JPWO2006006232A1 - Quantum cryptographic key distribution system - Google Patents

Abstract

In a quantum key distribution system, a plurality of users of three or more parties can easily and securely share a quantum encryption key, and a photon generator can be shared among a plurality of users. The transmission means (1) including the photon generation means (4) and the modulation means (5) is connected to each of two or more transmission paths (3a, 3b, 3c) connected to the transmission means (1). Two or more receiving means (2a, 2b, 2c) provided with demodulating means (7a, 7b, 7c), so that a pulse transmitted to each transmission path does not contain two or more photons. The modulation system information selected by the transmission means is exchanged between the transmission means and at least one reception means, and the demodulation system information selected by each reception means and the photon arrival / absence information are received by each reception means. The quantum cryptography key is shared when all of the receiving means receive photons and all of the demodulation systems selected by each receiving means match the modulation system selected by the transmitting means. .

Description

  The present invention relates to a quantum key distribution system, and more particularly to a quantum key distribution system for sharing a quantum key among three or more users.

  The encryption key distribution system is a system whose purpose is to share a random bit string (encryption key) between transmission / reception means. 2. Description of the Related Art In recent years, quantum key distribution technology that can distribute an encryption key that is guaranteed to be secure, regardless of the computational capability of an attacker, has attracted attention.

  As a conventional technique related to quantum key distribution, a quantum key distribution technique using a single photon light source known as a BB84 method is widely known (for example, Non-Patent Documents 1 and 2). The quantum key distribution technology disclosed in these documents utilizes the uncertainty (randomness) of quantum signal bits, avoiding eavesdropping by an eavesdropper, and transmitting means (sender) and receiving means (reception) Key).

  In quantum cryptography communication, which is the basis of the quantum cryptography key distribution technology, photons (photons) are used as communication means, and 1-bit information is transmitted by one photon so that a quantum effect such as an uncertainty principle occurs. At this time, when the eavesdropper selects an arbitrary base without knowing the quantum state such as the polarization and phase, and measures photons, the quantum state changes. Therefore, the receiving side can recognize whether or not the transmission data has been wiretapped by confirming the change in the quantum state of the photon. That is, the quantum key distribution technology according to the prior art has a feature that the quantum key sharing and the eavesdropper detection are physically guaranteed.

C. H. Bennett, and G.B. Brassard ,: Quantum Cryptography: Public Key Distribution and Coin Tossing, In Proceedings of IEEE Conference on Computers, System and SignalProceeding. 175-179 (DEC. 1984). C. H. Bennett, “Quantum Cryptography Using Any Two State States,” Phys. Rev. Lett. 68, 3121 (1992).

  However, in the BB84 method disclosed in Non-Patent Documents 1 and 2 and the like, a transmitter needs to accurately transmit a minute optical signal of 1 photon or less per pulse, and thus a complicated and expensive photon generator. There was a problem of having to have.

Further, the BB84 system has a problem that each user who uses the system must have a photon generator.
Furthermore, although the BB84 system provides a mechanism for sharing a quantum encryption key between two parties, there is a problem that it is difficult to share an encryption key between three or more parties.

  In view of such circumstances, the present invention enables multiple users of three or more parties to share a quantum encryption key easily and securely, and allows sharing of a photon generator among multiple users or multiple systems. An object of the present invention is to provide a quantum key distribution system.

  In the quantum key distribution system according to the present invention, in the quantum key distribution system for sharing the quantum key, the photon generating means and the modulation means having two or more selectable modulation systems are provided. A transmission means, two or more transmission paths connected to the transmission means, and a demodulation means connected to each of the two or more transmission paths and having two or more selectable demodulation systems Two or more receiving means, and are controlled so that two or more photons are not included in the pulse transmitted to each transmission line, and the transmission means and the two or more receiving means The modulation system information selected by the transmission unit is exchanged with at least one reception unit, and the demodulation system information selected by each of the two or more reception units and the photon arrival / absence information are those of the reception unit. The All of the two or more receiving means receive photons, and all of the demodulation systems selected by each of the two or more receiving means match the modulation system selected by the transmitting means. In this case, the quantum encryption key is shared.

  According to the present invention, the pulse transmitted to each transmission path is controlled so as not to include two or more photons, and the transmission means and at least one receiving means of the two or more receiving means The modulation system information selected by the transmission means is exchanged, and the demodulation system information selected by each of the two or more reception means and the photon arrival / absence information are exchanged between the reception means, and two or more receptions are received. Quantum cryptography between two or more receiving means when all of the means receive photons and all of the demodulation systems selected by each of the two or more receiving means match the modulation system selected by the transmitting means The key is shared.

  According to the quantum key distribution system of the present invention, all of the two or more receiving means receive the photons, and all of the demodulation systems selected by each of the two or more receiving means and the modulation selected by the transmitting means Since the quantum encryption key is shared when the system matches, there is an effect that a plurality of users including the sender can easily and safely share the quantum encryption key.

FIG. 1 is a conceptual diagram showing a configuration of the quantum key distribution system according to the first exemplary embodiment of the present invention. FIG. 2 is a conceptual diagram showing a configuration of the quantum key distribution system according to the second exemplary embodiment of the present invention. FIG. 3 is a conceptual diagram showing a configuration of a quantum key distribution system according to the third exemplary embodiment of the present invention. FIG. 4 is a conceptual diagram showing the configuration of the quantum key distribution system according to the fourth embodiment of the present invention. FIG. 5 is a conceptual diagram showing the configuration of the quantum key distribution system according to the fifth embodiment of the present invention. FIG. 6 is a conceptual diagram showing a configuration example of a quantum key distribution system according to the prior art using the BB84 method. FIG. 7 is a diagram showing a configuration example when a phase modulator is used for each of the modulation means 5 and the demodulation means 7 of the quantum cryptography key distribution system shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Transmission means 2, 2a, 2b, 2c, 2d, 2e Reception means 3a, 3b, 3c Transmission path 4 Photon generator 5 Modulation means 6a, 6b Signal processing means 7 Demodulation means 8 Light receiving means 9 Information exchange means 10 Error rate detection Means 11, 11a, 11b Demultiplexing means 12a, 12b, 12c Variable attenuator 13a, 13b Phase modulator 14a, 14b Multiplexing means

  Exemplary embodiments of a quantum key distribution system according to the present invention will be described below in detail with reference to the accompanying drawings. In addition, this invention is not limited by this embodiment.

Embodiment 1 FIG.
First, the quantum key distribution system according to the first exemplary embodiment of the present invention will be described. FIG. 1 is a conceptual diagram showing a configuration of the quantum key distribution system according to the first exemplary embodiment of the present invention. The quantum cryptography key distribution system of this embodiment is connected to a transmission means 1 used on the transmission side, three transmission paths 3a, 3b, 3c connected to the transmission means 1, and a transmission path on the reception side. Receiving means 2a, 2b, 2c. The transmitting means 1 has a photon generator 4, a modulating means 5, and a demultiplexing means 11, and each receiving means includes a demodulating means 7 and a light receiving means 8. Here, as the modulation means 5 and the demodulation means 7, a phase modulator, a polarization modulator, or the like can be used. As the photon generator 4, for example, a light source such as a laser diode is generally used. The transmission wavelength may be any wavelength as long as it is suitable for the transmission path 3. As the demultiplexing unit 11, for example, an optical coupler or the like can be used. As the transmission lines 3a, 3b, and 3c, various optical signal transmission lines such as single-mode optical fibers and dispersion-shifted optical fibers can be used. Further, the transmission path is not necessarily wired, and may be a wireless transmission path such as spatial transmission.

  Next, in order to facilitate understanding of the quantum key distribution system according to the present invention described above, the operation will be described by taking a quantum key distribution system according to the prior art using the BB84 method as an example.

  FIG. 6 is a conceptual diagram showing a configuration example of a quantum key distribution system according to the prior art using the BB84 method. In the figure, the transmission means 1 includes a photon generator 4, a modulation means 5, and a signal processing means 6a. Here, description will be made assuming that a phase modulator is used as the modulation means 5. In the transmission means 1, either the [0, π] system or the [π / 2, 3π / 2] system is selected for each pulse, and a signal of “0” or “1” is transmitted on the transmission line 3. Sent to. In signal transmission, two modulation systems are used, and only one photon is used per pulse. On the other hand, the receiving means 2 includes a demodulating means 7, a light receiving means 8, and a signal processing means 6b. In the receiving means 2, the received pulse is observed using an arbitrary demodulation system, and it is determined whether it is “0”, “1”, or “no photon”. Further, in the transmission means 1 and the reception means 2, the selected modulation / demodulation system information and the presence / absence information on the arrival of the pulse are exchanged by the information exchange means 9 such as a public line that may be wiretapped. In a lossy transmission line, many pulses result in “no photons”. Only when a photon is received by the receiving means 2 and the system is matched between transmission and reception, the bit string is shared between the two. The shared bit string is used as a quantum encryption key after error detection and error correction by the error rate detection means 10.

  FIG. 7 is a diagram showing a configuration example when a phase modulator is used for each of the modulation means 5 and the demodulation means 7 of the quantum key distribution system shown in FIG. In the figure, the modulation means 5 is constituted by a demultiplexing means 11a, a phase modulator 13a, and a multiplexing means 14a, and the demodulation means 7 is constituted by a demultiplexing means 11b, a phase modulator 13b, and a multiplexing means 14b. . When the transmission means 1 selects the [0, π] system (“π” measurement system), the phase modulator 13a generates a phase modulation amount “0” for the signal “0”, and the signal “0”. For 1 ”, a phase modulation amount“ π ”is generated. When the transmission means 1 selects the [π / 2, 3π / 2] system (“π / 2” measurement system), the phase modulator 13a uses the phase modulation amount “π / 2” for the signal “0”. And a phase modulation amount “3π / 2” is generated for the signal “1”. On the other hand, when the receiving means 2 selects the [0, π] system (“π” measurement system) as the demodulator, the phase modulator 13b generates the phase modulation amount “0” and [π / 2, 3π / 2] system (“π / 2” measurement system) is selected, the phase modulator 13b generates a phase modulation amount “π / 2”. Only when the transmission unit 1 and the reception unit 2 select the same modulation / demodulation system, the light receiving unit 8 can detect a correct signal.

  Since only one photon per pulse is used as in the above-described process, in order to eavesdrop on the signal sent to the communication path, the eavesdropper once extracts and observes the photon, and then returns the photon to the communication path again. I have to bring it back. However, due to the uncertainty principle of quantum mechanics, it is impossible to observe without disturbing the quantum state of photons, so in the above protocol, if there is an eavesdropper, an error occurs with a probability of 1/4 on the receiving side. It will be. For example, when a [0, π] system signal “0” is transmitted from the transmission means 1, the probability that an eavesdropper selects the wrong [π / 2, 3π / 2] system is ½, Sometimes the polarization direction observed by the receiving means 2 changes to [π / 2, 3π / 2]. On the other hand, when the signal in the polarization direction is observed in the [0, π] system, which is the same measurement system used in the transmission means 1, in the receiving means 2, the signal “0” and the signal “1” are displayed. Are observed with equal probability, the probability that the reception means 2 observes the signal “0” transmitted by the transmission means 1 as the signal “1” is ¼.

  If the number of check bits m is made sufficiently large, the error rate will surely approach 25% according to the law of large numbers, so the probability that wiretapping cannot be detected can be made small enough to be ignored. If the number of modulation / demodulation systems that can be selected is 3 or more, the number of check bits m required to prevent eavesdropping can be reduced. Thus, the presence of an eavesdropper can always be detected by performing error detection between the transmission side and the reception side. Note that the BB84 system has a significant meaning that the sharing of the quantum cryptography key and the detection of an eavesdropper are physically guaranteed by quantum mechanical complementarity, which is the principle of quantum mechanics.

  Next, returning to FIG. 1, the operation of the quantum key distribution system according to this embodiment will be described in the case where a phase modulator is used as the modulation means 5. In the transmission means 1, the same signal in which either the [0, π] system or the [π / 2, 3π / 2] system phase modulation system is selected for each pulse is transmitted to the three transmission lines 3a, 3b, and 3c. Transmitted almost simultaneously. What is important here is that, in any transmission path, the number of photons per pulse is controlled to be 1 or less. In the receiving means 2a, 2b and 2c, the received pulse is independently observed in an arbitrary demodulation system, and it is determined whether it is “0”, “1” or “no photon”. In the receiving means 2a, 2b, 2c, the selected demodulation system information and the presence / absence information of the arrival of the pulse are exchanged with each other. Further, these pieces of information are not measurement result information measured by each receiving means, and can be transmitted and received using a public line because the security of quantum encryption key sharing is not impaired even if eavesdropped.

  Only when all of the receiving means 2a, 2b and 2c receive photons, and all the receiving systems selected by the three parties match the modulation system of the transmitting means, the bit string is shared among the three parties. The bit string shared by the receiving means is used as a quantum encryption key between the three parties after error detection and error correction are performed as necessary. The feature is that the three parties sharing the quantum encryption key do not need to have a photon generator.

  At this time, the transmission unit 1 does not necessarily need to know the demodulation system selected by the reception units 2a, 2b, and 2c. Further, the transmission unit 1 does not necessarily need to exchange the selected modulation / demodulation system information with all the reception units 2a, 2b, and 2c. That is, any one of the reception units 2a, 2b, and 2c only needs to know the modulation system information of the transmission unit 1. Further, the transmission unit 1 does not need to know the quantum encryption key shared by the reception units 2a, 2b, and 2c.

  Although FIG. 1 shows an example in which the number of reception means is three, the number of reception means may be two or four or more.

  Also, only when all receiving means receive photons and all the receiving means adopt the same demodulation system, bit information is shared among the receiving means. The encryption key sharing speed decreases. However, the quantum cryptography key sharing speed is irrelevant to the information transmission speed, and in many cases, the quantum cryptography key sharing speed is not so important as long as the quantum cryptography key sharing speed is a certain level or higher.

  As described above, according to the quantum cryptography key distribution system of this embodiment, the pulse transmitted to each transmission path is controlled so as not to include two or more photons. The modulation system information selected by the transmission unit is exchanged with at least one of the above reception units, and the demodulation system information selected by each of the two or more reception units and the photon arrival / absence information are obtained. Exchanged between each of the reception means, all of the two or more reception means receive photons, and all of the demodulation systems selected by each of the two or more reception means and the modulation system selected by the transmission means are Since the quantum encryption key is shared when they match, a plurality of users including the sender can share the quantum encryption key easily and securely.

Embodiment 2. FIG.
FIG. 2 is a conceptual diagram showing a configuration of the quantum key distribution system according to the second exemplary embodiment of the present invention. The difference between the quantum cryptographic key distribution system shown in FIG. 1 and FIG. 1 is that variable attenuators 12 a, 12 b, and 12 c are provided at the three outputs of the demultiplexing means 11. In addition, about another structure, it is the same as that of Embodiment 1, or is equivalent, The same code | symbol is attached | subjected and shown about these parts.

  As described above, in the quantum key distribution system, it is important to control the number of photons per pulse to be 1 or less. Therefore, in the quantum key distribution system shown in FIG. 2, the variable attenuators are set to 12a, 12b, and 12c so that the optical signals transmitted to the transmission lines 3a, 3b, and 3c do not exceed one photon per pulse. Use to control. In addition, since noise on the transmission path affects the results measured by the receiving means 2a, 2b, and 2c and the transmission result on the transmission side, a variable attenuator is provided for each of the transmission paths 3a, 3b, and 3c. 12a, 12b, and 12c are inserted so that the attenuation can be controlled according to the transmission path characteristics of each transmission path. In addition, what is necessary is just to comprise so that the feedback circuit which monitored light intensity etc. may be used for control of the variable attenuators 12a, 12b, 12c, for example.

  As described above, according to the quantum key distribution system of this embodiment, since the variable attenuator is further provided in two or more transmission paths connected to the transmission means, each transmission path It is possible to control the number of photons per pulse of the optical signal transmitted to each so as not to exceed 1, so that the quantum encryption key can be shared reliably and stably.

Embodiment 3 FIG.
FIG. 3 is a conceptual diagram showing a configuration of a quantum key distribution system according to the third exemplary embodiment of the present invention. 1 differs from the quantum key distribution system shown in FIG. 1 in that the transmission unit 1 and the reception unit 2 include signal processing units 6a and 6b, respectively, and a part of the transmission unit 1 and the reception unit 2 (FIG. In the third example, the receiving means 2c) is provided with an error rate detecting means 10 that shares its function. In addition, about another structure, it is the same as that of Embodiment 1, or is equivalent, The same code | symbol is attached | subjected and shown about these parts.

  In the quantum key distribution system shown in FIG. 3, an error between transmission and reception is detected by the error rate detection means 10. By detecting an error between the transmission unit and the reception unit, it is possible to share the quantum encryption key while reliably detecting the presence of an eavesdropper based on the same principle as the conventional technology such as the BB84 method. However, the major difference from the prior art is that error detection only needs to be performed by the transmission means and a part of the reception means. Since it is not necessary for the transmitting means to know the demodulation system selected by all the receiving means, the transmitting means need not know the quantum encryption key shared by the receiving means.

  It should be noted that although all receiving means can perform error detection with the transmitting means, it is not essential that all receiving means perform error detection with the transmitting means. Further, it does not prevent the receiving means from performing error detection.

  As described above, according to the quantum key distribution system of this embodiment, all of the two or more receiving units receive photons, and each of the two or more receiving units selects all of the demodulation systems selected. Since the error rate of the bit string shared between the transmission means and two or more recipients is detected only when the modulation system selected by the transmission means matches, the presence of an eavesdropper is assured. It is possible to share the quantum cryptography key while detecting it.

Embodiment 4 FIG.
FIG. 4 is a conceptual diagram showing the configuration of the quantum key distribution system according to the fourth embodiment of the present invention. The difference between the quantum key distribution system shown in FIG. 3 and FIG. 3 is that the receiving means 2a, 2b, 2c are provided with respective signal processing means 6b-1, 6b-2, 6b-3, and the receiving means 2a, 2b, Only 2c (not using transmission means) is provided with an error rate detection means 10 that shares its function. In addition, about another structure, it is the same as that of Embodiment 3, or is equivalent, and attaches | subjects and shows these parts about these parts.

  In the quantum key distribution system shown in FIG. 4, the transmission means 1 transmits transmission modulation system information and does not receive any information such as demodulation information and photon reception from the reception means. In addition, by detecting an error between receiving means, it is possible to reliably detect the presence of an eavesdropper based on the same principle as that of the prior art such as the BB84 method, and to perform quantum cryptography with a receiver using three or more receiving means. Key sharing is possible. However, unlike the prior art, it is not necessary for the transmitting means to know the demodulation system of the receiving means, and error detection only needs to be performed between the receiving means. Further, the transmission means may not know the quantum encryption key shared by the reception means. Because of such characteristics, the sender using the transmission means may not be an absolutely reliable target. If such a feature is used, for example, a quantum encryption key can be shared only by an arbitrary receiver using a quantum communication channel provided by a third party.

  As described above, according to the quantum key distribution system of this embodiment, all of the two or more receiving units receive photons, and each of the two or more receiving units selects all of the demodulation systems selected. Since the error rate of the bit string shared between two or more recipients is detected only when the modulation system selected by the transmission means matches, if error detection is performed only between the reception means, In addition, since it is not necessary for the transmission means to know the quantum encryption key shared by the reception means, the quantum encryption key can be shared only by an arbitrary receiver.

Embodiment 5 FIG.
FIG. 5 is a conceptual diagram showing the configuration of the quantum key distribution system according to the fifth embodiment of the present invention. The difference between the quantum key distribution system shown in FIG. 1 and FIG. 1 is that the number of receiving means is five. In addition, about another structure, it is the same as that of Embodiment 3, or is equivalent, and attaches | subjects and shows these parts about these parts.

  In the quantum key distribution system shown in FIG. 5, the demodulating system information mutually selected by the receiving means 2a, 2b, and 2c and the presence / absence information of the arrival of pulses are exchanged, and these three parties share the quantum encryption key. Is done. At substantially the same time, the receiving means 2d and 2e also exchange the demodulated system information selected with each other and the presence / absence information of the arrival of the pulse, and the quantum encryption key is shared between these two parties. However, as is apparent from the above-described principle of quantum communication, even if the receiving means 2a, 2b, 2c and the receiving means 2d, 2e receive an optical pulse transmitted by the same transmitting means 1, the receiving means 2a, The quantum encryption keys shared by 2b and 2c and the quantum encryption keys shared by the receiving means 2d and 2e can be different from each other. Therefore, a plurality of groups can share the quantum encryption key almost simultaneously using the optical pulse transmitted by the same transmission means.

  Further, according to such a configuration, the optical pulse transmitted from the transmission means having the photon generator can be shared by a plurality of users and a plurality of systems, so that the system can be constructed at low cost and easily.

  In the system of FIG. 5 as well, the presence of an eavesdropper can be reliably detected by performing error detection for each group, similarly to the quantum key distribution system of Embodiment 3 shown in FIG. It becomes.

  As described above, according to the quantum key distribution system of this embodiment, since the photon generation means is shared by a plurality of quantum encryption key distribution systems, photons are generated among a plurality of users or a plurality of systems. The devices can be shared, and the system configuration can be made inexpensive and simple.

  As described above, the quantum cryptographic key distribution system according to the present invention is useful for sharing a quantum cryptographic key among a plurality of users or a plurality of systems, in particular, when the sender is not an absolutely reliable target, It is suitable as a quantum key distribution system that can flexibly respond to various needs such as when it is desired to share a quantum key individually for each of a plurality of user groups.

Claims (7)

In a quantum key distribution system for sharing a quantum key,
Transmitting means comprising photon generating means and modulation means having two or more selectable modulation systems;
Two or more transmission lines connected to the transmission means;
Two or more receiving means comprising demodulating means connected to each of the two or more transmission paths and having two or more selectable demodulation systems;
With
The pulse transmitted to each transmission path is controlled so as not to include two or more photons,
Modulation system information selected by the transmission means is exchanged between the transmission means and at least one of the two or more reception means,
The demodulation system information selected by each of the two or more receiving means and the photon arrival / absence information are exchanged between the receiving means,
The quantum cryptography when all of the two or more receiving means receive photons, and all of the demodulation systems selected by each of the two or more receiving means match the modulation system selected by the transmitting means. A quantum key distribution system characterized by sharing a key.   The quantum cryptography key distribution system according to claim 1, further comprising a variable attenuator inserted between the transmission unit and two or more transmission paths connected to the transmission unit.   Only when all of the two or more receiving means receive photons and all of the demodulation systems selected by each of the two or more receiving means match the modulation system selected by the transmitting means. 2. The quantum encryption key according to claim 1, further comprising: an error rate detection unit configured to share a bit string between the transmission unit and the two or more receivers and detect an error rate of the shared bit string. Delivery system.   Only when all of the two or more receiving means receive photons and all of the demodulation systems selected by each of the two or more receiving means match the modulation system selected by the transmitting means. The quantum key distribution system according to claim 1, further comprising error rate detection means for sharing a bit string between two or more recipients and detecting an error rate of the shared bit string.   2. The quantum key distribution system according to claim 1, wherein the photon generation means is shared by a plurality of quantum key distribution systems.   The quantum key distribution system according to claim 1, wherein a phase modulator is used as the modulation system and the demodulation system. 2. The quantum key distribution system according to claim 1, wherein a polarization modulator is used as the modulation system and the demodulation system.

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