JP6910994B2 - Communication equipment, communication methods, programs and communication systems - Google Patents

Description

Embodiments of the present invention relate to communication devices, communication methods, programs and communication systems.

With the progress of information and communication technology, various data have come to be exchanged, and ensuring the confidentiality and safety of the information to be transmitted has become a major issue. Among them, quantum cryptography communication technology is expected to be put into practical use as a cryptographic technology that cannot be deciphered even if the computing power of a computer is improved.

Quantum cryptography communication devices include a type that encodes key information into the phase difference between two pulses generated by an interferometer (phase encoding type) and a type that encodes into the polarization of photons. As a phase-encoded quantum cryptography communication device, a device for improving the key generation speed by utilizing an optical path selection by polarization in an interferometer on the receiving device (receiver) side has been proposed. By correcting the rotation of polarized light generated by the temperature change and vibration of the communication path, the optical path selection can be performed stably.

UK Pat. No. 2399220 UK Pat. No. 2,404103

However, when correcting the rotation of polarized light, in the prior art, rapid correction of polarized light change (polarized light correction) when a large and fast disturbance occurs in the communication path due to strong wind or the like, and polarization in a steady state where no disturbance occurs. Stabilization of correction was not compatible.

The communication device of the embodiment includes an estimation unit and a determination unit. The estimation unit estimates the amount of polarization rotation of photons used in quantum cryptography communication. The determination unit is a condition used for estimating the amount of rotation of polarized light, and determines the observation condition regarding the observed amount of photons.

The functional block diagram of the communication system of 1st Embodiment. The functional block diagram of the photon transmitting part and the photon receiving part of 1st Embodiment. The functional block diagram of the control part of the 1st Embodiment. The flowchart of the control process in 1st Embodiment. FIG. 5 is a functional block diagram of a photon transmitting unit and a photon receiving unit of the first modification. The functional block diagram of the photon transmitter and the photon receiver of the second embodiment. The functional block diagram of the control part of the 2nd Embodiment. The flowchart of communication processing in 2nd Embodiment. The flowchart of the control process in 2nd Embodiment. The functional block diagram of the photon transmitting unit and the photon receiving unit of the third embodiment. The functional block diagram of the control part of the 3rd Embodiment. The flowchart of the control process in 3rd Embodiment. The hardware block diagram of the main part of the apparatus of 1st to 3rd Embodiment.

A preferred embodiment of the communication device according to the present invention will be described in detail below with reference to the accompanying drawings.

(First Embodiment)
In the prior art, the observation time of the signal for correcting the polarization is constant. Therefore, the rapid correction of the polarization change at the time of disturbance and the stabilization of the polarization correction at the steady state are not compatible. The communication system of the first embodiment is a phase-encoded quantum cryptography communication system in which a high key distribution speed can be obtained by optical path selection by polarization. In the first embodiment, when the rotation of polarized light is fast and large due to disturbance or the like, the observation time is shortened, that is, the control cycle is shortened. This makes it possible to achieve both rapid polarization correction during disturbance and stable polarization correction during steady operation. Further, by shortening the control cycle only during a disturbance, it is possible to reduce the burden on the device due to unnecessarily frequent control. The steady state means a state in which the rotation of polarized light does not occur, or a state in which the rotation of polarized light is within a predetermined range.

Next, a configuration example of the communication system of the first embodiment will be described. FIG. 1 is a block diagram showing a functional configuration example of the communication system of the first embodiment. As shown in FIG. 1, the communication system has a configuration in which a transmitting device 100 and a receiving device 200 (an example of a communication device) are connected via a quantum communication path 11 and a classical communication path 12.

First, the transmission device 100 will be described. The transmission device 100 includes a photon transmission unit 110, a key generation unit 120, and a storage unit 131.

The photon transmission unit 110 transmits photons used for quantum cryptography communication to the receiving device 200. For example, the photon transmission unit 110 transmits a photon indicating a quantum signal used for sharing an encryption key by a quantum key distribution (QKD) technique. Quantum key distribution is a technique for securely sharing an encryption key by using a single photon continuously transmitted between a transmitting device 100 and a receiving device 200 connected by an optical fiber. Details of the function of the photon transmitter 110 will be described later.

The key generation unit 120 controls the operation of the photon transmission unit 110 to generate an encryption key. For example, the key generation unit 120 executes a quantum key distribution shifting process, an error correction process, and an confidentiality enhancement process to generate an encryption key to be shared with the receiving device 200.

The storage unit 131 stores various information used in various processes executed by the transmission device 100. For example, the storage unit 131 stores the encryption key information generated by the confidentiality enhancement processing.

Next, the receiving device 200 will be described. The receiving device 200 includes a photon receiving unit 210, a key generating unit 220, and a storage unit 231.

The photon receiving unit 210 receives photons used for quantum cryptography communication from the transmitting device 100. For example, the photon receiving unit 210 receives a photon indicating a quantum signal used for sharing an encryption key by a quantum key distribution technique. Details of the function of the photon receiver 210 will be described later.

The key generation unit 220 controls the operation of the photon receiving unit 210 and generates an encryption key based on the received photon information. For example, the key generation unit 220 executes a quantum key distribution shifting process, an error correction process, and a confidentiality enhancement process to generate an encryption key to be shared with the transmission device 100.

The storage unit 231 stores various information used in various processes executed by the receiving device 200. For example, the storage unit 231 stores the encryption key information generated by the confidentiality enhancement processing.

The quantum communication path 11 is a photon transmission path (optical communication path). The quantum communication path 11 is not limited to the optical fiber and may be a free space. The classical communication path 12 is used, for example, for transmitting and receiving control information for synchronizing between the transmitting device 100 and the receiving device 200. The classical channel 12 usually uses an optical fiber different from that of the quantum channel 11. A standard single-mode optical fiber is often used for the quantum communication path 11 and the classical communication path 12, but a multimode optical fiber may be used.

Each of the above units (key generation unit 120, key generation unit 220) is realized by, for example, one or a plurality of processors. For example, each of the above parts may be realized by causing a processor such as a CPU (Central Processing Unit) to execute a program, that is, by software. Each of the above parts may be realized by a processor such as a dedicated IC (Integrated Circuit), that is, hardware. Each of the above parts may be realized by using software and hardware in combination. When a plurality of processors are used, each processor may realize one of each part, or may realize two or more of each part.

The storage units 131 and 231 can be composed of any commonly used storage medium such as a flash memory, a memory card, a RAM (Random Access Memory), an HDD (Hard Disk Drive), and an optical disk.

Next, the details of the photon transmitting unit 110 and the photon receiving unit 210 will be described. FIG. 2 is a block diagram showing a functional configuration example of the photon transmitting unit 110 and the photon receiving unit 210.

The photon transmitter 110 includes an unpolarized light source 101, a polarization beam splitter 102, a phase modulator 103, a combiner 104, and a delay circuit 105.

The unpolarized light source 101 is a light source that emits unpolarized light (photons) that becomes a quantum signal.

The polarization beam splitter 102 splits the light emitted from the unpolarized light source 101 into two lights having different polarizations. For example, the polarization beam splitter 102 sends a photon having a first polarization to a first optical path and a photon having a second polarization orthogonal to the first polarization to a second optical path. The first optical path is an optical path including the phase modulator 103. The second optical path is an optical path including the delay circuit 105.

The phase modulator 103 modulates the phase of the photon that has advanced to the first optical path. For example, the phase modulator 103 modulates the phase of a photon so as to have a phase difference randomly selected from four types of phase differences with respect to a photon traveling to the second optical path according to an instruction from the key generation unit 120. ..

The delay circuit 105 is a circuit for delaying a photon that has advanced to the second optical path to reach the combiner 104.

The combiner 104 combines and outputs photons that have traveled through the first optical path and the second optical path.

The polarizing beam splitter 102, the phase modulator 103, the combiner 104, and the delay circuit 105 correspond to an interferometer for encoding information into light.

The photon receiving unit 210 includes a polarization rotating device 201, a polarization beam splitter 202, a phase modulator 203, a delay circuit 204, a beam splitter 205, photon detectors 206 and 207, and a control unit 208. There is.

The polarization rotating device 201 corrects the rotation of polarized light generated in the quantum communication path 11. For example, the polarization rotation device 201 corrects the rotation of polarization according to the control signal from the control unit 208.

The polarization beam splitter 202 sends a photon having a first polarization to a third optical path and a photon having a second polarization to a fourth optical path. The third optical path is an optical path including the delay circuit 204. The fourth optical path is an optical path including the phase modulator 203.

The phase modulator 203 modulates the phase of the photon that has advanced to the fourth optical path. For example, the phase modulator 203 modulates the phase of a photon by a phase randomly selected from two phases according to an instruction from the key generation unit 220.

The delay circuit 204 is a circuit for delaying a photon that has traveled to the third optical path to reach the beam splitter 205.

The beam splitter 205 selects the optical path of the photon according to the result of interference between the photon that has traveled to the third optical path and the photon that has traveled to the fourth optical path. For example, the beam splitter 205 sends the interfering photons to the photon detector 206 and the non-interfering photons to the photon detector 207.

The polarizing beam splitter 202, the phase modulator 203, the delay circuit 204, and the beam splitter 205 correspond to an interferometer that changes light information into a readable state.

The photon detectors 206 and 207 detect the photon and read the state of light. The observation signal, which is the detection result of the photon detectors 206 and 207, is output to the control unit 208.

The control unit 208 outputs the information of the observation signal to the key generation unit 220, and further controls the correction of the rotation of the polarized light by the polarization rotating device 201. For example, the control unit 208 inputs observation signals from the photon detectors 206 and 207 and outputs them to the key generation unit 220. Furthermore, the amount of polarization rotation is estimated from the observed signal. The control unit 208 outputs the control conditions used for the rotation correction control to the polarization rotation device 201 according to the estimation result. The control unit 208 determines the observation conditions to be used for the next estimation from the observation signal.

FIG. 3 is a block diagram showing an example of a detailed functional configuration of the control unit 208. As shown in FIG. 3, the control unit 208 includes a polarization estimation unit 251, a polarization correction output unit 252, a condition determination unit 253, and a signal output unit 254.

The polarization estimation unit 251 estimates the amount of polarization rotation of photons used in quantum cryptography communication. For example, the polarization estimation unit 251 estimates the amount of polarization rotation based on the change in the observed amount of photons used for quantum cryptography communication (the number of photons observed for a certain period of time, etc.).

The polarization correction output unit 252 outputs, for example, an amount for correcting the rotation of polarized light (polarization correction amount) to the polarization rotation device 201 with reference to the amount of rotation of polarization.

The condition determination unit 253 determines the observation conditions related to the photon observable amount, which is the condition used for the estimation by the polarization estimation unit 251 according to the photon observable amount used for the quantum cryptography communication (the number of photons observed for a certain period of time, etc.). do. The observation conditions are, for example, the observation time for observing photons, the number of photon counts, and the like. The condition determination unit 253 reduces the observed amount of photons used for estimation by shortening the observation time or reducing the number of counts, for example, when the observed amount of photons fluctuates greatly. Reducing the observable amount of photons corresponds to shortening the time required for estimation, and finally shortening the control cycle of polarization correction. The photon observable used by the condition determination unit 253 does not have to be the same as the observable amount estimated by the polarization estimation unit 251.

The signal output unit 254 receives light information (observation signal) from the photon detectors 206 and 207 and outputs (transmits) it to the key generation unit 220.

The control unit 208 is realized by, for example, one or more processors. For example, the control unit 208 may be realized by causing a processor such as a CPU to execute a program, that is, by software. The control unit 208 may be realized by a processor such as an FPGA or a dedicated IC that enables high-speed control, that is, hardware. The control unit 208 may be realized by using software and hardware in combination. When a plurality of processors are used, each processor may realize one of each unit of the control unit 208, or may realize two or more of each unit.

Next, the control process by the communication system according to the first embodiment configured in this way will be described. The control process is a process of controlling the correction of the rotation of polarized light based on the observed amount of photons. The control process of the present embodiment includes, for example, a process (communication process) of transmitting and receiving photons used for sharing an encryption key by a quantum key distribution technique. FIG. 4 is a flowchart showing an example of the control process according to the first embodiment.

The unpolarized light source 101 emits photons (step S101). The polarization beam splitter 102 sends a photon having a first polarization to the first optical path and a photon having a second polarization orthogonal to the first polarization to the second optical path.

The phase modulator 103 encodes the phase of the photon that has advanced to the first optical path (step S102). For example, the phase modulator 103 modulates the phase of a photon traveling in the first optical path so as to have a phase difference randomly selected from four ways with respect to the photon traveling in the second optical path. The photon that has advanced to the second optical path is sent to the combiner 104 by the delay circuit 105 with a delay with respect to the photon that has advanced to the first optical path. The combiner 104 combines the photons that have traveled to the first optical path and the photons that have traveled to the second optical path.

The photons combined by the combiner 104 pass through the quantum communication path 11 and are transmitted to the photon receiving unit 210 (step S103). The polarization rotation device 201 of the photon receiving unit 210 corrects the rotation of the polarization generated in the quantum communication path 11 according to the control signal from the control unit 208 (step S104).

The polarization beam splitter 202 sends a photon having a first polarization to a third optical path and a photon having a second polarization to a fourth optical path. The phase modulator 203 decodes the phase of the photon that has advanced to the fourth optical path (step S105). For example, the phase modulator 203 modulates the phase of a photon advancing in the fourth optical path by a phase randomly selected from two ways. The photon that has advanced to the third optical path is sent to the beam splitter 205 at the same timing as the photon that has advanced to the fourth optical path by the delay circuit 204 having the same length as the delay circuit 105. As a result, the photon that has traveled to the third optical path and the photon that has traveled to the fourth optical path interfere with each other.

The beam splitter 205 selects the photon optical path according to the result of the interference. The photon detector 206 or 207 reads out light information (signal) by detecting photons (step S106). The signal output unit 254 of the control unit 208 receives light information from the photon detectors 206 and 207 and transmits it to the key generation unit 220.

The polarization estimation unit 251 of the control unit 208 uses photons for use in quantum cryptography communication as observation signals, observes the total number of photons detected by the photon detectors 206 and 207 for a certain period of time, and rotates the polarization from the detected total value. Estimate the amount (step S107). The polarization correction output unit 252 of the control unit 208 transmits a control signal for instructing the polarization correction amount for correcting the polarization rotation amount to the polarization rotation device 201.

The condition determination unit 253 of the control unit 208 uses photons for use in quantum cryptography communication as observation signals, observes the total number of photons detected by the photon detectors 206 and 207 for a certain period of time, and from the detected total value, the next As the observation condition used for the estimation of, for example, the next observation time is determined (step S108).

The processes from step S101 to step S108 are repeatedly executed. In the next cycle, the total number of photons observed during the observation time determined in step S108 is used to estimate the amount of polarization rotation (step S107).

Next, the details of the method of estimating the amount of polarization rotation by the polarization estimation unit 251 will be described.

The rotation of polarized light in the quantum communication path 11 is caused by, for example, a change in the refractive index inside the optical fiber due to the influence of vibration, temperature change, and vibration of the fictitious fiber due to wind.

When the polarization is not corrected, the ratio of the photons of the first polarization to the fourth optical path and the ratio of the photons of the second polarization to the third optical path increase in the polarization beam splitter 202. In that case, the timing at which the first polarized photon and the second polarized photon reach the beam splitter 205 is not simultaneous, and interference does not occur. As a result, appropriate information cannot be retrieved, and the encryption key generation rate eventually decreases. In order to prevent this, it is necessary to correct the polarization before entering the polarization beam splitter 202 of the photon receiving unit 210.

In order to correct the polarization, it is necessary to estimate the amount of rotation of polarization. The polarization estimation unit 251 extracts only the photons that have reached the beam splitter 205 at the same time as an observation signal, acquires this as an observation signal for a certain period of time, and uses the rate of decrease of the observed amount (number of photons) per unit time from the reference value. Estimate the amount of polarization rotation. The reference value is set from, for example, the observed amount when the polarized light measured in advance is not rotating, or the maximum value when the observed amount for a long period of time is measured. The rate of decrease with respect to the reference value has a relationship shown by the following equation (1) with respect to the amount of rotation of polarized light θ. The polarization estimation unit 251 calculates (estimates) the polarization rotation amount θ corresponding to the reduction rate according to the equation (1).
Rate of decrease from the reference value (%) = (1-0.5 × cosθ) × 100 ・ ・ ・ (1)

The method for estimating the amount of rotation of polarized light by the polarized light estimation unit 251 is not limited to this. For example, the polarization estimation unit 251 may set a step size proportional to the amount of decrease from the reference value and estimate the amount of polarization rotation which is a stepwise value.

Instead of the rate of decrease from the reference value, only the photons that did not reach the beam splitter 205 at the same time may be extracted as an observation signal, and the amount of increase of the observation signal from zero may be used for estimation. In this case, the amount of increase of the observed signal from zero has the relationship shown in the following equation (2) with respect to the reference value. The polarization estimation unit 251 calculates (estimates) the polarization rotation amount θ corresponding to the increase amount according to the equation (2).
Increase amount = reference value x 0.5 x sin θ ... (2)

The polarization correction output unit 252 corrects the rotation by transmitting a control signal for instructing the polarization correction amount for correcting the estimated polarization rotation amount to the polarization rotation device 201. The control unit 208 may use a feedback control method such as PID (Proportional Integral Differential) control.

The polarization estimation unit 251 is based on an output signal from a sensor that detects factors affecting the refractive index of the quantum communication path 11, such as a wind sensor installed near the optical fiber and an acceleration sensor attached to the optical fiber. The amount of rotation of polarized light may be estimated.

Next, the details of the method for determining the observation conditions used for estimation by the condition determination unit 253 will be described.

For example, the condition determination unit 253 extracts only photons that reach the beam splitter 205 at the same time as an observation signal, and shortens the observation time when the observation amount per fixed time falls below a threshold value (for example, 75% of the steady state). To decide.

Very few photons are used in the quantum signal, and the average number of photons is set to be 1 photon or less. Therefore, a certain observation time is required for the polarization estimation unit 251 to accurately estimate the amount of rotation of polarized light. In particular, when a pseudo single photon is generated using laser light, the average number of photons may be set to 0.5 photon or less. In order to estimate the amount of polarization rotation from the change in the number of photons, it is necessary to collect 1000 to 10000 photons. Since noise photons called dark counts may be included in the count, it is necessary to observe the photons for a longer time. Therefore, the observation time is set to, for example, 100 milliseconds. In this case, since the correction amount is output to the polarization control device after the observation time has elapsed, the polarization control cycle is 100 milliseconds or more.

On the other hand, for example, when an optical fiber installed between utility poles is used, the fiber may sway quickly and greatly due to the influence of a strong wind, so that the polarization control may not be in time and the influence of uncorrectable polarization rotation may be large. Specifically, there is a significant decrease in the encryption key generation rate. In such a case, even if the estimation accuracy of the polarization rotation amount is lowered, the polarization correction is made to follow the polarization rotation due to the wind by shortening the time for estimating the polarization rotation amount and shortening the polarization control cycle. be able to. According to the present embodiment, the condition determination unit 253 determines to shorten the observation time when it confirms a certain decrease in the observation signal. For example, by determining the observation time as short as 5 milliseconds, quick feedback correction is possible. It should be noted that shortening the observation time used for estimation is synonymous with reducing the number of counts of observation signals (photons) used for estimation.

In the case of a strong wind due to the approach of a low pressure system, the observed signal does not continue to decrease, but the observed amount fluctuates greatly depending on the strength of the wind, which lasts for tens of minutes to several tens of hours. Therefore, in order to distinguish it from a decrease due to a failure of a device other than the wind, the condition determination unit 253 sets the observation time when the standard deviation of the observed amount for a certain period is equal to or more than the threshold value (for example, 10% of the steady state). You may decide to shorten it. It should be noted that the rotation of polarized light due to vibrations other than wind and temperature changes can be distinguished because the changes are slower than those of strong winds.

(Modification example 1)
The polarization rotating device 201 may be installed on the photon transmitter side. FIG. 5 is a block diagram showing a functional configuration example of the photon transmitting unit 110b and the photon receiving unit 210b according to the modified example 1 configured in this way.

The photon transmission unit 110b of the first modification is different from the photon transmission unit 110 of the first embodiment in that the polarization rotating device 201 is further provided. The photon receiving unit 210b of the first modification is different from the photon receiving unit 210 of the first embodiment in that it does not include the polarization rotating device 201.

In this modification, the control unit 208 transmits a control signal to the polarization rotation device 201 in the photon transmission unit 110b, for example, via the classical communication path 12.

According to this modification, for example, a photon whose rotation has been corrected by the polarization rotating device 201 can be configured to pass through an attenuator that optimizes the intensity of the quantum signal. Therefore, it is possible to avoid the loss of photons due to the polarization rotating device 201. The modification of installing the polarization rotating device on the photon transmitting unit side can also be applied to each of the following embodiments.

(Modification 2)
In the second modification, a method of determining the initial value of the observation conditions used for estimation will be described according to the length (communication distance) of the quantum communication path 11. In this modification, the condition determination unit 253 determines the initial value of the observation condition based on the observed amount of photons observed for a certain period of time in the steady state.

Here, the count rate is defined as the bit count of photons per second. The photon emitted from the photon transmitter 110 must have an intensity of 1 photon or less in order to prevent eavesdropping. On the other hand, in the middle of the quantum communication path 11, the quantum state cannot be detected and amplified. Further, as the quantum communication path 11 is longer, the number of photons gradually decreases due to absorption and scattering by impurities in the optical fiber which is the quantum communication path 11. For example, when a general single-mode fiber in the communication wavelength band is used, there is an attenuation of 0.2 decibels per kilometer. Therefore, the distance dependence of the count rate when only the loss in the optical fiber is considered is expressed by the following equation (3).
Distance L count rate =
Count rate with zero distance x 10 ^ (-0.2 x L / 10) ... (3)

For example, if the quantum channel 11 is 10 kilometers and the observed signal count rate is 10 megabits per second, assuming that the only cause of attenuation is the loss in the fiber, the quantum channel 11 counts at 30 kilometers. Is expected to decay to 4 megabits per second.

As described above, it is necessary to collect a certain amount of photons in order to estimate the amount of polarization rotation from the change in the number of photons. From the above equation (3), the observation time T required to obtain a constant photon bit count C is expressed by the following equation (4).
Observation time = C / Count rate at zero distance x 10 ^ (0.2 x L / 10) ... (4)

For example, assuming that the required bit count is 10000, the condition determination unit 253 determines 1 millisecond when the quantum communication path 11 is 10 kilometers, and the quantum communication path 11 when the quantum communication path 11 is 30 kilometers. 2.5 milliseconds, which is 2.5 times 10 kilometers, is determined as the initial value of the observation time used for estimation.

In this way, the condition determination unit 253 indicates that the larger the observed amount of photons in the steady state (the shorter the distance of the quantum channel 11), the smaller the observed amount used for estimation is the initial value of the observation condition. To determine. According to the second modification, for example, when the communication distance is short, the observation time is short, and when the communication distance is long, the observation time is long, so that stable polarization correction in a steady state becomes possible. The modification that determines the initial value of the observation condition can also be applied to each of the following embodiments.

As described above, according to the first embodiment, by shortening the observation time only when the rotation of polarized light occurs quickly and greatly, rapid polarization correction in strong wind and stable polarization correction in steady state are performed. Can be compatible with each other. Further, by shortening the control cycle only in strong winds, it is possible to reduce the burden on the device due to unnecessarily frequent control.

(Second embodiment)
The communication system of the second embodiment estimates the rotation to the polarization of the photon used in the quantum cryptography communication by using the observed amount of the photon (reference light) different from the photon used in the quantum cryptography communication.

FIG. 6 is a block diagram showing a functional configuration example of the photon transmitting unit 110-2 and the photon receiving unit 210-2 according to the second embodiment. FIG. 7 is a block diagram showing an example of a detailed functional configuration of the control unit 208-2.

As shown in FIG. 6, the photon transmitter 110-2 includes an unpolarized light source 101, a polarization beam splitter 102, a phase modulator 103-2, a combiner 104, a delay circuit 105, and a reference light source 106-. 2 and a combiner 107-2 are provided.

The photon receiving unit 210-2 includes a polarization rotating device 201, a polarization beam splitter 202, a phase modulator 203-2, a delay circuit 204, a beam splitter 205, photon detectors 206 and 207, and a control unit 208-. 2 and. As shown in FIG. 7, the control unit 208-2 includes a polarization estimation unit 251-2, a condition determination unit 253, a polarization correction output unit 252, and a signal output unit 254.

The photon transmitter 110-2 of the second embodiment adds the reference light source 106-2 and the combiner 107-2, and the function of the phase modulator 103-2 is the function of the first embodiment. It is different from the photon transmitter 110. The photon receiving unit 210-2 of the second embodiment is different from the photon receiving unit 210 of the first embodiment in the functions of the phase modulator 203-2 and the control unit 208-2 (polarization estimation unit 251-2). There is. Other configurations and functions are the same as those in FIG. 2, which is a block diagram of the communication system according to the first embodiment. Therefore, the same reference numerals are given, and the description thereof will be omitted here.

The reference light source 106-2 is a light source that emits reference light used for estimating the amount of rotation of polarized light. The unpolarized light source 101 may also serve as the reference light source 106-2.

The combiner 107-2 is used to incident the reference light emitted from the reference light source 106-2 into the same path as the photons emitted from the unpolarized light source 101.

The phase modulator 103-2 is different from the phase modulator 103 of the first embodiment in that it further has a function of modulating the phase of the reference light. For example, the phase modulator 103-2 modulates the phase of the photon of the reference light that has traveled to the first optical path so that it has a fixed phase difference with respect to the photon of the reference light that has traveled to the second optical path.

The phase modulator 203-2 is different from the phase modulator 203 of the first embodiment in that it further has a function of modulating the phase of the reference light. For example, the phase modulator 203-2 modulates the phase so that the photons of the reference light traveling in the fourth optical path interfere with the reference light traveling in the third path, and as a result, all the photons proceed to the photon detector 206.

The polarization estimation unit 251-2 estimates the amount of polarization rotation of photons used in quantum cryptography communication using reference light.

Next, the communication processing by the communication system according to the second embodiment configured in this way will be described. The communication process is a process of transmitting and receiving photons used for sharing an encryption key by, for example, quantum key distribution technology. FIG. 8 is a flowchart showing an example of communication processing in the second embodiment.

Since steps S201 to S206 are the same processes as steps S101 to S106 in the communication system according to the first embodiment, the description thereof will be omitted. As described above, in the second embodiment, the detected quantum signal is not used for estimating the amount of rotation of polarized light.

Next, the control process by the communication system according to the second embodiment will be described. FIG. 9 is a flowchart showing an example of the control process according to the second embodiment.

The combiner 107-2 incidents the reference light emitted from the reference light source 106-2 onto the same interferometer as the photon indicating the quantum signal (step S301). The polarization beam splitter 102 sends the reference light having the first polarization to the first optical path, and sends the reference light having the second polarization orthogonal to the first polarization to the second optical path.

The phase modulator 103-2 modulates the phase of the photon that has advanced to the first optical path (step S302). For example, the phase modulator 103-2 modulates the phase of a photon traveling in the first optical path so as to have a fixed phase difference with respect to the photon traveling in the second optical path. The photon that has advanced to the second optical path is sent to the combiner 104 by the delay circuit 105 with a delay with respect to the photon that has advanced to the first optical path. The combiner 104 combines the photons that have traveled to the first optical path and the photons that have traveled to the second optical path.

The photons combined by the combiner 104 pass through the quantum communication path 11 and are transmitted to the photon receiving unit 210-2 (step S303). The polarization rotation device 201 of the photon receiving unit 210-2 corrects the rotation of the polarization generated in the quantum communication path 11 according to the control signal from the control unit 208 (step S304).

The polarization beam splitter 202 sends a photon having a first polarization to a third optical path and a photon having a second polarization to a fourth optical path. The phase modulator 203-3 modulates the phase of the photon that has advanced to the fourth optical path (step S305). For example, the phase modulator 203-2 modulates the phase of a photon that has advanced to the fourth optical path so that all photons advance to the photon detector 206. The photon that has advanced to the third optical path is sent to the beam splitter 205 at the same timing as the photon that has advanced to the fourth optical path by the delay circuit 204 having the same length as the delay circuit 105. As a result, the photon that has traveled to the third optical path and the photon that has traveled to the fourth optical path interfere with each other.

All the reference light that interferes with the beam splitter 205 goes to the photon detector 206 if there is no rotation of polarized light. The photon detector 206 detects the reference light that interferes with the beam splitter 205 (step S306).

The polarization estimation unit 251-2 of the control unit 208-2 estimates the polarization rotation amount of the photon used in the quantum signal using the detected photon of the reference light (step S307). For example, the polarization estimation unit 251-2 acquires only the reference light that has reached the beam splitter 205 at the same time and has reached the photon detector 206 as an observation signal for a certain period of time, and the amount of decrease in the observation amount per unit time of the observation signal. The amount of polarization rotation is estimated from. Even the reference light passes through the same path as the photon used as the quantum signal, and by transmitting it alternately with the quantum signal, for example, an amount close to the amount of rotation of the polarization of the photon used as the quantum signal is estimated. be able to. The polarization correction output unit 252 transmits a control signal for instructing the polarization correction amount for correcting the estimated polarization rotation amount to the polarization rotation device 201.

The condition determination unit 253 of the control unit 208-2 determines, for example, the observation time of the next cycle as the observation condition used for estimation (step S308). For example, the condition determination unit 253 extracts only the photon of the reference light that has reached the photon detector 206 as an observation signal, and estimates when the observation amount per fixed time falls below the threshold value (for example, 75% of the steady state). Decide to shorten the observation time used. It may be decided to take out only the photon of the reference light that has reached the photon detector 207 as an observation signal and shorten the observation time used for estimation when the observation amount per fixed time exceeds the threshold value.

In order to suppress the noise of quantum signal detection, it is desirable that the reference light is combined at the timing when the quantum signal is not transmitted. That is, it is desirable that the communication process of FIG. 8 is executed at a timing different from that of the control process of FIG. While the reference light can be stronger than the quantum signal, inserting too much reference light reduces the time slot in which the quantum signal can be inserted. The timing of communication processing and control processing may be determined in consideration of these factors.

As described above, in the second embodiment, by shortening the observation time of the reference light only when the rotation of the polarized light occurs quickly and greatly, the rapid polarization correction in the strong wind and the stable polarization correction in the steady state are performed. Can be compatible with each other.

(Third Embodiment)
In the third embodiment, the estimation of the amount of polarization rotation using the reference light is realized by a method different from that of the second embodiment.

FIG. 10 is a block diagram showing a functional configuration example of the photon transmitting unit 110-3 and the photon receiving unit 210-3 according to the third embodiment. FIG. 11 is a block diagram showing an example of a detailed functional configuration of the control unit 208-3.

As shown in FIG. 10, the photon transmitter 110-3 includes an unpolarized light source 101, a polarization beam splitter 102, a phase modulator 103, a combiner 104, a delay circuit 105, and a reference light source 106-3. , And a combiner 107-3.

The photon receiving unit 210-3 includes a polarization rotating device 201, a polarization beam splitter 202, a phase modulator 203, a delay circuit 204, a beam splitter 205, photon detectors 206 and 207, and a signal output unit 24-3-3. The control unit 208-3, the demultiplexer 209-3, and the polarization rotation amount measuring device 211-3 are provided. As shown in FIG. 11, the control unit 208-3 includes a polarization estimation unit 251-3, a condition determination unit 253, and a polarization correction output unit 252.

In the photon transmission unit 110-3 of the third embodiment, the reference light source 106-3 and the combiner 107-3 are added, and the signal output unit 254-3 is independent of the control unit 208-3. This is different from the photon transmitter 110 of the first embodiment. The photon receiving unit 210-3 of the third embodiment adds the function of the control unit 208-3 (polarization estimation unit 251-3), the demultiplexer 209-3, and the polarization rotation amount measuring device 211-3. This is different from the photon receiving unit 210 of the first embodiment. Other configurations and functions are the same as those in FIG. 2, which is a block diagram of the communication system according to the first embodiment. Therefore, the same reference numerals are given, and the description thereof will be omitted here.

Reference light source 106-3 is a light source that emits reference light used for estimating the amount of rotation of polarized light. The unpolarized light source 101 may also serve as the reference light source 106-3.

The combiner 107-3 is used to incident the reference light emitted from the reference light source 106-3 into the same path as the photons emitted from the unpolarized light source 101.

The demultiplexer 209-3 demultiplexes the photon indicating the quantum signal and the reference light. For example, the demultiplexer 209-3 distinguishes between a photon indicating a quantum signal and a reference light by wavelength, sends a photon indicating a quantum signal to a polarization rotating device 201, and sends a reference light to a polarization rotation amount measuring device 211-3. ..

The polarization rotation amount measuring device 211-3 measures the polarization rotation amount of the reference light. For example, the polarization rotation amount measuring device 211-3 extracts the reference light as an observation signal, acquires the polarization rotation amount from the reference polarization for a certain period of time, and measures the average polarization rotation amount per unit time. The polarization rotation amount measuring device 211-3 may be configured by a polarization measuring device, or may be configured by a combination of a polarization beam splitter and a photon detector.

The polarization estimation unit 251-3 estimates the polarization rotation amount of the photon used for quantum cryptography communication by using the output signal (observation signal) from the polarization rotation amount measuring device 211-3. The amount of polarization rotation of the reference light may be used as it is as an estimated value, or when the wavelengths of the photon and the reference light used for quantum cryptography communication are different, the corrected value may be used.

Next, the control process by the communication system according to the third embodiment configured in this way will be described. Since the communication process is the same as that of the second embodiment, the description thereof will be omitted. FIG. 12 is a flowchart showing an example of the control process according to the third embodiment.

The combiner 107-3 incidents the reference light emitted from the reference light source 106-3 into the same path as the photon indicating the quantum signal (step S401). The incident reference light passes through the quantum communication path 11 and is transmitted to the photon receiving unit 210-3 (step S402). The demultiplexer 209-3 of the photon receiving unit 210-3 demultiplexes the reference light into the optical path toward the polarization rotation amount measuring device 211-3 (step S403). The polarization rotation amount measuring device 211-3 measures the polarization rotation amount of the reference light (step S404). Based on the measured values of the polarization rotation amount measuring device 211-3, the polarization estimation unit 251-3 of the control unit 208-3 estimates the polarization rotation amount generated in the photons used for quantum cryptography communication. Based on the estimated value, the polarization correction output unit 252 transmits a control signal for instructing the polarization correction amount for correcting the measured polarization rotation amount to the polarization rotation device 201. The polarization rotating device 201 corrects the polarization according to this control signal when the communication process is executed.

The condition determination unit 253 of the control unit 208-3 determines, for example, the observation time of the next cycle as the observation condition of the rotation according to the measurement result of the polarization rotation amount measuring device 211-3 (step S405).

In order to suppress the noise of quantum signal detection, a reference light source 106-3 that emits reference light having a wavelength different from that of a photon indicating a quantum signal is used, and a demultiplexer 209-3 has a mechanism for distinguishing and extracting light according to wavelength. It is desirable to prepare. When reference lights of different wavelengths are used, the quantum signal and the reference light may reach the demultiplexer 209-3 at the same timing. When the reference light having the same wavelength as the signal light is used, it is necessary to reach the demultiplexer 209-3 at different timings in order to distinguish by the arrival time.

Similar to the second embodiment, the third embodiment also shortens the observation time of the reference light for estimating the amount of polarization rotation only when the polarization rotation occurs quickly and greatly, so that the light can be quickly rotated in a strong wind. It is possible to achieve both polarization correction and stable polarization correction during steady operation.

As described above, according to the first to third embodiments, it is possible to achieve both rapid polarization correction when a large and fast disturbance occurs and stabilization of polarization correction in a steady state.

Finally, an example of the hardware configuration of the main part of each device (transmitting device, receiving device) of the first to third embodiments will be described.

FIG. 13 is a diagram showing an example of a hardware configuration of a main part of the apparatus of the first to third embodiments. The devices of the first to third embodiments include a control device 51, a main storage device 52, an auxiliary storage device 53, an optical processing device 54, and a communication I / F 55. The control device 51, the main storage device 52, the auxiliary storage device 53, the optical processing device 54, and the communication I / F 55 are connected via the bus 61.

The control device 51 executes the program read from the auxiliary storage device 53 to the main storage device 52. The control device 51 is, for example, a CPU. The main storage device 52 is a memory such as a ROM (Read Only Memory) and a RAM. The auxiliary storage device 53 is a memory card, an HDD, or the like.

The optical processing device 54 transmits or receives the above-mentioned single photon via the quantum communication path 11. The communication I / F 55 transmits or receives control information and the like via a classical communication path 12 such as an optical fiber and Ethernet (registered trademark).

The program executed by the apparatus of the first to third embodiments is a file in an installable format or an executable format and can be read by a computer such as a CD-ROM, a memory card, a CD-R, and a DVD. It is stored on a storage medium and provided as a computer program product.

Further, the program executed by the devices of the first to third embodiments may be stored on a computer connected to a network such as the Internet and provided by downloading via the network. Further, the program executed by the devices of the first to third embodiments may be configured to be provided via a network such as the Internet without being downloaded.

Further, the program executed by the apparatus of the first to third embodiments may be configured to be provided by incorporating it in a ROM or the like in advance.

The program executed by the transmitters of the first to third embodiments has a module configuration including functions that can be realized by the program among the functional configurations of the transmitters of the first to third embodiments described above. There is. Further, the program executed by the receiving device of the first to third embodiments has a module configuration including functions that can be realized by the program among the functional configurations of the receiving device of the first to third embodiments described above. It has become.

As for the function realized by the program, the function realized by the program is loaded into the main storage device 52 when the control device 51 reads the program from the storage medium such as the auxiliary storage device 53 and executes the program. That is, the function realized by the program is generated on the main storage device 52.

Note that some of the functions of the devices of the first to third embodiments may be realized by hardware such as an IC. The IC is, for example, a processor that executes dedicated processing. Further, when each function is realized by using a plurality of processors, each processor may realize one of each function, or may realize two or more of each function.

Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other embodiments, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are also included in the scope of the invention described in the claims and the equivalent scope thereof.

11 Quantum communication path 12 Classical communication path 51 Control device 52 Main storage device 53 Auxiliary storage device 54 Optical processing device 55 Communication I / F
61 Bus 100 Transmitter 101 Unpolarized light source 102 Polarized beam splitter 103, 103-2 Phase modulator 104 Combiner 105 Delay circuit 106-2, 106-3 Reference light source 107-2, 107-3 Combiner 110 Photon transmission Unit 120 Key generation unit 131 Storage unit 200 Receiver 201 Polarized rotating device 202 Polarized beam splitter 203, 203-2 Phase modulator 204 Delay circuit 205 Beam splitter 206, 207 Photon detector 208, 208-2, 208-3 Control unit 209-3 Demultiplexer 210 Photon receiver 211-3 Polarization rotation amount measuring device 220 Key generation unit 231 Storage unit 251, 251-2, 251-3 Polarization estimation unit 252 Polarization correction output unit 253 Condition determination unit 254 Signal output unit

Claims (20)

An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is provided with a determination unit that determines the observation conditions related to the observation amount of photons, which is a condition used for estimating the amount of rotation of polarized light .
When the standard deviation of the amount of photons observed for a certain period of time is equal to or greater than the first threshold value, the determination unit makes the observed amount smaller than the case where the standard deviation of the amount of photons observed for a certain period of time is smaller than the first threshold value. Determine the observation conditions that indicate
Communication device. An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is provided with a determination unit that determines the observation conditions related to the observation amount of photons, which is a condition used for estimating the amount of rotation of polarized light.
The determination unit indicates that when the amount of photons observed for a certain period of time is smaller than the second threshold value, the observed amount is smaller than when the amount of photons observed for a certain period of time is equal to or greater than the second threshold value. To decide,
Communication equipment. The estimation unit estimates the amount of polarization rotation of the photon used in the quantum cryptography communication based on the change in the observed amount of the photon.
The communication device according to claim 1 or 2. The estimation unit estimates the polarization rotation amount of the photon used in the quantum cryptography communication based on the change in the observed amount of the photon used in the quantum cryptography communication.
The communication device according to claim 3. The estimation unit estimates the polarization rotation amount of the photon used in the quantum cryptography communication based on the change in the observed amount of the photon different from the photon used in the quantum cryptography communication.
The communication device according to claim 3. An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is provided with a determination unit that determines the observation conditions related to the observation amount of photons, which is a condition used for estimating the amount of rotation of polarized light.
The determination unit determines the initial value of the observation condition indicating that the larger the amount of photons observed for a certain period of time in the steady state, the smaller the observed amount.
Communication equipment. An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is provided with a determination unit that determines the observation conditions related to the observation amount of photons, which is a condition used for estimating the amount of rotation of polarized light.
The estimation unit estimates the amount of polarization rotation of the photon based on an output signal from a sensor that detects a factor affecting the refractive index of the optical communication path used for transmitting the photon.
Communication equipment. The observation condition is the observation time of photons or the count number of photons.
The communication device according to any one of claims 1 to 7. An estimation step for estimating the amount of polarization rotation of photons used in quantum cryptography,
A condition used for estimating the amount of rotation of polarized light, which includes a determination step for determining an observation condition regarding an observation amount of photons.
In the determination step, when the standard deviation of the amount of photons observed for a certain period of time is equal to or greater than the first threshold value, the observed amount is made smaller than when the standard deviation of the amount of photons observed for a certain period of time is smaller than the first threshold value. Determine the observation conditions that indicate
Communication method. An estimation step for estimating the amount of polarization rotation of photons used in quantum cryptography,
A condition used for estimating the amount of rotation of polarized light, which includes a determination step for determining an observation condition regarding an observation amount of photons.
The determination step indicates that when the amount of photons observed for a certain period of time is smaller than the second threshold value, the observed amount is made smaller than when the amount of photons observed for a certain period of time is equal to or greater than the second threshold value. To decide,
Communication method. An estimation step for estimating the amount of polarization rotation of photons used in quantum cryptography,
A condition used for estimating the amount of rotation of polarized light, which includes a determination step for determining an observation condition regarding an observation amount of photons.
The determination step determines the initial value of the observation condition indicating that the larger the amount of photons observed for a certain period of time in the steady state, the smaller the observed amount.
Communication method. An estimation step for estimating the amount of polarization rotation of photons used in quantum cryptography,
A condition used for estimating the amount of rotation of polarized light, which includes a determination step for determining an observation condition regarding an observation amount of photons.
The estimation step estimates the amount of polarization rotation of the photon based on an output signal from a sensor that detects a factor affecting the refractive index of the optical communication path used for transmitting the photon.
Communication method . Computer,
An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is a condition used for estimating the amount of rotation of polarized light, and functions as a determinant for determining the observation condition regarding the amount of photon observation.
When the standard deviation of the amount of photons observed for a certain period of time is equal to or greater than the first threshold value, the determination unit makes the observed amount smaller than the case where the standard deviation of the amount of photons observed for a certain period of time is smaller than the first threshold value. Determine the observation conditions that indicate
program. Computer,
An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is a condition used for estimating the amount of rotation of polarized light, and functions as a determinant for determining the observation condition regarding the amount of photon observation.
The determination unit indicates that when the amount of photons observed for a certain period of time is smaller than the second threshold value, the observed amount is smaller than when the amount of photons observed for a certain period of time is equal to or greater than the second threshold value. To decide,
program. Computer,
An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is a condition used for estimating the amount of rotation of polarized light, and functions as a determinant for determining the observation condition regarding the amount of photon observation.
The determination unit determines the initial value of the observation condition indicating that the larger the amount of photons observed for a certain period of time in the steady state, the smaller the observed amount.
program. Computer,
An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is a condition used for estimating the amount of rotation of polarized light, and functions as a determinant for determining the observation condition regarding the amount of photon observation.
The estimation unit estimates the amount of polarization rotation of the photon based on an output signal from a sensor that detects a factor affecting the refractive index of the optical communication path used for transmitting the photon.
Program . A communication system including a transmitting device and a receiving device.
The transmitter is
A photon transmitter for transmitting photons used for quantum cryptography communication to the receiving device is provided.
The receiving device is
An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is provided with a determination unit that determines the observation conditions related to the observation amount of photons, which is a condition used for estimating the amount of rotation of polarized light.
When the standard deviation of the amount of photons observed for a certain period of time is equal to or greater than the first threshold value, the determination unit makes the observed amount smaller than the case where the standard deviation of the amount of photons observed for a certain period of time is smaller than the first threshold value. Determine the observation conditions that indicate
Communications system. A communication system including a transmitting device and a receiving device.
The transmitter is
A photon transmitter for transmitting photons used for quantum cryptography communication to the receiving device is provided.
The receiving device is
An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is provided with a determination unit that determines the observation conditions related to the observation amount of photons, which is a condition used for estimating the amount of rotation of polarized light.
The determination unit indicates that when the amount of photons observed for a certain period of time is smaller than the second threshold value, the observed amount is smaller than when the amount of photons observed for a certain period of time is equal to or greater than the second threshold value. To decide,
Communications system. A communication system including a transmitting device and a receiving device.
The transmitter is
A photon transmitter for transmitting photons used for quantum cryptography communication to the receiving device is provided.
The receiving device is
An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is provided with a determination unit that determines the observation conditions related to the observation amount of photons, which is a condition used for estimating the amount of rotation of polarized light.
The determination unit determines the initial value of the observation condition indicating that the larger the amount of photons observed for a certain period of time in the steady state, the smaller the observed amount.
Communications system. A communication system including a transmitting device and a receiving device.
The transmitter is
A photon transmitter for transmitting photons used for quantum cryptography communication to the receiving device is provided.
The receiving device is
An estimation unit that estimates the amount of polarization rotation of photons used in quantum cryptography communication,
It is provided with a determination unit that determines the observation conditions related to the observation amount of photons, which is a condition used for estimating the amount of rotation of polarized light.
The estimation unit estimates the amount of polarization rotation of the photon based on an output signal from a sensor that detects a factor affecting the refractive index of the optical communication path used for transmitting the photon.
Communication system .

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