JP4421977B2 - Quantum cryptographic communication device - Google Patents

Description

  The present invention relates to a quantum cryptography communication device, and more particularly, to a quantum cryptography communication device capable of supplying a quantum cryptography key to a transmitter and a receiver that are separated from each other by a longer distance than before.

  In recent years, research on quantum cryptography communication in which safety is physically guaranteed by using light of one photon level has been advanced. Quantum cryptography is one of encryption methods for supplying a secret key for performing cryptographic communication between two communication devices located at distant points, and is also called quantum key distribution (Patent Document 1). Although there are various methods for quantum key distribution, here, a differential phase shift quantum key distribution method similar to the present invention will be described (Non-patent Document 1).

  FIG. 3 shows a basic configuration of a conventional differential phase shift quantum key distribution device. The transmitter 1 transmits a coherent optical pulse train having a constant interval that is randomly phase-modulated with 0 or π as an average of less than one photon per pulse. This 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 photons are detected is quite probabilistic and uncertain until detected.

  The pulse train 2 transmitted from the transmitter 1 reaches the receiver 4 through the transmission path 3. The receiver 4 branches the received pulse train into two by an optical branching device (optical coupler) 5, adds a delay to one by an optical delay circuit (optical delay line) 6, and then a 2 × 2 optical coupler (optical multiplexing). Combine) again by 7). Photon detector A 8 and photon detector B 9 are provided at the two output ports of the multiplexing coupler 7, respectively.

  Here, it is assumed that the delay time given to one of the branching / combining circuits 5 to 7 is equal to the time interval of the input pulse train. If it does in this way, in the multiplexing coupler 7, the front and back pulses will be 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. If the phase difference is 0 as a result of the interference, the detector A 8 detects the photon, and if the phase difference is π, the detector B 9 detects the photon.

  Using the above configuration, the transmitter 1 and the receiver 4 can obtain a secret key by the following procedure. First, the receiver 4 detects photons by the above reception configuration. At this time, the detected time and the detector (either 8 or 9) are recorded. After the required number of photons are transmitted and received, the receiver 4 informs the transmitter 1 of the photon detection time. The transmitter 1 can know which detector has detected the photon by the receiver 4 from the notified detection time and its own phase modulation data.

  Therefore, if the detector A 8 is preliminarily determined as the bit “0” and the detector B 9 is the bit “1”, the transmitter 1 and the receiver 4 can obtain the same bit string. In this procedure, only the photon detection time is notified from the receiver 1 to the transmitter 4, and no bit information is output to the outside. Therefore, the bit information is not eavesdropped by another person through another receiver (not shown). Also, since the light that is being transmitted is an average of less than one photon per pulse, other receivers cannot branch part of the signal to obtain bit information. Because the photon is not divided into two, when another receiver detects the photon by branching, the photon does not reach the receiver 4, but the bit string transmitted by the transmitter 1 and the bit string received by the receiver 4 This is because does not match. Thus, the bit string obtained by the transceivers 1 and 4 by the above configuration and procedure is a bit string that is not wiretapped from the outside. Therefore, this bit string is used as a secret key for generating / reproducing encrypted data.

JP 2004-187268 A K. Inoue, E. Waks, and Y. Yamamoto, “Differential-phase-shift quantum key distribution using coherent light”, Physical Review A, vol. 68, paper number 022317 (2003).

In the conventional differential phase shift quantum key distribution system as described above, a single-photon level optical pulse is transmitted and received in order to avoid eavesdropping due to branching. By the way, there is a loss in the actual transmission path 3, and for this reason, some of the photons are lost in the transmission path 3 and do not reach the receiver 4. If the transmission line 3 is long, the loss is large, and the probability that a photon reaches the receiver 4 is small. On the other hand, in general, the photon detectors 8 and 9 may operate as if the photons were detected even though no photons were actually input. The malfunction of the photon detectors 8 and 9 can be ignored if there are a large number of input photons. However, if the number of input photons is small, the number of erroneous signals due to malfunction increases relatively, and correct photon detection cannot be performed. That is, if the transmission distance is long, the device does not operate correctly. That is, the transmittable distance is limited by the attenuation of the transmission path.

  In normal optical communication, it is possible to reinforce a weakened signal using an optical amplifier or a repeater via photoelectric conversion, but in the case of quantum key distribution, an optical signal of one photon level Therefore, when trying to directly amplify the light, the original optical signal is buried in the spontaneous emission light of the optical amplifier. In addition, even when attempting to regenerate a signal through photoelectric conversion, what is transmitted by quantum key distribution is an optical signal in which it is probabilistic which pulse will detect a photon. It is impossible to regenerate the pulse train by photoelectric conversion.

  As described above, conventional signal relay means cannot be applied to quantum key distribution according to the prior art, and there is a problem that the transmittable distance is limited due to transmission path loss.

  The present invention has been made in view of the above points, and its purpose is to supply a quantum encryption key to a transmitter and a receiver that are separated from each other by a distance longer than a conventional limit distance that enables quantum key distribution. An object of the present invention is to provide a quantum cryptography communication device capable of performing the above.

In order to achieve the above-described object, the invention of claim 1 is a quantum cryptography communication apparatus that supplies an encryption key for encrypting and decrypting data transmitted and received between a transmitter and a receiver. A transmitter that outputs a pulse train; a quantum correlation photon pair generator that outputs a photon pair consisting of a signal photon and an idler photon with quantum correlation at the same time interval as the optical pulse train; and the optical pulse train and the signal from the transmitter A repeater to which photons are input; and a receiver to which the idler photons are input. The repeater has 2 × 2 input / output terminals, and an optical pulse train from the transmitter at the same time. Is input to the first input terminal, the photon train of the signal photons is input to the second input terminal, and the first photon detector is connected to the first output terminal of the optical coupler. When A second photon detector connected to a second output terminal of the optical coupler, and the repeater detects at least one of the first and second photon detectors continuously. The time is notified to both the transmitter and the receiver, and the transmitter is informed whether one photon detector continuously detects photons or two photon detectors alternately detect photons. It is characterized by notifying.

Here, the receiver has a branching means for branching the photon string of the input idler photons into two paths, and one of the photon strings branched by the branching means is 1 for the other of the branched photon strings. A photon train having a delay means for delaying by pulses and a 2 × 2 input / output terminal, inputting the other of the photon trains branched by the branch device to the first input terminal, and delayed by the delay means To the second input terminal, a third photon detector connected to the first output terminal of the second optical coupler to detect a photon string, and the second photocoupler. A fourth photon detector connected to a second output terminal of the optical coupler for detecting a photon train, and the receiver is configured to detect the photon train at a time corresponding to a detection time notified from the repeater. If the third photodetector detects photons, the bit “0”, the fourth If the photodetector is a photon detection bits to generate a bit "1", the detection time of the photons generated the bit can be characterized in that informing the transmitter.

The transmitter includes means for generating a coherent optical pulse train at regular intervals, means for modulating the phase of the optical pulse by 0 or π, and the modulated optical pulse at an average of less than one photon per pulse. and means for transmitting to a transmission path to said relay apparatus, the transmitter records the phase modulation data, and detection time notified from the repeater, the detection time notified from the receiver, and The bit information generated by the receiver is detected from the own phase modulation data, and the bit information is used as its own bit information.

  The quantum correlation photon pair generator may include means for generating a pump photon, and generate a signal photon and an idler photon from the pump photon as an optical parametric process as the photon pair.

  In general, the transmission distance of a signal transmission apparatus is determined by the difference between transmission power and minimum reception sensitivity. Assuming that the photon generation efficiency of the quantum correlation photon pair generator used in the present invention is the same as that of the transmitter, the transmission distance from the quantum correlation photon pair generator to the receiver is the transmitter / receiver when there is no relay node. Can be the same as the distance between. In addition to this, the repeater (relay node) according to the present invention is adopted in the present invention, so that the distance from the transmitter to the repeater is added to the distance from the quantum correlation photon generator to the repeater. The effective transmission distance from the transmitter to the receiver. Therefore, when the present invention is applied, the distance between the transmitter and the receiver can be made significantly longer than before.

  The best mode for carrying out the present invention will be described below in detail with reference to the drawings.

  As described in detail below, the present invention inserts a quantum correlation photon pair generator and a relay node between a transmitter and a receiver based on the differential phase shift quantum key distribution described in the prior art. By doing so, the distance between the transceivers is increased.

  FIG. 1 shows a configuration of a part relating to a quantum correlation photon pair generator and a relay node of a quantum cryptography communication apparatus according to an embodiment of the present invention. A quantum correlation photon pair generator 13 and a relay node (repeater) 14 are arranged in the transmission path between the transceivers 1 and 4. The relay node 14 is connected to a 2 × 2 optical coupler 15 as a multiplexing means, two photon detectors 16 and 17 connected to the output port of the coupler, and two photon detectors 16 and 17 for transmission. An information exchange device 18 as means for exchanging information between the machine 1 and the receiver 4 is provided. The information exchange device 18 is composed of a CPU and the like. In FIG. 1, the quantum correlation photon pair generator 13 and the relay node 14 are depicted as being arranged close to each other for the sake of explanation, but in actuality, they are connected via a transmission line that is significantly longer than the conventional one. It shall be.

The transmitter 1 phase-modulates the light from the coherent pulse light source 21 by | 0, π | for each pulse, attenuates it to an average of less than 1 photon / pulse, and sends it to the transmission line. More specifically, the transmitter 1 includes a coherent pulse light source 21 that generates a coherent optical pulse train at regular intervals, a phase modulation circuit 22 that modulates the phase of the optical pulse with 0 or π, and a modulated optical pulse. An attenuation circuit 23 for transmitting to the repeater 14 with an average of less than one photon per pulse, and an information exchange device 24 as means for exchanging information between the relay node 14 and the receiver 4 connected to the phase modulation circuit 22 I have. The information exchange device 24 is composed of a CPU and the like.

The receiver 4 divides the input optical pulse train into two branches, and multiplexes them again after giving one pulse a delay for one pulse. Photon detectors 8 and 9 are provided at two output ends of the multiplexing coupler 7, respectively. More specifically, the receiver 4 includes an optical splitter (optical coupler) 5 that branches an input photon sequence of an idler photon, which will be described later, into two paths, and a photon sequence branched from one of the branched photon sequences. An optical delay circuit (optical delay line) 6 that is delayed by one pulse with respect to the other of the first and second input / output terminals, and the other of the branched photon strings is input to the first input terminal, 2 × 2 optical coupler for inputting a photon sequence delayed to the second input terminal (optical multiplexer) 7, photon detection for detecting the first is connected to the output terminal photons column of the optical coupler A photon detector B 9 connected to the second output terminal of the optical coupler A 8 and detecting the photon train; and connected to the two photon detectors 8 and 9 between the transmitter 1 and the relay node 14. And an information exchange device 25 as a means for exchanging information between them. The information exchange device 25 is composed of a CPU and the like.

  The information exchange device (CPU) 18 of the relay node 14 records the time when the photon was detected and which photon detector detected, and the photon detection time is transmitted to the transmitter 1 and the receiver 4 as will be described later. Both of them are notified to the transmitter 1 whether one photon detector continuously detects photons or two photon detectors alternately detect photons. The information exchange device (CPU) 24 of the transmitter 1 records the phase modulation data and, as will be described later, the detection information from the repeater 14, the time information from the receiver 4, and its own phase modulation data. Then, the bit information generated by the receiver 4 is sensed and the bit information is operated as its own bit information. The information exchange device (CPU) 25 of the receiver 4 records the photon detection time and the detected detector, and, as will be described later, at the time corresponding to the detection time notified from the repeater 14, the photodetector A 8. If the photon is detected, the bit is “0”, and if the photo detector B 9 is detecting the photon, the bit is generated as the bit “1”, and the transmitter 1 is informed of the detection time of the photon that generated the bit. do.

  The quantum correlation photon pair generator 13 has a pump light source 19 and an optical nonlinear medium 20, and is a device that always outputs two correlated photons simultaneously. Specifically, the quantum correlation photon pair generator 13 outputs two photons, that is, a signal photon and an idler photon, which will be described later, using, for example, an optical parametric process. The light from the pump light source 19 is input to the optical nonlinear medium 20, the signal photon and idler photon output from the optical nonlinear medium 20 are transmitted to the relay node 14, and the latter is transmitted to the receiver 4.

A type of optical nonlinear phenomena and said optical parametric process, for example, if you enter the signal light of the pump light and the optical frequency f _s of the optical frequency f _p in a second-order optical nonlinear crystal 20, nonlinear polarization P = c ₂ E _p E _s ^* from the frequency of the light frequency _f i _{= f} p -f _s is generated _{(c 2:} nonlinear susceptibility, _{E p:} pump light electric field, _{E s:} signal light field, * is a complex conjugate). Conventionally, the newly generated light is called idler light. Quantitatively speaking, this is a phenomenon in which an idler photon is generated from a pump photon and a signal photon, and there is a relationship as shown in FIG. This relationship can be regarded as a process in which one pump photon (f _p ) disappears and one signal photon (f _s ) and one idler photon (f _i ) are generated. The signal light input works to promote this process, but if the pump light is sufficiently strong, this process can occur spontaneously without the signal light input.

  That is, a signal photon and an idler photon are generated from an input of only pump light. At this time, in order to satisfy energy conservation, signal photons and idler photons are always generated in pairs. A pair of photons having such a correlation is called a quantum correlation photon pair.

The quantum correlation photon pair generator 13 in FIG. 1 is a device that generates such a correlated photon pair. When a correlated photon pair is generated using an optical parametric process, the following relationships are established between the phases of the interacting pump light, signal light, and idler light. As described above, classically, idler light is generated from nonlinear polarization of P = c ₂ E _p E _s ^* . Looking at the phase of this expression, there is a relationship of φ _i = φ _p −φ _s (φ _i : phase of idler polarization wave = idler light phase, φ _p : pump light phase, φ _s : signal light phase) I understand. This inter-system is also established in the quantum correlation photon pair generated by the optical parametric process.

  In the configuration of FIG. 1, the quantum correlation photon pair generator 13 emits the quantum correlation photon pair as described above, the signal photons are sent to the relay node (repeater) 14, and the idler photons are sent to the receiver 4. The Here, it is assumed that the transmitted photons are pulse trains having the same time interval as the transmission pulse train from the transmitter. Here, it is assumed that the number of photon pairs of signal photons and idler photons to be transmitted is one pair or less per pulse.

  The relay node 14 receives a transmission signal from the transmitter 1 and a signal photon from the quantum correlation photon pair generator 13. The input transmission signal and signal photon are combined by the 2 × 2 optical coupler 15. Photon detectors 16 and 17 are arranged at two output ports of the optical coupler 15, respectively.

With the above configuration, the secret key can be supplied to the transceivers 1 and 4. The principle and procedure will be described below using equations. When the quantum mechanical representation is used, the state | Ψ _t > of the transmission signal from the transmitter 1 input to the relay node 14 is expressed as the following equation (1).

Here, | t _j > is a state where one photon exists at time t _j , a exp (iθ _j ) is a probability amplitude (complex number) of state | t _j >, a is an amplitude part (real number), and θ _j is Represents the phase. N is the number of pulses. The subscript t is used to mean the target state to be transferred to the receiver 4. Hereinafter, the state sent from the transmitter 1 is called a target state. On the other hand, the state | ψ _e > emitted from the quantum correlation photon pair generator 13 is expressed by the following equation (2).

Here, the subscripts s and i represent a signal photon and an idler photon, respectively. Since a quantum correlation photon pair is always generated in a pair, it is expressed as | t _j > _s | t _j > _i as one quantum state in the pair. a exp (iφ _j ) is the probability amplitude (complex number) of the state | t _j > _s | t _j > _i , where a is the amplitude part (real number) and φ _j is the phase. Since this is output as a periodic pulse, the output state from the correlated photon pair generator 13 is expressed as the sum of each pair.

  In the present invention, the transmission distance is increased by causing the target state and the quantum correlation photon pair state to interact with each other. In order to describe such a situation, the target state and the correlated photon pair state are collectively regarded as one state, and the entire state | Ψ> is expressed as the following equation (3).

Here, attention is paid to the case where photons are detected at the time t _j and the time t _{j + 1} in the relay node 14 . When a state related to such a photon detection event is written out, the following equation (4) is obtained.

The above equation (4) is obtained by substituting the terms relating to t _j and t _{j + 1} of the above equations (1) and (2) into the above equation (3). Furthermore, Δ = θ _{j + 1} −θ _j , δ = φ _{j + 1} -Φ _j is replaced. Here, φ and δ are considered. φ is the phase of the signal photon state multiplied by the idler photon state (formula (2)). Therefore, φ = φ _s + φ _i . By the way, as described above, the relationship of φ _i = φ _p −φ _s is established in the photon pair emitted from the quantum correlation photon pair generator 13. Therefore, it is φ = φ _p. Here, it is assumed that the coherence time of the pump light in the quantum correlation photon pair generator 13 is sufficiently longer than the time interval of the pulse train. That is, the phase of pump light that generates adjacent photon pairs is assumed to be constant. With this setting, φ (t _j ) = φ (t _{j + 1} ), and thus δ = 0. Substituting this, equation (4) becomes the following equation (5).

  The above equation (5) represents the overall state in the stage before the target state and the signal photon are input to the optical coupler 15 in the relay node 14. In the optical coupler 15, the target state and the signal photon are branched to two output ports, respectively. The state change due to the branching is expressed by the following equations (6a) and (6b).

Here, | d ₁ , t _j > represents a state in which one photon is output to the output port to the photon detector 16 at time t _j , and | d ₂ , t _j > represents one photon to the photon detector 17. Represents the state of being output to the output port at time t _j . Substituting the above equations (6a) and (6b) into the above equation (5), and further extracting the state in which photons are continuously detected at times t _j and t _{j + 1} , the following equation (7) is obtained.

Here, it is assumed that the target photon and the signal photon cannot be distinguished in the photon detection. Then, the subscripts t and s that distinguish the target photon state and the signal photon state are eliminated, and the above equation (7) is rewritten as the following equation (8).

In the above equation (8), the photon detector 16 continuously detects photons (terms where | d ₁ , t _j > | d ₁ , t _{j + 1} >), and the photon detector 17 continues. A state in which photons are detected (terms where | d ₂ , t _j > | d ₂ , t _{j + 1} > are applied), a state in which photon detectors 16 and 17 alternately detect photons (| d ₁ , t _j > | D ₂ , t _{j + 1} > or | d ₂ , t _j > | d ₁ , t _{j + 1} >)).

  Next, consider each of these states. When the state in which one photon detector continuously detects photons is extracted from the above equation (8), the following equation (9) is obtained.

  As described in the section of the prior art, in the differential phase shift quantum key distribution, the transmission light pulse is phase-modulated by 0 or π. Therefore, Δ = 0 or ± π. When this is substituted into the above equation (9), the following equations (10a) and (10b) are obtained for each case.

Here, attention is paid to the state of idler photons. In the above equation (10a), a term (| t _j > _i + | t _{j + 1} > _i ) is multiplied as a common term. This is because when the single photon detectors time t _j, successively with t j + ₁ to detect a photon, the time t _j, the probability amplitude of the idler photons t j + ₁ represents that always the same phase . On the other hand, the term (| t _j > _i− | t _{j + 1} > _i ) is multiplied as a common term in the above equation (10b). This is one of the photon detectors time t _j, when detecting photons in succession with t j + _1, the time t _j, the probability amplitude of the idler photons t j + ₁ represents that the always antiphase .

  Similarly, the state in which the two photon detectors 16 and 17 alternately detect photons is expressed by the following equations (11a) and (11b).

In the above equation (11a), when Δ = 0 and the two photon detectors 16 and 17 detect photons alternately, the probability amplitudes of idler photons at times t _j and t _{j + 1} are always in opposite phases. Represents that. In the above equation (11b), when Δ = ± π and the two photon detectors 16 and 17 detect photons alternately, the probability amplitudes of idler photons at times t _j and t _{j + 1} are always in phase. Represents that.

When the receiver 4 detects a photon with a configuration similar to that described in the section of the prior art (FIG. 3), if two consecutive pulses are in phase, the photon detector 8 detects the photon and the two consecutive pulses are reversed. If it is in phase, the photon detector 9 detects the photon. Considering this reception operation and the above case classification, the photon detection mode of the receiver 4 is as follows.
(I) If the relay node 14 detects photons continuously with one photon detector (16 or 17) and Δ = 0, the receiver 4 detects photons with photon detector A8;
(Ii) If the relay node 14 detects photons continuously with one photon detector and Δ = ± π, the receiver 4 detects photons with the photon detector B 9
(Iii) If the relay node 14 detects photons alternately by the two photon detectors 16 and 17, and Δ = 0, the receiver 4 detects photons by the photon detector B 9;
(Iv) If the relay node 14 detects photons alternately by the two photon detectors 16 and 17, and Δ = ± π, the receiver 4 detects photons by the photon detector A8;
Will do.

  Using the above operating characteristics, the transmitter 1 and the receiver 4 can obtain an encryption key in the following procedure.

  The transmitter 1 and the quantum correlation photon generator 13 transmit the target pulse train and the quantum correlation photon pair pulse train, respectively, and the relay node 14 and the receiver 4 perform photon detection with the receiving configuration of FIGS.

  When the photon detector (16, 17) continuously detects photons, the relay node 14 notifies the transmitter 1 and the receiver 4 of the time. In this case, the relay node 14 informs the transmitter 1 whether one photon detector continuously detects photons or whether two photon detectors alternately detect photons.

  When the detector A 8 detects photons at the time corresponding to the detection time notified from the relay node 14, the receiver 4 uses the bit “0”, and if the detector B 9 detects photons, the bit “1”. ". Furthermore, the receiver 4 informs the transmitter 1 of the detection time of the photon that generated the bit.

  The transmitter 1 can know the bit information generated by the receiver 4 from the detection information from the relay node 14, the time information from the receiver 4, and its own modulation data. Therefore, it is used as its own bit information.

  Through the above procedure, the transceivers 1 and 4 obtain the same bit string. At this time, the bit information itself is not sent to the outside. Therefore, this bit string can be used as a secret key.

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

It is a block diagram which shows the structure of the principal part of the quantum cryptography communication apparatus in one Embodiment of this invention. It is a conceptual diagram explaining the principle of a secondary optical parametric process. It is a block diagram which shows the basic composition of the quantum cryptography apparatus by the conventional differential phase shift quantum key distribution system.

Explanation of symbols

1 Transmitter 2 Pulse train 3 Transmission path 4 Receiver 5 Optical splitter (optical coupler)
6 Optical delay circuit (optical delay line)
7 Optical multiplexer (2 × 2 optical coupler)
8 Photon detector A
9 Photon detector B
13 Quantum correlation photon pair generator 14 Relay node (repeater)
15 Optical coupler C
16 Photon detector 1
17 Photon detector 2
18, 24, 25 Information exchange device (CPU)
DESCRIPTION OF SYMBOLS 19 Pump light source 20 Optical nonlinear medium 21 Coherent pulse light source 22 Phase modulation circuit 23 Attenuation circuit

Claims (4)

In a quantum cryptography communication device that supplies an encryption key for encrypting and decrypting data transmitted and received between a transmitter and a receiver,
A transmitter that outputs optical pulse trains at regular intervals;
A quantum correlation photon pair generator for outputting a photon pair consisting of a signal photon and an idler photon with quantum correlation at the same time interval as the optical pulse train;
A repeater to which the optical pulse train from the transmitter and the signal photon are input;
A receiver to which the idler photon is input,
The repeater is
Light having a 2 × 2 input / output terminal, and inputting the optical pulse train from the transmitter to the first input terminal at the same time and inputting the photon train of the signal photons to the second input terminal With the coupler,
A first photon detector connected to a first output terminal of the optical coupler;
A second photon detector connected to a second output terminal of the optical coupler;
When at least one of the first and second photon detectors detects photons continuously, the repeater informs both the transmitter and the receiver of the time, and one photon detector A quantum cryptography communication device that informs the transmitter whether photons are detected continuously or two photon detectors detect photons alternately. The receiver
Branching means for branching the photon train of the input idler photons into two paths;
Delay means for delaying one of the photon trains branched by the branching means by one pulse with respect to the other of the branched photon trains;
A 2 × 2 input / output terminal; the other one of the photon strings branched by the branching means is input to the first input terminal; and the photon string delayed by the delaying means is input to the second input terminal. A second optical coupler for input;
A third photon detector connected to the first output terminal of the second optical coupler for detecting a photon string;
A fourth photon detector connected to a second output terminal of the second optical coupler for detecting a photon train;
The receiver detects bit “0” if the third photodetector detects photons at a time corresponding to the detection time notified from the repeater, and the fourth photodetector detects photons. 2. The quantum cryptography communication device according to claim 1, wherein a bit is generated as bit “1” and the transmitter is notified of the detection time of the photon that generated the bit. The transmitter is
Means for generating a coherent optical pulse train at regular intervals;
Means for modulating the phase of the optical pulse by 0 or π;
Means for sending the modulated optical pulses to the transmission path to the repeater at an average of less than one photon per pulse;
The transmitter records phase modulation data, and the bit generated by the receiver from the detection time notified from the repeater, the detection time notified from the receiver, and its own phase modulation data. 3. The quantum cryptography communication apparatus according to claim 2 , wherein information is detected and the bit information is used as its own bit information.   4. The quantum correlation photon pair generator includes means for generating a pump photon, and generates a signal photon and an idler photon from the pump photon by an optical parametric process as the photon pair. The quantum cryptography communication device according to any one of the above.

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