JP4417360B2 - Quantum communication system - Google Patents

Description

  The present invention relates to a quantum communication system. More specifically, the present invention relates to a quantum communication system that detects wiretapping by detecting a relative phase difference of a phase-modulated optical pulse train.

  Conventionally, as a cryptographic technique, an encryption method based on mathematical calculation difficulty of a mathematical formula (for example, a long time is required for calculation for decryption) has been used, but recently, a single photon level is used. Research on quantum cryptography that uses light to guarantee physical security is ongoing.

  Quantum cryptography is a cryptographic technique that uses the theory of quantum mechanics, and is known as the ultimate cryptographic technique in which the content becomes meaningless even if eavesdropping, and the eavesdropping can be understood.

  In the field of quantum communication, there is known a system that supplies a secret key for performing cryptographic communication between two parties present at points distant from each other, and this system is also called a quantum key distribution system.

  There are various types of quantum key distribution systems. In this specification, a “differential phase shift quantum key distribution system” (see Non-Patent Document 1) will be described as a prior art.

  FIG. 1 shows a basic configuration of a “differential phase shift quantum key distribution system” of the prior art.

  The transmitter 100 transmits a coherent optical pulse train having a constant interval arbitrarily modulated by 0 or π to the receiver 110 via the transmission line 120 at an average of less than 1 photon per pulse (for example, 0.1 photon / pulse). Send it out. The state where the average number of photons is less than 1 can be realized by greatly attenuating normal laser light.

  When a photon is detected from such an optical pulse train, the detection result is that a photon is detected with a certain pulse and nothing is detected with a certain pulse. The pulse at which a photon is detected is uncertain until it is measured.

  As shown in FIG. 1, the optical pulse train transmitted from the transmitter 100 reaches the receiver 110 via the transmission path 120. First, the receiver 110 uses the optical branching means 111 to split the optical pulse train received from the transmitter 100 into two so that the energy pulse is equally divided, and the branched optical pulse trains are divided into the long path 115 and the short path. Send to path 116. In the long path 115, a certain delay (in this specification, time T) is added to the optical pulse train. Thereafter, the optical pulse train that has passed through the long path 115 and the short path 116 is combined again by the 2 × 2 multiplexing coupler 112. The 2 × 2 multiplexing coupler 112 has two input terminals (connected to the long path 115 and the short path 116, respectively), and receives an optical pulse train that has passed through the long path 115 and the short path 116. The 2 × 2 multiplexing coupler 112 also includes two output terminals, each of which is connected to the photon detector 1 (113) and the photon detector 2 (114).

  It is assumed that the constant delay time T given to one optical pulse train in the long path 115 is equal to the constant interval T of the optical pulse train input from the transmitter 100 to the receiver 110 via the transmission path 120 (FIG. In the 2 × 2 multiplexing coupler 112, the preceding and succeeding pulses overlap and are multiplexed.

  The optical pulse train input to the receiver 110 is phase-modulated by 0 or π. Therefore, if the propagation phase of the branching / combining circuit in the receiver 110 is appropriate, the phase difference between the overlapping pulses is 0 or π.

  As a result of multiplexing by the 2 × 2 multiplexing coupler 112, both pulses interfere. If the phase difference is 0, the photon detector 2 (114) detects a photon, and if the phase difference is π, the photon detector 1 (113) Detect photons.

  Using the above configuration, the transmitter 100 and the receiver 110 obtain a secret key by the following procedure.

  First, the receiver 110 detects a photon from a pulse transmitted from the transmitter 100 through the transmission path 120 with the above configuration. At this time, the receiver 110 records the time when the photon was detected and the detector. After the predetermined number of photons are received, the receiver 110 notifies the transmitter 100 of the time when the photons are detected (photon detection time).

  In the transmitter 100, the receiver 110 detects a photon by either the photon detector 1 (113) or the photon detector 2 (114) from the notified photon detection time and the phase modulation data of the transmitter itself. Can know.

  Therefore, if the photon detector 1 (113) detects a photon as a bit “0” and the photon detector 2 (114) detects a photon as a bit “1” in advance, The receiver 110 can obtain the same bit string on both sides.

  In the above procedure, since only the photon detection time is notified from the receiver 110 to the transmitter 100, the bit information does not go out of the receiver 100 and is not wiretapped.

  Further, another conventional technique will be described. There is a method called an impersonation method, which is a more advanced wiretapping method.

  FIG. 2 is an explanatory diagram of spoofing eavesdropping, which is a conventional technique.

  An eavesdropper receives a transmission signal transmitted by the transmitter 200 in the middle of the transmission path 220 with the same device configuration as the original receiver 210, and sends a dummy signal to the original receiver 210 based on the reception result. Transmission is performed using the optical transmitter 231. If the eavesdropper can correctly receive the transmission signal, the dummy signal is the same as the original transmission signal, and information can be obtained without the receiver 210 noticing the wiretapping action.

  However, in the “differential phase shift quantum key distribution system”, since the transmission signal is an optical pulse train having an average of less than one photon per pulse (for example, 0.1 photon / pulse), even if such a transmission signal is received. Photons are detected only once per 10 pulses.

  Therefore, the eavesdropper can know the phase difference between the two pulses corresponding to the time when the photon is detected, but cannot detect the phase difference in other cases.

  When an eavesdropper tries to send a dummy signal based on such a detection result, for a pulse whose phase difference cannot be detected, the phase selected by guessing is allocated and retransmitted (spoofing wiretapping 1), or no signal is sent. There is no choice but to use either method (spoofing spoofing 2).

  In the former case (spoofing spoofing 1), when the receiver 210 detects the phase difference selected by the guess, it is highly likely that the signal is different from the signal sent by the transmitter 200. In the latter case (spoofing spoofing 2), signal mismatch still occurs. The reason is that in this case, the eavesdropper sends an optical pulse train including two consecutive pulses corresponding to the time when the photon is detected, but it is an empty pulse other than the two consecutive isolated pulses. That is, when the receiver 210 receives two isolated continuous pulses, photons can be detected at three times in the stage of output from the branching / combining circuit (see FIG. 2).

  Referring to FIG. 2, in FIG. 2, among the optical pulse trains branched by the optical branching unit 211, an optical pulse train that is not delayed (the optical pulse train described below among the optical pulse trains in the receiver 210). ) And the delayed optical pulse train (the optical pulse train described above among the optical pulse trains in the receiver 210) are illustrated in time series (the right side is temporally earlier).

  In FIG. 2, three times when both optical pulse trains overlap are shown, but when a photon is detected at the middle time (referred to as the second time), the detection result is two pulses. According to the phase difference, photons are detected by a detector as intended by the transmitter 200.

  However, when a photon is detected at the first or third time that is before or after the second time of the time when two optical pulse trains overlap, there is no other party to interfere with, so the phase of one pulse ( 0 or π), photons are detected by the photon detector 1 or the photon detector 2.

  Therefore, when the receiver 210 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 transmitter 200.

  In this manner, when spoofing is performed, a bit mismatch (bit error) occurs between the transmitter 200 and the receiver 210. Therefore, the transmitter 200 and the receiver 210 obtain a secret key according to a normal procedure, and then perform a verification check using some test bits. If the system is operating normally, the bit information of both will match, but if there is spoofing, there will be bits that do not match.

  More specifically, the probability that the receiver 210 will detect a photon at the first or third time is ½, and the probability that the bits obtained from this photon detection do not match is further ½ of that. Therefore, bit mismatch (bit error) occurs at a rate of 1/4. If there is a bit error, it is determined that the system is not operating normally, and the secret key is discarded. 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.

  As described above, even if a spoofing eavesdropping is conventionally performed, since the transmission signal is less than one photon per pulse, the eavesdropper cannot transmit an accurate dummy signal, and a bit error occurs between the transmitter and the receiver. Was able to be determined.

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

  In the above description, it was explained that the obtained secret key is secure on the assumption that the performance of the transceiver is perfect. In practice, however, bit errors inherent in the system occur due to incomplete transceiver performance.

  In such a case, a part of the key may be eavesdropped due to the bit error.

  For example, assume that the original bit error rate of the system was e. On the other hand, it is assumed that the eavesdropper performs the above-described spoofing eavesdropping 2 only on a part of the transmission signal. When the wiretapping rate is x, the bit error rate generated thereby is x × 1/4 = x / 4.

  Here, when x / 4 <e, that is, when the bit error rate inherent in the system is higher than the bit error rate caused by eavesdropping, the transmitter and the receiver have the fluctuation due to the bit error inherent in the system. It cannot be distinguished from an increase in errors due to eavesdropping, and it is difficult to notice eavesdropping. That is, the key information corresponding to the ratio x may be stolen by an eavesdropper without the transmitter and the receiver noticing.

  In actual devices, the occurrence of bit errors due to imperfections in the transmitter and receiver is inevitable. In particular, in the case of a quantum key distribution system, since one photon is a carrier of information propagation, it is impossible to take a method of discriminating “0” or “1” by the number of photons as in a normal digital optical communication system. Bit errors are likely to occur.

  Therefore, if the bit error due to eavesdropping and the original bit error due to the system cannot be distinguished as described above, it is difficult to notice the eavesdropping, so there is a high possibility that part of the key will be stolen by spoofing.

  To prevent this, a system that can detect spoofing eavesdropping without relying on bit errors is desired.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a quantum communication system that determines spoofing by a method different from a bit error.

In order to solve the above-described problems, a quantum communication system of the present invention is a quantum communication system including a transmitter and a receiver, and the transmitter includes a light source that transmits an optical pulse train composed of pulses having a constant time interval. , Including a phase modulator that phase-modulates the optical pulse train by 0 or π, and at least two or more consecutive time-modulated optical pulse trains, each having a null pulse, and μ is a constant value of 0 <μ <1 , 1 and attenuating means for delivering a pulse per average μ pulse train of photons, and transmission means for transmitting the information of the time position of successive at least two, receivers, optical pulse train transmitted from the damping means of the transmitter Branching means for receiving the first optical pulse train and the second optical pulse train, delay means for delaying the first optical pulse train for a time equal to a fixed time interval, and the delayed first light The multiplexing means for combining the pulse train and the second optical pulse train to generate the combined optical pulse train, and the relative phase difference between the delayed first optical pulse train and the second optical pulse train is 0 First photon detection means for detecting photons in some cases, and second photon detection for detecting photons when the relative phase difference between the delayed first optical pulse train and second optical pulse train is π Means, receiving means for receiving information on at least two consecutive time positions, and in a time slot corresponding to the time position received from the transmitter via the transmitting means in the combined optical pulse train, And determining means for determining whether or not it has been detected.

The communication method in the quantum communication system comprising a transmitter, and the receiver is an optical pulse train having a constant time interval that is phase-modulated by 0 or [pi, including empty successive pulses at least two, mu is A step of receiving an average μ photon pulse train per pulse , which is a constant value of 0 <μ <1, and branching the pulse train into a first optical pulse train and a second optical pulse train; And a step of generating a combined optical pulse train and receiving information on the time positions of at least two or more consecutive empty pulses from the transmitter. And a step of detecting photons from each time slot of the combined optical pulse train, and no photons are detected at the time positions of at least two consecutive received empty pulses. Characterized by comprising a step of determining whether the detected photons from the time slot corresponding optical pulse train.

  According to the present invention, it is possible to discover the presence / absence of spoofing from the presence / absence of photon detection in a specific time slot. Since it is not a mechanism for detecting eavesdropping from the bit error rate as in the prior art, there is an effect of preventing partial eavesdropping that is difficult to notice due to a bit error due to incompleteness of the apparatus.

  FIG. 3 is a configuration diagram of a quantum communication system according to an embodiment of the present invention.

  In FIG. 3, the transmitter 300 includes a light source 301, a phase modulator 302, an attenuation unit 303, and a control unit CPU 304.

  The light source 301 is connected to the phase modulator 302. The phase modulator 302 is connected to the attenuating means 303. That is, the phase modulator 302 exists between the light source 301 and the attenuation means 303.

  The control unit CPU 304 is a CPU that performs main control of the entire transmitter 300. The transmitter 300 stores various data in addition to a light source 301, a phase modulator 302, an attenuating means 303, a ROM (not shown) that stores a control program of the control unit CPU 304, and a temporary work area. A memory (not shown) such as a RAM to be used is also provided. With such a configuration, the control unit CPU 304 can perform main control of the entire transmitter 300.

  The light source 301 transmits an optical pulse train at a constant time interval T, and the optical pulse train is input to the phase modulator 302.

  The phase modulator 302 phase-modulates each pulse included in the optical pulse train input from the light source 301 with 0 or π, and then delivers the optical pulse train to the attenuation means 303. That is, the phase of each pulse included in the optical pulse train is 0 or π.

  As the attenuating means 303, any means may be used as long as it is a means for largely attenuating light incident from a light source such as a laser beam such as an ND filter (Neutral Density Filter). In the present invention, an optical pulse train of less than one photon per pulse is transmitted from the transmitter 300, but an optical pulse train of less than one photon per pulse greatly attenuates ordinary laser light and realizes a state where the average number of photons is less than one. can do.

  More specifically, “an optical pulse train of less than one photon per pulse” refers to a predetermined number of optical pulse trains, including two consecutive empty pulses, and the number of photons contained in the optical pulse train is This refers to a state where there are fewer than the number of pulses.

  Further, in this specification, “empty pulse” refers to a pulse having zero photons. The light source 301 transmits an optical pulse train at a constant time interval T (at this time, each pulse may include a photon), but the photon is included by passing through the attenuation means 303. No pulse is generated. Such a pulse is defined as “empty pulse” in order to indicate that nothing is included, that is, it is merely empty.

  Thereafter, the transmitter 300 transmits the optical pulse train that has passed through the attenuation means 303 to the receiver 310.

  The receiver 310 as a photon detection device includes an optical branching means C1 (311), a 2 × 2 multiplexing coupler C2 (312), a photon detector 1 (313), a photon detector 2 (314), a control unit CPU315, A path 316 and a short path 317 are provided.

  The optical branching means C1 (311) is connected to the long path 316 and the short path 317.

  The 2 × 2 multiplexing coupler C2 (312) has two input terminals and two output terminals. The input terminal is connected to the long path 316 and the short path 317, and the output terminal is connected to the photon detector 1 (313) and the photon detector 2 (314).

  The 2 × 2 multiplexing coupler C2 (312) receives an optical pulse train from the long path 316 at one input terminal, and receives an optical pulse train from the short path 317 at the other input terminal. Thereafter, the 2 × 2 multiplexing coupler C2 (312) outputs a photon from one output terminal to the photon detector 1 when the phase difference between the two optical pulse trains is π, and when the phase difference is zero. Photons are output from the other output terminal to the photon detector 2.

  For the receiver 310, the control unit CPU 315 performs main control of the entire receiver 310. The transmitter 300 is similar to the transmitter 300 in that it has a ROM (not shown) storing a control program of the control unit CPU 315 and a memory (not shown) that performs the same function as the memory of the transmitter 300.

  The light input to the receiver 310 is split in energy equally (for example, 50 to 50) into the long path 316 and the short path 317 by the optical branching means C1 (311). The optical pulse train branched to the long path 316 is delayed by a fixed delay time T, and then combined again with the optical pulse train that has passed through the short path 317 by the 2 × 2 multiplexing coupler C2 (312). Here, it is assumed that the constant delay time T is equal to the pulse interval T of the input optical pulse train as shown in FIG.

  Under the above configuration, the transmitter 300 generates an optical pulse train having a constant interval (pulse interval) T arbitrarily phase-modulated by 0 or π at an average of less than one photon (for example, 0.1 photon / pulse). Send it out.

  Further, the transmitter 300 makes the coherence time of the optical pulse train sufficiently longer than the pulse interval T, and mixes two consecutive empty pulses in the optical pulse train at an arbitrary time position.

  In the following description, for convenience of explanation, the description will be made under the above conditions, but the present invention is not intended to be limited to the above conditions.

  FIG. 3 shows an example in which the third and fourth pulses are empty pulses.

  The receiver 310 receives the optical pulse train transmitted from the transmitter 300 via the optical transmission path 320 by the optical branching means C1 (311).

  The optical pulse train branched into the long path 316 and the short path 317 in energy equally (for example, 50 to 50) by the optical branching means C1 (311) is combined by the 2 × 2 multiplexing coupler C2 (312). The optical pulse train that is waved and combined is detected by the photon detector 1 (313) or the photon detector 2 (314) according to the phase difference between the pulses as will be described later.

  When the circuit of the receiver 310 is configured as described above, the 2 × 2 multiplexing coupler C2 (312) of the receiver 310 causes the front and rear pulses to overlap and cause interference. That is, each pulse of the optical pulse train transmitted from the transmitter 300 is transmitted at a constant time interval T, and the optical pulse train that has passed through the long path 316 is delayed by a fixed delay time T. The optical pulse train that has passed through 316 is combined with the optical pulse train that has passed through the short path 317 with a delay of exactly one pulse. This state is shown in detail in FIG.

  If the phase difference between the pulses is 0 as a result of the interference in the 2 × 2 multiplexing coupler C2 (312), the photon detector 2 (314) detects the photon, and if it is π, the photon detector 1 (313). Detects photons.

  However, since the light transmitted from the transmitter 300 is less than one photon on average per pulse, photons are rarely detected.

  When the photon is detected, the control unit CPU 315 stores the time when the photon is detected (photon detection time) in a memory or the like (not shown) provided in the receiver 310.

  FIG. 4 shows how the received pulses overlap when the empty pulses described in FIG. 3 are transmitted in succession (the third and fourth pulses are empty pulses).

  In FIG. 4, pulses of hatched rectangles (1, 2, 5, 6) and blank rectangles (3, 4) are shown, but hatched rectangles (1, 2, 5, 6) may have photons. A pulse (hereinafter referred to as a signal pulse) is represented, while a blank rectangle (3, 4) represents an empty pulse.

  As shown in the figure, since there are empty pulses, there are cases where one of the overlapping pulses is a signal pulse and the other is an empty pulse (t2, t4), and both are empty pulses (t3).

  In the former case (t2, t4), since there is no interfering partner, the photon is detected by the photon detector 1 (313) when the phase of the signal pulse is π, or when the phase of the signal pulse is 0 Is detected by the photon detector 2 (314). On the other hand, in the latter case (t3), no photons are detected.

Using the above configuration and operation characteristics, the transmitter 300 and the receiver 310 obtain a common bit by the following procedure. As described above, the control units CPU 304 and 315 also control the transmitter 300 and the receiver 310 to obtain a common bit.
(1) The transmitter 300 transmits an optical pulse train having a predetermined length to the receiver 310.
(2) The receiver 310 notifies the transmitter 300 of a time slot (photon detection time) in which a photon is detected.
(3) The transmitter 300 notifies the receiver 310 of the time positions of the empty pulses (in the above example, the time positions of the third and fourth pulses).
(4) The transmitter 300 determines whether the photon detector 1 or the photon detector 2 has detected a photon in the receiver 310 from its own phase modulation data and the photon detection time received from the receiver 310. However, when the signal pulse and the empty pulse overlap (t2 and t4 in FIG. 4), as described above, either the photon detector 1 or the photon detector 2 detects the photon according to the phase of the signal pulse itself.
(5) The receiver 310 determines whether the event of detecting the photon is based on the result of interference between the signal pulses based on the time position information of the empty pulse transmitted from the transmitter 300. It is determined whether or not the result of overlapping. More specifically, the control unit CPU 315 of the receiver 310 reads a determination program from a memory (not shown) in the receiver 310 and makes a determination from the information on the time position and the event of detecting the photon. .
(6) The transmitter 300 and the receiver 310 set the bit “0” if the photon detector 1 (313) has detected the photon generated from the interference between the signal pulses, and the photon detector 2 ( If it is according to 314), bit “1” is assigned. In this case, since the detector that has detected the photon is determined, the transmitter 300 and the receiver 310 obtain the same bit value. This is a secret key bit.

  The transmitter 300 and the receiver 310 in the above (2) and (3) respectively notify the time position of the empty pulse and the photon detection time, but in consideration of the presence of an eavesdropper, the notification is sent to the transmission line 320. It can be performed via a different transmission path (not shown).

  Hereinafter, a mechanism for detecting spoofing eavesdropping in the system having the above-described configuration and procedure will be described with reference to FIG.

  In spoofing eavesdropping, an eavesdropper usually detects photons by a receiving circuit similar to the receiver 310 in the middle of the optical transmission line 320 (the flow of detecting photons overlaps with the above description and is omitted). Then, based on the photon detection result, a dummy signal is transmitted so that the regular receiver 310 has the same detection result, so that the receiver 310 does not notice wiretapping.

  However, in this embodiment, the transmitter 300 transmits an optical pulse train whose transmission level is an average of less than one photon per pulse (for example, 0.1 photon / pulse). In this case, an eavesdropper detects a photon only once in an average of 10 slots.

  The eavesdropper can send out two consecutive pulses whose phases are set so that the regular receiver 310 detects the photon with the photon detector 1 or the photon detector 2 for the slot in which the photon is detected. On the other hand, since it is not known which slot of the photon detector 1 or the photon detector 2 should be able to detect the slot where the photon is not detected, nothing is sent. Thereby, two isolated continuous pulses are sent to the receiver.

  More specifically, two consecutive empty pulses 3 and 4 enter the receiving circuit of the eavesdropper 230 from the transmitter 300. Then, the receiver circuit of the eavesdropper 230 may detect photons in the time slot (arrow A in FIG. 5) where the signal pulse 2 and the empty pulse 3 overlap.

  However, an eavesdropper cannot distinguish whether this is due to interference between signal pulses or whether one is a detection result of only signal pulses. Therefore, the eavesdropper 230 generates two continuous pulses at the time positions 2 and 3 and transmits the pulse train to the regular receiver 310, assuming that the phase between the pulses 2 and 3 is measured.

  When two consecutive pulses at time positions 2 and 3 are input to the receiver 310, photons may be detected in the time slot where the pulses 3 and 4 overlap (arrow B in FIG. 5). However, as described with reference to FIG. 4, when the third and fourth optical pulse trains transmitted from the transmitter 300 are empty pulses, the receiver 310 has a time for the third and fourth pulses to overlap. No photons are detected in the slot.

  That is, the example of FIG. 5 shows that a photon is detected in a time slot that is known on the transmitter side that a photon is not necessarily detected when there is no eavesdropping.

  Therefore, if there is such photon detection, the quantum communication system according to the present invention knows that there has been spoofing.

  That is, the quantum communication system of the present invention controls the multiplexing at the receiver 310 side by controlling at which time position in the optical pulse train transmitted from the transmitter 300 side the empty pulses are continuously generated. The presence or absence of eavesdropping can be determined based on whether or not photons are detected in the time slot in which the empty pulse is generated in the subsequent optical pulse train.

  Further, in order to clarify the difference from the “differential phase shift quantum key distribution system” which is the prior art, a detailed description will be given. What is normally transmitted in the “differential phase shift quantum key distribution system” is “a coherent optical pulse train having an average number of photons of about 0.1 / pulse”. For example, when the time slots from No. 1 to No. 10 are considered, there is a possibility that a photon exists in any time slot before the number of photons is measured. Photons are detected only in one slot and the other slots are simply observed as empty. When considered from the receiver side, the “differential phase shift quantum key distribution system” of the present invention and the prior art seem to be the same, but conversely, when considered from the transmitter side, the “empty pulse” The transmitter knows that a photon should not be detected in a time slot, and differs from the prior art in that information on whether or not a photon was detected in that time slot is useful for finding eavesdropping.

  This makes it possible to detect spoofing eavesdropping without resorting to bit errors that are the subject of the present invention.

  In the description of the above-described embodiment, for the sake of convenience of description, the case where two consecutive empty pulses are third and fourth has been described. However, the present invention is not limited to the above-described embodiment.

  That is, in order for the quantum communication system of the present invention to detect eavesdropping, photons are detected in the optical pulse train after being combined when being combined by the 2 × 2 combining coupler C2 (312) of the receiver 310. A time slot that does not occur (in the example of FIG. 4, a time slot in which the third and fourth pulses overlap) is formed, and this time slot matches the time position at which the transmitter generated an empty pulse. It only has to be.

  Therefore, for example, three consecutive empty pulses may be used, or more consecutive empty pulses may be used.

It is a figure which shows the structure of the differential phase shift quantum key distribution system which is a prior art. It is a figure which shows the spoofing eavesdropping method which is a prior art. It is a figure which shows the structure of the quantum communication system which concerns on the Example of this invention. FIG. 4 is a diagram illustrating how pulses overlap in the receiver of FIG. 3. The top shows a pulse train that has passed through a long path, and the bottom shows a pulse train that has passed through a short path. It is a figure explaining the mechanism of the spoofing eavesdrop discovery in this invention.

Explanation of symbols

100, 200, 300 Transmitter 110, 210, 310 Receiver 111, 211, 311 Optical branching means 112, 312 2 × 2 multiplexing coupler 113, 313 Photon detector 1
114, 314 Photon detector 2
115, 316 Long path 116, 317 Short path 120, 220, 320 Transmission path 230 Eavesdropper 231 Optical transmitter 301 Light source 302 Phase modulator 303 Attenuating means 304, 315 Controller CPU

Claims (2)

A quantum communication system comprising a transmitter and a receiver,
The transmitter is
A light source that transmits an optical pulse train composed of pulses at regular time intervals;
A phase modulator for phase modulating the optical pulse train with 0 or π;
Attenuating means for transmitting a phase-modulated optical pulse train as a pulse train of average μ photons per pulse, including empty pulses at at least two consecutive time positions, where μ is a constant value of 0 <μ <1 ;
Transmitting means for transmitting the information of the time position that is continuous at least two or more, and
The receiver
Branching means for receiving the transmitted optical pulse train from the attenuation means of the transmitter and branching it into a first optical pulse train and a second optical pulse train;
Delay means for delaying the first optical pulse train by a time equal to the fixed time interval;
A multiplexing unit configured to combine the delayed first optical pulse train and the second optical pulse train to generate a combined optical pulse train;
First photon detection means for detecting photons when the relative phase difference between the delayed first optical pulse train and the second optical pulse train is zero;
Second photon detection means for detecting photons when the relative phase difference between the delayed first optical pulse train and the second optical pulse train is π;
Receiving means for receiving at least two or more consecutive time position information;
Determining means for determining whether a photon is detected in a time slot corresponding to the time position received from the transmitter via the transmitting means in the multiplexed optical pulse train; A characteristic quantum communication system. A communication method in a quantum communication system comprising a transmitter and a receiver,
An optical pulse train having a constant time interval that is phase-modulated by 0 or π, and includes at least two consecutive empty pulses, where μ is a constant value of 0 <μ <1, and an average μ photon per pulse Receiving a pulse train and branching into a first optical pulse train and a second optical pulse train;
Delaying the first optical pulse train for a time equal to the predetermined time interval, and then combining the second optical pulse train to generate a combined optical pulse train;
Receiving information on the time position of at least two consecutive empty pulses from the transmitter;
Detecting photons from each time slot of the combined optical pulse train;
Determining whether a photon has been detected from a corresponding time slot of the combined optical pulse train based on the fact that no photon is detected at the time position of the received at least two consecutive empty pulses; A communication method comprising:

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