JP6658102B2 - Quantum key distribution system and time synchronization method - Google Patents

Description

  The present invention relates to a quantum key distribution system using quantum entangled photon pairs that can synchronize time information when detecting photons between a transmitter and a receiver, and a time synchronization method suitable for use in this system.

  In order to realize secure encrypted communication without information leakage, the sender and receiver share the encryption key used for encrypting and decrypting information without the third party such as an eavesdropper knowing it. It is essential.

  Quantum key distribution systems are attracting attention as cryptographic key distribution systems that guarantee 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 been active in recent years. .

  The quantum key distribution system is realized using, for example, a single photon light source. Single-photon light sources ideally produce one photon per pulse. In a quantum key distribution system using a single photon light source, one of the two parties sharing the encryption key generates a single photon and sends it to the other, and the other receives the photon with a single photon detector. After that, the final encryption key is shared through the process of exchanging observation base information, correcting errors, and concealing amplification.

  In addition, the quantum key distribution system can be realized using a quantum entangled light source. A entangled light source ideally produces one set of entangled photons per pulse. In a quantum key distribution system using a quantum entangled light source, two parties sharing an encryption key share an encryption key by receiving one of the photons constituting a quantum entangled photon pair with a single photon receiver.

  In a quantum key distribution system using a single photon light source or a quantum entangled light source, time synchronization between two parties sharing an encryption key is essential. That is, when sharing the encryption key, it is essential that the two parties sharing the encryption key know each other's time origin and adjust the clock. In a quantum key distribution system using a single photon light source, at least one of the two parties sharing the encryption key needs to know the relative relationship between the time at which one photon is sent and the time at which the other photon is received. Also, in a quantum key distribution system using a entangled light source, one photon forming one set of photons is formed by the time when one of the two parties sharing the encryption key receives light, and one set of photons is formed. At least one of the two parties sharing the encryption key needs to know the relative relationship of the time when the other photon is received by the other party sharing the encryption key.

  If time synchronization between the two parties sharing the encryption key is not performed, no correlation can be obtained between the key information obtained by the two parties even if the two parties attempt to share the encryption key. As a result, it becomes impossible for two persons sharing the encryption key to share the encryption key. Therefore, in the conventional quantum key distribution system, in order to know the time origin, a light source different from the light source used for the quantum key distribution is prepared, and the pulse light generated by the other light source is shared with the encryption key. To one of them. Then, the other of the two parties sharing the encryption key can know the transmission time and the reception time of the pulse light, so that the transmission delay time generated in the transmission path or the like can be known. As a result, the two parties can understand each other's time. The origin can be matched (for example, see Non-Patent Document 1 or 2).

Alexander Treiber et al. , "A fully automated entanglement-based quantum cryptography system for telecom fiber networks", New Journal of Physics 11 (2009) 04, 2009 M. Sasaki et al. , “Field test of quantum key distribution in the Tokyo QKD Network”, Optics Express vol. 19, no. 11, pp. 10387-10409 (2011)

  However, in the above-described conventional time synchronization method, a light source not used for quantum key distribution and its receiver are prepared. For this reason, the burden on the sharer of the encryption key increases, such as increasing the number of components of the system. As a result, the cost, power consumption, and the like of this system increase.

  Further, in synchronizing the time information, if the transmission and reception of the pulse light is performed at a light intensity of a normal laser light level, such that the pulse light is not detected by the single photon detector used for the quantum key distribution, for example, It is necessary to shift the wavelength significantly. Further, there is a concern that noise light such as Raman scattered light may be generated due to propagation of such high-intensity light through the optical fiber transmission line. In order to avoid such a problem, the burden on the sender and receiver is further increased.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a quantum key distribution system capable of synchronizing time information more easily, and a time suitable for use in this system. It is to provide a synchronization method.

  In order to achieve the above object, a quantum key distribution system according to the present invention includes a light source unit, a first detection unit, a second detection unit, a time synchronization unit, a first signal processing unit, And two signal processing units.

The light source generates quantum entangled photon pairs. The first detection unit receives one photon of the quantum entangled photon pair during a first detection time T from the first start time t _0a to the first end time t _0a + T, and A first raw key A including first time information indicating the time of reception and first quantum information indicating the quantum state of the photon is obtained. The second detector detects the second start time t _0a + ΔT _{delay. ave−} Δτ, the second end point time t _0a + ΔT _delay. During the second detection time T + 2Δτ up to _ave + T + Δτ, the other photon of the quantum entangled photon pair is received, second time information indicating the time at which the photon was received, and second time information indicating the quantum state of this photon A second raw key B including quantum information is obtained. The time synchronization unit, the correction minimum propagation delay time difference start time _{t c t 0a} + _{ΔT delay. ave−} Δτ to the maximum propagation delay time difference t _0a + ΔT _{delay. ave +} varied between .DELTA..tau, for each of the correction start time t _c, obtains the simultaneous detection number of first time information and second time information, a correction start time t _c to simultaneous detection count is maximum the difference _t c _{-t 0a} time origin information [Delta] t _delay of the first start time _{t 0a.} Obtained as _opt . The first signal processing unit receives the first raw key A from the first detection unit, and the second signal processing unit receives the second raw key B from the second detection unit. The first and second signal processing units _provide the time origin information ΔT _{delay. After} the first quantum information and the second quantum information are time-synchronized using _opt , an encryption key is generated.

The time synchronization method according to the present invention includes the following steps. First, the first detector receives one photon of the quantum entangled photon pair during the first detection time T from the first start time t _0a to the first end time t _0a + T. , The first raw key A including the first time information and the first quantum information, and obtaining the second starting time t _0a + ΔT _{delay. ave−} Δτ, the second end point time t _0a + ΔT _delay. During a second detection time T + 2Δτ up to _ave + T + Δτ, a second raw key B including second time information and second quantum information obtained by receiving the other photon of the quantum entangled photon pair is obtained. I do. Next, the correction start time _t minimum propagation delay time difference _{c t 0a} + _{ΔT delay. ave−} Δτ to the maximum propagation delay time difference t _0a + ΔT _{delay. ave +} varied between .DELTA..tau, for each of the correction start time t _c, obtains the simultaneous detection number of first time information and second time information, a correction start time t _c to simultaneous detection count is maximum the difference _t c _{-t 0a} time origin information [Delta] t _delay of the first start time _{t 0a.} Obtained as _opt . Next, the time origin information ΔT _{delay. After} the first quantum information and the second quantum information are time-synchronized using _opt , an encryption key is generated.

  According to the quantum key distribution system and the time synchronization method of the present invention, time synchronization is performed using a raw key used for generating an encryption key. Therefore, a separate light source for time synchronization is not required. Therefore, time synchronization can be performed with a lower cost and a simple configuration.

It is a schematic diagram of a quantum key distribution system. It is a schematic diagram which shows an example of a structure of the detection part with which a quantum key distribution system is provided. FIG. 3 is a schematic diagram for explaining an outline of a time synchronization method. FIG. 3 is a schematic diagram for explaining a time synchronization method of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shapes, sizes, and arrangements of the components are only schematically shown to the extent that the present invention can be understood. Also, numerical conditions and the like 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 present invention.

(Quantum key distribution system)
An embodiment of a quantum key distribution system according to the present invention will be described with reference to FIGS. FIG. 1 is a schematic diagram of a quantum key distribution system. FIG. 2 is a schematic diagram illustrating a configuration example of a detection unit included in the quantum key distribution system.

  The quantum key distribution system includes a light source unit 12, first and second detection units 22 and 32, and first and second signal processing units 24 and 34.

  The light source unit 12 is a quantum entangled light source that generates entangled light composed of signal photons and idler photons. For convenience, it is assumed that signal photons are sent to the first detection unit 22 and idler photons are sent to the second detection unit 32. Here, an optical path 21 from the light source unit 12 to the first signal processing unit 24 and an optical path 31 from the light source unit 12 to the second signal processing unit 34 are quantum channels such as an optical fiber communication network and a spatial optical system. It is. In the following description, a device on the first side including the first detection unit 22 and the first signal processing unit 24 is referred to as a transmission-side device 20, and the second detection unit 32 and the second signal processing unit 34 The device on the second side including the above may be referred to as a receiving device 30.

  The first and second detectors 22 and 32 each include a single photon detector that detects signal photons and idler photons.

  The first and second detectors 22 and 32 can be configured similarly to a receiver used for quantum key distribution or the like.

  Here, an example in which a polarization entangled photon pair is used as the entangled photon pair will be described. A polarization entangled photon pair refers to a photon pair in which the polarization of each photon is not determined, but the relationship of the polarization measurement results (relationship such as parallel to each other, orthogonal to each other) is determined. That is, the polarization quantum entangled photon pair is a correlated photon pair in which a plurality of combinations of polarizations are superimposed on the photon pair, and there is a polarization correlation between the photon pairs.

  The first detector 22 includes a half mirror 501, a wave plate 502, first and second polarization beam splitters 503 and 504, and first to fourth single-photon detectors 505 to 508.

  The signal photon arriving at the first detection unit 22 is input to the first input terminal 501-1 of the half mirror 501, and is output from one of the second output terminal 501-2 and the third output terminal 501-3. Since there is basically one incoming photon, this photon is output to only one of the two output terminals 501-2 and 501-3. Which output terminal is output corresponds to the selection of the observation base in the quantum key distribution technique.

  Thereafter, when the signal photon is output from the second output end 501-2 of the half mirror 501, it is input to the first input end 503-1 of the first polarization beam splitter 503, and the signal photon is output according to the polarization state. The signal is output from one of the two output terminals 503-2 and the third output terminal 503-3, and finally received by one of the first single photon detector 505 and the second single photon detector 506. .

  When the signal photon is output from the third output end 501-3 of the half mirror 501, the second polarized beam is passed after passing through the half-wave plate or the quarter-wave plate 502. The signal is input to the first input terminal 504-1 of the splitter 504, and is output from one of the second output terminal 504-2 and the third output terminal 504-3 according to the polarization state of the input signal. The light is received by one of the photon detector 507 and the fourth single photon detector 508.

  As a result, signal photons are detected by any of the four single photon detectors 505-508. In the quantum key distribution mechanism, for example, when a signal photon is detected by the first or third single photon detector 505 or 507, a bit “1” is assigned, and the second or fourth single photon detector 506 is assigned. Alternatively, when a signal photon is detected at 508, a random bit string is generated by assigning a bit “0”. Also, when a signal photon is detected by the first or second single photon detector 505 or 506, the first observation base is assumed to be selected, while the third or fourth single photon detector 507 is selected. , 508, it is assumed that the second observation base has been selected.

  The second detector 32 differs from the first detector 22 only in that it detects idler photons instead of signal photons. The configuration and operation of the second detection unit 32 are substantially the same as those of the first detection unit 22, and thus redundant description will be omitted.

  The first and second signal processing units 24 and 34 are signal processing circuits that generate a final encryption key used for encrypted communication. The first and second signal processors 24 and 34 are connected by the classical channel 11. In the communication on the classical channel 11, a mechanism such as message authentication is used so that the content is not falsified.

  The first signal processing unit 24 obtains the time (Ta) at which the signal photon was received, the observation base (Ba) at that time, and the bit value (Ma) from the detection result of the first detection unit 22. Here, the time Ta, the observation base Ba, and the bit value Ma are collectively referred to as a first raw key A.

  The second signal processing unit 34 obtains the time (Tb) at which the idler photon was received, the observation base (Bb) at that time, and the bit value (Mb) from the detection result of the second detection unit 32. Here, the time Tb, the observation base Bb, and the bit value Mb are collectively referred to as a second raw key B.

  The first and second signal processing units 24 and 34 exchange information of the raw key A and the raw key B by bidirectional communication via the classical channel 11, and generate a shift key by exchanging observation base information. Furthermore, a key distillation process based on error correction and confidential amplification is executed to generate a final encryption key.

  In this configuration example, a time synchronization unit 26 is provided in the first signal processing unit 24. The time synchronization unit 26 may be provided in either the first signal processing unit 24 or the second signal processing unit 34. The time synchronizing unit 26 also functions as a time correlator that matches the times using the time Ta and the time Tb. Note that the above-mentioned condition that “the time information matches” means that after the difference in the propagation delay time of the photons in the quantum channels 21 and 31 is corrected, as described later, the photons are not necessarily received. It does not mean that the actual times match.

The time synchronizing unit 26 calculates the time origin information ΔT _{delay. As} the optimum value of the difference in the propagation delay time between the first detector 22 that receives the signal light and the second detector 32 that receives the idler light _{. Get opt} . This time origin information ΔT _{delay. The} process of obtaining _opt will be described later.

The first and second signal processing units 24 and 34 _{generate the} time origin information ΔT _delay. The difference in the propagation delay time of photons in the quantum channels 21 and 31 is corrected using _opt . After that, the first and second signal processing units 24 and 34 execute a series of processes necessary for encryption key generation, such as error rate estimation, shift key generation, error correction, and security enhancement. This final encryption key is shared by the first device (sending device 20) and the second device (receiving device 30).

(Outline of time synchronization method)
To help understand the time synchronization method of the present invention, an outline of the time synchronization method will be described with reference to FIG. FIG. 3 is a schematic diagram for explaining the outline of the time synchronization method.

  In general, the locations of the transmitting device 20 and the receiving device 30 with respect to the light source unit 12 are not symmetric. That is, the distances from the light source unit 12 to the first detection unit 22 and the second detection unit 32 are not the same. Therefore, for the signal photons and the idler photons simultaneously generated in the light source unit 12, the time until the signal photons are received by the first detection unit 22 and the time until the idler photons are received by the second detection unit 32. Has a time difference.

As shown in FIG. 3, it is assumed that the distance over which the idler photon propagates is longer, and the propagation delay time difference between the signal photon and the idler photon is ΔT _delay . When the quantum channels 21 and 31 are formed of optical fibers, ΔT _delay is approximately 5 μsec per 1 km of the transmission distance.

The transmitting device 20 and the receiving device 30 need to know the ΔT _delay with an accuracy of at most one time slot or less. If the correction amount of the propagation delay time is shifted by one time slot or more, the first raw key A obtained by the first detection unit 22 and the second raw key A obtained by the second detection unit 32 B's synchronization cannot be guaranteed. As a result, there is no correlation between the first raw key A and the second raw key B, and the encryption key cannot be shared. Therefore, for example, assuming a 1 GHz clock system, it is necessary to know ΔT _delay with a time accuracy of 1 nsec or less.

If ΔT _delay can be known with the required time accuracy, the propagation delay time difference ΔT _delay is corrected to one of the time Ta included in the first raw key A and the time Tb included in the second raw key B. Then, the synchronization between the first raw key A and the second raw key B, that is, time synchronization can be achieved.

Here, the propagation time of light in the transmission paths used for the quantum channels 21 and 31 changes due to the influence of the temperature dependence of the refractive index and the like. It is known that when the quantum channels 21 and 31 are optical fibers, when the temperature of the optical fiber of 1 km changes by 1 ° C., the propagation time changes by about 31 psec. If the difference in transmission distance is 20 km, and an optical fiber is installed between frames and a temperature fluctuation of 40 ° C. is expected due to fluctuations in the outside air, ΔT _delay fluctuates by about 24.8 nsec. Become.

That is, even if there is a large fluctuation of about 24.8 nsec in ΔT _delay , it is necessary to perform time synchronization with an accuracy of 1 nsec or less.

Basically, signal photons and idler photons used in quantum key distribution are extremely weak light of 1 photon or less per time slot, and these lights are attenuated due to transmission loss, detection efficiency of a single photon detector, and the like. Therefore, the first detection unit 22 and the second detection unit 32 detect the frequency only once or less per several hundred time slots. In such a system, it is very difficult to know exactly the ΔT _delay including the temporal change reflecting the temperature change and the like as described above.

(Conventional example of time synchronization method)
In the conventional quantum key distribution system, a pulse light having a strong light intensity, which is different from that used for quantum key distribution, is prepared in the first device that shares the encryption key, and this pulse light is transmitted to the first device. Send to the device on the second side. The second-side device knows the time at which the first-side device transmitted the pulse light and the second-side device received the pulse light with respect to the received pulse light having a high light intensity. ΔT _delay including its time-dependent change can be easily known. As a result, time synchronization can be established between the first raw key A and the second raw key B.

When the time is synchronized, for example, the device on the first side obtains the time acquired during the first detection time T from the appropriate first start time t _0a to the first end time t _0a + T. Ta, the observation base Ba, and the bit value Ma are recorded as the first raw key A. On the other hand, the second side unit, from a second start time _{t 0a} + ΔT _delay plus the propagation delay time difference [Delta] T _delay to the first start time _{t 0a,} to a second end point time _{t 0a} + ΔT _delay + _T No. The time Tb, the observation base Bb, and the bit value Mb acquired during the second detection time T are recorded as the second raw key B.

Then, the first and second signal processing units 24 and 34 collate the first raw key A and the second raw key B by using the classical channel 11, and use the above-mentioned t _0a + ΔT _delay to transmit / receive After correcting the time origin, the final encryption key is shared through the processes of error rate estimation, shift key generation, error correction, and security enhancement.

The method of time synchronization of the quantum key distribution system using a single photon light source is also the same as that of the quantum key distribution system using the quantum entangled photon pair described above, and is different from that used for quantum key distribution. A method of preparing pulsed light and knowing ΔT _delay from the pulsed light is used. On the other hand, the time synchronization method using such a method has problems such as an increase in cost and a complicated system configuration, as described above.

(Time synchronization method of the present invention)
In contrast to the above-described conventional time synchronization method, the time synchronization method of the present invention is performed as follows. An embodiment of the time synchronization method of the present invention will be described with reference to FIG. FIG. 4 is a schematic diagram for explaining the time synchronization method of the present invention.

In the time synchronization method of the present invention, first, a raw key acquisition process is performed. In the raw key obtaining process, the first detecting unit 22 obtains the time Ta, which is obtained during the first detecting time T from the appropriate first starting time t _0a to the first ending time t _0a + T. The observation base Ba and the bit value Ma are recorded as the first raw key A. In the second detection unit 32, the second start time t _0a + ΔT _{delay. ave−} Δτ, the second end point time t _0a + ΔT _{delay. The} time Tb, the observation base Bb, and the bit value Mb acquired during the second detection time T + 2Δτ up to _ave + T + Δτ are recorded as the second raw key B.

Here, the minimum propagation delay time difference of the system in consideration of the temperature change or the like is represented by ΔT _{delay. min} (= ΔT _delay.ave −Δτ), and the maximum propagation delay time difference is represented by ΔT _{delay. max} (= ΔT _delay.ave + Δτ). ΔT _{delay. ave} is the average propagation delay time difference, and the minimum propagation delay time difference ΔT _{delay. min} and the maximum propagation delay time difference ΔT _delay. Obtained as the median of _max . Δτ is referred to as a surplus time. Average propagation delay time difference ΔT _{delay. The ave} and the surplus time Δτ are appropriately set to optimal values according to the difference in the propagation distance from the light source unit 12 to the first detection unit 22 and the second detection unit 32, the environment in which the quantum channel is installed, and the like. You.

  This raw key obtaining process can be performed in the same manner as a conventionally known raw key obtaining process, except that the second detection time T + 2Δτ is longer than the first detection time T.

The portion of the second raw key B obtained by the second-side device excluding the bit value Mb is sent to the first-side device via the classical channel 11.

  Subsequent to the raw key obtaining process, a time origin information obtaining process is performed.

The time origin acquisition process, the time synchronization unit 26, the corrected time _{t c t 0a} + _{ΔT delay. ave-} Δτ to t _0a + ΔT _{delay. ave +} varied between .DELTA..tau, obtains the simultaneous detection times for each of the correction start time t _c, the difference t _c -t _0a time origin of the correction start time t _c and t _0a simultaneous detection number is maximum Information ΔT _delay. stored as _opt .

The acquisition of the number of simultaneous detections is performed, for example, as follows. First, from the time Tb contained in the second raw key B, extracted from the corrected start time t _c correction end point time t _{c +} first detection time T equal part time acquired during the time TTb to T I do. Next, the partial time TTb and the time Ta are collated by the time synchronizing unit 26, and the number of times that the detection times match each other (the number of simultaneous detections) is obtained. Here, “the detection times match” means that there is a combination of x and y that satisfies Tax = Tby (x and y are integers of 1 or more).

Subsequent to the time origin acquisition step, an encryption key generation step is performed. In the encryption key generation process, the time origin information ΔT _{delay. Using the opt} , the first and second signal processing units 24 and 34 correct the time origin between the first raw key A and the second raw key B. Thereafter, a final encryption key is generated through a process of estimating an error rate, generating a shift key, correcting an error, and enhancing security. This encryption key generation step can be performed in the same manner as a conventionally known encryption key generation step.

  The effect of the present invention is caused by the quantum mechanical effect of the correlated photon pair that is the source of the entangled photon pair.

  The correlated photon pair is a photon pair having a 100% correlation with the generation time, wavelength, polarization, or the like when a pair of signal light and idler light is generated. In the present invention, time synchronization is used. That is, if signal light is generated, idler light is generated at the same time, and vice versa.

  Therefore, the signal light and the idler light that form a pair are basically simultaneously detected with a delay time difference of zero. When there is a propagation delay time difference due to a transmission path or the like, both are detected based on the propagation delay time difference.

  On the other hand, the phenomenon that both are detected other than the propagation delay time difference is the phenomenon that one of each pair of photons generated in a different time slot is detected, or the noise signal of a single photon detector or the like is detected. These phenomena occur accidentally.

  Assuming that the distribution of the logarithm of the photon is a Poisson distribution, the probability Pcc of simultaneously counting the (normal) signal light and the idler light forming the former pair is given by the following equation (1).

P _cc = pα _sig α _id (1)
Here, p is the average number of log photons per time slot, α _sig and α _id are the attenuation rates of signal photons and idler photons due to propagation loss, detection efficiency of a single photon detector, and the like, respectively.

On the other hand, the probability _Pacc of simultaneous counting due to the latter accidental phenomenon is given by the following equation (2).

P _acc = (pα _sig + d _sig ) (pα _id + d _id ) (2)
Here, d _x (x = sig, id) is a dark current detection rate of the single photon detectors constituting the first detector 22 and the second detector 32.

The condition under which t _c is set so as to give the maximum simultaneous count is the same as when the probability of simultaneous count is P _cc + P _acc . At this time, the propagation delay time difference t _0a -t _c provided when comparing Ta and TTb is equal to the propagation of the actual system when these first raw key A and second raw key B are obtained. This corresponds to the delay time difference. Therefore, by using this time correction, it becomes possible to synchronize the time between the sender and the receiver.

On the other hand, when the correction start time t _c is deviated from the optimum value of the above, the probability of simultaneous count becomes P _acc contingent count only. That is, it is lower by P _acc than under the optimum condition.

In the present invention, it is changed correction start time t _c, since there is no change in it TTb comparing the time synchronization unit 26 is time information between the integration time T, the result, the simultaneous counts The probability is proportional to the simultaneous count calculated by the time synchronization unit 26. Therefore, by comparing the number of simultaneous counting, it is possible to easily detect the correct start time t _c which gives the optimal propagation delay time difference.

When the propagation delay time changes due to a change in the temperature of the transmission line or the like, the above-mentioned optimum t _c changes. However, in the present invention, the above-described optimum the a _{t c} can be easily detected.

  As described above, in the present invention, the optimum propagation delay time difference can be determined using the raw key information itself, so that a separate pulse light for time synchronization as in the conventional example is unnecessary.

The minimum propagation delay time difference determined by the system requirements is _defined as ΔT _{delay. ave-} Δτ, and the maximum propagation delay time difference is ΔT _{delay. ave} + Δτ, the minimum propagation delay time difference ΔT _{delay. ave−} Δτ to the maximum propagation delay time difference ΔT _delay. By changing the correction start time _{t c} in until _ave + .DELTA..tau, simultaneous counting probability _P cc _{+ P acc} provide optimal propagation delay time difference [Delta] T _{delay. opt} can always be found.

It is difficult to apply the present invention to a quantum key distribution system using a single photon light source. This is because, in the case of a quantum key distribution system using a single-photon source, also by changing the above-mentioned correction start time t _c, the simultaneous count rate because the change does not occur.

  Here, in the present invention, the time at which the second detection unit acquires the second raw key B is T + 2Δτ, which is longer than the acquisition time T in the conventional example. As a result, the key generation efficiency is lower in principle than the conventional example.

  However, in reality, for example, as described in the above-described example, when the difference in the transmission distance is 20 km and the temperature change of the optical fibers that are the quantum channels 21 and 31 is at most about 40 ° C., the propagation delay The fluctuation amount of the time difference, that is, the value of 2Δτ is only about 24.8 nsec at most. Since the reduction rate of the key rate is T / (T + 2Δτ), if T = 1 sec, the reduction in the key generation efficiency in this case can be almost ignored.

Further, even when the propagation delay time in each of the quantum channels 21 and 31 greatly fluctuates, ΔT _{delay. ave} and Δτ may be set.

  As described above, this time synchronization method does not require a separate pulse light source or the like, and enables time synchronization between the transmitter and the receiver. For this reason, it is possible to provide a quantum key distribution system using a quantum entangled light source with a lower cost and a simple configuration.

Reference Signs List 12 light source unit 20 transmission side device 21, 31 optical path 22, 32 detection unit 24, 34 signal processing unit 26 time synchronization unit 30 reception side device 501 half mirror 502 wavelength plate 503, 504 polarization beam splitter 505, 506, 507, 508 single One-photon detector

Claims (2)

A light source unit for generating a entangled photon pair,
During a first detection time T from a first start time t _0a to a first end time t _0a + T, one photon of the quantum entangled photon pair is received, and a time indicating the time at which the photon was received. 1 and a first raw key A including first quantum information including first observation base information indicating an observation base of the photon and first bit value information indicating an observation value. A first detector , and
A first signal processing unit that receives the first raw key A from the first detection unit
A first side device having:
Second start time t _0a + ΔT _{delay. ave−} Δτ, the second end point time t _0a + ΔT _delay. During the second detection time T + 2Δτ up to _ave + T + Δτ, the other photon of the quantum entangled photon pair is received, second time information indicating the time at which the photon was received , and the observation base of the photon are indicated. A second detector that obtains a second raw key B including second quantum information including second observation base information and second bit value information indicating an observation value ;
A second signal processing unit that receives the second raw key B from the second detection unit
A second side device having:
A classical channel for transmitting the second time information and the second observation basis information included in the second signal processing unit to the first signal processing unit;
Wherein it provided in the first signal processing unit, the correction start time _t minimum propagation delay time difference _{c t 0a} + _{ΔT delay. ave−} Δτ to the maximum propagation delay time difference t _0a + ΔT _{delay. ave +} varied between .DELTA..tau, for each of the correction start time t _c, we obtain the simultaneous detection count of said second time information and the first time information, the simultaneous detection count is maximum correction start time t _c and the first start time _t the difference _t c _{-t 0a} time origin information [Delta] t _delay of _0a. and time synchronization unit to get as _opt
With
The first signal processing unit of the time origin information [Delta] T _delay. The first and second signal processing units generate an encryption key after time-synchronizing the first quantum information and the second quantum information using _opt. system. A light source unit, a first detection unit, a second detection unit, a first signal processing unit, a second signal processing unit, and a time synchronization unit provided in the first signal processing unit. A time synchronization method performed in a quantum key distribution system comprising:
The first between the first start time t _0a of the first detection time T until the first end point time t _{0a + T} in the detection unit, entangled photon pairs one photon of which the light source unit generates , A first raw key including first quantum information including first time information, first observation base information indicating an observation base, and first bit value information indicating an observation value. A, and the second detection unit obtains the second start time t _0a + ΔT _{delay. ave−} Δτ, the second end point time t _0a + ΔT _delay. During a second detection time T + 2Δτ up to _ave + T + Δτ, second time information, second observation base information indicating the observation base, and observation value obtained by receiving the other photon of the quantum entangled photon pair Obtaining a second raw key B including second quantum information consisting of second bit value information indicating
The first signal processing unit receives the first raw key A from the first detection unit, and the second signal processing unit transmits the second raw key B from the second detection unit. Receiving the
Transmitting the second time information and the second observation base information included in the second signal processing unit to the first signal processing unit;
A step in which the time synchronization unit acquires the first time information and the second time information;
Minimum propagation delay time correction start time _{t c} difference _{t 0a} + _{ΔT delay. ave−} Δτ to the maximum propagation delay time difference t _0a + ΔT _{delay. ave +} varied between .DELTA..tau, for each of the correction start time t _c, we obtain the simultaneous detection count of said second time information and the first time information, the simultaneous detection count is maximum correction start time t _c a difference _t c _{-t 0a} time origin information [Delta] t _delay of the first start time _{t 0a.} obtaining as an _opt ;
The first signal processing unit _{generates the} time origin information ΔT _delay. after time-synchronizing the first quantum information and the second quantum information using _opt , the first and second signal processing units generate an encryption key. Time synchronization method.

Images