JP4076532B2 - Quantum cryptography communication device and secret key generation method using quantum cryptography communication device - Google Patents

Description

  The present invention relates to a quantum key distribution technique using a single photon pair having a quantum correlation, and in particular, based on a differential phase shift quantum key distribution technique, the quantum encryption key can be distributed over a long distance. The present invention relates to a quantum cryptography communication device and a secret key generation method using the quantum cryptography communication device.

  Various methods have been proposed as a method for realizing the quantum cryptography system. Here, a method called general differential phase shift quantum key distribution according to the present invention will be described. This method is a method that does not relay quantum cryptography proposed in the present invention, that is, between Alice (first device) and Bob (second device) without going through Charlie (third device). In this method, the encryption key is directly generated (Non-Patent Document 1).

  FIG. 7 shows the configuration of a conventional differential phase shift quantum key distribution system. A light source 611 on the side of Alice (first device) 610 transmits coherent light pulses at regular intervals. In Alice 610, the phase of each optical pulse is phase-modulated to 0 or π by the phase modulator 613, and the intensity of the phase-modulated optical pulse is reduced to less than one photon per pulse by the attenuator (attenuator) 615. And is sent to the optical fiber 620.

  Bob (second device) 630 measures the phase difference between two successive optical pulses using an optical path difference interferometer 631 corresponding to the interval between the optical pulses transmitted through the optical fiber 620. The interferometer 631 includes an entrance-side beam splitter 632, a pair of total reflection mirrors 633 and 634, and an exit-side beam splitter 635. When the phase difference between successive optical pulses is 0, the first terminal In 636, when the phase difference is π, the interferometer 631 is adjusted so that light is output to the second terminal 637. When the interferometer 631 is adjusted in this way, when the phase difference of the optical pulse is 0, the photon is detected by the first photon detector (B1) 638, and the phase difference of the optical pulse is π. The photon is detected by the second photon detector (B2) 639.

  Using the setup as described above, Alice 610 uses the phase modulator 613 to randomly phase modulate each optical pulse to 0 or π, and stores information on how to modulate the information on the Alice side (shown in the figure). Do not record). Bob 630 records the time at which the photon was detected and information on which photon detector 638, 639 detected the photon in the storage means (not shown) on the Bob side. Bob 630 then transmits the time at which the photon was detected to Alice 610.

  Bob 630 detects “0” when the photon is detected by the first photon detector (B1) 638 from the information as to which photon detector 638, 639 detects the photon, and detects the second photon. If it is detected by the device (B2) 639, a key bit is created as “1”. In addition, Alice 610 uses the photon detection time transmitted from Bob 630, and how the optical pulse corresponding to the photon detection time is recorded from its own recording (recording information on how the optical pulse was modulated). You can know if it was modulated. Alice 610 has the same key as Bob 630 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”. A bit can be created.

  As described above, Alice 610 and Bob 630 only reveal the photon detection time of Bob 630, and information about the bit (which photon detector 638, 639 detected the photon and how the photon is modulated). Key bits can be shared without revealing).

  The above is the operation principle of the conventional differential phase shift quantum key distribution.

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

  In a quantum cryptography system using photons, weak light that is 1 photon or less per pulse is used to prevent eavesdropping. When this weak light is transmitted through a fiber, attenuation is accompanied, and thus there is a limit to the distance that light that can be transmitted to transmit an encryption key can reach.

  As described above, the conventional quantum key distribution system has a loss of a transmission path for transmitting photons, so that the probability of detecting photons decreases as the transmission distance increases, and it becomes difficult to generate a correct secret key. There were difficulties. As a result, in order to generate a correct encryption key, the delivery distance has to be set within a certain distance that is relatively short.

  An object of the present invention is to solve such a problem of the prior art, and in the differential phase shift quantum key distribution system, by using two or more light sources that generate entanglement photon pairs, An object of the present invention is to provide a quantum cryptography communication device having a longer transmission distance than that of the prior art and a secret key generation method using the quantum cryptography communication device.

To achieve the above object, a quantum cryptography communication apparatus of the present invention, quantum cryptography communication to share a quantum cryptographic key with the first equipment and the second equipment by the quantum key distribution system using entangled photon pairs A third device connected as a relay node to an intermediate position between the first device and the second device, and a first light source connected to an intermediate position between the first device and the third device When the second device and a second light source connected to an intermediate position between the third device, and the first light source, one pulse per a light pulse of a predetermined interval in synchronization with the second light source Quantum entangled photon pairs with an average number of photons of less than 1 are generated at predetermined time intervals, one photon of the generated entangled photon pairs is in the first device, and the other one is in the third device. and means for sending said second light source is synchronized with the first light source Entangled photon pairs in an average photon number less than 1 per one pulse as an optical pulse of a predetermined interval occurs at a predetermined time interval, the second device one photon of the entangled photon pairs were the occurrence of other Means for sending one photon to the third device, wherein the third device detects the respective photons sent from the first light source and the second light source, and the optical multiplexer; Two photon detector groups connected to each of the output terminals of the detector to detect the combined photons, and the fact that any one of the photon detector groups successively detects the photons and its The photon detection group of time or both of them includes a transmission means for transmitting the time statement and the time when photons were detected to the first device and the second device, alternately and continuously , One device has an optical path difference corresponding to the predetermined time interval and 2 Interferometer having an output path, stores information about whether the photon detectors installed in each of the output path, either the time at which the photon detector detects the photon-photon detector detects the photon Storage means for transmitting, transmitting means for transmitting the time when photons are detected to the second device, receiving means for receiving information sent from the second device and information sent from the third device, A key bit of a shared quantum encryption key that becomes a secret key using the photon detection information sent from the third device and a photon corresponding to the photon detection information using an event detected by both the first device and the second device. and a cryptographic generation means for generating said second device, an interferometer having said optical path difference corresponding to a predetermined time interval and two output paths, and installed in each of the output paths photon inspection and detector, the photon detector photons Sent from the first device, storage means for storing information about the photon time and which photon detector detected the photon, transmission means for sending the photon detection time to the first device, and Receiving means for receiving information and information sent from the third device, and photon detection information sent from the third device and the corresponding photons are received by both the first device and the second device. Cryptography generating means for generating a key bit of a shared quantum encryption key to be a secret key using the detected event .

In addition, the first light source and the second light source may be configured by a pump light source and an optical nonlinear medium, respectively, and generate a photon pair by an optical parametric process.

  The interferometers included in the first device and the second device each include an optical demultiplexer having an input terminal to which the photons are input and two output terminals, and two of the optical demultiplexers. And an optical multiplexer having a pair of optical paths composed of a long optical path and a short optical path connected to one output terminal, an input terminal connected to the pair of optical paths, and two output terminals. The light path difference between the long light path and the short light path may be equal to the distance that light propagates at the predetermined time interval.

In order to achieve the above object, the secret key generation method of the present invention includes a computer control that controls the quantum cipher key that is a secret key for encrypting and decrypting data using the quantum cipher communication device of the present invention. A first step in which the first light source and the second light source are synchronized to supply photons to the first device, the second device, and a third device ; and The fact that three devices continuously detected photons with one of the photon detector groups and the time, or the fact that photons were detected alternately with two photon detector groups and the time, the first device and The second step of communicating to the second device, the time at which the first device and the second device detected a photon with each photon detector, and which photon detector detected the photon. Each time, and the time when the photon was detected A third step of transmitting / receiving to the first device, the first device and the second device, the event that the third device detected a photon from the transmitted time information, and a photon corresponding to the event detected by the first device and the And a fourth step of generating a key bit of the quantum encryption key using an event detected by both of the second devices.

  Here, the fourth step includes a step of determining whether or not the third device continuously detects photons by only one of the photon detector groups in the first device; When photons are continuously detected by only one photon detector group and the photons are detected by the first photon detector of the first device, the key bit is set to “0” and the photons are When detected by the second photon detector of the first device, the step of setting the key bit to “1” and when the third device detects photons alternately by the two photon detector groups, the photon Is detected by the first photon detector of the first device, the key bit is “0”, and when the photon is detected by the second photon detector of the first device, the key bit is “ 1 ″, and the fourth step includes the step of: The step of determining whether the third device continuously detects photons by only one of the photon detector groups, and the third device continuously detects photons by only one of the photon detector groups. In this case, when the photon is detected by the first photon detector of the second device, the key bit is set to “0”, and when the photon is detected by the second photon detector of the second device. When the key bit is set to “1” and when the third device detects photons alternately by two photon detector groups, the photon is detected by the first photon detector of the second device. Includes a step of setting the key bit to “1” and setting the key bit to “0” when the photon is detected by the second photon detector of the second device. it can.

  In the present invention, the light transmitted from the first light source that generates the entanglement photon pair provided between the first device (Alice) and the third device (Charlie), the third device (Charlie), and the second device ( Since the quantum encryption key is generated by using the light transmitted from the second light source that generates the entanglement photon pair provided in the middle of Bob), the light is transmitted directly from Alice to Bob. In comparison, the delivery distance over which sufficient light can reach can be increased as a total, for example, approximately four times that of the conventional one. Thus, according to the present invention, there is an effect that it is possible to provide a quantum cryptography system having a longer transmission distance than before.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the present invention, the expression “two groups of photon detectors” and “one group of photon detectors” includes the case where the configuration of “group” is “one”. The first embodiment of the present invention shows a case where two groups of photon detectors included in the third device have one photon detector included in each group.

(First embodiment)
FIG. 1 mainly shows the configuration of an optical system according to the first embodiment to which the quantum cryptography communication apparatus of the present invention is applied. As shown in FIG. 1, this communication system includes Alice (first device) 110, optical fiber transmission line 120, Bob (second device) 130, Charlie (third device) 140, and one pulse as a light pulse at regular intervals. It has first and second entanglement light sources 150 and 160 that generate entanglement photon pairs (quantum entangled photon pairs) synchronously at a predetermined time interval with an average photon number of less than 1.

  Alice (first device) 110 measures the phase difference between two consecutive optical pulses using interferometer 111 having an optical path difference corresponding to the interval between the optical pulses generated by light source 150 (the predetermined time interval). . The interferometer 111 includes an entrance-side beam splitter (optical demultiplexer) 112, a pair of mirrors 113 and 114, and an exit-side beam splitter 115, and the phase difference between successive optical pulses is zero. The light is output to the first output terminal 116 of the exit-side beam splitter (optical multiplexer) 115 and, when the phase difference is π, the light is output to the second output terminal 117 of the exit-side beam splitter 115. Has been adjusted. That is, a pair of optical paths composed of a long optical path and a short optical path connected to the two output terminals of the entrance-side beam splitter 112 are configured, and the exit-side beam splitter 115 is connected to the long optical path and the short optical path. However, the difference in the distance between the long light path and the short light path is set equal to the distance that the light propagates at the predetermined time interval. When the interferometer 111 is adjusted in this way, when the phase difference of the optical pulse is 0, the photon is detected by the first photon detector (A1) 118 connected to the first output terminal 116, and the optical pulse When the phase difference is π, photons are detected by the second photon detector (A2) 119 connected to the second output terminal 117. Further, Alice 110 stores the storage time (1101 in FIG. 2) for storing the time when the photon is detected and the information of which photon detector is the detected photon detector, and the time when the photon is detected. And a transmission / reception unit (1102 in FIG. 2).

  Bob (second device) 130 has the same configuration as Alice (first device) 110, and includes an optical path difference interferometer 131 corresponding to the interval between the light pulses generated by light source 160 (the predetermined time interval). Used to measure the phase difference between two successive light pulses. The interferometer 131 includes an entrance-side beam splitter (optical demultiplexer) 132, a pair of mirrors 133 and 134, and an exit-side beam splitter (optical multiplexer) 135. When the phase difference is 0, light is output to the first output terminal 136 of the exit-side beam splitter 135, and when the phase difference is π, light is output to the second output terminal 137 of the exit-side beam splitter 135. Has been adjusted. That is, a pair of optical paths composed of a long optical path and a short optical path connected to the two output terminals of the entrance-side beam splitter 132 are configured, and the exit-side beam splitter 135 is connected to the long optical path and the short optical path. However, the difference in the distance between the long light path and the short light path is set equal to the distance that the light propagates at the predetermined time interval. When the interferometer 131 is adjusted in this way, when the phase difference of the optical pulse is 0, the photon is detected by the first photon detector (B1) 138 connected to the first output terminal 136, and the optical pulse When the phase difference of π is π, photons are detected by the second photon detector (B2) 139 connected to the second output terminal 137. Further, Bob 130 uses a storage unit (1301 in FIG. 2) for storing the time when the photon is detected and information on which photon detector the detected photon detector is, and the time when the photon is detected. And a transmission / reception unit (1302 in FIG. 2).

  The Charlie (third device) 140 is a 50:50 optical coupler (photocombiner) 141 having 2 × 2 input / output terminals for synthesizing respective photons (idlers) sent from the light sources 150 and 160, and the synthesized result. And a pair of photon detectors 142 and 143 for detecting photons. Further, the Charlie 140 includes a recording unit (1401 in FIG. 2) that records a photon detection time at which photons are continuously detected and information on which photon detector that has detected these photons is, A transmitter (FIG. 5) that transmits to Alice 110 and Bob 130 the fact that photons were continuously detected by one of the photon detectors and the time, or the fact that photons were detected alternately by the two photon detectors and the time. 2 1402).

  The first light source 150 is installed between Alice 110 and Charlie 140, and the second light source 160 is installed between Charlie 140 and Bob 130. Both light sources 150 and 160 are synchronized with each other. The light sources 150 and 160 include, for example, a pump light source (1501 and 1601 in FIG. 2) and an optical nonlinear medium (1502 and 1602 in FIG. 2), and generate photon pairs by an optical parametric process.

  FIG. 2 shows the system configuration of the quantum cryptography communication apparatus of the present invention. As shown in the figure, the third device (Charlie) 140 has a recording unit 1401 that records the photon detection time and the photon detector (one of 142 or 143) that has detected the photon. Further, in the third device 140, a group of photon detectors (one of 142 and 143) connected to one output terminal of the optical combiner 141 having two output terminals continuously detected photons. The fact and the time, or the fact that the group of photon detectors connected to each of the output terminals of the optical multiplexer 141 alternately detected the photon and the time, are shown in the first device (Alice) 110 and The second apparatus (Bob) 130 has a transmission unit 1402 that transmits through the information transmission path 170 that has been prevented from being falsified by existing technology.

  Each of the first device 110 and the second device 130 includes a storage unit 1101 that stores information on when each photon was detected and which photon detector detected the photon. Further, the first device 110 and the second device 130 transmit / receive the times at which each photon is detected through the information transmission path 170, and also receive the information transmitted by the third device 1102 and 1302 , And encryption key generation units 1103 and 1303 that generate key bits of a common encryption key using the detected photon detection time, the received photon detection time, and the information received from the third device 140.

  The first light source 150 and the second light source 160 are composed of pump light sources 1501 and 1601 and optical nonlinear media 1502 and 1602, respectively, and generate photon pairs by an optical parametric process.

  In the arrangement described above, the entanglement photon pair [(quantum entangled) photon pair] from the first light source 150 and the second light source 160 as the light pulses of a constant interval is obtained from Alice 110 and Charlie 140, and Charlie 140 and Bob 130. Each is sent through the optical fiber transmission line 120. At this time, the average number of photons contained in the light pulse is less than 1 per pulse. Further, it is assumed that the first light source 150 and the second light source 160 transmit light pulses in synchronization. Here, as the light sources 150 and 160 for generating the entanglement photon pair, for example, a light source by parametric conversion is assumed. In the parametric conversion, the pump light generates two photon pairs called a signal and an idler. Here, the signal of the first light source 150 is sent to Alice 110, the signal of the second light source 160 is sent to Bob 130, Assume that both idlers of the first light source 150 and the second light source 160 are sent to Charlie 140.

  Next, a method for describing the state of a photon will be described. The photon state can be expressed as a Hilbert space vector called a ket vector. A ket vector is represented as | a>. Here, a is a symbol that distinguishes the difference from other ket vectors.

  In quantum mechanics, a state in which two or more states are superimposed is allowed, and the state is described as the sum of the ket vectors. For example, the superimposed state of the state | a> and the state | b> is described as 1 / √2 (| a> + | b>). (1 / √2 is a constant for normalization.)

  A state obtained by integrating the two states is described as a tensor product of the ket vector. For example, the state that the state | a> and the state | b> are integrated is

Is described. However,

Can be simply abbreviated as | a> | b>.

Using the above-mentioned ket vector, the quantum mechanical state | Ψ ₀ > of the photon pair output from the light sources 150 and 160 can be described as the following equation (1).

_{However, | t j> s1, |} t j> s2, | t j> i1, | t j> i2 , the signal of the first light source 150, respectively in this order, sent at time t _j, the second light source 160 The signal, the idler of the first light source 150, and the idler of the second light source 160 are shown. φ _j ¹ and φ _j ² represent the initial phases of the photon pairs transmitted from the first light source 150 and the second light source 160. | A ₁ | ² and | a ₂ | ² represent probabilities that the state is observed.

The above state | Ψ ₀ > is transmitted from the light sources 150 and 160 to the Alice (first device) 110, Charlie (third device) 140, and Bob (second device) 130 via the fiber transmission line 120, respectively. Will be sent. Assuming that the propagation times of the signal and idler are the same, let φ _a and φ _b be the sum of the propagation phases received between Alice 110 and Charlie 140 and between Charlie 110 and Bob 130. The state | Ψ ₁ > of the photon pair when arriving at each of Alice 110, Charlie 140, and Bob 130 can be described as the following equation (2).

Next, paying attention only to the case where Charlie 140 detects photons transmitted at successive times of times t _j and t _{j + 1} , the state | Ψ ₂ > of the photon pair at the time involved in this detection is It can be described as equation (3).

However, assuming that the pump light used for the parametric conversion has a coherent time sufficiently longer than the interval between the optical pulses, two consecutive optical pulses become coherent. Therefore, φ _j = φ _{j + 1} was set.

  When the above formula (3) is expanded and divided into signals and idlers, the following formula (4) is obtained.

  Here, in Charlie 140, the two idlers are put in the optical coupler 141 and combined. That is, the state of each idler is changed by the optical coupler 141 as shown in the following equations (5) and (6).

However, | C1, t _j > _i1 represents the idler state when the first photon detector (C1) 142 detects the idler transmitted from the first light source 150 at time t _j . | C2, t _j > _i1 similarly represents the state when the idler is detected by the second photon detector (C2) 143.

Also, | C1, t _j > _i2 represents a state where the first photon detector (C1) 142 detects the idler transmitted from the second light source 160 at time t _j . Similarly, | C2, t _j > _i2 represents a state when the idler is detected by the second photon detector (C2) 143. Therefore, the state of the idler | Ψ ₃ > after these two idlers are put into the optical coupler 141 can be described as the following equation (7).

  Here, when Charlie 140 focuses on a state in which photons are detected only by the first photon detector (C1) 142 or the second photon detector (C2) 143 at successive times, the above equation (7) becomes It can deform | transform like Formula (8).

  Further, assuming that Charlie 140 cannot distinguish between idler 1 and idler 2 during photon detection, the second term in the above equation (8) (from the first + sign at the left end in the description of the above equation, 2 at the left end) The term up to immediately before the first + symbol) and the third term (from the second + symbol at the left end to the point immediately before the third + symbol at the left end in the description above) are 0 respectively. (8) can be transformed as the following equation (9).

Next, Alice 110 and Bob 130 pass their respective signals through their interferometers 111 and 131. Here, the change in the state of each signal when passing through the interferometers 111 and 131 will be described. First, it is assumed that a signal of | t _j > _s1 from the first light source 150 is input to the interferometer 111 on the Alice 110 side. The signal | by t _{j> s1} beam splitter 112 is installed at the entrance of the interferometer 111, as shown in the following equation (10), its status is changed.

However, | S _a , t _j > _s1 represents a state in which the photon _travels on the short path (the straight path in FIG. 1) of the interferometer 111. | L _a , t _j > s 1 represents a state in which the photon has _traveled the long path (path with reflection in FIG. 1) of the interferometer 111.

  The state of the photon before the photon propagates through each path of the interferometer 111 and enters the beam splitter 115 installed on the exit side is described as a superposition of the two states described in the following equations (11) and (12). it can.

Here, φ _Sa and φ _La represent propagation phases when photons travel along the short side and long side paths of the interferometer 111, respectively. In addition, Δt represents a difference in time required for propagation between the short side and the long side.

  Furthermore, the state of the photon after passing through the beam splitter 115 installed at the exit can be described as the following equations (13) and (14).

Here, | A1, t> _s1 represents the state of the photon when the photon (signal) is detected by the first photon detector (A1) 118 of Alice 110. | A2, t> _s1 similarly represents the state of the photon when the photon (signal) is detected by the second photon detector (A2) 119 of Alice 110. Further, since the propagation delay time of the interferometer 111 is equal to the optical pulse interval, it can be set to t _j + Δt → t _{j + 1} .

As can be seen from the above description, since the state after the photon passes through the interferometer 111 is a superposition state of these, the state of the photon put in the Alice side interferometer 111 | t _j > _s1 is It changes as the following equation (15).

Here, when φ _La −φ _Sa = Δφ _a , the above equation (15) is

It becomes.

When the propagation phase difference of the interferometer 111 is adjusted to 0, that is, Δφ _a = 0, the above equation (16) can be described as the following equation (17) using a certain real number φ _A.

_{Similarly, | t j + 1> s1} , | t j> s2, | signal t _{j +} 1> _s2 (time t _{j + 1} first light source 150 that is sent to, the sent at time t _j The states of the photons after the signals from the two light sources 160 and the signals from the second light source 160 transmitted at time t _{j + 1} ) pass through the corresponding interferometers 111 and 131, respectively, are expressed by the following equations (18) to (20). It can be written as

Here, | B1, t> _s1 represents the state of the photon when the photon is detected by the first photon detector (B1) 138 of Bob 130. | B2, t> _s1 similarly represents the state of the photon when the photon is detected by the second photon detector (B2) 139 of Bob 130. In addition, it is assumed that Δφ _b = 0 is adjusted as described above.

  From the above discussion, the state of the photon (signal) after passing through the interferometers 111 and 131 can be described as the following equation (21).

Next, focusing on only the state involving the signal transmitted at time t _{j + 1} , the above equation (21) can be described as the following equation (22).

In the above equation (22), | C1, t _j > | C1, t _{j + 1} > + | C2, t _j > | C2, t _{j + 1} > is the first photon detector (C1) of Charlie 140 142, or the second photon detector (C2) 143, shows the state of a photon (idler) when photons are detected continuously at times t _j and t _{j + 1} . Also, | A2, t _{j + 1} > _s1 | B2, t _{j + 1} > _s2 + | A1, t _{j + 1} > _s1 | B1, t _{j + 1} > _s2 is the second photon detector on the Alice 111 side. (A2) When a photon (signal) is detected at 119, a photon is detected by the second photon detector (B2) 139 on the Bob 130 side, and conversely, the first photon detector (A1 on the Alice 110 side) ) 118 indicates that a photon is detected by the first photon detector (B1) 130 on the Bob 130 side. That is, when Charlie 140 detects photons at successive times t _j and t _{j + 1} in the same photon detector (142 or 143), the photons observed by Alice 110 and Bob 130 at time t _{j + 1} It can be seen that there is a correlation as described above.

  Similarly, paying attention to the case where Charlie 140 continuously detects photons on one side of each of photon detectors (C1, C2) 142, 143, a result like the following formula (23) from the above formula (21): Is obtained.

That is, as shown by | A2, t _{j + 1} > _s1 | B1, t _{j + 1} > _s2 + | A1, t _{j + 1} > _s1 | B2, t _{j + 1} > _s2 , When photons are detected by the second photon detector (A2) 119, photons are detected by the first photon detector (B1) 138 on the Bob 130 side, and conversely, the first photon detector on the Alice 110 side. When a photon is detected at (A1) 118, a correlation is obtained such that a photon is detected by the second photon detector (B2) 139 on the Bob 130 side.

  In this way, the encryption key is generated between Alice 110 and Bob 130 by performing the following procedure shown in the flowchart of FIG.

  As shown in FIG. 3, Charlie 140 records, in recording unit 1401, the photon detection time at which photons are continuously detected and the information on which photon detector is the photon detector that has detected the photons (step 201). .

  Next, the Charlie 140 uses the transmitter 1402 to continuously detect the photon with any one of the photon detectors and the time, or with the fact that the photons are detected alternately with the two photon detectors and the time. Is communicated to Alice 110 and Bob 130 through the information transmission path 170 that has been prevented from being tampered with by the existing technology (step 202).

  On the other hand, Alice 110 and Bob 130 record in their storage units 1101 and 1301 the time when the photon was detected by their photon detector and the information on which photon detector the photon was detected by (step 203). ).

  Next, Alice 110 and Bob 130 transmit and receive the time at which photons could be detected through the information transmission path 170 (step 204).

  Alice 110 and Bob 130 are the events in which Charlie 130 continuously detects photons from the transmitted time information (the fact that either one of the photon detectors continuously detects photons, or two photon detectors). And the fact that both Alice 110 and Bob 130 detected the corresponding photon (information on which photon detector was used to detect the photon) as follows: A key bit is generated (step 205).

(A) When Charlie 140 detects photons continuously with only one photon detector,
Based on the correlation expressed by the above equation (22), when Alice 110 detects the photon by the first photon detector (A1) 118, the key bit is set to “0” and the photon is set to the second photon. When detected by the photon detector (A2) 119, the key bit is set to “1”. Similarly, Bob sets the key bit to “0” when the photon is detected by the first photon detector (B1) 138, and detects the photon by the second photon detector (B2) 139. The key bit is set to “1”.

(B) If Charlie 140 detects photons one by one with an alternating photon detector,
Based on the correlation expressed by the above equation (23), when Alice 110 detects the photon by the first photon detector (A1) 118, the key bit is set to “0” and the photon is set to the second photon. When detected by the photon detector (A2) 119, the key bit is set to “1”. On the other hand, Bob 130 sets the key bit to “1” when the photon is detected by the first photon detector (B1) 138, and detects the photon by the second photon detector (B2) 139. The key bit is set to “0”.

  In this way, Alice 110 and Bob 130 generate the key bits, so that the key bits of Alice 110 and Bob 130 are the same from the correlation shown in Equation (22) and Equation (23) above. It turns out that it becomes.

  As noted above, Alice 110 and Bob 130 publish only the time when photons were detected and not which photon detector detected the photons. It can be seen that secret key bits can be shared between Alice 110 and Bob 130.

  In the present embodiment, data is encrypted from Charlie 140 to Alice 110 and Bob 130 by executing the above processing using the light sources 150 and 160 that generate two entanglement photon pairs. It is possible to supply information that is a source of creation of an encryption key for decryption and generate and share a secret key bit between Alice 110 and Bob 130.

  The flowchart of FIG. 4 shows the details of the procedure executed by Alice 110. The flowchart of FIG. 5 shows the details of the procedure executed by Bob 130. The procedure shown in FIG. 4 and FIG. 5 expresses the contents described in the items (a) and (b) described in step 205 above as a control flow by dividing them into Alice 110 and Bob 130. Since there is no particular change, detailed description thereof is omitted to avoid duplication of explanation.

(Second Embodiment)
In the first embodiment of the present invention described above, it is assumed that Charlie 140 can measure the arrival time of photons included in two consecutive pulses by the same photon detector 142 or 143. However, in reality, the repetition rate of detection by the photon detector is slower than the rate (transmission speed) of the optical pulse, and it may be possible that continuous photons cannot be detected by the same photon detector. In such a case, as will be described in detail below with reference to FIG. 6, two photon detectors are used for each photon included in two consecutive pulses, so that they are included in two consecutive pulses. It is possible to measure the arrival time of the photon to be detected more reliably in terms of probability.

  That is, in the second embodiment of the present invention, instead of the first photon detector (C1) 142 shown in FIG. 1, a 50:50 optical coupler 144 and two first photon detectors (C1) 142 shown in FIG. A one-photon detector (C1-1) 145 and a second photon detector (C1-2) 146 are installed and operated. Similarly, instead of the photon detector (C2) 143 shown in FIG. 1, as shown in FIG. 6, a 50:50 optical coupler 147 and two third photon detectors (C2-1) 148 and A fourth photon detector (C2-2) 149 is installed and operated.

  When Charlie 140 detects photons at successive times using the first photon detector (C1-1) 145 and the second photon detector (C1-2) 146 using the above-described setup, This corresponds to a case where photons are continuously detected by the single photon detector (C1) 142. Similarly, when Charlie 140 detects a photon at a continuous time with the third photon detector (C2-1) 145 and the fourth photon detector (C2-2) 146, the above-mentioned one photon detector ( C2) corresponds to the case where photons are continuously detected at 143.

  By configuring as described above, the above-described secret key generation method of the present invention can be realized even when a photon detector whose operation speed is slower than the rate of optical pulses is used.

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

For example, in the above embodiment, the first light source 150 and the second light source 160 use the photon pair generated at the same time t = t _j . However, for example, in the above-described expression (2), | t _j > _s1 , | t _j > _i1 , | t _j > _s2 , and | t _j > _i2 are set as shown in the following expression (24). Thus, using the signal photon (s1) and idler photon (i1) generated at time (tj) by the first light source 150, the signal photon (s2) and idler generated at time (tj ') by the second light source 160 are used. The same can be said for photons (i2).

  Therefore, photon pairs generated at different times by the two light sources 150 and 160 may be used. In addition, this makes it unnecessary for the optical path length between the first light source 150 and the third device (Charlie) 140 and the optical path length between the second light source 160 and the third device (Charlie) 140 to be equal. For the same reason, the optical path length between the first light source 150 and the first device 110 and the optical path length between the first light source 150 and the third device 240 do not have to be equal, and photons generated at the same time. It is only necessary to detect one photon of each pair. In addition, the relationship between the second light source, the second device, and the third device is the same, and the optical path length between the second light source 160 and the second device 130 and between the second light source 160 and the third device 240 are the same. It is not necessary to be equal to the optical path length, as long as one photon of a photon pair generated at the same time can be detected.

  There is a limitation on the arrangement (optical path length) of the device because the optical path difference between the long optical path and the short optical path of the interferometer 111 (131) is a predetermined amount at which the light source 150 (160) generates a photon pair. It is only a distance corresponding to the time interval.

  In addition, the present invention supplies a software program (in this embodiment, a program corresponding to the flowcharts shown in FIGS. 3 to 5) for realizing the functions of the above-described embodiment directly or remotely to a system or apparatus. And the case where the computer of the system or apparatus is also achieved by reading and executing the supplied program. In that case, as long as it has the function of a program, the form does not need to be a program.

  Accordingly, since the functions of the present invention are implemented by computer, the program code installed in the computer also implements the present invention. That is, the claims of the present invention include the computer program itself for realizing the functional processing of the present invention.

It is a block diagram which mainly shows the structure of the optical system of the quantum cryptography communication system in the 1st Embodiment of this invention. It is a block diagram which shows the system configuration | structure of the quantum cryptography communication system in the 1st Embodiment of this invention. It is a flowchart which shows the whole procedure of the secret key generation method in the 1st Embodiment of this invention. It is a flowchart which shows the procedure of the private key generation | occurrence | production which Alice (1st apparatus) performs in the 1st Embodiment of this invention. It is a flowchart which shows the procedure of the private key generation | occurrence | production which Bob (2nd apparatus) performs in the 1st Embodiment of this invention. It is a block diagram which shows the structure of the quantum cryptography communication system in the 2nd Embodiment of this invention. It is a block diagram which shows the structure of the conventional differential phase shift quantum delivery system.

Explanation of symbols

110 Alice (first device)
111, 131 Interferometer 112, 132 Entrance side beam splitter 113, 114, 133, 134 Mirror 115, 135 Exit side beam splitter 116, 117, 136, 137 Output terminal 118, 138 First photon detector (A1, B1) )
119,139 Second photon detector (A2, B2)
120 Optical fiber transmission line 130 Bob (second device)
140 Charlie (third device)
141 Coupler 142 First Photon Detector (C1)
143 Second photon detector (C2)
144, 147 coupler
145 First photon detector (C1-1)
148 Second photon detector (C1-2)
148 Third Photon Detector (C2-1)
149 Fourth photon detector (C2-2)
150 First entanglement light source 160 Second entanglement light source 170 Information transmission path 610 Alice (first device)
611 Coherent light source 613 Phase modulator 615 Attenuator 620 Optical fiber 630 Bob (second device)
631 Interferometer 632 Entrance-side beam splitter 633, 634 Mirror 635 Exit-side beam splitter 636, 637 Output terminal 638 First photon detector (B1)
639 Second photon detector (B2)
1101, 1301 Storage unit 1102, 1302 Transmission / reception unit 1103, 1303 Encryption key generation unit 1401 Recording unit 1402 Transmission unit 1501, 1601 Pump light source 1502, 1602 Optical nonlinear medium

Claims (5)

A quantum cryptography communication apparatus to share a quantum cryptographic key with the first equipment and the second equipment by the quantum key distribution system using entangled photon pairs,
A third device connected as a relay node at an intermediate position between the first device and the second device;
A first light source connected to an intermediate position between the first device and the third device;
The second device and a second light source connected to an intermediate position between the third device, and the first light source, the second light source in synchronization with one pulse per mean photon as the light pulse of predetermined intervals A quantum entangled photon pair is generated at a predetermined time interval with a number less than 1 and one photon of the generated entangled photon pair is sent to the first device and the other one photon is sent to the third device. Having means ,
The second light source generates quantum entangled photon pairs at a predetermined time interval with an average number of photons per pulse as a light pulse at regular intervals in synchronization with the first light source, and the generated entangled photon pair Means for sending one of the photons to the second device and the other one to the third device;
The third device includes an optical multiplexer that detects each photon transmitted from the first light source and the second light source, and a combined photon connected to each of both output terminals of the optical multiplexer. The two photon detector groups for detecting the photon and either one of the photon detector groups successively detect the fact that the photon is detected and the time, or both the photon detection groups are alternately continuous. And a transmission means for transmitting the timing when the photon is detected and its time to the first device and the second device ,
Wherein the first device is an interferometer having said optical path difference corresponding to a predetermined time interval and two output paths, and a photon detector installed in each of the output path, the photon detector detects the photon Sent from the second device, storage means for storing information about the photon time and which photon detector detected the photon, transmission means for sending the photon detection time to the second device, and Receiving means for receiving information and information sent from the third device, and photon detection information sent from the third device and the corresponding photons are received by both the first device and the second device. A cipher generation means for generating a key bit of a shared quantum encryption key to be a secret key using the detected event ,
The second device is an interferometer having said optical path difference corresponding to a predetermined time interval and two output paths, and a photon detector installed in each of the output paths, the photon detector photons Storage means for storing information about the detected time and which photon detector has detected the photon, transmitting means for transmitting the time at which the photon was detected to the first device, and sent from the first device Receiving means for receiving incoming information and information sent from the third device, photon detection information sent from the third device, and photons corresponding thereto, both of the first device and the second device. And a cipher generation means for generating a key bit of a shared quantum cryptography key that becomes a secret key using the event detected by . 2. The quantum cryptography communication device according to claim 1 , wherein each of the first light source and the second light source includes a pump light source and an optical nonlinear medium, and generates a photon pair by an optical parametric process. The interferometers included in the first device and the second device each include an optical demultiplexer having an input terminal to which the photons are input and two output terminals,
A pair of optical paths consisting of a long optical path and a short optical path connected to the two output terminals of the optical demultiplexer;
An optical multiplexer having an input terminal and two output terminals connected to the pair of optical paths;
3. The quantum cryptography communication device according to claim 1, wherein an optical path difference between the long optical path and the short optical path is equal to a distance in which light propagates at the predetermined time interval. A method for generating, under computer control, the quantum encryption key that serves as a secret key for encrypting and decrypting data using the quantum cryptography communication device according to any one of claims 1 to 3. ,
A first step in which the first light source and the second light source are synchronized to supply photons to the first device, the second device, and a third device;
The fact that the third device continuously detects photons in any one of the photon detector groups and the time, or the fact that photons are detected alternately in the two photon detector groups and the time, A second step of communicating with the device and the second device;
The first device and the second device respectively record the time when the photon was detected by each photon detector and the information on which photon was detected by the photon detector. A third step of transmitting and receiving;
Both the first device and the second device have detected an event that the third device has detected a photon from the transmitted time information, and the corresponding photon has been detected by the first device and the second device. And a fourth step of generating a key bit of the quantum cryptography key using an event. A secret key generation method using a quantum cryptography communication device. In the first device, the fourth step includes:
Determining whether the third device continuously detects photons by only one of the photon detector groups; and
When the third device detects photons continuously with only one of the photon detector groups and the photon is detected with the first photon detector of the first device, the key bit is set to “0”. When the photon is detected by the second photon detector of the first device, the key bit is set to “1”;
When the third device detects photons alternately by two photon detector groups and the photon is detected by the first photon detector of the first device, the key bit is set to “0”, If a photon is detected by the second photon detector of the first device, the key bit is set to “1”, and
Furthermore, the fourth step includes the second device,
Determining whether the third device continuously detects photons by only one of the photon detector groups; and
When the third device detects photons continuously by only one of the photon detector groups and the photon is detected by the first photon detector of the second device, the key bit is set to “0”. When the photon is detected by the second photon detector of the second device, the key bit is set to “1”;
When the third device detects photons alternately by two photon detector groups, when the photon is detected by the first photon detector of the second device, the key bit is set to “1”, 5. The secret key generation method according to claim 4 , further comprising a step of setting a key bit to “0” when a photon is detected by a second photon detector of the second device.

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