JP6060737B2 - Quantum key distribution receiver - Google Patents

Description

  The present invention relates to a quantum key distribution receiver for receiving a photon for generating an encryption key, which is used in a quantum key distribution system using a entangled photon pair.

  In order to realize secure encrypted communication without information leakage, the key used for encryption and decryption of information is shared only by the sender and receiver without being known to a third party such as an eavesdropper. It is essential.

  The quantum key distribution system is attracting attention as an encryption key distribution system that guarantees the ultimate unconditional security in accordance with the laws of physics, and research and development aimed at application to future high-security information communication systems has become active. (For example, refer to Patent Document 1 or 2).

  Here, a quantum key distribution system using the polarization state of photons will be briefly described.

  In the quantum key distribution system using the polarization state of photons, the sender can select, for example, longitudinal (V) polarized light, lateral (H) polarized light, right oblique 45 degree polarized light, and left oblique 45 degree polarized light as the photon polarization state. Select one at random and send it. A case in which longitudinally polarized light or laterally polarized light is selected and transmitted is referred to as an H / V transmission base, and a case in which right oblique 45 degree polarized light or left oblique 45 degree polarized light is selected and transmitted is referred to as a diagonal transmission base.

The receiver can be an H / V receiver base that measures whether the transmitted photons are longitudinally or laterally polarized,
A diagonal reception base for measuring whether the polarization is 45 ° right oblique polarization or 45 ° oblique left polarization is selected at random, and the polarization of the incoming photon is measured. This measurement system can be realized by an optical system using a polarizing beam splitter and a wave plate.

  Here, the H / V base and the diagonal base are non-orthogonal. For this reason, if longitudinally polarized light or laterally polarized light is measured on the H / V reception base, a deterministic measurement result can be obtained as to whether it is longitudinally polarized light or laterally polarized light. However, if it is measured on the diagonal reception base, it can be known only with a probability of 50% whether it is longitudinally polarized light or transversely polarized light, and a definite result cannot be obtained.

  For example, if the sender sends a photon on the H / V transmission basis and the receiver selects and measures the H / V reception basis, the receiver can know the polarization state of the photon transmitted by the sender in a definite manner. . At this time, for example, if the vertical polarization state is bit “1” and the horizontal polarization state is bit “0”, the sender and the receiver can share a random bit string.

  On the other hand, if the sender sends a photon on the H / V transmission base and the receiver selects and measures the diagonal reception base, the receiver can know only the polarization state of the photon transmitted by the sender only probabilistically. At this time, the sender and receiver cannot share a random bit string.

  Using this mechanism, the sender / receiver gets the measurement result of the required number of bits, and then teaches the bases selected from each other, and uses only the bit value when the bases match, so that the sender / receiver is the same Random bit strings can be shared. The quantum key distribution system uses this bit string as an encryption key.

  Here, in order not to allow wiretapping of the encryption key, it is necessary to use one or less photons for each bit. For this reason, in order to realize a quantum key distribution system using photons, a single photon detector capable of detecting single photon level photons is required. As this single photon detector, a semiconductor avalanche photodiode, a detector using a superconductor, and the like have been reported.

  As a quantum key distribution receiver using such a single photon detector, there are known what is called active modulation and what is called passive modulation.

  With reference to FIG. 5, an active modulation type receiver and a passive modulation type receiver will be described. FIG. 5A is a schematic diagram of an active modulation type receiver. FIG. 5B is a schematic diagram of a passive modulation type receiver.

  The active modulation type receiver 14 uses a polarization modulator 50 and a random number generator 52 to randomly select a reception base. As the polarization modulator 50, a half-wave plate with a variable optical axis is used. The polarization modulator 50 switches between a state in which the two optical axes are matched with the longitudinal polarization direction and the transverse polarization direction and a state in which the two optical axes are rotated by 22.5 degrees in accordance with the random number generated by the random number generator 52. .

  The polarization beam splitter 30 is disposed after the polarization modulator 50, and the first single photon detector 41 and the second single photon detector 42 are disposed at the two output ends thereof.

  In the active modulation type receiver 14, the state in which the optical axis of the polarization modulator 50 matches the longitudinal polarization direction and the lateral polarization direction corresponds to the selection of the H / V reception base, which is 22.5 degrees. The rotated state corresponds to the selection of the diagonal reception base.

  The passive modulation type receiver 16 uses a half mirror 20 instead of the polarization modulator and the random number generator. Since the number of incoming photons is basically one, the photons input to the first input / output end 20a of the half mirror 20 are output only to either the second input / output end 20b or the third input / output end 20c. Not.

  Photons output from the third input / output terminal 20 c of the half mirror 20 are sent to the first polarization beam splitter 31. The case where a photon reaches the first polarization beam splitter 31 corresponds to the selection of the H / V reception base. The first polarization beam splitter 31 sends vertically polarized light to the first single photon detector 41 and sends horizontally polarized light to the second single photon detector 42.

  On the other hand, photons output from the second input / output terminal 20 b of the half mirror 20 are sent to the second polarization beam splitter 32 via the polarization converter 22. In the polarization converter 22, light having 45 ° right-polarized light and 45 ° left-polarized light is converted into vertically polarized light or horizontally polarized light. The case where a photon reaches the second polarization beam splitter 32 corresponds to the selection of the diagonal reception base. The second polarizing beam splitter 32 sends the vertically polarized photons to the third single photon detector 43 and sends the horizontally polarized photons to the fourth single photon detector 44.

JP 2012-049890 A JP 2012-004955 A

  In the case of an active modulation type receiver, two single photon detectors are required, which is half of the passive modulation type receiver, but a random number generator is required. Since the eavesdropping becomes possible if the random number information of the random number generator is known to a third party, this absolute secrecy is necessary. Moreover, if the same random number is used repeatedly, it becomes possible for an eavesdropper to estimate the reception base, and the possibility of eavesdropping arises. Using a more complex random number generator will increase the cost and size of the system.

  On the other hand, in the case of a passive modulation type receiver, since it does not have a random number generator, a simpler device configuration is possible and confidentiality is maintained. However, since four single photon detectors are required, the cost and size of the system are increased.

  The present invention has been made in view of the above-described problems. An object of the present invention is to provide a quantum key distribution receiver that does not require a random number generator and operates with two single photon detectors.

  In order to achieve the above-described object, a quantum key distribution receiver according to the present invention includes a half mirror having first to third input / output ends, a polarization converter, and first to third input / output ends. The first polarized light, which is linearly polarized light having a predetermined polarization direction, input to the first input / output terminal is output from the second input / output terminal, and the orthogonally polarized light having the polarization direction orthogonal to the predetermined polarized light is output to the first input / output terminal. 1 to 1 which output from the first input / output terminal the specified polarized light input to the second input / output terminal and the orthogonally polarized light input to the third input / output terminal. 4 polarization beam splitters, and first and second single photon detectors.

The input photons are either the first and second paths from the half mirror to the first single photon detector, or the third and fourth paths from the half mirror to the second single photon detector. either through one of the paths, Ru is detected in one of the 1-2 single photon detector. There is a delay time difference of T / n (n is an integer of 2 or more) between photons passing through the first path and the second path with respect to the period T of photons arriving at the quantum key distribution receiver. Given. Further, a delay time difference of T / n is given between photons passing through the third path and the fourth path.

  According to the quantum key distribution receiver of the present invention, no random number generator is required, and two single photon detectors operate. For this reason, it is possible to reduce the size and cost of the system.

It is a schematic block diagram of a 1st receiver. It is a schematic diagram for demonstrating operation | movement of a 1st receiver. It is a schematic block diagram of a 2nd receiver. It is a schematic diagram for demonstrating operation | movement of a 2nd receiver. It is a schematic block diagram of the conventional receiver.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

  In this example, the prescribed polarization light that is linearly polarized light in the prescribed polarization direction is referred to as longitudinal (V) polarization, and the orthogonal polarization light in the polarization direction orthogonal to the V polarization that is the prescribed polarization light is described as transverse (H) polarization. To do.

(Configuration of first receiver)
With reference to FIG. 1, a first embodiment (hereinafter also referred to as a first receiver) of a quantum key distribution receiver according to the present invention will be described. FIG. 1 is a schematic configuration diagram of a first receiver.

  The first receiver 10 includes a half mirror 20, a polarization converter 22, first to fourth polarization beam splitters 31 to 34, an optical delay circuit 24, a first single photon detector 41, and a second single photon. A detector 42 is provided.

  The half mirror 20 includes first to third input / output ends 20a to 20c. The light input to the first input end 20a of the half mirror 20 is output from one of the second input / output end 20b and the third input / output end 20c.

  Each of the first to fourth polarizing beam splitters 31 to 34 includes first to third input / output terminals. The V-polarized light input to the first input / output ends 31a to 34a is output from the second input / output ends 31b to 34b. Also, the H-polarized light input to the first input / output terminals 31a to 34a is output from the third input / output terminals 31c to 34c. This input / output characteristic is reversible. That is, the V-polarized light input to the second input / output ends 31b to 34b is output from the first input / output ends 31a to 34a, and the H-polarized light input to the third input / output ends 31c to 34c. Are output from the first input / output terminals 31a to 34a.

  The second input / output end 20 b of the half mirror 20 is connected to the first input / output end 34 a of the fourth polarization beam splitter 34. The third input / output end 20 c of the half mirror 20 is connected to the first input / output end 31 a of the first polarization beam splitter 31 via the polarization converter 22 and the optical delay circuit 24.

  The second input / output end 31b of the first polarizing beam splitter 31, the second input / output end 32b of the second polarizing beam splitter 32, the third input / output end 31c of the first polarizing beam splitter 31, and the third A third input / output end 33c of the second polarization beam splitter 33, a third input / output end 32c of the second polarization beam splitter 32, a third input / output end 34c of the fourth polarization beam splitter 34, and a third The second input / output end 33b of the polarizing beam splitter 33 and the second input / output end 34b of the fourth polarizing beam splitter 34 are connected to each other. The first input / output end 32a of the second polarization beam splitter 32 is connected to the first single photon detector 41, and the first input / output end 33a of the third polarization beam splitter 33 is the second input. A single photon detector 42 is connected.

  As the polarization converter 22, a half-wave plate in which two optical axes are rotated 22.5 degrees from H-polarized light and V-polarized light can be used. The half-wave plate converts right oblique 45 degree polarized light into V polarized light and left oblique 45 degree polarized light into H polarized light.

  In addition, when using right-handed circularly polarized light and left-handed circularly polarized light instead of right oblique 45 degree polarized light and left oblique 45 degree polarized light, a quarter wavelength plate can be used as the polarization converter 22. In this case, the optical axis of the quarter wavelength plate is set so as to convert clockwise circularly polarized light into longitudinally polarized light and counterclockwise circularly polarized light into laterally polarized light.

  Both the light output from the second input / output end 20b of the half mirror 20 and the light output from the third input / output end 20c of the half mirror 20 are input to the first single photon detector 41. Is done. The second single photon detector 42 also has both the light output from the second input / output end 20b of the half mirror 20 and the light output from the third input / output end 20c of the half mirror 20. Is entered. An optical delay circuit 24 is used to give a predetermined time difference between the lights output from the second input / output terminal 20b and the third input / output terminal 20c of the half mirror 20. If a predetermined time difference is given due to the difference in the length of the optical path, the optical delay circuit 24 may be omitted.

(Operation of the first receiver)
The operation of the first receiver will be described with reference to FIGS. FIG. 2 is a diagram for explaining the operation of the first receiver, with the horizontal axis taking time.

  Here, an example will be described in which one of photons of V polarization, H polarization, right oblique 45 degree polarization, and left oblique 45 degree polarization arrives at the receiver in the period T. Here, it is assumed that a time slot of period T / 2 is set, and photons arrive at the receiver in even-numbered time slots (FIG. 2A).

  Photons input to the first input / output terminal 20a of the half mirror 20 are output from either one of the second and third input / output terminals 20b and 20c in a probability manner. Here, when a photon is output from the second input / output terminal 20b, this corresponds to the selection of the H / V base as the reception base. On the other hand, when a photon is output from the third input / output terminal 20c, this corresponds to the selection of the diagonal base as the reception base.

  The V-polarized light output from the second input / output end 20b of the half mirror 20 serves as the third path (III), the first input / output end 34a and the second input / output end of the fourth polarization beam splitter 34. 34 b and the second input / output end 33 b and the first input / output end 33 a of the third polarization beam splitter 33, and then input to the second single photon detector 42. On the other hand, the H-polarized light output from the second input / output end 20b of the half mirror 20 serves as the first path (I) and the first input / output end 34a and the third input / output end of the fourth polarization beam splitter 34. The light is input to the first single photon detector 41 through the output end 34 c, the third input / output end 32 c and the first input / output end 32 a of the second polarization beam splitter 32.

  The right oblique 45 degree polarized light and the left oblique 45 degree polarized light output from the second input / output terminal 20b of the half mirror 20 are probabilistically separated by the fourth polarization beam splitter 34, and the first and first Two single photon detectors 41 and 42 are input. When a photon is detected by the first single photon detector 41, it is observed that a V-polarized photon has arrived at the first receiver 10, and a photon is detected by the second single photon detector 42. In this case, it is observed that H-polarized photons have arrived at the first receiver 10.

  The 45 ° right-polarized light output from the third input / output terminal 20c of the half mirror 20 is converted into V-polarized light by the polarization converter 22 as the second path (II), and then the first polarizing beam splitter. 31 through the first input / output end 31a and the second input / output end 32b, the second input / output end 32b of the second polarization beam splitter 32, and the first input / output end 32a. Input to the detector 41. On the other hand, the 45 ° left-polarized light output from the third input / output terminal 20c of the half mirror 20 is converted into H-polarized light by the polarization converter 22 as the fourth path (IV), and then the first polarized light. The first input / output end 31a and the third input / output end 31c of the beam splitter 31 and the third input / output end 33c and the first input / output end 33a of the third polarizing beam splitter 33 are passed through the second input / output end 33a. This is input to the one-photon detector 42.

  Further, the V-polarized light and H-polarized light output from the third input / output terminal 20 c of the half mirror 20 are converted into right oblique 45 ° polarized light and left oblique 45 ° polarized light by the polarization converter 22. Thereafter, the light is stochastically separated by the first polarization beam splitter 31 and input to one of the first and second single photon detectors. When the first single photon detector 41 detects a photon, it is observed that a 45-degree right-polarized photon has arrived at the first receiver 10, and the second single photon detector 42 detects the photon. Is detected, it is observed that left-handed 45-degree polarized photons have arrived at the first receiver 10.

  Here, the optical path length from the first input / output end 31a of the first polarizing beam splitter 31 to the input to the first and second single photon detectors 41 and 42, and the fourth polarizing beam splitter. It is assumed that the optical path lengths from the first input / output end 34a of 34 to the first and second single photon detectors 41 and 42 are all set equal. Further, photons passing through the path from the second input / output end 20b of the half mirror 20 to the first input / output end 34a of the fourth polarization beam splitter 34, and the first input / output end 20c of the half mirror 20 to the first A delay time difference of T / 2 is given as a predetermined delay time difference to the photons passing through the path to the first input / output end 31a of the polarizing beam splitter 31.

  As a result, for photons detected by the first single photon detector 41, a delay of T / 2 between the photons passing through the first path (I) and the photons passing through the second path (II). A time difference is given. Similarly, for photons detected by the second single photon detector 42, a delay of T / 2 between the photons passing through the third path (III) and the photons passing through the fourth path (IV). A time difference is given.

  Thus, for example, the photons detected by the first single photon detector 41 in the even-numbered time slot are photons passing through the first path (I), and the detection of this photon is the H / V reception. This corresponds to selecting a base and observing an H-polarized photon (FIG. 2B). At this time, in the even-numbered time slot, the photon detected by the second single photon detector 42 is a photon passing through the third path (III), and the detection of this photon is performed by the H / V reception basis. Corresponds to observing V-polarized photons (FIG. 2D).

  Also, in the odd-numbered time slot, the photon detected by the first single photon detector 41 is a photon passing through the second path (II), and the detection of this photon selects the diagonal reception base. This corresponds to the observation of 45-degree polarized photons on the right (FIG. 2C). Similarly, in the odd-numbered time slot, the photon detected by the second single-photon detector 42 is a photon passing through the fourth path (IV), and the detection of this photon selects the diagonal reception base. This corresponds to the observation of 45-degree polarized photons on the left (FIG. 2E).

(Configuration of second receiver)
With reference to FIG. 3, a second embodiment of the quantum key distribution receiver according to the present invention (hereinafter also referred to as a second receiver) will be described. FIG. 3 is a schematic configuration diagram of the second receiver.

  The second receiver 12 includes a half mirror 20, a polarization converter 22, first to fourth polarization beam splitters 31 to 34, a first single photon detector 41, and a second single photon detector 42. Configured.

  The second receiver 12 is different in connection relationship between the first receiver 10 and each component, and each component can be configured in the same manner as the first receiver 10. Therefore, the overlapping description about each component is abbreviate | omitted.

  The second input / output end 20 b of the half mirror 20 is connected to the first input / output end 34 a of the fourth polarization beam splitter 34. The third input / output end 20 c of the half mirror 20 is connected to the first input / output end 31 a of the first polarization beam splitter 31 via the polarization converter 22.

  The second input / output end 31b of the first polarization beam splitter 31, the second input / output end 33b of the third polarization beam splitter 33, the third input / output end 31c of the first polarization beam splitter 31, and the third The third input / output end 33c of the polarization beam splitter 33, the third input / output end 32c of the second polarization beam splitter 32, the third input / output end 34c of the fourth polarization beam splitter 34, and the second The second input / output end 32b of the polarizing beam splitter 32 and the second input / output end 34b of the fourth polarizing beam splitter 34 are connected to each other. The first input / output end 32a of the second polarization beam splitter 32 is connected to the first single photon detector 41, and the first input / output end 33a of the third polarization beam splitter 33 is the second input. A single photon detector 42 is connected.

  Of the light output from the second input / output terminal 20 b of the half mirror 20, both H-polarized light and V-polarized light are input to the first single photon detector 41. Here, the optical path connecting the second input / output end 32 b of the second polarization beam splitter 32 and the second input / output end 34 b of the fourth polarization beam splitter 34, and the third path of the second polarization beam splitter 32. There is an optical path length difference corresponding to a predetermined optical delay time difference between the input / output end 32 c and the optical path connecting the third input / output end 34 c of the fourth polarization beam splitter 34.

  The second single photon detector 42 receives both 45 ° right-polarized light and 45 ° left-polarized light out of the light output from the third input / output terminal of the half mirror 20. Here, the optical path connecting the second input / output end 31 b of the first polarization beam splitter 31 and the second input / output end 33 b of the third polarization beam splitter 33, and the third path of the first polarization beam splitter 31. There is an optical path length difference corresponding to a predetermined optical delay time difference between the input / output end 31 c and the optical path connecting the third input / output end 33 c of the third polarization beam splitter 33.

(Operation of the second receiver)
The operation of the second receiver will be described with reference to FIGS. 3 and 4. FIG. 4 is a diagram for explaining the operation of the second receiver, with the horizontal axis taking time.

  Here, an example will be described in which one of photons of V polarization, H polarization, right oblique 45 degree polarization, and left oblique 45 degree polarization arrives at the receiver in the period T. Here, it is assumed that a time slot having a period T / 2 is set, and photons arrive at the receiver in even-numbered time slots (FIG. 4A).

  Photons input to the first input / output terminal 20a of the half mirror 20 are output from either one of the second and third input / output terminals 20b and 20c in a probability manner. Here, when a photon is output from the second input / output terminal 20b, this corresponds to selection of the H / V base as the reception base. On the other hand, when a photon is output from the third input / output terminal 20c, this corresponds to the selection of the diagonal base as the reception base.

  The V-polarized light output from the second input / output end 20b of the half mirror 20 serves as the second path (II), the first input / output end 34a and the second input / output end of the fourth polarization beam splitter 34. 34 b and the second input / output end 32 b and the first input / output end 32 a of the second polarization beam splitter 32, and then input to the first single photon detector 41. On the other hand, the H-polarized light output from the second input / output end 20b of the half mirror 20 serves as the first path (I) and the first input / output end 34a and the third input / output end of the fourth polarization beam splitter 34. The light is input to the first single photon detector 41 through the output end 34 c, the third input / output end 32 c and the first input / output end 32 a of the second polarization beam splitter 32.

  The right oblique 45 degree polarized light and the left oblique 45 degree polarized light output from the second input / output terminal 20b of the half mirror 20 are probabilistically separated by the fourth polarization beam splitter 34, and the first and first The signal is input to the first single photon detector 41 via one of the two paths (I and II).

  The 45 ° right oblique polarization output from the third input / output terminal 20c of the half mirror 20 is converted into V polarization by the polarization converter 22 as the fourth path (IV), and then the first polarization beam splitter. 31 through the first input / output end 31a and the second input / output end 32b, the second input / output end 33b and the first input / output end 32a of the third polarization beam splitter 33, and the second single photon. Input to the detector 42. On the other hand, the 45 ° left-polarized light output from the third input / output terminal 20c of the half mirror 20 is converted into H-polarized light by the polarization converter 22 as the third path (III), and then the first polarized light. The first input / output end 31a and the third input / output end 31c of the beam splitter 31 and the third input / output end 33c and the first input / output end 33a of the third polarizing beam splitter 33 are passed through the second input / output end 33a. This is input to the one-photon detector 42.

  Further, the V-polarized light and H-polarized light output from the third input / output terminal 20 c of the half mirror 20 are converted into right oblique 45 ° polarized light and left oblique 45 ° polarized light by the polarization converter 22. Thereafter, the light is stochastically separated by the first polarization beam splitter 31 and input to the second single-photon detector 42 via any one of the third and fourth paths (III and IV).

  Here, a delay time of T / 2 is given as a predetermined delay time difference between the photons passing through the first path (I) and the photons passing through the second path (II), and the third path (III) A delay time of T / 2 is also given as a predetermined delay time difference between the photon passing through and the photon passing through the fourth path (IV).

  As a result, for photons detected by the first single photon detector 41, a delay of T / 2 between the photons passing through the first path (I) and the photons passing through the second path (II). A time difference is given. Similarly, for photons detected by the second single photon detector 42, a delay of T / 2 between the photons passing through the third path (III) and the photons passing through the fourth path (IV). A time difference is given.

  Therefore, detection of photons by the first single photon detector 41 in even-numbered time slots corresponds to selecting an H / V reception basis and observing H-polarized photons (FIG. 4B )). Similarly, detection of photons by the second single photon detector 42 in even-numbered time slots corresponds to selecting a diagonal reception base and observing photons with 45 ° polarization to the left (FIG. 4). (D)).

  In addition, detection of photons by the first single photon detector 41 in the odd-numbered time slot corresponds to selecting an H / V reception base and observing V-polarized photons (FIG. 4C )). Similarly, detection of a photon by the second single photon detector 42 in an odd-numbered time slot corresponds to selecting a diagonal reception base and observing a photon with 45 ° right polarization (FIG. 4). (E)).

  According to the first and second receivers described above, the single key photon detector operates with two units, which does not require a random number generator and can be reduced in size and cost. Sharing is possible.

  In the first and second receivers described above, a time difference of T / 2 is given. In this case, each single photon detector can detect photons in both the even and odd time slots. On the other hand, if the time difference to be given is T / n and n is set to an integer larger than 2, a time slot in which no photons are detected can be provided. Therefore, the time difference to be given is preferably T / n (n is an integer of 2 or more).

  As for the polarization state of the photon, any one of longitudinally polarized light, laterally polarized light, right oblique 45 degree polarized light, and left oblique 45 degree polarized light may be used instead of right oblique 45 degree polarized light and left oblique 45 degree polarized light. Whether to use polarized light and counterclockwise circularly polarized light may be appropriately selected according to the setting. The receiver is preferably composed of a polarization maintaining optical system, but may be a pseudo polarization maintaining optical system by appropriately providing a polarization controller. Whether optical coupling uses a spatial optical system, an optical waveguide such as an optical fiber, or the combination of these may be selected as appropriate according to the setting.

10, 12, 14, 16 Receiver
20 half mirror
22 Polarization converter
24 Optical delay circuit 30, 31, 32, 33, 34 Polarization beam splitter 41, 42, 43, 44 Single photon detector 50 Polarization modulator 52 Random number generator

Claims (3)

Half mirror,
A polarization converter;
First to fourth polarizing beam splitters;
Ru and first and second single-photon detector to a quantum key distribution for a receiver,
The input photons include first and second paths respectively passing through the two polarization beam splitters from the half mirror to the first single photon detector, and the second single from the half mirror. Detected by one of the first and second single photon detectors via one of the third and fourth paths, each passing through two polarization beam splitters, to the photon detector,
For the period T of photons arriving at the quantum key distribution receiver,
A delay time difference of T / n (n is an integer of 2 or more) is given between photons passing through the first path and the second path,
Wherein the third path and between the photons through the fourth path, Erareru delay time difference T / n is given
Quantum key distribution for the receiver which is characterized a call.
The half mirror includes first to third input / output ends,
Each of the first to fourth polarizing beam splitters has first to third input / output ends, and receives the second predetermined polarized light, which is linearly polarized light having a predetermined polarization direction, input to the first input / output end. Output from the input / output terminal of the first, and the orthogonally polarized light in the polarization direction orthogonal to the specified polarized light is output from the third input / output terminal, and the specified polarized light input to the second input / output terminal, Outputting the orthogonally polarized light input to the third input / output terminal from the first input / output terminal;
A second input / output end of the half mirror is connected to a first input / output end of the fourth polarizing beam splitter;
The third input / output end of the half mirror is connected to the first input / output end of the first polarization beam splitter via the polarization converter,
The second input / output end of the first polarization beam splitter, the second input / output end of the second polarization beam splitter, the third input / output end of the first polarization beam splitter, and the third polarization A third input / output end of the beam splitter, a third input / output end of the second polarization beam splitter, a third input / output end of the fourth polarization beam splitter, and a third input / output end of the third polarization beam splitter. A second input / output terminal and a second input / output terminal of the fourth polarizing beam splitter are connected to each other;
A first input / output end of the second polarizing beam splitter is connected to the first single photon detector;
A first input / output end of the third polarizing beam splitter is connected to the second single photon detector;
The optical path length from the first input / output end of the first polarizing beam splitter to the input to the first or second single photon detector, and the first input of the fourth polarizing beam splitter. The optical path lengths from the output end to the first and second single photon detectors are all equal,
The optical path length from the second input / output end of the half mirror to the first input / output end of the fourth polarization beam splitter, and from the third input / output end of the half mirror to the first polarization beam splitter. 2. The quantum key distribution receiver according to claim 1, wherein there is an optical path length difference corresponding to a delay time difference of T / n between the optical path length to the first input / output terminal. The half mirror includes first to third input / output ends,
Each of the first to fourth polarizing beam splitters has first to third input / output ends, and receives the second predetermined polarized light, which is linearly polarized light having a predetermined polarization direction, input to the first input / output end. Output from the input / output terminal of the first, and the orthogonally polarized light in the polarization direction orthogonal to the specified polarized light is output from the third input / output terminal, and the specified polarized light input to the second input / output terminal, Outputting the orthogonally polarized light input to the third input / output terminal from the first input / output terminal;
A second input / output end of the half mirror is connected to a first input / output end of the fourth polarizing beam splitter;
The third input / output end of the half mirror is connected to the first input / output end of the first polarization beam splitter via the polarization converter,
The second input / output end of the first polarization beam splitter, the second input / output end of the third polarization beam splitter, the third input / output end of the first polarization beam splitter, and the third polarization. A third input / output end of the beam splitter, a third input / output end of the second polarization beam splitter, a third input / output end of the fourth polarization beam splitter, and a second input / output end of the second polarization beam splitter. A second input / output terminal and a second input / output terminal of the fourth polarizing beam splitter are connected to each other;
A first input / output end of the second polarizing beam splitter is connected to the first single photon detector;
A first input / output terminal of the third polarizing beam splitter is connected to the second single photon detector;
From the first input / output end of the first polarizing beam splitter, through the third input / output end of the first polarizing beam splitter and through the third input / output end of the third polarizing beam splitter, the second input / output end. From the first input / output end of the first polarization beam splitter to the second input / output end of the first polarization beam splitter, the third There is an optical path length difference corresponding to a delay time difference of T / n between the optical path length from the second input / output terminal of the polarizing beam splitter to the input to the second single photon detector.
From the first input / output end of the fourth polarizing beam splitter, through the third input / output end of the fourth polarizing beam splitter and the third input / output end of the second polarizing beam splitter, the first input / output end From the optical path length until it is input to the single photon detector and the first input / output end of the fourth polarization beam splitter, the second input / output end of the fourth polarization beam splitter, There is an optical path length difference corresponding to a delay time difference of T / n between the optical path length from the second input / output terminal of the polarization beam splitter to the input to the first single photon detector. The quantum key distribution receiver according to claim 1, wherein:

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