JP4608412B2 - Quantum secret key distribution system and quantum secret key distribution method - Google Patents

Description

  The present invention relates to a secret key distribution technique used for cryptographic communication, and more particularly to a quantum secret key distribution technique.

  In recent years, various methods have been proposed as a method for realizing a quantum cryptography system. Here, a method called differential phase shift quantum key distribution will be described.

  FIG. 1 shows a basic configuration of a conventional differential phase shift quantum key distribution system. The transmitter (Alice) 20 phase-modulates a coherent optical pulse train from the coherent pulse light source 22 randomly with 0 or π by the phase modulator 24, and averages less than one photon per pulse by the attenuator 26. The signal is attenuated and sent to the transmission line 30.

  The receiver (Bob) 40 measures the phase difference between two successive optical pulses using an interferometer having an optical path difference equal to the pulse interval of the received pulse train. This interferometer combines an optical coupler 44a for bifurcating a received pulse train, delay means 45 for providing a time delay equal to the pulse interval of the received pulse train between the branched paths, and optical pulses from the branched paths 2 X2 optical coupler 44b. The output of the 2 × 2 optical coupler 44b is provided with photon detectors B1 and B2. When the phase difference between two consecutive light pulses is 0, the photon detector B1 detects the photon, and the phase difference is In the case of π, photons are detected by the photon detector B2.

  Using the above setup, Alice and Bob can share a secret key as follows. Alice randomly modulates each pulse of the coherent optical pulse train to 0 or π by the phase modulator 24 and records how it is modulated. Bob receives the optical pulse train and records the time when the photon was detected and which photon detector detected it. Bob then notifies Alice when the photon was detected.

  From the information on which photon detector has detected the photon, Bob sets “0” when the photon is detected by the photon detector B1 and “1” when the photon is detected by the photon detector B2. Create a bit to be a secret key. On the other hand, Alice can know how the optical pulse corresponding to the photon detection time notified from Bob is modulated from its own record. Therefore, if Alice is modulated so that the phase difference of the optical pulse becomes 0, and “1” when modulated so that the phase difference becomes π, Alice has the same secret key bit as Bob. Can be created.

  As described above, Alice and Bob share the secret key without revealing information about the bit (which photon detector was detected or how it was modulated) by only revealing Bob's photon detection time. can do.

K. Inoue, E. Waks, and Y. Yamamoto, “Differential-phase-shift quantum key distribution using coherent light,” Physical Review A68, 022317 (2003).

  The conventional quantum cryptography system targets the distribution of a secret key between two parties (Alice and Bob). In the delivery of the same secret key between three or more parties, the delivery of the secret key between the two parties must be applied to each pair, and there is a problem that it takes time and effort. Therefore, it is convenient if the same secret key can be delivered at one time among three or more parties.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a quantum secret key distribution system and a quantum secret key distribution method capable of easily distributing a secret key among three or more parties. Is to provide.

In order to achieve the above object, the present invention provides a quantum secret key distribution system according to claim 1, wherein a transmitter for transmitting an optical pulse after performing differential phase shift modulation, and the transmission A branching unit for branching the optical pulse transmitted from the device, and a first receiver for receiving one of the optical pulses branched by the branching unit, detecting the modulation phase, and providing time information when the detection was successful And a second receiver that receives the other of the optical pulses branched by the branching means, detects a modulation phase, and provides time information when the detection is successful, the first and second receptions. Among the time information provided from the transmitter, for each optical pulse transmitted from the transmitter, the time information that the first and second receivers have both successfully detected is extracted as common time information, and the common time is extracted. Information at each time Based on the tone phase, the transmitter, each of said first receiver and said second receiver and generating the same secret key data.

The invention according to claim 2 is a quantum secret key distribution system, wherein a transmitter for transmitting an optical pulse after differential phase shift modulation and a branch for branching the optical pulse transmitted from the transmitter into n branches And n receivers that respectively receive the optical pulses branched by the branching means, each of which detects the modulation phase of the optical pulse and provides time information of successful detection Among the time information provided from the n receivers, the time information that all the n receivers have successfully detected for each optical pulse transmitted from the transmitter is a common time. The information is extracted as information, and each of the transmitter and the n receivers generates the same secret key data based on the modulation phase at each time of the common time information.

  According to a third aspect of the present invention, in the quantum secret key distribution system according to the first or second aspect, together with the common time information, time information when at least one of the plurality of receivers has been successfully detected is detected time. Extracted as information, and based on the modulation phase at each time of the detection time information, at least two of the transmitter and the successfully detected receiver respectively generate the same secret key data .

According to a fourth aspect of the present invention, in the quantum secret key distribution system according to any one of the first to third aspects, the average number of light pulses transmitted from the transmitter is less than one photon per pulse. It is characterized by.

The invention according to claim 5 is a quantum secret key distribution method, comprising: a transmission step of transmitting an optical pulse by differential phase shift modulation from a transmitter; and a branching of the optical pulse transmitted from the transmitter A first detection step for detecting a modulation phase by receiving one of the branched optical pulses by a first receiver, and time information for successful detection in the first detection step. The first information providing step, the second detection step of receiving the other of the branched optical pulses by the second receiver and detecting the modulation phase, and the detection succeeded in the second detection step. Of the time information provided by the second time information providing step for providing time information and the time information provided by the first and second information providing steps, for each optical pulse transmitted from the transmitter in the transmission step, Based on the extraction step of extracting, as common time information, time information that both the first and second receivers have successfully detected in the first and second detection steps, and the modulation phase at each time of the common time information The transmitter, the first receiver, and the second receiver each include a generation step of generating the same secret key.

The invention according to claim 6 is a quantum secret key distribution method, wherein a transmission step of transmitting an optical pulse by differential phase shift modulation from a transmitter and transmitting the optical pulse transmitted from the transmitter by n A branching step for branching, a step for receiving the n-branched optical pulses by n receivers, respectively, a detection step for detecting a modulation phase, and a time at which detection was successful in each of the detection steps An information providing step for providing information; and among the provided time information, a time at which all of the n receivers have successfully detected the optical pulse transmitted from the transmitter in the transmission step in the detection step. An extraction step for extracting information as common time information, and based on the modulation phase at each time of the common time information, the transmitter and the n receivers Respectively it is characterized in that it comprises the step of generating the same secret key data.

The invention according to claim 7 is the quantum secret key distribution method according to claim 5 or 6 , wherein at least one of the plurality of receivers successfully extracts time information as detection time information; And at least two of the transmitter and the successfully detected receiver each generate the same secret key data based on the modulation phase at each time of the detection time information. To do.

The invention according to claim 8 is the quantum secret key distribution method according to any one of claims 5 to 7 , wherein an optical pulse transmitted from the transmitter is an average of less than one photon per pulse. It is characterized by.

  According to the present invention, a common secret key can be distributed among three parties at once. At that time, it is also possible to deliver a secret key between the two parties. Further, since the transmitter can be shared among many people, there is an advantage that the entire system can be simplified.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 2 shows a configuration example of a quantum secret key distribution system according to an embodiment of the present invention. This quantum secret key distribution system 100 is branched by a transmitter (Alice) 200 that phase-modulates and transmits a coherent optical pulse train, and a 50:50 optical coupler (splitter) 320 that branches the transmitted optical pulse train. It comprises a receiver (Bob 1) 400 that receives and demodulates one of the optical pulse trains, and a receiver (Bob 2) 600 that receives and demodulates the other of the branched optical pulse trains.

The transmitter 200 includes a coherent pulse light source 220 that outputs a coherent optical pulse train, a phase modulator 240 that modulates the phase difference between two successive optical pulses, and the intensity of the optical pulse train is less than one photon on average per pulse. And an attenuator 260 for attenuating. The receiver 400 also combines an optical coupler 440a that bifurcates the received optical pulse train, a delay means 450 that delays one of the branched optical pulse trains by a time equal to the pulse interval, and combines the branched optical pulse trains for interference. An interferometer including a 2 × 2 optical coupler 44 0 b. In addition, the receiver 400 includes two photon detectors B11 and B12 that detect the interfering light pulses. The receiver 600 has basically the same configuration as the receiver 400, and includes an interferometer including an optical coupler 640a, a delay unit 650, and a 2 × 2 optical coupler 640b, and photon detectors B21 and B22. I have.

  The coherent pulse light source 220 of the transmitter outputs a coherent optical pulse train at regular intervals. The phase modulator 240 phase-modulates each pulse of this optical pulse train with 0 or π at random. The attenuator 260 attenuates the phase-modulated optical pulse train to be less than one photon per pulse. The attenuated optical pulse train is transmitted to the optical transmission line 300, branched by the splitter 320, and transmitted to the receivers 400 and 600 via the optical transmission lines 340 and 360, respectively. Here, since the optical pulse train is attenuated by the attenuator 260 so that the average is less than one photon per pulse, some of the optical pulses are detected by only one of the receivers 400 and 600 with a certain probability. Or it may be detected by both receivers 400 and 600 with a certain probability.

  The receiver 400 divides the received optical pulse train into two by the optical coupler 440a, gives a time delay equal to the pulse interval of the optical pulse train to one of the branched optical pulse trains by the delay means 450, and re-combines it by the multiplexing coupler 440b. Make it interfere. With this interferometer, the phase difference between two successive optical pulses can be measured. Specifically, when the phase difference between two consecutive light pulses is 0, a photon is detected by the photon detector B11, and when the phase difference is π, a photon is detected by the photon detector B12. Adjust the interferometer so that.

  Similarly, the receiver 600 causes the optical pulse train received via the interferometer including the optical coupler 640a, the delay unit 650, and the multiplexing coupler 640b to interfere. Here, when the phase difference between two consecutive light pulses is 0, the photon detector B21 detects the photon, and when the phase difference is π, the photon detector B22 detects the photon. Adjust the interferometer.

  Using the above setup, Alice, Bob 1 and Bob 2 can share a secret key as follows. Alice randomly modulates each pulse of the coherent optical pulse train to 0 or π by the phase modulator 240 and records how it is modulated. Bob 1 and Bob 2 each receive an optical pulse train and record the time at which a photon was detected for each pulse and which photon detector detected it. Bob 1 and Bob 2 then notify Alice of the time at which the photon was detected. Alice extracts the time information of the optical pulses detected by both (Bob 1 and Bob 2) from the time information notified from Bob 1 and Bob 2 as common time information. , Notify Bob 1 and Bob 2.

  Bob 1 extracted the record corresponding to the common time information notified from Alice, and the photon was detected by the photon detector B11 from the information of which photon detector was detected by the photon at each time. In this case, a bit serving as a secret key is created as “0” and “1” when detected by the photon detector B12. Similarly, Bob 2 extracts the record corresponding to the common time information notified from Alice, and from the information on which photon detector has detected the photon at each time, the photon is detected by the photon detector B21. A bit serving as a secret key is created as “0” when detected and “1” when detected by the photon detector B22. On the other hand, Alice can know how the optical pulse is modulated at each time of the common time information from its own record. Therefore, Alice is the same as Bob 1 and Bob 2 when the optical pulse is modulated so that the phase difference is 0, and when it is modulated so that the phase difference is π, it is “1”. Secret key bits can be created.

  As described above, Alice, Bob 1, and Bob 2 only disclose the photon detection time of Bob 1 and Bob 2, and information about the bits (which photon detector detected and how it was modulated). You can share your secret key without revealing it.

  Furthermore, using the above-described configuration, a secret key that is shared by the three parties can be generated, and a secret key that is shared by the two parties Alice and Bob 1 and Alice and Bob 2 can also be generated. That is, in order for Alice to generate a secret key with Bob 1, only Bob 1 needs to generate a secret key using time information when a photon is detected. In order for Alice to generate a secret key with Bob 2, only Bob 2 needs to generate a secret key using time information when a photon is detected.

(Other examples)
In the above embodiment, the distribution of the secret key between the three parties has been described. However, it is obvious that the present invention can be extended to the distribution of the secret key between four or more parties. Specifically, the optical pulse train from one transmitter is branched into n, and the optical pulse train is transmitted to n receivers. Then, by providing the time information when each receiver detects the light pulse and extracting the record corresponding to the time information common to all, a secret key common to all can be created.

  As mentioned above, in view of many possible embodiments to which the principle of the present invention can be applied, the examples described here are merely illustrative and do not limit the scope of the present invention. For example, in the above embodiment, the differential phase shift modulation has been described as an example. However, the principle of the present invention is applied to other modulation schemes for creating a secret key by exchanging information such as photon detection time. You can also. Therefore, the configuration and details of the embodiment illustrated here can be changed without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

It is a functional block diagram which shows the basic structural example of the quantum secret key distribution system between the two parties by the conventional differential phase shift modulation. It is a functional block diagram which shows the structural example of the quantum secret key distribution system between the three parties by one Example of this invention.

Explanation of symbols

10 Differential Phase Shift Quantum Key Distribution System 20 Transmitter (Alice)
22 Coherent pulse light source 24 Phase modulator 26 Attenuator 30 Transmission path 40 Receiver (Bob)
44a, 44b Optical coupler 45 Delay means B1, B2 Photon detector 100 Quantum secret key distribution system 200 Transmitter (Alice) 200
220 Coherent pulse light source 240 Phase modulator 260 Attenuator 300, 340, 360 Transmission path 320 Optical coupler (splitter)
400 Receiver (Bob 1)
440a, 440b Optical coupler 450 Delay means B11, B12 Photon detector 600 Receiver (Bob 2)
640a, 640b Optical coupler 650 Delay means B21, B22 Photon detector

Claims (8)

A quantum secret key distribution system,
A transmitter for differential phase shift modulating and transmitting an optical pulse;
Branching means for branching the optical pulse transmitted from the transmitter;
A first receiver that receives one of the optical pulses branched by the branching means, detects a modulation phase, and provides time information of successful detection;
A second receiver that receives the other of the optical pulses branched by the branching means, detects a modulation phase, and provides time information when the detection is successful, and from the first and second receivers, Among the provided time information, for each optical pulse transmitted from the transmitter, the time information that the first and second receivers have both successfully detected is extracted as common time information, and the common time information A quantum secret key distribution system, wherein each of the transmitter, the first receiver, and the second receiver generates the same secret key data based on a modulation phase at each time. A quantum secret key distribution system,
A transmitter for differential phase shift modulating and transmitting an optical pulse;
Branching means for branching an optical pulse transmitted from the transmitter into n branches;
N receivers that respectively receive the optical pulses branched by the branching means, each of which detects the modulation phase of the optical pulse and provides time information of successful detection; The time information provided from the n receivers is extracted as common time information for each optical pulse transmitted from the transmitter, with the time that all the n receivers have successfully detected. The quantum secret key distribution system, wherein each of the transmitter and the n receivers generates the same secret key data based on a modulation phase at each time of the common time information. In the quantum secret key distribution system according to claim 1 or 2,
Along with the common time information, time information at which at least one of the plurality of receivers succeeds in detection is extracted as detection time information, and the transmitter and the detection are performed based on the modulation phase at each time of the detection time information. A quantum secret key distribution system, wherein at least two successful receivers each generate the same secret key data. In the quantum secret key distribution system according to any one of claims 1 to 3 ,
An optical pulse transmitted from the transmitter has an average of less than one photon per pulse, and is a quantum secret key distribution system. A quantum secret key distribution method comprising:
A transmission step of transmitting an optical pulse from a transmitter by differential phase shift modulation; and
A branching step for branching an optical pulse transmitted from the transmitter;
A first detection step in which one of the branched optical pulses is received by a first receiver to detect a modulation phase;
A first information providing step of providing time information of successful detection in the first detection step;
A second detection step of detecting the modulation phase by receiving the other of the branched optical pulses with a second receiver;
A second time information providing step of providing time information successfully detected in the second detecting step;
Of the time information provided by the first and second information providing steps, for each optical pulse transmitted from the transmitter in the transmission step, the first and second detection steps in the first and second detection steps. An extraction step for extracting time information that the two receivers have successfully detected as common time information;
And a generation step in which each of the transmitter, the first receiver, and the second receiver generates the same secret key based on the modulation phase at each time of the common time information. Quantum secret key distribution method. A quantum secret key distribution method comprising:
A transmission step of transmitting an optical pulse from a transmitter by differential phase shift modulation; and
A branching step of branching an optical pulse transmitted from the transmitter into n branches;
Receiving each of the n-branched optical pulses with n receivers, each detecting a modulation phase; and
An information providing step of providing time information of successful detection in each of the detection steps;
Of the provided time information, for the optical pulse transmitted from the transmitter in the transmission step, extraction is performed to extract time information in which all the n receivers have been successfully detected as common time information in the detection step. Steps,
And a step of generating the same secret key data by each of the transmitter and the n receivers based on a modulation phase at each time of the common time information. The quantum secret key distribution method according to claim 5 or 6 ,
Extracting time information at which at least one of the plurality of receivers has been successfully detected as detection time information; and
And at least two of the transmitter and the successfully detected receiver each generate the same secret key data based on the modulation phase at each time of the detection time information. Quantum secret key distribution method. In the quantum secret key distribution method according to any one of claims 5 to 7 ,
An optical pulse transmitted from the transmitter has an average of less than one photon per pulse, and the quantum secret key distribution method is characterized in that:

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