JP5571832B1 - Transmitter / receiver in quantum cryptography communication - Google Patents

Abstract

Kind Code: A1 To provide a means for improving the security of quantum cryptography by minimizing the amount of information that can be leaked outside from a quantum cryptography transmitting / receiving device.
A transmission device includes a signal generation unit that generates transmission signal light of quantum cryptography communication, and a control unit that controls whether light can be transmitted from the inside of the signal generation unit to the outside or vice versa. The signal generation unit includes a modulation unit that modulates the input pulsed light, and the control unit determines that the state of the modulation unit in a predetermined time period including the time at which the modulation unit modulates the input pulsed light is outside the signal generation unit. The light transmission is controlled so as not to be measured. The receiving device includes a signal receiving unit that receives the transmission signal light of the quantum cryptography communication, and a control unit that controls whether light can be transmitted from the inside of the signal receiving unit to the outside or vice versa. Controls whether or not light can be transmitted so that the light generated when receiving the transmission signal light at the signal receiving unit does not transmit in a predetermined time interval including the time when the light reaches the control unit.
[Selection] Figure 3

Description

  The present invention relates to improving the safety of a transmission / reception apparatus in quantum cryptography communication using an optical shutter.

Quantum cryptography is a general term for protocols that use quantum communication to supply random and uncorrelated bit sequences between senders and receivers, and various types of protocols have been proposed and implemented (DARPA). / SECOQC / Tokyo QKD Network). In many of these protocols, if the equipment is not incomplete, it can be unconditionally safe (even a third party who can perform all operations permitted by the laws of physics). Guaranteed). The security of quantum cryptography is as follows: An eavesdropper with the following capabilities: Any operation can be performed on the transmitted / received light.
・ The internal state of the sender and receiver (states generated using random numbers, observation results, etc.) is unknown.
This is proved when it is assumed that the measurement of the sender / receiver is an idealized device. However, when actually trying to implement these protocols, the possibility of numerous defects has been pointed out from the deviation from the ideal model. As one of the problems, it has been pointed out that the state of the sender / receiver can be observed from the outside, that is, the peep from outside can be observed.

  As a method for dealing with such a defect, for example, in Non-Patent Document 1, extraneous light is measured by measuring the intensity of light from the outside with an additional measuring device and further assuming an attack method from the outside. The upper limit of the amount of information that may leak to the outside is estimated, and when the estimated amount of information is large, no key is generated.

Christian Kurtsiefer, Patrick Zarda, Sonja Mayer & Harald Weinfurter, "The breakdown flash of silicon avalanche photodiodes-back door for evesdropper attacks," J. Mod. Opt, Vol. 48, Issue13, pp.2039-2047, 2001. A. Vakhitov, V. Makarov and D. R. Hjelme, "Large pulse attack as a method of conventional optical eavesdropping in quantum cryptography." J. Mod. Opt., Vol. 48,, pp. 2023-2038 (2001). C.H.Bennett, G. Brassard, "Quantum Cryptography: Public Key Distribution and Coin Tossing", Proceedings of IEEE International Conference on Computers Systems and Signal Processing, Bangalore India, pp 175-179, December 1984. A. K. Ekert, "Quantum cryptography based on Bell ’s theorem", Phys. Rev. Lett. 67, 661-663, 1991. C. H. Bennett, "Quantum cryptography using any two nonorthogonal states", Phys. Rev. Lett. 68, 3121-3124, 1992. K. Inoue, E. Waks, and Y. Yamamoto, "Differential-phase-shift quantum key distribution using coherent light", Phys. Rev. A. 68, 022317, 2003. A. Acin, N. Brunner, N. Gisin, S. Massar, S. Pironio, and V. Scarani, "Device-Independent Security of Quantum Cryptography against Collective Attacks", Phys. Rev. Lett. Vol. 98, 230501, 2007.

  With the method of Non-Patent Document 1, it is possible to estimate the amount of information that may leak outside, but it is not possible to prevent the information itself from leaking outside.

  The present invention provides a transmission / reception apparatus in which the amount of information leaked to the outside is extremely small.

  This is characterized in that it provides means for solving the above-described problem of peeping from the outside.

  Since the sender / receiver must transmit or receive signal light, the transmitter / receiver always has an input / output port. However, the input / output port is also a promising input / output port when an eavesdropper attempts to observe the internal state. However, the timing at which the input / output port must be opened for transmission and reception is not necessarily the same as the timing at which the eavesdropper can observe the internal state if the input / output port is opened. The present invention solves the peeping problem by designing a transceiver apparatus or the like so that these two timings are different and allowing light to pass through the window only in the former case.

  Here, the “internal state” refers only to an internal state that changes depending on the type of transmitted signal light or the type of received observation result.

It is the schematic which shows an example of the transmitter in the conventional quantum cryptography communication. It is a conceptual diagram which shows the principle of the transmitter of this invention. It is the schematic which shows an example of the transmitter in the quantum cryptography communication of this invention. It is the schematic which shows an example of the receiver in the conventional quantum cryptography communication. It is a conceptual diagram which shows the principle of the receiver of this invention. It is the schematic which shows an example of the receiver in the quantum cryptography communication of this invention.

[Opening and closing the window at the sender]
[Conventional example]
An example of a transmission apparatus in conventional quantum cryptography communication is shown in FIG.

  1 has a laser light source 1 that generates pulsed light, a beam splitter (BS1) 2 that splits pulsed light at 50:50, mirrors 3a and 3b, and an optical path difference of time difference τ. Two arms 4a and 4b, a phase modulator PM5 that modulates the phase of the light of the longer arm 4a, and a beam splitter (BS2) 6 that multiplexes the light from the two arms at 50:50 It has.

  The laser light source 1 emits pulse light having a relatively large intensity, not a single photon level. This pulsed light is branched by a 50:50 beam splitter (BS1) 2. The light that has passed through the shorter arm 4b of the interferometer is output as reference light from the beam splitter (BS1) 2. On the other hand, the light that has passed through the longer arm 4a of the interferometer is randomly phase-modulated by the phase modulator PM5 to one of the binary values {0, δ} with a probability of 1/2, and is output from the beam splitter (BS2) 6. Output as phase-modulated light. This phase-modulated light is delayed with respect to the reference light by a time difference τ according to the optical path difference between the two arms of the interferometer. As a result, two consecutive reference lights and phase-modulated lights having the same intensity and the same frequency are generated. Here, the light intensity and the phase modulation value δ are set in advance at the transmitter end so that the signal state spread due to quantum noise (quantum fluctuation) partially overlaps.

  The reference light and the phase-modulated light generated as described above are output to the outside of the transmission device through the output port 7.

[Principle of the present invention]
The internal state of the sender can be observed externally by mainly irradiating light (peepsing light) from the window and reflecting light reflected by the phase modulator in the transmitter coming out of the window. (Non-Patent Document 1, Non-Patent Document 2). When the reflected light is analyzed, the state of the phase modulator can be analyzed. Since the type of the signal light output from the transmission device is determined based on the degree of modulation by the phase modulator, the signal light output from the transmission device can be estimated by analyzing the state of the phase modulator.

  However, in order to estimate the signal light output from the transmission device from the information obtained by peeking at the phase modulator, the state of the phase modulator at the time when the phase modulator modulates the signal light is determined. It is necessary to observe.

  Light from the outside for observing the state of the phase modulator (hereinafter referred to as “seeing light”) is input from an output port (hereinafter referred to as “window”). That is, if the window is opened at the time when the modulation degree of the phase modulator is changed according to the type of signal light to be output, the state of the phase modulator can be viewed from the outside. In other words, if the window is closed at this time, information regarding the type of signal output from the transmission device will not be leaked even if it is viewed from the outside.

  FIG. 2 shows a conceptual diagram. For simplicity, it is assumed that the laser light source, the phase modulator, and the window are arranged one-dimensionally, the position of the laser light source is L0, the position coordinate of the phase modulator is L1, and the position coordinate of the window is L2. Further, assuming that the speed of light corresponds to a 45 ° oblique line in FIG. 2, time t1 when signal light is output from the laser light source, time t2 when signal light modulated from the phase modulator is output, and phase modulation The time t3 when the signal light output from the detector reaches the window, the time t0 when the peeping light from the outside reaches the window, and the time t3 when the peeping light from the outside reaches the phase modulator are illustrated. .

  In general, there is a physical distance between the laser light source, the phase modulator, and the window. Therefore, the time t2 when the signal light output from the laser light source reaches the phase modulator and the time t3 when the signal light reaches the window are not equal to each other.

  Similarly, the time t0 when the peeping light reaches the window and the time t2 when the peeking light reaches the phase modulator are not equal to each other. In order to obtain information on the signal light from the outside, the peeping light needs to reach the phase modulator at time t2. Therefore, the peeping light must reach the window at time t0 before that.

  On the other hand, the time t3 at which the signal light output from the phase modulator reaches the window is a time later than the time t0. Therefore, even if the window is closed at the time t0, transmission of the signal light is not hindered. .

  In other words, if the window is opened and closed such that the window is closed at time t0 and the window is opened at time t3, extra information regarding the type of signal output by the transmission device even if it is viewed from the outside. It is possible to realize a transmission device that does not leak.

[Example 1]
FIG. 3 shows a configuration according to this embodiment of the transmission apparatus of the present invention configured based on the above principle.

  The transmission apparatus according to this embodiment includes a signal generation unit 8, an intensity modulator 9, and a control unit 10.

  The signal generator 8 has the same configuration as that of the conventional transmission device for quantum cryptography communication shown in FIG. Compared with the conventional transmission apparatus, the transmission apparatus of the present embodiment has an intensity modulator 9 that changes the intensity of the signal light output from the signal generation section 8 outside the signal generation section 8 and transmits the intensity light in the intensity modulator. The difference is that the control unit 10 controls the intensity of the signal light. This intensity modulator serves as the window that can be opened and closed. Conventional quantum cryptography transmitters are not provided with an intensity modulator for purposes other than intensity modulation of signal light at this position.

  The intensity modulator transmits the signal light (reference light and phase modulation light) output from the signal generation unit with the transmittance input from the control unit and outputs the signal light.

  The control unit controls the transmittance of the intensity modulator, and outputs a control signal for controlling the transmittance of the intensity modulator to the intensity modulator.

  Changing the transmittance of the intensity modulator corresponds to opening and closing the window. If the transmittance is increased, the input / output port is opened, and the reference light and the phase-modulated light output from the signal generation device are output to the transmission path. On the other hand, if the transmittance is lowered, the input / output port is closed, and the reference light and the phase-modulated light output from the signal generation device are not leaked to the outside.

  When the transmittance is high, light from the outside is also transmitted, so that it can be said that the state of the phase modulator inside the transmission device is easily peeped from the outside. Conversely, when the transmittance is low, light from the outside is not transmitted, so that the state of the phase modulator inside the transmission device is not observed from the outside.

  The control unit includes the time t0 shown in the conceptual diagram of FIG. 2 and has a low transmittance of the intensity modulator during a predetermined time interval not including the time t3, includes the time t3, and includes the time t0. The transmittance is controlled so that the transmittance of the intensity modulator is high during a predetermined time interval in a range not including.

Since the physical distance between the laser light source, the phase modulator, and the intensity modulator does not change, the time difference between times t1 and t0 and the time difference between times t1 and t3 are fixed. The difference T _1,0 = t1-t0 between the time t1 when the light is output from the laser light source and the time t0, and the difference T _3,1 = t3-t1 between the time t1 and the time t3 when the light is output from the laser light source are: It can be obtained in advance as follows.
T _1,0 = T _3,2 -T _2,1
T _3,1 = T _3,2 + T _2,1
Here, T _2,1 is a value obtained by dividing the distance between the laser light source and the phase modulator by the speed of light, that is, the time required for the signal light output from the laser light source to reach the phase modulator. . T _{3 and 2} are values obtained by dividing the distance between the phase modulator and the intensity modulator by the speed of light, that is, until the signal light output from the phase modulator reaches the intensity modulator (window). It takes time.

In many cases, the protocol is designed so that the signal light of the transmission device is transmitted periodically at a predetermined time. That is, the time t1 is given in advance by design. Therefore, the control unit has a low transmittance of the intensity modulator during a predetermined time interval that includes the time t1−T ₁ , ₀ and does not include the time t1 + T ₃ , ₁ and the time t1 + T _{3 , 1} and the transmittance may be controlled so that the transmittance of the intensity modulator is high during a predetermined time interval in a range not including time t1- _T1,0 .

As described above, the time t1 is given by design, and the distance between the laser light source, the phase modulator, and the intensity modulator is fixed. Therefore, the time t2 and the time t3 are automatically obtained from the time t1. Therefore, the control unit may control the transmittance of the intensity modulator based on time t2 or time t3 instead of time t1. That is, the transmittance of the intensity modulator is low during a predetermined time interval that includes the time t2-T _3,2 and does not include the time t1 + T _3,2 , and includes the time t2 + T _3,2 . In addition, the transmittance may be controlled so that the transmittance of the intensity modulator is high during a predetermined time interval that does not include the time t2-T3, ₂ . Alternatively, wherein the time of the time t3-2 _{· T 3,2,} and, during a predetermined time interval ranges which do not include the time t3 has a low transmittance of the intensity modulator includes a time t3, and time t3- The transmittance may be controlled so that the transmittance of the intensity modulator is high during a predetermined time interval that does not include 2 · T ₃ , ₂ .

  Since the signal light of the transmission device is normally transmitted periodically, the signal light is light so that the time when the signal light transmitted one time slot before reaching the window does not become the same as the above-described time t0. And the physical positions of the laser light source, the phase modulator, and the intensity modulator are designed.

[Modification]
In the conventional example and the first embodiment, for the sake of simplicity, the example of the signal generation device including one phase modulator has been described. However, the configuration of the transmission device is not limited to that illustrated in FIG. Different. Depending on the protocol, the type of signal light may be changed using a device other than the phase modulator, or a device may be provided that changes a plurality of types of signal light.

  The above-described phase modulator is an example of “a space (place) where optical characteristics change”. In other words, it is the spatial position of the “modulator that modulates signal light”. As long as it modulates signal light, it may be a phase modulator, an intensity modulator, or another modulator. However, the modulator here is a modulator for generating signal light, that is, a modulator inside the signal generation unit. Peeping in the transmission device is performed by applying strong light from the outside to the “space (place) where the optical characteristics change” and analyzing the reflected light. "Space changing (place)", and light from the outside at the time when the signal light is modulated in the space for all "space (place) where the optical characteristics change" in the signal generator It is the essence of the present invention that the window is closed so that the space is not reached.

  The intensity modulator that functions as a window may be another mechanism as long as it can optically switch between transmitting and not transmitting light from the inside of the transmitting apparatus to the outside and from the outside to the inside of the transmitting apparatus. Absent.

  Known protocols for quantum cryptography include BB84 (Non-Patent Document 3), E91 (Non-Patent Document 4), B92 (Non-Patent Document 5), DPSQKD (Non-Patent Document 6), and the like.

[Opening and closing the window at the receiver]
[Conventional example]
A configuration example of a conventional receiving apparatus is shown in FIG.

  4 has a beam splitter (BS4) 12 that splits light input from the transmission device through the input port 11 at a ratio of 50:50, mirrors 13a and 13b, and has an optical path difference of a time difference τ and interference. The two arms 14a and 14b constituting the meter, the phase modulator 15 that modulates the phase of the light of the longer arm 14a, and the beam splitter (BS3) 16 that multiplexes the light from the two arms at 50:50 And photon detectors (A) 17 and (B) 18 for detecting light from the beam splitter BS1 at respective ports.

  Two consecutive reference beams and phase-modulated beams from the transmitter reach the input port 11 of the receiver via the transmission path. The reference light and the phase-modulated light input to the receiving device are branched by a 50:50 beam splitter (BS4) 12. The light that has passed through the longer arm 14a of the interferometer out of the branched reference light is subjected to a delay of time difference τ, and is randomly random with a probability ½ to one of binary values {0, −δ} by the phase modulator. Phase modulated. On the other hand, of the branched phase modulated light, the light that has passed through the short arm 14b of the interferometer is directly incident on the beam splitter (BS3) 16, and the light from the two arms of the interferometer is converted into the 50:50 beam splitter (BS3). ) 16 is combined.

  After transmitting the reference light from the transmitter, the phase-modulated light is transmitted with a time difference τ. First, reference light is incident on the beam splitter (BS4) 12 of the receiving apparatus, and then phase-modulated light is incident. The reference light is branched by the beam splitter (BS4) 12 and passes through the long arm 14a and the short arm 14b of the interferometer. The subsequent phase-modulated light is also branched by the beam splitter (BS4) 12 and passes through the long arm 14a and the short arm 14b of the interferometer. In this way, when two consecutive pulses (reference light and phase-modulated light having a delay of time difference τ with respect to the reference light) are branched and combined via the delay optical path in the receiving device, three times are obtained. Light is detected in the slot.

  In the first slot, the reference light that has passed through the short arm 14b is detected as light that has passed through the beam splitter (BS3) 16. In the second slot having a time difference τ from the first slot, light obtained by combining the reference light passing through the long arm 14a and the phase-modulated light passing through the short arm 14b by the beam splitter (BS3) 16 is detected. The In the third slot having a time difference τ from the second slot, the light in which the phase-modulated light passing through the long arm 14a passes through the beam splitter (BS3) 16 is detected. Here, the data in the first (first slot) and last (third slot) slots are discarded, and the detection result in the second slot where the reference light and the phase-modulated light interfere with each other is used as the received signal.

[Principle of the present invention]
The observation result of the signal received by the receiving device is observed to the outside by light leaking outside from the input port (window) generated when observing the signal light with the photon detector (non-patent) Reference 7).

  The light generated when the photon detector observes the signal light has a high correlation with the observation result. By observing this light from the outside, the information obtained by the photon detector should be estimated. Can do. In other words, if the window is closed at the time when the light generated when observing the signal light with the photon detector reaches the window, this light will not leak to the outside, so avoid being peeped. be able to.

A conceptual diagram is shown in FIG. For simplicity, it is assumed that the photon detector A and the window are arranged one-dimensionally, the position of the photon detector A is L3, and the position coordinates of the window are L4. Also, assuming that the speed of light corresponds to a 45-degree oblique line in FIG. 5, the time t4 when the signal light output from the transmission device reaches the window, the time t5 _A when the signal light reaches the photon detector _A , The time t6 _A when the light generated when observing the signal light with the photon detector A reaches the window is illustrated.

In general, there is a physical distance between the photon detector and the window. Therefore, the time t4 when the signal light of the transmission device reaches the window is not equal to the time t6 _A when the light generated when observing the signal light with the photon detector reaches the window. Therefore, though at the time t4 window opens, so that the window at time t6 _A is closed state, by opening or closing the window, without interfering with the reception of the signal light, the problem that is peeped outside Can be solved.

In the conceptual diagram of FIG. 5, only one of the photon detectors (photon detector A) constituting the receiving device is illustrated and described, but in practice, all the photon detectors in the receiving device are illustrated. On the other hand, the time at which the same window should be closed is controlled. In other words, so that the light generated upon the observed signal light with photon detector B is in a state of closed the time t6 window _B, to reach the window, open and close the window.

[Example 2]
FIG. 6 shows a configuration diagram of a receiving apparatus according to the second embodiment. The receiving apparatus of this embodiment includes a signal receiving unit 19, an intensity modulator 20, and a control unit 21.

  The signal receiver 19 has the same configuration as that of the conventional receiver for quantum cryptography communication shown in FIG. Compared with the conventional receiving apparatus, the receiving apparatus of this embodiment controls the intensity at the intensity modulator 20 that changes the intensity of the signal light input from the transmitting apparatus outside the signal receiving unit 8 and the intensity at the intensity modulator. The point which is provided with the control part 21 to perform is different. This intensity modulator plays the role of the above-described openable / closable window, and the conventional quantum cryptography receiver does not include an intensity modulator for purposes other than the intensity modulation of the signal light at this position.

  The intensity modulator transmits the signal light (reference light and phase-modulated light) input from the transmission device with the transmittance input from the control unit.

  The control unit controls the transmittance of the intensity modulator, and outputs a control signal for controlling the transmittance of the intensity modulator to the intensity modulator.

  Changing the transmittance of the intensity modulator corresponds to opening and closing the window. If the transmittance is increased, the input / output port is opened, and the light generated when observing the signal light with the photon detector of the signal receiver leaks out of the window and can be seen by a third party. Increases nature. On the other hand, if the transmittance is lowered, the input / output port is closed, and the light generated when the signal light is observed by the photon detector of the signal receiving unit is not leaked from the window.

The control unit is shown in the conceptual diagram of FIG.
(1) a time t4, and, between the range of the predetermined time interval which does not include the time t6 _A and the time t6 _B has a high transmissivity of the intensity modulator,
(2) a predetermined time interval in a range including time t6 _A and not including time t4; and
(3) a predetermined time interval in a range including time t6 _B and not including time t4;
During this period, the transmittance is controlled so that the transmittance of the intensity modulator is lowered.

  Signal light is periodically transmitted from the transmission device. Since the physical distance between the transmission device and the reception device is fixed, the time t4 is specified by the transmission period of the signal light of the transmission device. That is, it is given in advance by design.

Since the physical distance between the photon detector and an intensity modulator does not change, and the time difference T _A time t4 and time t6 _A, the time difference T _B at time t4 and the time t6 _B is fixed. Time T _A is a value obtained by dividing twice the speed of light in the distance between the photon detector A and an intensity modulator. Time T _B is a value obtained by dividing the speed of light to twice the distance between the photon detectors B and an intensity modulator.

Therefore, the control unit, each time t4, wherein the time t4, and the time t4 + T _A and the time t4 + T between the time the range of the predetermined time interval that does not include the _B has a high transmissivity of the intensity modulator, a time t4 + T _A It includes, and, as between the range of the predetermined time interval which does not include the time t4, wherein the time t4 + T _B, and, during a predetermined time interval ranges which do not include a time t4 so that the transmittance of the intensity modulator is lowered In addition, the transmittance is controlled.

Since the signal light from the transmission device is normally periodically input to the reception device, the time at which the signal light input after one time slot reaches the window is the same as the time t6 _A or t6 _B described above. It is assumed that the physical positions of the photon detectors A and B and the intensity modulator are designed so that the time does not come and the signal light emits light (the period when the signal light is received).

[Modification]
The above photon detector is an example of “space (location) where optical information may leak” by observing the received signal, and the received signal is observed using equipment other than the photon detector. In this case, the device becomes a “space (place) where optical information may be leaked”. The point of the present invention is that all of the “space (place) where optical information may leak” exists in the receiving apparatus, and the “space (place) where optical information may leak”. The light transmittance is controlled by an intensity modulator installed at the input / output port position so that the window is closed at the time when the light emitted when observing the received signal reaches the window. .

  Further, the configuration of the signal receiving unit of the receiving device is not limited to the above-described example, and differs depending on the quantum cryptography protocol and the like. Depending on the protocol, there may be provided two or more photon detectors or “a space (place) where optical information may leak” other than the photon detector. The present invention can be applied to such a configuration based on the same concept.

  As described above, a method for preventing active peeping is shown for the transmitting device, and a method for preventing passive peeping is shown for the receiving device, but this may be important in the opposite case. This is because, for example, when the base of observation is selected by changing the optical characteristics in the receiving device, it is necessary to prevent active peeping on the receiver side as well. Furthermore, there may be a case where a photon detector or a “space (place) where optical information may leak” other than the photon detector is provided inside the signal generation unit of the transmission device. Need to prevent passive peeping on the sender side in addition to active peeping. Also in these cases, the present embodiment can be obviously applied.

  Specifically, there may be a configuration in which a signal modulator is further provided with a phase modulator and an intensity modulator, and the received signal is observed after optically changing the observation base and intensity. In this case, in addition to “space (place) where optical information may leak”, the concept of “space (place) where optical characteristics change” shown in the transmission apparatus of the first embodiment is received. This is also applied to the device, and the transmittance for closing the window is prevented so that the peeping light from the outside does not reach the space at the time when the optical property changes in the “space (place) where the optical property changes”. Control is also performed.

1 Laser light source 2 Beam splitter (BS1)
3a, 3b, 13a, 13b Mirrors 4a, 4b, 14a, 14b Arms 5, 15 Phase modulator PM
6 Beam splitter (BS2)
7 Output port 8 Signal generation unit 9 Intensity modulator 10 Control unit 11 Input port 12 Beam splitter (BS4)
16 Beam splitter (BS3)
17 Photon detector (A)
18 Photon detector (B)
19 signal receiver 20 intensity modulator 21 controller

Claims (11)

A signal generator for generating transmission signal light of quantum cryptography communication;
A control unit that controls whether light can be transmitted from the inside of the signal generation unit to the outside or from the outside to the inside;
Including
The signal generation unit includes a modulation unit that modulates input pulsed light,
The control unit controls whether or not light can be transmitted so that the state of the modulation unit is not measured from outside the signal generation unit in a predetermined time interval including a time at which the modulated light is modulated by the modulation unit. A transmission apparatus characterized by the above. A signal generator for generating transmission signal light of quantum cryptography communication;
A window part through which light passes from the inside of the signal receiving unit to the outside, or from the outside to the inside;
Output from the signal generation unit so that the transmission signal light generated by the signal generation unit reaches the window unit so that the light is transmitted through the window unit and does not transmit light at a time other than the arrival time. A control unit for controlling whether or not light can be transmitted;
A transmission apparatus comprising: A signal generator for generating transmission signal light of quantum cryptography communication;
A window part through which light passes from the inside to the outside of the signal generator, or from the outside to the inside;
A control unit that controls whether or not light can pass through the window;
Including
The signal generation unit includes a modulation unit that modulates pulsed light,
The time pulse light in which the modulation part has output takes to reach the window portion and T _1,
When the time when the transmission signal light reaches the window portion is t1,
Wherein, the light is not transmitted at time t1-2 · T _1, so as to transmit light at time t1, transmission apparatus and controls whether the light transmission.   The transmission apparatus according to claim 3, wherein the window unit is an intensity modulator whose light transmittance changes, and the control unit controls the transmittance of the intensity modulator. A signal receiving unit that receives a transmission signal light of quantum cryptography communication; and
A control unit that controls whether light can be transmitted from the inside of the signal receiving unit to the outside, or from the outside to the inside;
Including
The control unit controls whether or not light can be transmitted so that light generated when the signal receiving unit receives transmission signal light does not transmit light in a predetermined time interval including a time when the signal reaches the control unit. A receiving apparatus. A signal receiving unit that receives a transmission signal light of quantum cryptography communication; and
A window part through which light passes from the inside of the signal receiving unit to the outside, or from the outside to the inside;
At the time when the light generated when the signal receiving unit receives the transmission signal light reaches the window portion, the light is transmitted through the window portion, and the light is not transmitted at a time other than the arrival time. A control unit for controlling whether or not the light output from the signal receiving unit is transmitted through the window unit;
A receiving apparatus comprising: A signal receiving unit that receives a transmission signal light of quantum cryptography communication; and
A window part through which light passes from the inside of the signal receiving unit to the outside, or from the outside to the inside;
A control unit that controls whether or not light can pass through the window;
Including
The signal receiving unit includes an observer that generates light when receiving the transmission signal light,
T ₂ is a time required for the transmission signal light input from the window to reach the observer,
When the time when the transmission signal light reaches the window portion is t2,
Wherein, the light is not transmitted at time t2 + ₂ · T 2, so as to transmit light at time t2, the receiving apparatus characterized by controlling the availability of light transmission.   The receiving apparatus according to claim 7, wherein the window unit is an intensity modulator whose light transmittance changes, and the control unit controls the transmittance of the intensity modulator. The signal receiving unit further includes a modulation unit that modulates the transmission signal light,
When the time required for the transmission signal light input from the window to reach the modulation unit is T ₃ , the control unit further prevents the light from transmitting at time t2-2 · T ₃ . The receiving apparatus according to claim 7, wherein whether or not light can be transmitted is controlled. A method of transmitting a transmission signal light of quantum cryptography communication by a transmission device,
After the pulse light is modulated in the transmission device, the time required for the modulated pulse light to be emitted out of the transmission device through the window is T ₄ ,
When the time when the pulsed light reaches the window portion is t4,
The transmitting device at time t4-2 · T ₄ does not emit light, to emit light at time t4, wherein the controller controls the window. A method of receiving a transmission signal light of quantum cryptography communication by a receiving device,
T ₅ is a time required for the transmitted signal light input to the receiving device through the window to reach the observer,
When the time when the transmission signal light reaches the window portion is t5,
Wherein, the light is not transmitted at time t5 + 2 · T _5, so that the light is transmitted at time t5, wherein the controlling the propriety of light transmission.

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