JP4358206B2 - Quantum cryptographic communication device and quantum cryptographic communication method - Google Patents

Description

  The present invention relates to a key distribution technique, and more particularly, to a quantum cryptography communication device and a quantum cryptography communication method that use a relative phase difference of a coherent optical pulse train and have an anti-theft function.

  In recent years, research on quantum cryptography that uses a single-photon level of light and is different from encryption methods that secure the amount of computation on the basis of safety and that is physically secure is being promoted. ing. Quantum cryptography is a system that supplies a secret key for performing cryptographic communication between two parties (two communication devices) that exist at distant points, and is generally called quantum key distribution. Although there are various types of quantum key distribution, here, a differential phase shift quantum key distribution system (see Non-Patent Document 1) will be described as a conventional technique.

  FIG. 3 shows a basic configuration of a conventional differential phase shift quantum key distribution system. The transmitter 30 randomly performs 0 or π phase modulation on pulses including an average of less than 1 photon (for example, 0.1 photon / pulse), records the pulse transmission slot time and phase modulation, and the phase modulation. The coherent optical pulse train 31 having a constant interval is sent out. The state where the average number of photons is less than one is realized by greatly attenuating normal laser light. When such a pulse train is photon detected, a photon is detected with a certain pulse, but nothing is detected with a certain pulse. At which pulse the photon is detected is quite probabilistic and is uncertain until it is measured.

  The pulse train 31 transmitted from the transmitter 30 is received by the receiver 33 via the transmission path 32. The receiver 33 branches the received pulse train into two by an optical branching means (branching device) 34, adds a delay to one of the pulse trains by a delay means (long path) 35, and then converts these two pulse trains into 2 × 2 Are combined again by the coupler 36. Photon detectors 37 and 38 are provided at two output terminals of the multiplexing coupler 36, respectively. The delay time given to one pulse train by the long path of the branching / combining circuits 34 to 36 is set equal to the time interval of the pulse train inputted to the receiver 33. That is, the long path has a delay time corresponding to the pulse interval. As a result, in the multiplexing coupler 36, adjacent pulses before and after are overlapped and combined. The input pulse train is phase-modulated by 0 or π. Therefore, if the propagation phase of the branching / combining path is appropriate, the phase difference between the overlapping pulses is 0 or π. As a result of the combination, both (the two pulses of the overlapping pulses) interfere, and if the phase difference is 0, the first detector 37 detects the photon, and if the phase difference is π, the second detector 38 detects the photon. become. In this way, photons are detected by different detectors according to the phase difference 0 and π of the pulse.

  Using the above configuration, the transmitter 30 and the receiver 33 obtain a secret key by the following procedure. First, the receiver 33 detects the photons transmitted from the transmitter 30 and passed through the transmission path 32 by the above reception mechanism. At this time, the detected time (slot time) and detector are recorded for each detected photon. After a predetermined number of photons have been transmitted and received, the receiver 33 informs the transmitter 30 of the photon detection time, which is the time at which the photon was detected. The transmitter 30 uses the detected photon detection time (slot time) and the phase modulation data (slot time and phase difference) of the transmitter itself to determine which detector (37, 38) the receiver 33 uses. You can know if it was detected. Here, an event in which a photon is detected by the first detector 37 is a bit “0”, an event in which a photon is detected by the second detector 38 is a bit “1”, and the event between the transmitter 30 and the receiver 33 is determined in advance. If agreed, the sender (transmitter 30) and the receiver (receiver 33) can obtain the same bit string (key bit).

  In the above procedure, the information notified from the receiver 33 to the transmitter 30, that is, from the receiver to the transmitter, is only the photon detection time, and the bit information is not sent outside the communication devices. Therefore, the bit information is not wiretapped from information notified from the receiver to the sender. In addition, since the signal transmitted through the transmission path 32 is light with an average of less than one photon per pulse, an eavesdropper cannot branch part of the signal on the transmission path 32 to obtain bit information. Because it is physically impossible for one photon to be divided into two, if the eavesdropper detects a photon by branching, the photon does not reach the receiver 33 and does not become a bit string of the sender and the receiver Because.

  As a more advanced wiretapping method, there is a method called an impersonation method. FIG. 4 is an explanatory diagram of spoofing for a conventional differential phase shift quantum key distribution system. An eavesdropper (wiretap) 43 receives a transmission signal transmitted by the transmitter 30 in the middle of the transmission path 32 with the same configuration 44 to 48 as that of the receiver 33, and results of reception by the photon detectors 47 and 48. Based on the above, the dummy signal is transmitted to the original receiver 33 using its own optical transmitter 49. If the eavesdropper 43 can correctly receive the transmission signal, the dummy signal is the same as the original transmission signal, and information (secret key) can be obtained without the receiver 33 noticing the wiretapping action.

  However, as described above, in the differential phase shift quantum key distribution system, the transmission signal is a pulse train having an average of less than 1 photon per pulse, for example, 0.1 photon / pulse. When such a signal is received, a photon is detected only once in 10 pulses. Accordingly, the eavesdropper 43 can detect the phase difference between the two pulses corresponding to the time when the photon is detected, but cannot detect any other phase difference. When trying to send a dummy signal based on such a detection result, for a pulse whose phase difference could not be detected, the phase selected by guess guess is allocated and retransmitted (hereinafter referred to as spoofing wiretapping 1), or no signal (Hereinafter referred to as spoofing eavesdropping 2), or either of these methods must be taken.

  When the former (spoofing spoofing 1) is adopted, when the receiver 33 detects the phase difference selected by the guess, it is different from the one sent by the transmitter 30. Even if the latter (spoofing spoofing 2) is adopted, the transmission / reception signal mismatch still occurs. The reason is that in this latter case, the eavesdropper 43 sends two isolated continuous pulses. When the receiver 33 receives two isolated pulses (see 50 in FIG. 4), photons can be detected at three times at the output stages 37 and 38 of the branching and multiplexing circuits 34 to 36 (53 in FIG. 4). 52).

  FIG. 4 shows a non-delayed pulse train 53 and a delayed pulse train 52 among the pulse trains branched by the optical branching means 34 in time series. In this figure, when a photon is detected at the middle time (referred to as the second time) of the time when both pulse trains overlap, the detection result follows the phase difference of two pulses, and the sender 30 intends Photons are detected by the detector as described.

  On the other hand, if a photon is detected at the first or third time, which is before or after the second time of the two pulse trains overlapping, there is no interfering party, so the photons are randomly It is detected by the first or second detector. Therefore, when the receiver 33 obtains the secret key bit from the photon detection result at the first or third time, the bit is different from that intended by the sender 30.

  Thus, when spoofing is performed, bit mismatch occurs between the transmitter and the receiver. Therefore, the transceivers 30 and 33 obtain a secret key according to a normal procedure, and then answer several test bits. If the system is operating normally, the bit information of both will match, but if there is spoofing, some bits will not match. If there is a mismatch bit, the system determines that the system is not normal and discards the secret key. In other words, if the test bits match, it can be determined that there was no wiretapping, and the secret key is guaranteed to be secure.

K. Inoue, E. Waks, Y. Yamamoto, “Differential-phase-shift quantum key distribution using coherent light”, 2003, Physical Review A, vol.68, Paper number 022317

  As is clear from the above, one of the important performances required for the quantum key distribution system is easy discovery of spoofing. In the differential phase shift quantum key distribution system, when spoofing is performed, a bit mismatch occurs in the transmission / reception organization according to the above-described principle, and an eavesdropping action is detected from this, but not all bits are mismatched.

  For example, when sending a dummy pulse train with guesswork (spoofing wiretapping 1), if the pulse detected by the receiver 33 is a photon detected by an eavesdropper 43, the bit mismatch does not occur. When two isolated pulses are transmitted (spoofing wiretapping 2), if the receiver 33 detects a photon at the second time, there is no bit mismatch.

  Here, it can be said that the higher the bit mismatch rate, the higher the eavesdropping detection capability, and the higher the degree of security. The bit mismatch rate for spoofing 1 can be increased by reducing the average number of photons. If the average number of photons is small, the probability that the eavesdropper 43 and the receiver 33 happen to detect the same pulse photonly decreases, that is, the correct answer rate decreases and the bit mismatch rate increases. On the other hand, the bit mismatch rate for spoofing eavesdropping 2 is constant regardless of the average number of photons, and the probability that the receiver 33 detects photons at the first or third time is ½, and is obtained from this photon detection. Since the probability that the bits do not match is further ½ of that, the overall bit mismatch rate is ¼.

  As a more secure quantum cryptography system, it is desired that the probability of bit mismatch caused by spoofing eavesdropping is higher, and thereby the detection probability of spoofing eavesdropping is higher.

  The present invention has been made in view of the above-described problems, and its purpose is to improve the safety of communication by improving the discovery probability of spoofing, and more specifically. An object of the present invention is to provide a quantum cryptography communication device and a quantum cryptography communication method that improve the bit mismatch rate due to spoofing.

In order to achieve the above object, the invention described in claim 1 comprises a transmitter for transmitting an optical pulse train of a constant time interval T, which is randomly phase-modulated with 0 or π, at an average power level of less than 1 photon per pulse; A quantum cryptography communication method using a quantum cryptography communication device comprising a receiver for receiving the optical pulse train transmitted from the transmitter, wherein the receiver receives the optical pulse train transmitted from the transmitter as 2 A branching splitter and one of two optical pulse trains branched by the splitter are equal to the pulse interval T of the optical pulse train from the transmitter with respect to the other optical pulse train, or an integral multiple thereof. A delay path that gives a time difference; and a multiplexer / demultiplexer that multiplexes the other optical pulse train branched by the demultiplexer and the optical pulse train delayed by the delay path, and divides the combined optical pulse train into two. Have said slow A plurality of delay multiplexing / demultiplexing units each consisting of a path and the multiplexer / demultiplexer are connected in cascade, and the time differences of the delay paths of the respective delay multiplexing / demultiplexing units are different from each other, and the method uses 0 or π When transmitting an optical pulse train having a constant time interval T that is randomly phase-modulated at a power level of less than one photon on average per pulse, the continuous pulse train is blocked into a plurality of predetermined pulses and assigned to each block. Any one of a plurality of predetermined phase patterns prepared in advance is randomly selected and assigned to each block, and each pulse is phase-modulated according to the assignment.

According to a second aspect of the present invention, in the quantum cryptography communication method according to the first aspect, the predetermined plurality of pulses serving as a unit block are four pulses, and the predetermined phase pattern is {0-0. -Π-0} and {π-π-π-0}.

The invention according to claim 3 is a transmitter that transmits an optical pulse train having a constant time interval T, which is randomly phase-modulated by 0 or π, at a power level of less than 1 photon on average per pulse, and transmitted from the transmitter. A quantum cryptography communication method using a quantum cryptography communication device including a receiver for receiving the optical pulse train, wherein the receiver transmits the optical pulse train transmitted from the transmitter to a first long path and a first Optical branching means for branching into two short paths, the first long path and the two optical pulse trains output from the first short path are combined, and the combined optical pulse train is defined as the second long path A first optical coupler having a 2 × 2 input / output terminal branched into two on a second short path, and the two optical pulse trains output from the second long path and the second short path are multiplexed. , 2 × 2 input / output terminals that divide the combined optical pulse train into two A second optical coupler and two photon detectors respectively connected to the output terminals of the second optical coupler, wherein the first long path is between the long path and the first short path. Is given a time difference equal to the pulse interval T of the optical pulse train from the transmitter, and the second long path has a pulse interval 2T that is twice the pulse interval T between the long path and the second short path. The method is configured to transmit an optical pulse train having a constant time interval T, which is randomly phase-modulated by 0 or π, at the power level of less than 1 photon on average per pulse. , A continuous pulse train is divided into a plurality of predetermined pulses, and one of a plurality of predetermined phase patterns prepared in advance assigned to each block is randomly selected and assigned to each block. Characterized by phase modulating each pulse in accordance with the allocation.
According to a fourth aspect of the present invention, in the quantum cryptography communication method according to the third aspect, the predetermined plurality of pulses serving as a unit block are four pulses, and the predetermined phase pattern is {0-0. -Π-0} and {π-π-π-0}.

  As described above, the present invention multistages the delay multiplexing / demultiplexing unit including the duplexer, the delay path, and the multiplexer in the receiver, so that the multiplexer just before the detector has more than two pulses. Since many pulses (two-pulse multiplexing is a prior art) are combined, the detection probability of pulse multiplexing having the correct phase generated by spoofing is reduced, and at the same time the bit mismatch rate is increased. There is an effect that the mismatch detection probability in the test bit increases and the probability of detecting eavesdropping increases.

  In addition, the present invention has an effect that the secret key bit generation rate can be improved by the phase modulation method that improves the secret key bit generation rate that is lowered by the multistage configuration.

  As described above, according to the present invention, it is possible to provide a quantum cryptography communication device and a quantum cryptography communication method that improve communication security by increasing the bit mismatch rate due to spoofing.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a configuration of a quantum cryptography communication system according to an embodiment of the present invention. In FIG. 1, the transmitter 10 includes a light source 11, a phase modulator 12, an attenuation unit 13, and a control unit (CPU) 14. As the attenuating means (optical attenuator) 13, any known means may be used as long as it can greatly attenuate light incident from the light source 11 such as an ND filter. The configuration and operation of the transmitter 10 are the same as in the prior art.

  A receiver 100 as a light detection device includes an optical branching means (C1) 101, two 2 × 2 optical couplers (C2, C3) 102 and 103, two photon detectors 104 and 105, and a control unit (CPU). ) 106.

  The control units 14 and 106 have an information processing function, and not only a dedicated device but also a commercially available computer can be used. The control units 14 and 106 include, for example, a processor that performs necessary calculations, determinations, control, and the like, a memory for recording a photon detection time, a detector (or phase), and the like, and another device ( A line control unit (modem) for exchanging information with a receiver or a transmitter, and a bus. The communication path 17 can be either wired or wireless, and is not limited to an optical communication path.

  The light input to the receiver 100 is branched into two by an optical branching unit (demultiplexer) 101, and after a delay time T is given to one light by a first delay path 107 having a long path, a short path 108 is obtained. The other light passing therethrough is recombined by the first optical coupler (multiplexer) 102. Here, the delay time T is set equal to the pulse interval of the input pulse train. The optical coupler 102 has two output terminals. The light output from the two output terminals is given a delay time of 2T (twice T) to one of the light by the second delay path 109 of the long path, and then passes through the short path 110. And the second optical coupler 103 are combined again. Photon detectors 104 and 105 are connected to the two output terminals of the optical coupler 103, respectively.

  Comparing the configuration of the present embodiment in FIG. 1 with the configuration of the conventional example in FIG. 3, in this embodiment, the delay multiplexing / demultiplexing unit including the duplexer, the delay path, and the multiplexer in the receiver is multistaged. It can be seen that (cascade connection, cascade).

  In the above configuration, the transmitter 10 transmits an optical pulse train having a constant interval (pulse interval) T that is randomly phase-modulated with 0 or π by passing a coherent optical pulse generated from the light source 11 through the phase modulator 12. By passing through the attenuating means 13, an average of less than one photon per pulse (for example, 0.1 photon / pulse) is transmitted to the optical transmission line 15. At the same time, the control unit 14 records the pulse transmission slot time and phase modulation. The coherence time of the optical pulse is set to be longer than 3 times the pulse interval (3T).

  The receiver 100 inputs the optical pulse train transmitted from the transmitter 10 through the optical transmission path 15 to the first optical branching means (C1) 101. The optical pulse train branched by the optical branching means 101 passes through two stages of delay paths 107 and 109 configured in a cascade form, and is combined by the final optical coupler (C3) 103, and by the photon detectors 104 and 105. Each is detected.

When the receiver circuit of the receiver 100 is configured in this way, the optical coupler 103 at the final stage
(I) a first pulse 121 that has passed through a first-stage long path 107-2 and a first-stage long path 109;
(Ii) the second pulse 122 that has passed through the first stage short path 108-the second stage length passage 109;
(Iii) a third pulse 123 that has passed through the first-stage long path 107-2 and the second-stage short path 110;
(Iv) a fourth pulse 124 that has passed through the first-stage short path 108-2 and the second-stage short path 110 ;
Will be combined at the same time. Here, the long path is a delay path that is delayed among the branched paths, and the short path is a path that is not delayed.

  As described above, when four pulses are combined at the same time in the optical coupler 103 in the final stage, the four pulses cause interference, and the photon detectors 104 and 105 at which output terminals detect the photons. Depends on the phase relationship of each pulse.

  When the photon detection event in the case of four-pulse multiplexing is described by an equation, the probability amplitude of the photon output to the first photon detector 104 is expressed by the following equation (1).

exp (iθ ₁ ) + exp (iθ ₂ ) −exp (iθ ₃ ) + exp (iθ ₄ ) (1)
The probability amplitude of the photons output to the second photon detector 105 is expressed by the following equation (2).

exp (iθ ₁ ) + exp (iθ ₂ ) + exp (iθ ₃ ) −exp (iθ ₄ ) (2)
Here, the first term represents the first pulse 121, the second term represents the second pulse 122, the third term represents the third pulse 123, the fourth term represents the fourth pulse 124, and θ _i (i = 1 to 1). 4) is the phase of each pulse. However, for the sake of simplicity, common items are omitted. The phase of each pulse is modulated by 0 or π. The probability amplitudes for the respective phase states are as shown in Table 1 below.

  The photon detection probability is given by the square of the absolute value of the probability amplitude. In Table 1, the probability amplitude of one is ± 4 and the other is 0 indicates that whenever a photon is detected, it is detected by a detector having an amplitude of ± 4. Hereinafter, this is called deterministic photon detection. When the probability amplitude is ± 2, it is probabilistic which photon is detected by which detection.

Using the characteristics shown in Table 1, the transmitter 10 and the receiver 100 can obtain a common bit by the following procedure.
(1) The transmitter 10 and the receiver 100 transmit and receive a pulse train having a necessary length with the above configuration. At the same time, the transmitter 100 records pulse transmission slot time and phase modulation by the control unit 14.
(2) The receiver 100 notifies the transmitter 10 of the time slot in which the photon is detected via the control unit 106 and the communication path 17. At the same time, the receiver 100 records the detection slot time and the detector in the control unit 106.
(3) The transmitter 10 refers to the received time slot data and the recorded phase modulation data (slot time and phase of pulse transmission) in the control unit 14 so that the detection event of the receiver 100 is detected. It is determined whether or not deterministic photon detection is performed, and the determination result is notified to the receiver 100 via the communication path 17.
(4) The transmitter 10 and the receiver 100, for an event that is deterministic photon detection, is a bit “0” if the event is due to the first detector 104, and a bit if it is due to the second detector 105. “1” is given. For deterministic photon detection, the transmitter 10 knows which detector has detected the photon, so that the transceivers 10 and 100 obtain the same bit value. This bit is a secret key bit.

  In the system according to the above configuration / procedure, the bit mismatch rate due to spoofing eavesdropping is higher than before. The reason will be described with reference to FIG.

  2, the internal configurations of the transmitter 10 and the receiver 100 are the same as those in FIG. Reference numeral 200 denotes an eavesdropper (eavesdropper) inserted in the middle of the optical transmission line 15, and includes substantially the same configurations 201 to 205 and 207 to 210 as the receiver 100, and further includes both photon detectors 204 and 205. The optical transmitter 220 is connected to the output terminal. Since the components 201 to 205 and 207 to 210 of the eavesdropper 200 correspond to the components 101 to 105 and 107 to 110 of the receiver shown in FIG. 1, detailed description of the configuration is omitted.

  In spoofing eavesdropping, the eavesdropper 200 detects photons by a receiving circuit similar to the receiver 100 in the middle of the optical transmission path 15. Based on the photon detection result, the eavesdropper 200 transmits a dummy signal via the optical transmission line 15 so that the regular receiver 100 has the same detection result. At this time, the pulse train 16 transmitted from the transmitter 10 is a pulse train having an average transmission level of less than one photon per pulse, for example, 0.1 photons / pulse, as described above. Therefore, the eavesdropper 200 detects a photon only once per average slot. The eavesdropper 200 sends out four consecutive pulses with a set phase for the slot in which the photon is detected, as in the case where the regular receiver 100 detects the photon with a predetermined detector, but what about the slot in which the photon is not detected? Do not send any. As a result, four isolated continuous pulses 120 are sent to the receiver 100.

  When the receiver 100 receives the isolated four consecutive pulses 120, in the photon detectors 104 and 105, as shown in FIG. 2, four consecutive pulse trains 131 to 135 shifted by one pulse are overlapped. In this case, photons can be detected in seven time slots. Which detector 104, 105 detects a photon depends on the way of interference in each slot, as described in Table 1 above. In the middle time slot (indicated by a circle in FIG. 2), all four pulses interfere, so that a photon is detected by the detector intended by the eavesdropper 200. On the other hand, in other time slots, since the number of interfering pulses is 3 or 2, or there is no interfering partner, photons are randomly detected for the detector regardless of the intention of the eavesdropper 200. Therefore, if the receiver 100 detects photons in these time slots and then generates a secret key bit according to the procedure (4) described above, the secret key bit may not match the key bit generated by the transmitter 10. Arise. As a result, an eavesdropping action can be found by subsequent test bit verification.

  The probability of detecting photons in the middle time slot is 1/4, and the probability of detecting photons in the other time slots is 1-1 / 4 = 3/4. Since half of the latter are bit mismatches, the bit mismatch rate is 3/8. On the other hand, the bit mismatch rate due to spoofing in the conventional system described with reference to FIG. 4 is 1/4. That is, in the present invention, the bit mismatch rate due to spoofing is much higher than that of the conventional system.

  Thus, according to the present invention, the detection probability of pulse multiplexing having the correct phase generated by spoofing is reduced, and at the same time, the bit mismatch rate is increased. As a result, the mismatch detection probability in the test bit increases and the probability of detecting eavesdropping increases.

  As described above, according to the present embodiment, quantum key distribution with an increased bit mismatch rate due to spoofing is realized, but a disadvantage that arises is that the generation rate of secret key bits is lowered. In this embodiment, a key (secret key) is generated from a deterministic photon detection event, and other photon detections are ignored. On the other hand, since all the detected photons contribute to the key bit generation in the conventional system, this embodiment has a lower key generation rate than the conventional system. As is generally done, when the pulse phases are randomly modulated with {0, π}, the 16 phase combination patterns shown in Table 1 are generated with equal probability. In this case, the probability that a photon is detected is given by the square of the probability amplitude shown in Table 1. When all the detection probabilities of the photon detection patterns shown in Table 1 are added, the number is 128. Of these, 64 are added for deterministic photon detection. That is, the key bit generation rate is 50%.

  However, as will be described below, this key generation rate can be increased by imposing conditions on the phase modulation pattern. For example, in the transmitter 10, a continuous pulse train is divided into four blocks, and either {0-0-π-0} or {π-π-π-0} pattern is randomly (randomly) assigned to each block. ), And each pulse is phase-modulated by the phase modulator 12 in accordance therewith. Here, for convenience, the former {0-0-π-0} is referred to as a pattern A, and the latter {π-π-π-0} is referred to as a pattern B.

  As a result of the phase modulation, the receiver 100 detects photons in accordance with the phase state of four consecutive pulses randomly selected from the pulse train. In this case, four continuous pulses are not always selected in units of blocks, and four pulses extending over blocks are selected. Now, since there are two types of block patterns A and B, it is sufficient to consider the case where two consecutive blocks are AA, AB, BA, and BB. . The relationship between the continuous four pulses selected from each combination and the detector capable of detecting photons is as shown in Table 2 below.

In Table 2, “1” or “2” is entered as deterministic photon detection, and “1, 2” is entered as probabilistic photon detection. In the case of deterministic photon detection, the | probability amplitude | ² is 16, and in the case of stochastic photon detection, 2 × 2 + 2 × 2 = 8. From Table 2, the rate at which it is deterministic when it detects a photon (detector column other than “1”) (16 if the detector column is deterministic with “1” or “2”, When “1, 2” is uncertain, it is 8.) is 92% (11/12%). As described above, when the condition of the phase modulation pattern is imposed, the secret key bit generation rate can be increased.

(Other embodiments)
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example, and the constituent members thereof are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, change in shape design, etc. are all included in the embodiments of the present invention.

  For example, in the receiver configuration of FIG. 1, the delay time T is given by the first-stage branching / combining circuit and the delay time 2T is given by the second-stage branching / combining circuit. The delay time 2T may be given by the first-stage branch / combining circuit, and the delay time T may be given by the second-stage branch / combining circuit.

It is a block diagram which shows the structure of the quantum cryptography communication system in one Embodiment of this invention. It is a block diagram for demonstrating impersonation eavesdropping with respect to the quantum cryptography communication system in one Embodiment of this invention. It is a block diagram which shows the basic composition of the conventional differential phase shift quantum key distribution system. It is a block diagram for demonstrating impersonation eavesdropping with respect to the differential phase shift quantum key distribution system of a prior art.

Explanation of symbols

10 Transmitter (sender)
DESCRIPTION OF SYMBOLS 11 Light source 12 Phase modulator 13 Optical attenuation means 14 Control part 15 Optical transmission path 17 Communication path 100 Receiver (receiver)
101 Optical branching means (optical demultiplexer)
102,103 2 × 2 optical coupler (optical multiplexer)
104, 105 Photon detector 106 Control unit 107, 109 Delay path (long path)
108, 110 Short path 200 Eavesdropper (Eavesdropper)
201 Optical branching means (optical demultiplexer)
202,203 2 × 2 optical coupler (optical multiplexer)
204, 205 Photon detector 207, 209 Delay path (long path)
208, 210 Short path 220 Optical transmitter

Claims (4)

A transmitter for transmitting an optical pulse train having a constant time interval T, which is randomly phase-modulated by 0 or π, at a power level of less than one photon on average per pulse, and a receiver for receiving the optical pulse train transmitted from the transmitter; A quantum cryptography communication method using a quantum cryptography communication device comprising:
The receiver
A duplexer for branching the optical pulse train transmitted from the transmitter;
A delay path that gives one of the two optical pulse trains branched by the duplexer a time difference equal to or an integral multiple of the pulse interval T of the optical pulse train from the transmitter with respect to the other optical pulse train;
A multiplexer / demultiplexer that multiplexes the other optical pulse train branched by the demultiplexer and the optical pulse train delayed by the delay path, and divides the combined optical pulse train into two branches;
A plurality of delay multiplexing / demultiplexing units composed of the delay path and the multiplexer / demultiplexer are connected in cascade, and the time differences of the delay paths of the delay multiplexing / demultiplexing units are different from each other ,
The method
When transmitting an optical pulse train having a constant time interval T, which is randomly phase-modulated by 0 or π, at the transmitter at an average power level of less than 1 photon,
Block a continuous pulse train into a predetermined number of pulses,
Randomly select any one of a plurality of predetermined phase patterns prepared for each block and assign them to each block.
Phase modulate each pulse according to the assignment
A quantum cryptography communication method characterized by the above. The quantum cryptography communication method according to claim 1 ,
The predetermined plurality of pulses serving as a unit block are four pulses, and the predetermined phase pattern is {0-0-π-0} and {π-π-π-0}. Quantum cryptography communication method. A transmitter for transmitting an optical pulse train having a constant time interval T, which is randomly phase-modulated by 0 or π, at a power level of less than one photon on average per pulse, and a receiver for receiving the optical pulse train transmitted from the transmitter; A quantum cryptography communication method using a quantum cryptography communication device comprising :
The receiver is
Optical branching means for branching the optical pulse train transmitted from the transmitter into a first long path and a first short path;
2 × 2 that multiplexes two optical pulse trains output from the first long path and the first short path, and divides the combined optical pulse train into a second long path and a second short path A first optical coupler having input / output terminals of
A second optical coupler having a 2 × 2 input / output terminal that multiplexes two optical pulse trains output from the second long path and the second short path, and divides the combined optical pulse train into two branches; ,
Two photon detectors respectively connected to the output terminals of the second optical coupler ,
The first long path gives a time difference between the long path and the first short path equal to the pulse interval T of the optical pulse train from the transmitter, and the second long path is Set to give a time difference between the second short path and the pulse interval 2T equal to twice the pulse interval T ;
The method
When transmitting an optical pulse train having a constant time interval T, which is randomly phase-modulated by 0 or π, at the transmitter at an average power level of less than 1 photon,
Block a continuous pulse train into a predetermined number of pulses,
Randomly select any one of a plurality of predetermined phase patterns prepared for each block and assign them to each block.
Phase modulate each pulse according to the assignment
A quantum cryptography communication method characterized by the above. In the quantum cryptography communication method according to claim 3,
The predetermined plurality of pulses serving as a unit block are four pulses, and the predetermined phase pattern is {0-0-π-0} and {π-π-π-0}. Quantum cryptography communication method.

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