Control device, quantum key distribution system, control method and program

Abstract

To provide a control device, a quantum key distribution system, a control method, and a program that improve the key generation speed while suppressing the increase in the error rate in quantum key distribution due to crosstalk.SOLUTION: A control device according to an embodiment includes a processing unit that calculates the generation speed of an encryption key on the basis of output information output from a QKD device that has a detector that detects photons used to generate an encryption key by QKD (Quantum Key Distribution), and controls a gate time width at which the photons are detected on the basis of the generation speed of the encryption key.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] An embodiment of the present invention relates to a control device, a quantum key distribution system, a control method, and a program. [Background technology]

[0002] With the advancement of information and communication technology, a wide variety of data is now exchanged, and ensuring the confidentiality and security of the transmitted information has become a major issue. Even with the advancement of computer computing power, quantum cryptography, which uses a shared key for encrypted communication via quantum key distribution, remains an unbreakable cryptographic technology, and its practical application is anticipated. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Photon level crosstalk between parallel fibers installed in urban areas, Mikio Fujiwara, Shigehito Miki, Taro Yamashita, Zhen Wang, and Masahide Sasaki, [online], [Retrieved February 29, 2024], Internet〈URL: https: / / opg.optica.org / oe / fulltext.cfm?uri=oe-18-21-22199&id=206129〉 [Non-patent document 2] PW Shor and J. Preskill, “Simple proof of security of the BB84 quantum key distribution protocol” Phys. Rev. Lett., vol. 85, pp.441-444, Jul 2000 [Non-patent document 3] ZLYuan, A.Plews, R.Takahashi, K.Doi, W.Tam, AWSharpe, ARDixon, E.Lavelle, JFDynes, A.Murakami, M.Kujiraoka, M.Lucamarini, Y.Tanizawa, H.Sato, and AJShields, “10 Mb / s quantum key distribution” Summary of the Invention [Problem to be solved by the invention]

[0004] However, with conventional technologies, it has been difficult to improve the key generation speed while suppressing the increase in the error rate in quantum key distribution due to crosstalk. [Means for solving the problem]

[0005] The control device of the embodiment includes a processing unit that calculates the generation speed of the encryption key based on output information output from a QKD device that has a detector that detects photons used to generate an encryption key by QKD (Quantum Key Distribution), and controls the gate time width at which the photons are detected based on the generation speed of the encryption key. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of a quantum key distribution system according to an embodiment. [Figure 2] 10 is a flowchart illustrating an example of a control method according to the embodiment. [Figure 3] 10A and 10B are diagrams for explaining an example of a control method for a gate time width according to the first modification of the embodiment. [Figure 4] FIG. 10 is a diagram showing an example of the configuration of a quantum key distribution system according to a second modification of the embodiment. [Figure 5] FIG. 10 is a diagram showing an example of the configuration of a quantum key distribution system according to a third modification of the embodiment. [Figure 6] FIG. 1 is a diagram showing an example of the hardware configuration of a QKD device according to an embodiment. [Figure 7] FIG. 2 is a diagram illustrating an example of a hardware configuration of a control device according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0007] Hereinafter, embodiments of a control device, a quantum key distribution system, a control method, and a program will be described in detail with reference to the accompanying drawings.

[0008] This embodiment relates to a quantum key distribution (QKD) device and a quantum cryptography communication system, and in particular to a QKD device and a quantum key distribution system that can suppress the impact on key generation speed of crosstalk, which is signal light for other communications that enters from a communication path.

[0009] In this embodiment, the quantum key distribution protocol is the BB84 protocol, and quantum key (encryption key) generation processing is performed by sifting processing and key distillation processing, and the key generation speed is calculated during the generation processing.

[0010] [Configuration example] 1 is a diagram showing an example of the configuration of a quantum key distribution system 100 according to an embodiment. The quantum key distribution system 100 according to the embodiment includes QKD devices 1a and 1b, and a control device 2.

[0011] The QKD device 1a is a quantum key distribution device that transmits photons (quantum signals) via a quantum communication channel, and the QKD device 1b is a quantum key distribution device that receives photons via a quantum communication channel.

[0012] Hereinafter, when there is no need to distinguish between QKD devices 1a and 1b, they will simply be referred to as QKD device 1. Note that QKD device 1 may have the functions of both transmitting and receiving photons. That is, QKD device 1a may also have the function of receiving photons, and QKD device 1b may also have the function of transmitting photons.

[0013] The QKD device 1 b comprises a detector 11 , a processing unit 12 and a communication unit 13 .

[0014] The detector 11 receives a quantum signal (photon signal) by detecting photons propagated through the quantum communication channel.

[0015] The processing unit 12 is realized by at least one processing device. This processing device includes, for example, a control device and an arithmetic unit, and is realized by analog or digital circuits, etc. The processing device may be a central processing unit (CPU), a general-purpose processor, a microprocessor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a combination thereof.

[0016] The processing unit 12 includes a sifting processing unit 121 and a key distillation unit 122 .

[0017] The sifting processing unit 121 exchanges control information with the QKD device 1a, thereby performing sifting to share encryption key information between the QKD devices 1a and 1b.

[0018] Based on the shared encryption key information, the key distillation unit 122 corrects errors in the encryption key information that occurred between the QKD devices 1a and 1b through error correction processing, and further performs privacy amplification processing, which is equivalent to information compression, to negate any information in the encryption key information that may have been eavesdropped on by an eavesdropper.

[0019] Through the processing of key distillation unit 122, QKD devices 1a and 1b are ultimately able to share an encryption key that is guaranteed not to have been intercepted. The processes from error correction to privacy amplification are collectively called key distillation processing.

[0020] The shared encryption key is used when carrying out encrypted data communication between the QKD devices 1a and 1b, or between applications running on devices connected to the QKD devices 1a and 1b, respectively.

[0021] The communication unit 13 is realized by a wired or wireless communication IF (Interface). For example, the communication unit 13 transmits and receives control information for the shifting process to and from the QKD device 1a.

[0022] Furthermore, for example, the communication unit 13 transmits and receives data to and from the control device 2. Specifically, the communication unit 13 receives data (hereinafter referred to as "output information") used to calculate the key generation speed from the sifting processing unit 121 and the key distillation unit 122, and transmits the output information to the control device 2. Then, when the communication unit 13 receives a gate time width, which will be described later, from the control device 2, the communication unit 13 inputs the gate time width to the detector 11.

[0023] (Performance / key generation speed) The key generation speed, which is the amount of shared encryption keys per unit time, corresponds to the operating speed performance of the QKD device 1. The more encryption keys that can be used, the faster and more secure encrypted communication becomes possible, so the higher the key generation speed, the higher the performance of the QKD device 1.

[0024] (safety) In quantum key distribution, the quantum used to share encryption keys follows the uncertainty principle, one of the fundamental principles of quantum mechanics, which states that its state changes when it is observed. Due to this property, if an eavesdropper observes a quantum containing encryption key information transmitted by QKD device 1a on a quantum communication channel, the quantum state changes, and QKD device 1b, which receives the quantum, can know that the quantum has been observed by the eavesdropper.

[0025] At this time, the change in the quantum state appears as the error rate (Quantum Bit Error Rate, QBER) of the link between the QKD devices 1a and 1b. If an eavesdropper attempts to eavesdrop on the quantum, the quantum state changes and the QBER increases, allowing the QKD devices 1a and 1b to detect the presence of the eavesdropper.

[0026] However, quantum key distribution measurement values ​​such as QBER fluctuate not only due to the presence of an eavesdropper, but also due to the presence of unrelated light in the optical fiber of the quantum communication path and external factors (environmental factors) such as temperature, and the QKD device 1 cannot distinguish between these differences. Therefore, it is generally considered to be on the safe side and assumes that all fluctuations in quantum key distribution measurement values ​​due to environmental factors are due to the presence of an eavesdropper, in other words, that information has been leaked to an eavesdropper, and therefore performs information compression using privacy amplification processing, so the higher the QBER, the slower the key generation speed.

[0027] (crosstalk) Optical fiber or free space (free space optical communication) is generally used for quantum communication channels. In the case of QKD devices1 that use optical fiber to transmit photons, the quantum signal optical fiber that transmits photons for quantum key distribution is often adjacent to optical fibers used for other communication purposes in the actual field. Therefore, depending on the installation environment, communication signal light from adjacent optical fibers can leak into the quantum signal optical fiber, which is called crosstalk (Figure 1).

[0028] Sources of crosstalk include data communications in adjacent optical fibers, as well as signals emitted by eavesdroppers into quantum communication channels (optical fibers or free space) in order to interfere.

[0029] When the quantum signal wavelength and the crosstalk wavelength are different, crosstalk can be reduced using a wavelength filter, but when the quantum signal wavelength and the crosstalk wavelength are the same, reduction using a wavelength filter is impossible. Furthermore, when the source of crosstalk is known and controllable by the installer of the QKD device 1, such as a classical signal (control signal) for quantum key distribution, it is possible to suppress it by time multiplexing. However, when the source of crosstalk is used for a completely different communication purpose than quantum key distribution, it is impossible to control. Furthermore, while it is effective to select an optical fiber for the quantum signal that is physically separated from the optical fiber that generates the crosstalk, there are situations in which the installer of the QKD device 1 cannot freely choose.

[0030] Next, a description will be given of the control device 2. The control device 2 includes a processing unit 21 and a communication unit 22.

[0031] The processing unit 21 is realized by at least one processing unit. This processing unit includes, for example, a control unit and an arithmetic unit, and is realized by analog or digital circuits, etc. The processing unit may be a central processing unit (CPU), a general-purpose processor, a microprocessor, a digital signal processor (DSP), an ASIC, an FPGA, or a combination thereof. The processing unit 21 includes a calculation unit 211 and a control unit 212.

[0032] The communication unit 22 is realized by a wired or wireless communication IF. The communication unit 22 transmits and receives data to and from the QKD device 1b. Specifically, when the communication unit 22 receives output information from the QKD device 1b, it inputs the output information to the calculation unit 211. Then, when the communication unit 22 receives a gate time width, which will be described later, from the control unit 212, it transmits the gate time width to the QKD device 1b.

[0033] Next, the calculation unit 211 and the control unit 212 will be described in detail. The calculation unit 211 calculates the key generation rate using output information of the QKD device 1b input from the communication unit 22. For example, the output information is sieve key information output from the sifting processing unit 121. For example, the sieve key information includes the sieve key generation rate (count rate) and the sample error rate (sample QBER). The sample error rate is the QBER calculated from a part of the sieve key.

[0034] Furthermore, for example, the output information is the QBER output from the key distillation unit 122. The encryption key generation speed based on the QBER output from the key distillation unit 122 is highly accurate because it is the value at the time when the generation of the final encryption key is completed, but it takes time to know the encryption key generation speed because it is necessary to wait until all key distillation processes are completed.

[0035] For example, calculation unit 211 calculates the key generation rate based on the QBER output from key distillation unit 122. Specifically, calculation unit 211 may calculate (update) the encryption key generation rate every time a QBER is output from key distillation unit 122. Furthermore, calculation unit 211 may calculate the key generation rate by averaging a plurality of encryption key generation rates acquired within a unit time.

[0036] On the other hand, when the calculation unit 211 simply calculates the key generation speed based on the output information from the sifting processing unit 121, the accuracy is reduced because it is not the final result, but there is an advantage that the information can be updated every time the sieve key processing is completed, so the frequency of feedback can be increased.

[0037] Hereinafter, the key generation rate calculated simply based on the output information from the sifting processing unit 121 will be referred to as a simple SKR (Secure Key Rate).

[0038] The simplified SKR using the sieve key generation rate (count rate) and the sample error rate (sample QBER) is approximately calculated based on the descriptions in, for example, Non-Patent Documents 2 and 3 (for example, SKR calculation formula (1) in Non-Patent Document 3).

[0039] Whether the calculation unit 211 uses the output information of the sifting processing unit 121 or the key distillation unit 122 to calculate the key generation speed is determined based on the settings of the calculation unit 211, for example.

[0040] The control unit 212 controls the gate time width of the detector 11 based on the key generation speed calculated by the calculation unit 211 .

[0041] [Example of quantum key distribution method] Fig. 2 is a flowchart showing an example of a quantum key distribution method according to an embodiment. Fig. 2 illustrates an example in which the processing unit 21 sets an initial value of the gate time width and gradually narrows the gate time width to a value shorter than the initial value by feedback control using simple SKR as the encryption key generation speed.

[0042] First, control unit 212 sets an initial value of the gate time width in detector 11 (step S1). Next, detector 11 detects photons that are emitted from QKD device 1a, propagate through the quantum communication path, and arrive at QKD device 1b (step S2). The photons are converted into electronic signals by detector 11, which has an initial value (a predetermined gate time width) set as the gate time width.

[0043] Next, the calculation unit 211 calculates the above-mentioned simplified SKR (step S3). Specifically, the communication unit 22 receives the sieve key generation speed (count rate) and the sample error rate as output information from the sifting processing unit 121, and inputs the output information to the calculation unit 211. The calculation unit 211 calculates the above-mentioned simplified SKR using the output information.

[0044] Next, the control unit 212 reduces the set value of the gate time width, thereby narrowing the width of the gate time during which photons are converted into electrical signals (step S4).

[0045] Next, detector 11 detects photons emitted from QKD device 1a, propagating through the quantum communication path, and arriving at QKD device 1b (step S5). Next, calculation unit 211 calculates the above-mentioned simplified SKR (step S6). Steps S5 and S6 are similar to steps S1 and S2 above, so detailed explanations will be omitted.

[0046] Next, the control unit 212 compares the simplified SKR calculated before lowering the set value of the gate time width in step S4 with the simplified SKR calculated in step S6, and determines whether the simplified SKR has improved (step S7).

[0047] If the simple SKR has improved (step S7, Yes), the process returns to step S4. That is, if the simple SKR after narrowing the gate time width is greater than the simple SKR before narrowing the gate time width, the control unit 212 controls the gate time width to be further narrowed.

[0048] If the simple SKR has not improved (step S7, No), the process proceeds to step S8. If the simple SKR has deteriorated (step S8, Yes), that is, if the simple SKR after narrowing the gate time width is smaller than the simple SKR before narrowing the gate time width, the control unit 212 widens the gate time width by returning it to the value before the change (step S9).

[0049] If the simple SKR has not deteriorated (step S8, No), that is, if the simple SKR has not changed, the control unit 212 performs control to maintain the gate time width without executing step S9.

[0050] The feedback control according to the cycle of steps S4 to S7 is repeatedly executed as long as an improvement in the simple SKR is expected.

[0051] When using a gate detector 11 to detect photons, the narrower the gate time, the less crosstalk there is, but at the same time, the less quantum signal is detected. Therefore, as shown in the flowchart in Figure 2, the control unit 212 feedback-controls the gate time width using the simplified SKR as an index to optimize the balance between the decrease in encryption key generation speed due to an increase in the error rate caused by crosstalk and the decrease in encryption key generation speed due to a decrease in quantum signal.

[0052] The index is not limited to the simple SKR, and the encryption key generation speed based on the QBER output from the key distillation unit 122 may be used.

[0053] As described above, in the control device 2 of the embodiment, the calculation unit 211 calculates the encryption key generation rate based on output information output from the QKD device 1b, which includes the detector 11 that detects photons used to generate an encryption key by QKD. Then, the control unit 212 controls the gate time width during which photons are detected, based on the encryption key generation rate.

[0054] As a result, the control device 2 of the embodiment can improve the key generation speed while suppressing an increase in the error rate of quantum key distribution due to crosstalk.

[0055] (Modification 1 of the embodiment) Next, a first modification of the embodiment will be described. In the description of the first modification, the same explanation as in the embodiment will be omitted, and only differences from the embodiment will be described. In the first modification, an example will be described in which the control device 2 determines a minimum value of the gate time width and controls the gate time width set in the detector 11 so that it does not become smaller than the minimum value.

[0056] 3 is a diagram for explaining an example of a method for controlling the gate time width in the first modification of the embodiment. In the example of FIG. 3, it is assumed that the crosstalk is a contiguous wave. In the case of the BB84 method, an avalanche photodiode (APD) for gate operation is generally used, and the error rate increases by the amount of crosstalk / 2 / (crosstalk and signal) within the gate time, and the key is compressed more than necessary, resulting in a decrease in the key generation speed.

[0057] The control unit 212 of the first modification performs control so that the gate time width does not become smaller than a predetermined minimum value when executing the process of gradually narrowing the gate time width through the cycle of steps S4 to S7 in Fig. 2. That is, the control unit 212 of the first modification ends the process of gradually narrowing the gate time width described above when further narrowing the gate time width would cause the gate time width to become smaller than the predetermined minimum value.

[0058] The predetermined minimum value is set to, for example, a value equivalent to two pulses of the photon signal, taking into consideration that it takes time for the photon signal to rise after the gate is opened.

[0059] According to the first modification, by setting the minimum value of the gate time width to a gate time width equivalent to two pulses of the signal, it is possible to ensure that one period of the pulse signal is detected.

[0060] (Modification 2 of the embodiment) Next, a second modification of the embodiment will be described. In the description of the second modification, the same explanation as in the embodiment will be omitted, and only the differences from the embodiment will be described. In the second modification, an example will be described in which the control of the gate time width by the control device 2 in the above-mentioned embodiment is performed by the QKD device 1b-2 (see FIG. 4) instead of the control device 2.

[0061] [Configuration example] 4 is a diagram showing an example of the configuration of a quantum key distribution system 100-2 according to a second modification of the embodiment. The quantum key distribution system 100-2 according to the second modification includes QKD devices 1a and 1b-2. The QKD device 1b-2 includes a detector 11, a processing unit 12-2, and a communication unit 13. The second modification is different from the above-described embodiment in that the processing unit 12-2 further includes a control unit 123.

[0062] The control unit 123 receives feedback on the encryption key generation speed from the key distillation unit 122, and performs feedback control to gradually narrow the gate time width using the encryption key generation speed as an index. Details of the feedback control to gradually narrow the gate time width are the same as those in the flowchart of the above-described embodiment (see FIG. 2), and therefore will not be described again.

[0063] According to the quantum key distribution system 100-2 of the second modification, the same effects as those of the quantum key distribution system 100 of the above-described embodiment can be obtained.

[0064] (Modification 3 of the embodiment) Next, a third modification of the embodiment will be described. In the description of the third modification, the same explanation as in the embodiment will be omitted, and only the differences from the embodiment will be described. In the third modification, an example will be described in which the control of the gate time width by the control device 2 in the above-mentioned embodiment is performed by the QKD device 1b-3 (see FIG. 5) instead of the control device 2.

[0065] [Configuration example] 5 is a diagram showing an example of the configuration of a quantum key distribution system 100-3 according to a third modification of the embodiment. The quantum key distribution system 100-3 according to the third modification includes QKD devices 1a and 1b-3. The QKD device 1b-3 includes a detector 11, a processing unit 12-3, and a communication unit 13. The third modification is different from the above-described embodiment in that the processing unit 12-3 further includes a control unit 123 and a calculation unit 124.

[0066] The control unit 123 receives feedback on the key generation speed from the key distillation unit 122, and performs feedback control to gradually narrow the gate time width using the key generation speed as an index. Details of the feedback control to gradually narrow the gate time width are the same as those in the flowchart of the above-described embodiment (see FIG. 2), and therefore will not be described here.

[0067] The calculation unit 124 calculates the key generation speed using the above-mentioned sieve key information input from the sifting processing unit 121. The method of calculating the key generation speed is the same as that of the calculation unit 211 in the above-mentioned embodiment, and therefore the explanation will be omitted.

[0068] According to the quantum key distribution system 100-3 of the third modification, the same effects as those of the quantum key distribution system 100 of the above-described embodiment can be obtained.

[0069] Finally, an example of the hardware configuration of the QKD device 1 and the control device 2 of the embodiment will be described.

[0070] [Example of hardware configuration] 6 is a diagram showing an example of the hardware configuration of the QKD device 1 of the embodiment. The QKD device 1 of the embodiment includes a processor 301, a main memory device 302, an auxiliary memory device 303, a display device 304, an input device 305, a quantum communication IF 306, and a classical communication IF 307.

[0071] The processor 301 , the main memory device 302 , the auxiliary memory device 303 , the display device 304 , the input device 305 , the quantum communication IF 306 and the classical communication IF 307 are connected via a bus 310 .

[0072] The processor 301 executes a program read from the auxiliary storage device 303 to the main storage device 302. The main storage device 302 is memory such as a ROM and a RAM. The auxiliary storage device 303 is a HDD, a memory card, or the like.

[0073] The display device 304 displays the status of the QKD device 1, etc. The input device 305 accepts input from the user. The display device 304 and the input device 305 may be realized by a touch panel or the like having a display function and an input function. Furthermore, the display device 304 and the input device 305 do not have to be provided in the QKD device 1. In this case, for example, the display function and input function of an external terminal connected to the QKD device 1 are used.

[0074] The quantum communication IF 306 is an interface for connecting to a QKD link through which photons are transmitted. The classical communication IF 307 is an interface for connecting to a transmission path or the like through which control signals are transmitted between the opposing QKD device 1.

[0075] 7 is a diagram showing an example of the hardware configuration of the control device 2 according to the embodiment. The control device 2 includes a processor 401, a main storage device 402, an auxiliary storage device 403, a display device 404, an input device 405, and a communication IF 406.

[0076] The processor 401 , the main memory device 402 , the auxiliary memory device 403 , the display device 404 , the input device 405 and the communication IF 406 are connected via a bus 410 .

[0077] The processor 401 executes a program read from the auxiliary storage device 403 to the main storage device 402. The main storage device 402 is a memory such as a ROM and a RAM. The auxiliary storage device 403 is a HDD, a memory card, or the like.

[0078] The display device 404 displays the status of the control device 2, etc. The input device 405 accepts input from a user. The display device 404 and the input device 405 may be realized by a touch panel or the like having a display function and an input function. The display device 404 and the input device 405 may not be provided in the control device 2. In this case, for example, the display function and the input function of an external terminal connected to the control device 2 are used.

[0079] The communication IF 406 is an interface for connecting to a transmission line.

[0080] The programs executed by the QKD device 1 and control device 2 of the embodiment are provided as a computer program product stored in an installable or executable format on a computer-readable storage medium such as a CD-ROM, memory card, CD-R, or DVD (Digital Versatile Disc).

[0081] In addition, the programs executed by the QKD device 1 and control device 2 of the embodiment may be stored on a computer connected to a network such as the Internet, and may be provided by being downloaded via the network.

[0082] Furthermore, the programs executed by the QKD device 1 and the control device 2 of the embodiment may be configured to be provided via a network such as the Internet without being downloaded.

[0083] Furthermore, the programs executed by the QKD device 1 and the control device 2 of the embodiment may be configured to be provided by being pre-installed in a ROM or the like.

[0084] The program executed by the QKD device 1 of the embodiment has a modular configuration that includes functions that can be realized by the program, among the functional configuration of the QKD device 1 of the embodiment. The functions realized by the program are loaded into the main memory device 302 by the processor 301 reading and executing the program from a storage medium such as the auxiliary memory device 303. In other words, the functions realized by the program are generated on the main memory device 302.

[0085] Similarly, the program executed by the control device 2 of the embodiment has a modular configuration including functions that can be realized by the program among the functional configuration of the control device 2 of the embodiment. The functions realized by the program are loaded into the main storage device 402 by the processor 401 reading and executing the program from a storage medium such as the auxiliary storage device 403. In other words, the functions realized by the program are generated on the main storage device 402.

[0086] It should be noted that some or all of the functions of the QKD device 1 and the control device 2 of the embodiment may be realized by hardware such as an IC (Integrated Circuit), etc. The IC is, for example, a processor that executes dedicated processing.

[0087] Furthermore, when each function is realized using a plurality of processors, each processor may realize one of the functions, or may realize two or more of the functions.

[0088] Furthermore, the control device 2 of the embodiment may operate in any manner. For example, the control device 2 may be operated as a cloud system on a network to control the gate time width of one or more QKD devices 1 connected via the network.

[0089] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims.

[0090] (Addendum) The above-described embodiments can be summarized as the following technical proposals.

[0091] Technical proposal 1 a processing unit that calculates a generation rate of the encryption key based on output information output from a QKD device that includes a detector that detects photons used to generate an encryption key by QKD (Quantum Key Distribution), and controls a gate time width during which the photons are detected based on the generation rate of the encryption key; A control device comprising: Technical proposal 2 The output information includes a generation rate of a sieve key output by a sifting process in QKD and a sample QBER (Quantum Bit Error Rate) calculated from a portion of the sieve key; the processing unit calculates the encryption key generation rate based on the sieve key generation rate and the sample QBER. The control device described in Technical Proposal 1. Technical proposal 3 the output information includes a QBER of the encryption key output by a key distillation process in QKD; The processing unit calculates the encryption key generation speed based on the QBER. The control device described in Technical Proposal 1. Technical proposal 4 the processing unit sets an initial value of the gate time width, and controls the gate time width to be narrowed to a value shorter than the initial value by feedback control based on a generation speed of the encryption key. A control device according to any one of technical proposals 1 to 3. Technical proposal 5 the processing unit performs control to further narrow the gate time width when the encryption key generation speed after narrowing the gate time width is higher than the encryption key generation speed before narrowing the gate time width. A control device according to any one of technical proposals 1 to 4. Technical plan 6 the processing unit terminates the control of narrowing the gate time width when the encryption key generation speed before narrowing the gate time width is the same as the encryption key generation speed after narrowing the gate time width. A control device according to any one of technical proposals 1 to 5. Technical proposal 7 when the encryption key generation speed after narrowing the gate time width is lower than the encryption key generation speed before narrowing the gate time width, the processing unit returns the gate time width to the value before narrowing and terminates the control of narrowing the gate time width. A control device according to any one of technical proposals 1 to 6. Technical proposal 8 the processing unit determines a minimum value of the gate time width and performs control to narrow the gate time width so that the gate time width does not become smaller than the minimum value. A control device according to any one of technical proposals 1 to 7. Technical proposal 9 The minimum value of the gate time width is a value equivalent to two pulses of the photon signal. A control device according to any one of technical proposals 1 to 8. Technical proposal 10 A control device according to any one of technical proposals 1 to 9; one or more of said QKD devices; A quantum key distribution system comprising: Technical proposal 11 Calculating a generation speed of the encryption key based on output information output from a QKD device including a detector for detecting photons used to generate the encryption key by QKD (Quantum Key Distribution); controlling a gate time width during which the photons are detected based on a generation speed of the encryption key; Control method. Technical proposal 12 On the computer, Calculating a generation speed of the encryption key based on output information output from a QKD (Quantum Key Distribution) device that includes a detector for detecting photons used to generate the encryption key by QKD; a gate time width during which the photons are detected is controlled based on the generation speed of the encryption key; program. [Explanation of symbols]

[0092] 1 QKD device 2. Control device 11 Detector 12 Processing section 13 Communications Department 21 Processing section 22 Communications Department 100 Quantum Key Distribution System 121 Shifting processing section 122 Key Distillation Department 123 Control Unit 124 Calculation Department 211 Calculation Department 212 Control Unit 301 processor 302 Main storage 303 Auxiliary storage device 304 Display device 305 Input Device 306 Quantum Communication Interface 307 Classical Communication IF 310 Bus 401 processor 402 Main storage 403 Auxiliary storage 404 Display device 405 Input Device 406 Communication Interface 410 Bus

Claims

1. a processing unit that calculates a generation rate of the encryption key based on output information output from a QKD device that includes a detector for detecting photons used to generate an encryption key by QKD (Quantum Key Distribution), and controls a gate time width during which the photons are detected based on the generation rate of the encryption key; A control device comprising:

2. The output information includes a generation rate of a sieve key output in a sifting process in QKD and a sample QBER (Quantum Bit Error Rate) calculated from a portion of the sieve key; the processing unit calculates the encryption key generation rate based on the sieve key generation rate and the sample QBER. The control device according to claim 1 .

3. the output information includes a QBER of the encryption key output by a key distillation process in QKD; the processing unit calculates the encryption key generation speed based on the QBER. The control device according to claim 1 .

4. the processing unit sets an initial value of the gate time width, and controls the gate time width to be narrowed to a value shorter than the initial value by feedback control based on a generation speed of the encryption key. The control device according to any one of claims 1 to 3.

5. the processing unit performs control to further narrow the gate time width when the encryption key generation speed after narrowing the gate time width is higher than the encryption key generation speed before narrowing the gate time width. The control device according to claim 4.

6. the processing unit terminates the control of narrowing the gate time width when the encryption key generation speed before narrowing the gate time width is the same as the encryption key generation speed after narrowing the gate time width. The control device according to claim 4.

7. when the encryption key generation speed after narrowing the gate time width is lower than the encryption key generation speed before narrowing the gate time width, the processing unit returns the gate time width to the value before narrowing and terminates the control of narrowing the gate time width. The control device according to claim 4.

8. the processing unit determines a minimum value of the gate time width and performs control to narrow the gate time width so that the gate time width does not become smaller than the minimum value. The control device according to claim 4.

9. the minimum value of the gate time width is a value corresponding to two pulses of the photon signal; The control device according to claim 8.

10. A control device according to any one of claims 1 to 3; one or more of said QKD devices; A quantum key distribution system comprising:

11. Calculating a generation speed of the encryption key based on output information output from a quantum key distribution (QKD) device that includes a detector for detecting photons used to generate the encryption key by QKD; controlling a gate time width during which the photons are detected based on a generation speed of the encryption key; Control method.

12. On the computer, Calculating a generation speed of the encryption key based on output information output from a quantum key distribution (QKD) device that includes a detector for detecting photons used to generate the encryption key by QKD; a gate time width during which the photons are detected is controlled based on the generation speed of the encryption key; program.

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