. Further, since the bit strings that can be determined from the information shared by the quantum cryptography transmission device 10 and the quantum cryptography reception device 20 are the same, the quantum cryptography transmission device 10 and the quantum cryptography reception device 20 can obtain information of the same bit sequence. The information on the bit strings generated by the quantum cryptography transmission device 10 and the quantum cryptography reception device 20 is used as a quantum cryptography key that is shared between the two devices. The generated quantum cryptography key is output to an information processing apparatus provided with the quantum cryptography transmission device 10 and the quantum cryptography reception device 20, respectively, and used for browsing confidential information.

  The key distillation processing unit 16 and the key distillation processing unit 23 also perform error correction processing and confidentiality enhancement processing when generating a bit string for the quantum encryption key. The error correction process is a process for correcting an error that occurs when a bit string is generated due to a transmission error on the optical network 31 or the like. For example, the occurrence location of the error can be detected by calculating the parity for each predetermined block of the bit string generated by the key distillation processing unit 16 and the key distillation processing unit 23 and exchanging the parity information. For the block in which an error has occurred, the quantum cryptography transmission device 10 again sends an optical pulse to which information for generating a quantum cryptography key is added to the quantum cryptography reception device 20 via the optical network 31. The key distillation processing unit 16 and the key distillation processing unit 23 share information on the phase modulation amount and the detection result via the communication network 32, and can allocate the bits in the same manner as usual, thereby correcting the error.

  The confidentiality enhancement process may be performed by, for example, combining two bit strings generated by the key distillation processing unit 16 and the key distillation processing unit 23 by a predetermined method, calculating an exclusive OR, and generating a bit string. it can. The key distillation processing unit 16 and the key distillation processing unit 23 use a bit string generated by calculating an exclusive OR as a quantum encryption key. When such processing is performed, since the bit string to be generated cannot be generated correctly unless the information of the two blocks is obtained correctly, the secrecy is improved.

  Information on the number of photons of the optical pulse at each time detected by the quantum cryptography reception device 20 is shared with the quantum cryptography reception device 10. Based on information on which of the signal light and the intensity of the two decoy lights stored in the quantum cryptography receiving device 10 and information on the number of photons detected by the quantum cryptography receiving device 20 are obtained on the optical network 31. A third party eavesdropping action, that is, verification of information leakage is performed.

The quantum cryptography communication system according to the present embodiment uses a decoy method, and the decoy modulator modulates an optical pulse so that an optical pulse transmitted from the quantum cryptography transmission device becomes one of three different intensities. The pulse is intensity modulated. In the present embodiment, the intensity modulation for obtaining optical pulses having different intensities for the decoy method is performed by branching the optical pulse into two waveguides and causing the optical pulses subjected to phase modulation to interfere with each other. . In the present embodiment, among the optical pulses having different intensities, the output intensity of the optical pulse having the highest intensity used as the signal light is set to other than the maximum output of the decoy modulator. Since the signal light has the highest frequency and the high intensity, the intensity measured by the light detection unit can be regarded as the intensity of the signal light. By comparing the intensity detected by the light detection unit with a predetermined set value assumed from the setting of the intensity of the signal light, it is possible to detect whether or not the phase modulation amount has changed in the decoy modulation unit. In the decoy modulation unit of the present embodiment, the change in the output intensity with respect to the variation in the phase modulation amount is larger in the case other than the maximum output, compared to the case of the maximum output. Therefore, by setting the intensity to other than the maximum output, the detection sensitivity when detecting the variation of the phase modulation amount based on the result of the intensity measurement is improved. In addition, by setting the intensity of the signal light that is the target for measuring the output intensity so that the intensity change with respect to the phase difference is the largest, the effect of improving the detection accuracy of the variation in the phase modulation amount is the highest. Become. From the above, the quantum cryptography transmission device according to the present embodiment can detect the variation of the phase modulation amount with high accuracy, and can improve the accuracy of correction of the phase modulation amount based on the result. As a result, in the quantum cryptography communication system according to the present embodiment, it is possible to sufficiently suppress fluctuations in output intensity when intensity modulation is performed on an optical pulse. The quantum cryptography transmission device according to the second embodiment includes a decoy modulation unit. As a two-electrode modulator LN intensity modulator. Instead of such a configuration, a decoy modulation unit that generates three types of light pulses by arranging two intensity modulators having only one modulation unit that can be binarized in series. It can also be configured.

  In the quantum cryptography transmission device of the second embodiment, the variable optical attenuator is provided as the optical attenuator at the subsequent stage of the decoy modulator. However, the decoy modulator and the optical attenuator may be integrated. it can. In addition, when intensity modulation performed by phase modulation and interference effect is performed in the decoy modulation unit, the intensity modulation is performed on the optical pulse so that the average photon number of the optical pulse of each intensity of the decoy method is obtained. Can do. That is, when the intensity modulation is performed on the optical pulse by the decoy modulation unit, the intensity modulation is performed so that the intensity of the output optical pulse has the average number of photons as shown in FIG. When the intensity modulation is performed in the decoy modulation unit so that the average photon number of the light pulses of each intensity of the decoy method is obtained, a test light pulse is used to determine the correction amount of the phase modulation amount. In such a case, a test light pulse having a predetermined intensity is output from the decoy modulation unit, and the phase modulation amount is corrected based on the result of measuring the intensity by the photodetector. By using such a configuration instead of the configuration of the second embodiment, the quantum cryptography transmission device does not need to include the decoy modulation unit and the optical attenuation unit independently, and only includes the decoy modulation unit. Therefore, the apparatus configuration can be simplified.

  Further, the quantum cryptography transmission device according to the second embodiment includes a decoy modulation unit after the encoding unit. Instead of such a configuration, a coding unit may be provided after the decoy modulation unit. That is, after the intensity modulation is performed by the modulation unit for decoy so that the optical pulse has a predetermined intensity for the decoy method, the code information of the quantum encryption key based on a predetermined algorithm may be added to the optical pulse. . When intensity modulation comes first, a configuration in which a light detection unit is provided after the decoy modulation unit and the phase modulation amount is corrected based on the measurement result of the intensity of the optical pulse can be adopted.

  Although the quantum cryptography communication system according to the second embodiment distributes the quantum cryptography key using the phase coding scheme and the decoy scheme based on the BB84 protocol, other protocols and coding schemes may be used. As another coding method, for example, Time-bin coding or polarization coding can be used. When other coding schemes are used, the encoding unit and the decoding unit are configured according to the coding scheme.

  A part or all of the above-described embodiment can be described as in the following supplementary notes, but is not limited thereto.

(Appendix 1)
Intensity modulation means for applying intensity modulation to the input optical pulse and outputting it as an output pulse;
Detecting means for detecting the intensity of the output pulse;
Control means for correcting the modulation amount based on the intensity of the output pulse, and controlling the modulation amount of the intensity modulation means so that the output intensity is selected from the intensity set in a plurality of stages other than the maximum output; A modulation device comprising:

(Appendix 2)
The modulation apparatus according to appendix 1, wherein the intensity modulation means performs intensity modulation using a predetermined phase modulation method and an optical interference effect.

(Appendix 3)
Attenuating means for attenuating the output pulse so that the average number of photons contained in one light pulse is less than or equal to a predetermined number;
3. The modulation apparatus according to claim 1, wherein the detection unit measures the intensity of the output pulse before attenuation is performed by the attenuation unit.

(Appendix 4)
The intensity modulation means uses the output pulse having an intensity such that an average number of photons contained in one optical pulse is equal to or less than a predetermined number when the intensity modulation is performed on the optical pulse. 3. The modulation device according to either 1 or 2.

(Appendix 5)
The intensity modulation means applies intensity modulation to the input optical pulse for testing so that the intensity is a predetermined intensity other than the maximum output to produce the output pulse, and the detection means applies intensity modulation to the optical pulse for testing. The modulation apparatus according to any one of appendices 1 to 4, wherein the intensity of the applied output pulse is detected.

(Appendix 6)
The control means controls the intensity at which the fluctuation of the output intensity with respect to the fluctuation of the phase modulation amount of the intensity modulation means is maximized as the maximum intensity among the intensity set in a plurality of stages. 5 to 5. The modulation device according to any one of 5 to 5.

(Appendix 7)
7. The modulation device according to any one of appendices 1 to 6, wherein the highest frequency among the strengths set in a plurality of stages is selected most frequently.

(Appendix 8)
A modulation device according to any one of appendices 1 to 7,
A light source that generates the light pulses;
A transmission device comprising:

(Appendix 9)
The transmission device according to attachment 8,
A communication system, comprising: a receiving unit that receives the output pulse; and a photon detecting unit that detects the number of photons of the received output pulse.

(Appendix 10)
Apply intensity modulation to the input optical pulse and output it as an output pulse.
Detecting the intensity of the output pulse;
A modulation method comprising: correcting a modulation amount based on the intensity of the optical pulse, and controlling the modulation amount so that an output intensity selected from intensities set in a plurality of stages other than the maximum output is obtained.

(Appendix 11)
The modulation method according to appendix 9, wherein intensity modulation is performed using a predetermined phase modulation method and an optical interference effect.

(Appendix 12)
The output pulse is branched to detect the intensity of the output pulse, and the output pulse on the side where the intensity is not detected is attenuated so that the average number of photons contained in one optical pulse is a predetermined number or less. The modulation method according to any one of appendix 10 or 11, wherein:

(Appendix 13)
Either of the appendix 10 or 11, wherein when the intensity modulation is performed on the optical pulse, the output pulse has an intensity such that the average number of photons contained in one optical pulse is equal to or less than a predetermined number. The modulation method described.

(Appendix 14)
Intensity modulation is applied to the input optical pulse for test so that the intensity is a predetermined intensity other than the maximum output to produce the output pulse, and the intensity of the output pulse in which the intensity modulation is applied to the optical pulse for test is detected. 14. The modulation method according to any one of supplementary notes 10 to 13, wherein:

(Appendix 15)
15. The supplementary note 11 to 14, wherein the modulation amount is controlled by setting the strength at which the fluctuation of the output strength with respect to the variation of the phase modulation amount is the maximum among the strengths set in a plurality of stages. Modulation method.

(Appendix 16)
16. The modulation method according to any one of appendices 10 to 15, wherein the frequency of selecting the maximum intensity among the intensities set in a plurality of stages is the highest.

  The present invention relates to a quantum cryptography communication system, and is particularly applicable to a quantum cryptography communication system using a decoy method.

DESCRIPTION OF SYMBOLS 10 Quantum cryptography transmitter 11 Light source part 12 Encoding part 13 Modifier for decoy 14 Optical attenuation part 15 Optical detection part 16 Key distillation process part 20 Quantum cryptography receiver 21 Decoding part 22 Photon detection part 23 Key distillation process part 31 Optical network 32 communication network 41 control unit 42 correction amount control unit 43 first phase modulation unit 44 second phase modulation unit 101 intensity modulation unit 102 detection unit 103 control unit

Claims (10)

Intensity modulation means for applying intensity modulation to the input optical pulse and outputting it as an output pulse;
Detecting means for detecting the intensity of the output pulse;
Control means for correcting the modulation amount based on the intensity of the output pulse, and controlling the modulation amount of the intensity modulation means so that the output intensity is selected from the intensity set in a plurality of stages other than the maximum output; A modulation device comprising:   2. The modulation apparatus according to claim 1, wherein the intensity modulation means performs intensity modulation using a predetermined phase modulation method and an optical interference effect. Attenuating means for attenuating the output pulse so that the average number of photons contained in one light pulse is less than or equal to a predetermined number;
The modulation device according to claim 1, wherein the detection unit measures the intensity of the output pulse before the attenuation unit performs attenuation.   The intensity modulation means uses the output pulse having an intensity such that an average number of photons contained in one optical pulse is equal to or less than a predetermined number when the intensity modulation is performed on the optical pulse. Item 3. The modulation device according to any one of Items 1 and 2.   The control means controls the intensity at which the fluctuation of the output intensity with respect to the fluctuation of the phase modulation amount of the intensity modulation means becomes maximum as the maximum intensity among the intensity set in a plurality of steps. 5. The modulation device according to any one of 2 to 4. A modulation device according to any one of claims 1 to 5;
A light source that generates the light pulses;
A transmission device comprising: A transmission device according to claim 6;
A communication system comprising: a receiving unit that receives the output pulse; and a photon detecting unit that detects the number of photons of the received output pulse. Apply intensity modulation to the input optical pulse and output it as an output pulse.
Detecting the intensity of the output pulse;
A modulation method comprising: correcting a modulation amount based on the intensity of the optical pulse, and controlling the modulation amount so that an output intensity selected from intensities set in a plurality of stages other than the maximum output is obtained.   9. The modulation method according to claim 8, wherein intensity modulation is performed using a predetermined phase modulation method and an optical interference effect.   9. The modulation method according to claim 8, wherein the modulation amount is controlled with the intensity at which the fluctuation of the output intensity with respect to the fluctuation of the phase modulation quantity is maximized among the intensity set in a plurality of stages.

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