JP4134271B1 - Quantum cryptography receiver, quantum cryptography transmitter, and optical pulse phase modulation method - Google Patents

Abstract

Optical pulses 31x to 34x indicate forward optical pulses. The optical pulses 31y to 34y indicate a case where the optical pulses 31x to 34x travel on the return path after the period c has elapsed. Therefore, the optical pulses 31y to 34y are optical pulses 31x to 34x, respectively. Focus on the light pulse 31x. Receiving side control unit _{E B} at time "t-2-c", the phase modulation amount theta _'B1 to the optical pulse 31x by PMB, stored in the reception buffer. Then, the control of the receiving controller E _B, PMB at time "t-2" after the period c, based on the phase modulation amount theta _'B1 stored in the reception buffer, an optical pulse 31y phase Modulate. Thus, PMB is 'by phase-modulating the optical pulses 31y based on the _B1, the phase modulation amount θ of the optical pulse 31x in the forward path' phase modulation amount θ can be canceled effect of _B1. The optical pulses 32x to 34x are the same as the optical pulse 31x.
[Selection] Figure 4

Description

  The present invention relates to a quantum cryptography reception device, a quantum cryptography transmission device, and an optical pulse phase modulation method in Plug & Play quantum cryptography.

  The challenges of quantum cryptography are communication speed and communication distance. Moreover, the two are not independent. As long as they use a single photon or comparable weak light, the two are in a trade-off relationship. This is because the light intensity on the transmission side is determined to be one photon (level). For this reason, if the communication distance is increased, the number of photons that can be reached decreases due to transmission loss on the communication path, resulting in a slower communication speed. In the case of a normal single mode optical fiber, a loss of about 20 dB (= 1/100) occurs at a communication distance of 100 km, so that even if the transmitter emits 100 photons, the receiver can receive only one. In other words, in order for the receiver to receive at an average rate of 1 bps (bits / second), the sender must output at a rate of at least 100 bps. Moreover, this is a case where only the loss of the optical fiber is considered. In practice, various loss factors such as detection efficiency of the detector, loss of the modulator, and Fresnel reflection at the connection end face are added. For this reason, the net transmission rate of the sender is generally required to be 10,000 times or more faster than the reception rate of the receiver. If the reception rate is 1 Kbps, the transmission rate is 10 Mbps, that is, 10 MHz or more. If the reception rate is 1 Mbps, the transmission rate is required to be 10 GHz or more. In other words, even at present, the device operates at a gigahertz level, and even if a photon is transmitted, only a bit rate on the receiving side can be obtained.

  On the other hand, the general view is that further speedup is necessary for practical use. Therefore, research is being actively conducted to fundamentally review the method itself, to cope with higher speeds and to improve efficiency. However, these studies often presuppose the use of stronger light rather than single photons as information carriers. Since the quantum cryptosystem called the BB84 protocol was proposed, its security has been demanded by the “existence” of a single photon. However, many methods that are not based on that premise have been proposed, and those that cannot be evaluated with the same earthwork are mixedly discussed. Of course, I think there is a great possibility that a protocol that will become the mainstream of future quantum cryptography will appear from among these methods, but even the unconditional security of single-photon quantum cryptography using the BB84 protocol is several years ago. Unfortunately, there is no guarantee that these newly proposed quantum ciphers are secure at the moment, given that they have finally been proved.

  This time, the inventors propose a mounting method for speeding up the Plug & Play quantum cryptography without departing from the conventional security framework. As a result, according to this mounting method, there is a possibility that the drive frequency can be increased to the same extent as that of the one-way quantum cryptography. Of course, regarding safety, it is based on the “conventional BB84 protocol + single photon property”, and it is considered that there is no problem.

(Conventional method)
A conventional method of the Plug & Play quantum cryptography (for example, Patent Document 1) will be described with reference to FIG. The Plug & Play quantum cryptography shown in FIG. 8 is a system that automatically compensates for the influence of polarization fluctuations, birefringence, and the like that occur in a communication path when photons reciprocate through the same communication path. As the name suggests, the Plug & Play quantum cryptography is an excellent method that can be stably operated simply by connecting them. For the reasons described above, many quantum cryptography experiments using fibers that have been performed all over the world have been performed using the Plug & Play method. The operation of the Plug & Play quantum cryptography will be described with reference to FIG.

  As shown in FIG. 8, the conventional Plug & Play quantum cryptography system includes a quantum cryptography transmission device 100 on the transmission side that transmits quantum cryptography, and a quantum cryptography reception device 200 on the reception side that receives quantum cryptography. The quantum cryptography transmitter 100 and the quantum cryptography receiver 200 are connected by a communication optical fiber 16 that is a long-distance optical fiber. The quantum cryptography transmission device 100 may be referred to as a base A. The quantum cryptography transmission device 100 is referred to as “Alice” in this field. The quantum cryptography receiving device 200 may be referred to as a base B. The quantum cryptography receiving device 200 is called “Bob” in this field.

The quantum cryptography transmission device 100 includes a transmission-side control device E _A (1), an attenuator 2, a phase modulator 3 (sometimes referred to as PMA3), a Faraday mirror 4 (sometimes referred to as FM4), a photodetector 5, and a coupler. 6 is provided.

The quantum cryptography receiving device 200 includes a receiving side control device E _B (7), a phase modulator 8 (sometimes referred to as PMB8), a laser 9, a polarization beam splitter 10 (sometimes referred to as PBS10), a coupler 11, and a photon detector. 12, a photon detector 13, a circulator 14, and a delay generating optical fiber 15.

  The operation of the method of FIG. 8 will be described. The light pulse emitted from the laser 9 passes through the circulator 14 and enters the coupler 11. The incident optical pulse is transmitted by the coupler 11 through a short path S (path S indicated by a broken line in FIG. 8) and a long path L (path L indicated by a dotted line in FIG. 8) provided with a delay generating optical fiber 15. ) And is divided into two light pulses. In this case, although not shown in FIG. 8, generally, these two light pulses are incident on the PBS 10 after their polarizations are adjusted. When viewed on the time axis, the two optical pulses are divided into a preceding “optical pulse a” and an “optical pulse b” that follows the optical pulse 15 with a delay through the optical fiber 15 for delay generation. That is, the light pulse in the forward path is not one but goes to the base A as two light pulses. Since they are single photons, these two optical pulses are in a superposition state of the two pulses in terms of quantum mechanics.

  The PBS 10 is a device that divides the beam according to the polarization state of the photon. In this case, the optical pulse a that has been adjusted to the vertical polarization through the path S passes through the communication optical fiber 16 as it is (the polarization of the horizontal polarization). Light cannot pass). On the other hand, the light pulse b passing through the path L is incident on the other port of the PBS 10 after the polarization is adjusted in the lateral direction. Since the PBS 10 reflects only light polarized in the horizontal direction, the light pulse b that has passed through the path L is sent to the communication optical fiber 16 (it cannot pass in the vertical direction).

An optical pulse a and an optical pulse b having two different polarization modes are input to the quantum cryptography transmission device 100 through the communication optical fiber 16 at different timings. The optical pulse a and the optical pulse b input through the communication optical fiber 16 are mixed by the coupler 6 and divided. One of the light pulses a and b divided by the coupler 6 is still high-intensity light and is detected by the photodetector 5. According to the optical pulse detection timing of the optical detector 5, the PMA 3 modulates only the optical pulse b of the optical pulses a and b. The other optical pulses a and b divided by the coupler 6 pass through the attenuator 2, pass through the PMA 3, and bounce off at the Faraday mirror 4 (the photodetector 5 is used for timing synchronization as described above). . The Faraday mirror 4 is a mirror that rotates the polarization of photons by 90 degrees in a reciprocating manner. The light bounced off by the Faraday mirror 4 bounces when the plane of polarization is rotated horizontally when it is incident, and vertically when it is horizontally polarized when incident. Of the two optical pulses a and b reflected by the Faraday mirror 4 and passing through the PMA 3, the PMA 3 is controlled by the control device E _A (1) so as to apply phase modulation (θ _A ) only to the second optical pulse b. Timing is adjusted. The preceding optical pulse a and the optical pulse b phase-modulated by the PMA 3 pass through the attenuator 2 again and are weakened. At this point, these optical pulses a and b are single photons or less weak light. Then, it passes through the coupler 6 and returns to the base B through the communication optical fiber 16. At this time, the double pulse is of course unchanged.

If the communication optical fiber 16 is an optical fiber that maintains the polarization state of the photon, the optical pulse a that precedes in time on the forward path through the path S is changed into laterally polarized light by the FM 4 and is at the head. Returning, this time, it is guided to a port different from the forward path by the PBS 10, passes through the path L, and undergoes phase modulation (θ _B ) of the PMB 8.

On the other hand, the light pulse b (θ _A ) which travels in the forward path through the path L, flies as horizontally polarized light, is converted into longitudinally polarized light by the FM4, undergoes phase modulation by the PMA3, and returns with a slight delay. Guided to path S.

Accordingly, the preceding optical pulse a passing through the path S on the forward path passes through the path L on the return path (path S → FM → L). Further, the light pulse b (θ _A ) that has passed through the path L on the forward path passes through the path S on the return path (path L → FM → S). These optical pulses a (θ _B ) and optical pulses b (θ _A ) have the same time to reach the coupler 11, and the photons interfere with each other according to the phase modulation amounts (θ _A and θ _B ) affected on the optical path. Wake up.

The modulation amount by PMA3 and theta _B modulation amount by θ _A, PMB8, also photons through the path S → FM → L, the upper detector 12 to come state back to the time _t | ψ slH>, route L If the state of passing through FM → S and returning to the lower detector 13 at the same time t is | ψ _lsL >, the state of the photon after passing through the coupler 11 | ψ〉 is given by This is a superposition of two states as shown in 1).

However, c is a constant and Δθ = θ _A −θ _B.
That is, the state of the photon changes depending on the values of θ _A and θ _B that are independently determined by the sender and the receiver, the port output from the coupler 11 is changed, and the receiving detector is changed accordingly. This photon interference effect allows key sharing between the sender and receiver. That is, the operation of the phase modulation at the bases A and B is reflected as a result. This allows key sharing between bases. This is an outline of a conventional Plug & Play quantum cryptography as disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-517499.

  The system shown in FIG. 8 is not so difficult to realize as the past results show when the driving speed of the entire system is not so high and the width of the optical pulse is sufficiently narrow everywhere.

  However, the Plug & Play quantum cryptography is considered to be difficult to increase in speed compared to the one-way quantum cryptography. One of the reasons is that when optical pulses are generated continuously at high speed, the optical system reciprocates, so that the forward pulse and the backward pulse are mixed and the modulation timing control becomes strict. is there.

  In particular, when driven at high speed, it is difficult to control the phase modulation timing in the PMB 8 of the receiver called the base B (Bob) in FIG.

  FIG. 9 is a diagram for explaining that it is difficult to control the phase modulation timing in the PMB 8 when the quantum cryptography receiving device 200 as the base B is driven at high speed. With reference to FIG. 9, the difficulty of the phase modulation timing control in the PMB 8 will be described. In FIG. 9, light pulses 51 to 53 indicate forward light pulses. Optical pulses 54 to 57 indicate optical pulses on the return path. Originally, the PMB 8 performs phase modulation on the return optical pulse 54 and the like, and does not perform phase modulation on the outgoing optical pulse 51 and the like. One reason for this is that if phase modulation is performed at a strong light level, information may be leaked by dividing the photons. In this case, if the system (base A and base B) is increased to a high-speed drive of the GHz level or higher with the aim of improving the performance of the quantum cryptography, the interval between the light pulses is reduced by the inverse number. Therefore, the optical pulse must be controlled at a time of nanosecond (ns) or picosecond (ps) level. In general, the Plug & Play system is realized as the past results show, when the drive speed of the whole system is not so fast and the width of the light pulse is kept sufficiently narrow everywhere on the path by dispersion compensation etc. It's not so difficult to do.

  However, when the driving speed is about 1 GHz, the pulse interval D shown in FIG. 9 is about 1 ns. Then, for example, a forward optical pulse that is not subject to phase modulation by the PMB8 and a return (return) optical pulse that is subject to phase modulation by the PMB8 intersect in 1 ns. For example, the forward light pulse 51 exists between the return light pulses 56 and 57. That is, the electrical pulse width B for performing phase modulation is required to be narrower than 1 ns, for example, several hundred ps. It is necessary to set by adjusting the length of the fiber or the like so that the pulses of several hundreds of ps do not overlap with each other as shown in FIG. In other words, it is necessary to perform adjustment by adjusting the length of the fiber or the like so that only the optical pulse on the return path that is the target of phase modulation by the PMB 8 is phase-modulated by the PMB 8.

  Thus, even if the initial setting is made so that only the optical pulse of the return path is phase-modulated so that the optical pulse of the outward path and the optical pulse of the return path do not overlap, the length of the optical path L is reduced with the ambient temperature change. It will change. As a result, the arrival timing of the optical pulse on the return path is shifted. For this reason, a situation occurs in which round-trip light pulses overlap, resulting in unintended modulation. That is, even the forward optical pulse that is not subject to phase modulation by the PMB8 is phase-modulated by the PMB8. In the case of FIG. 9, for example, the optical pulse 56 and the optical pulse 51 overlap, and when the backward optical pulse 56 is modulated, the forward optical pulse 51 that is not scheduled for phase modulation is also phase-modulated. End up.

The coefficient of thermal expansion of the fiber varies depending on the material. For example, when the length is 100 km, the temperature changes by about 70 cm when the temperature changes by 1 ° C. It takes 3.5 ns for light to travel through a 70 cm fiber. In the case of high-speed driving at a driving speed of GHz level, it is fatal that a change of 3.5 ns occurs at a temperature change of only 1 ° C even though the control at the picosecond level is necessary as described above. In fact, manual control is not possible.
JP-T-2000-517499, pages 1 to 26, FIG. 2, FIG. 3, and FIG. JP-A-2003-32249, pages 1 to 9, FIG. 2, FIG. 3, FIG. 4 and FIG. PCT / JP2004 / 007001

  In such a plug & play quantum cryptography, a method of controlling by dividing and assigning time slots has also been proposed (Patent Document 2) for the problem of serious mixing of optical pulse trains with different traveling directions. This is expressed as a so-called intermittent mode. This method is a feature of the Plug & Play method, and a light pulse is continuously transmitted by a certain amount on the side having the light source and the detector, that is, on the site B side, and is turned back on the site A side. This is a method of controlling so that the next pulse is not issued at the time when the signal returns. Therefore, there is no worry that round-trip photons overlap. For example, if the distance between bases is 50 km, the time for light to travel back and forth on this communication path is about 500 μs (the speed of light in the fiber is 1 meter / 5 nanoseconds). Therefore, if light is emitted from a 500 μs laser (500,000 pulses are output if it is a 1 GHz pulse laser), only the detector operates for the next 500 μs, and then the process of emitting 500 μs light is repeated, as described above. It is avoided that round-trip light pulses overlap. Certainly, if this method is used, safety problems can be cleared, and there is also an advantage that the influence of backscattering due to the close relationship between the light source and the photodetector can be eliminated. However, there is a problem that the efficiency is halved as compared with a case where pulses are continuously transmitted.

  Further, Patent Document 3 discloses an improvement plan in which a detour is installed in the reception-side modulator by devising an optical system, and only the return pulse is guided to the modulator. However, there has been a problem that the bit rate is reduced to 1/10 than before due to an increase in loss due to the addition of extra components.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a method for alleviating the difficulty of phase modulation timing control while ensuring the security of quantum cryptography.

The quantum cryptography receiving device of the present invention is:
In a quantum cryptography receiving device that receives the quantum cryptography by the plug and play method from a quantum cryptography transmission device that transmits the quantum cryptography by a plug and play method in the quantum cryptography method,
A reception-side phase modulation unit that receives an optical pulse reflected by the quantum cryptography transmission device and phase-modulates the received optical pulse;
A reception-side phase modulation amount storage unit that stores a phase modulation amount of an optical pulse phase-modulated in the past by the reception-side phase modulation unit;
The reception-side phase modulation unit is
When the optical pulse is phase-modulated, the optical pulse is phase-modulated based on the phase modulation amount stored in the reception-side phase modulation amount storage unit.

The reception-side phase modulation amount storage unit is
The reception-side phase modulation unit stores a corresponding modulation amount that is a phase modulation amount corresponding to an optical pulse to be phase-modulated,
The reception-side phase modulation unit is
When the optical pulse is phase-modulated, the optical pulse is phase-modulated based on the corresponding modulation amount stored in the reception-side phase modulation amount storage unit.

The corresponding modulation amount stored in the reception-side phase modulation amount storage unit is
A quantum cryptography receiving apparatus in which the optical pulse corresponding to the corresponding modulation amount is self for a preset period with respect to the time at which the reception-side phase modulation unit attempts to phase-modulate the optical pulse corresponding to the corresponding modulation amount The phase modulation amount at the past time is the optical pulse reciprocation period indicating the period from the time when the light is transmitted to the time when the light is reflected by the quantum cryptography transmitter and returned to the quantum cryptography receiver itself.

The quantum cryptography receiver further includes:
A reception-side quantum error rate detection unit that detects a quantum error rate in quantum cryptography communication,
The reception-side phase modulation unit is
Based on the phase modulation amount stored in the reception-side phase modulation amount storage unit when the optical pulse is phase-modulated when the detection result of the quantum error rate by the reception-side quantum error rate detection unit exceeds a predetermined value. The optical pulse is phase-modulated.

The quantum cryptography transmission device of this invention is
In a quantum cryptography receiving device that receives quantum cryptography by a plug and play method in a quantum cryptography method, in a quantum cryptography transmission device that sends the quantum cryptography by the plug and play method,
A transmission-side phase modulation unit that receives an optical pulse transmitted from the quantum cryptography reception device and phase-modulates the received optical pulse;
A transmission-side phase modulation amount storage unit that stores a phase modulation amount of an optical pulse phase-modulated in the past by the transmission-side phase modulation unit;
The transmission-side phase modulation unit is
When the optical pulse is phase-modulated, the optical pulse is phase-modulated based on the phase modulation amount stored in the transmission-side phase modulation amount storage unit.

The optical pulse phase modulation method of the present invention comprises:
In a phase modulation method of optical pulses performed by a quantum cryptography receiving device that receives the quantum cryptography by the plug and play method from a quantum cryptography transmission device that transmits the quantum cryptography by a plug and play method in the quantum cryptography method,
The receiving phase modulation unit
Receiving an optical pulse reflected by the quantum cryptography transmitter, phase modulating the received optical pulse,
The receiving-side phase modulation amount storage unit
Storing a phase modulation amount of an optical pulse phase-modulated in the past by the reception-side phase modulation unit;
The reception-side phase modulation unit is
When the optical pulse is phase-modulated, the optical pulse is phase-modulated based on the phase modulation amount stored in the reception-side phase modulation amount storage unit.

The optical pulse phase modulation method of the present invention comprises:
In a phase modulation method of optical pulses performed by a quantum cryptography transmission device that transmits the quantum cryptography by the plug and play method to a quantum cryptography reception device that receives the quantum cryptography by a plug and play method in the quantum cryptography method,
The transmission-side phase modulation unit
Receiving an optical pulse transmitted from the quantum cryptography receiver, phase-modulating the received optical pulse,
The transmission-side phase modulation amount storage unit is
Stores the phase modulation amount of the optical pulse phase-modulated in the past by the transmission-side phase modulation unit,
The transmission-side phase modulation unit is
When the optical pulse is phase-modulated, the optical pulse is phase-modulated based on the phase modulation amount stored in the transmission-side phase modulation amount storage unit.

  According to the present invention, it is possible to provide a method for alleviating the difficulty of phase modulation timing control while ensuring the security of quantum cryptography.

Embodiment 1 FIG.
FIG. 1 is a diagram illustrating a configuration of the quantum cryptography system according to the first embodiment. FIG. 1 shows the configuration of a quantum cryptography transmission device 100 and a quantum cryptography reception device 200 in the Plug & Play quantum cryptography of the first embodiment. In FIG. 1, the same components as those in FIG. 8 are denoted by the same reference numerals. The plug & play quantum cryptography of FIG. 1 differs from the conventional plug & play quantum cryptography of FIG. 8 in the configuration in that the transmission side control device E _A (1) of the quantum cryptography transmission device 100 (base A) is the transmission side buffer 101. (Transmission-side phase modulation amount storage unit) and the reception-side control device E _B (7) of the quantum cryptography reception device 200 includes a reception-side buffer 201 (reception-side phase modulation amount storage unit).

FIG. 2 is a diagram in which the quantum cryptography transmission device 100 is extracted from FIG. As shown in FIG. 2, the quantum cryptography transmission device 100 includes a transmission-side control device E _A (1), an attenuator 2, a PMA 3 (transmission-side phase modulation unit), a Faraday mirror 4, a photodetector 5, a coupler 6, and A transmission-side buffer 101 (transmission-side phase modulation amount storage unit) is provided.

FIG. 3 is a diagram in which the quantum cryptography receiving device 200 is extracted from FIG. As shown in FIG. 3, the quantum cryptography receiving device 200 includes a receiving side control device E _B (7) (receiving side quantum error rate detecting unit), a phase modulator 8 (receiving side phase modulating unit), a laser 9, a PBS 10, A coupler 11, a photon detector 12, a photon detector 13, a circulator 14, a delay generating optical fiber 15, and a reception side buffer 201 (reception side phase modulation storage unit) are provided.

  FIG. 4 is a diagram for explaining the phase modulation operation of the PMB 8 of FIG. The feature of the quantum cryptography system in FIG. 1 is that the PMB 8 performs phase modulation on the optical pulse based on the past phase modulation amount stored in the reception-side buffer 201. The phase modulation operation of the PMB 8 will be described with reference to FIG. In FIG. 4, optical pulses 31x to 34x indicate forward optical pulses. Optical pulses 31y to 34y indicate cases where the optical pulses 31x to 34x travel on the return path, and the optical pulses 31y to 34y are the same as the optical pulses 31x to 34x, respectively. That is, the light pulse 31y is a light pulse 31y after the period c has elapsed and the forward light pulse 31x becomes a return light pulse. The same applies to the optical pulses 32y to 34y. FIG. 4 shows the current phase modulation data 38 and the phase modulation data 37 before 500 μs (modulation data preceding the phase modulation data 38 by the period c) for each optical pulse. 500 μs is the time required for the optical pulse to reciprocate between the base B and the base A when the length of the communication optical fiber 16 is 50 km (optical pulse reciprocation period).

First, regarding the timing of modulation by the PMB 8, the normal operation in the case where the forward optical pulse and the backward optical pulse do not overlap follows the conventional Plug & Play system.
That is, the difference Δθ between θ _A and θ _B , which is the modulation amount between the bases A and B,
That is, Δθ = | θ _A −θ _B |
Accordingly, the photon detector 12 or the photon detector 13 reacts and accumulates data.

  On the other hand, FIG. 4 shows a countermeasure when the optical pulses are overlapped in the round trip at the timing of modulation by the PMB8. A normal state (a state in which optical pulses in the forward path and the backward path do not overlap during modulation by the PMB8 as shown in FIG. 9) and a state in which timings overlap as shown in FIG. 4 (a forward path and a return path in the modulation by the PMB8) Is detected as an increase in the quantum error rate (QBER) due to a change in the modulation amount (a change in the modulation amount due to the modulation to the optical pulse in the forward path that should not be modulated). Is possible.

In the quantum cryptography system according to the first embodiment, the reception-side buffer 201 included in the control device E _B (7) stores the modulation amount θ ′ _B (corresponding modulation amount) that the forward optical pulse would have been received by the PMB 8. The forward path modulation by the PMB 8 is not planned, and this modulation amount θ ′ _B is the modulation amount that the optical pulse on the return path should receive. The PMB 8 adds the modulation amount θ ′ _B to modulate the optical pulse on the return path.

This will be specifically described with reference to FIG. The state in which the optical pulses of the forward path and the backward path overlap during modulation by the PMB8 as shown in FIG. 4 can be detected by the control device E _B (7) as an increase in the quantum error rate (QBER). The control device E _B (7) always stores the phase modulation amount by the PMB 8 in the reception buffer 201, and overwrites the phase modulation amount when it is determined that the quantum error rate (QBER) exceeds a predetermined threshold.
That is,
In FIG.
At time “t-2-c”, the control device E _B (7) stores the phase modulation amount (θ ′ _B1 ) of the optical pulse 31x by the PMB8 in the reception-side buffer 201.
Similarly,
The control device E _B (7) stores the phase modulation amount (θ ′ _B2 ) to the optical pulse 32x by the PMB 8 in the reception-side buffer 201 at time “t-1-c”.
Similarly,
The control device E _B (7) stores the phase modulation amount (θ ′ _B3 ) to the optical pulse 33x by the PMB 8 in the reception side buffer 201 at time “t−c”.
Similarly,
At time “t + 1-c”, the control device E _B (7) stores the phase modulation amount (θ ′ _B4 ) to the optical pulse 34 x by the PMB 8 in the reception side buffer 201.
Similarly, the control device E _B (7) stores all the phase modulation amounts in the reception-side buffer 201.
And
By the control of the control device E _B (7),
The PMB 8 phase-modulates the optical pulse 31y based on the phase modulation amount (θ ′ _B1 ) stored in the reception-side buffer 201 at time “t−2” after the period c. That is, when the original modulation amount is (θ _B1 ), the PMB 8 phase-modulates the optical pulse 31 y on the return path with a phase modulation amount that can completely overwrite the phase modulation (θ ′ _B1 ) on the optical pulse 31 x on the forward path. . In other words, the strong light in the forward path can be eavesdropped because of the strong light rather than the single photon, but cannot be eavesdropped on the return path because of the single photon level.
Similarly,
The PMB 8 performs phase modulation on the optical pulse 32y based on the phase modulation amount (θ ′ _B2 ) stored in the reception-side buffer 201 at time “t−1” after the period c.
Similarly,
The PMB 8 phase-modulates the optical pulse 33y based on the phase modulation amount (θ ′ _B3 ) stored in the reception-side buffer 201 at time “t” after the period c.
Similarly,
The PMB 8 performs phase modulation on the optical pulse 34 y based on the phase modulation amount (θ ′ _B4 ) stored in the reception buffer 201 at time “t + 1” after the period c.

That is, the reception-side buffer 201 is a time before the time T (the optical pulse reciprocation period) required for the optical pulse to reciprocate in the communication path with respect to the time T when the PMB 8 attempts to phase-modulate the optical pulse on the return path. The phase modulation data (corresponding modulation amount) at a certain time “Tc” is stored. Conceptually, the time required for the light pulse to travel through the communication path 36 indicated by a broken line in FIG. 4 is the period c. The PMB 8 has a phase modulation amount “θ _B + θ ′ _B ” for canceling the phase modulation amount received by the optical pulse in the forward path based on θ ′ _Bi stored in the reception buffer 201 (or depending on the design of the optical system. In “θ _B −θ ′ _B ”), phase modulation is performed on the optical pulse in the return path.

Thus, from θ ′ _Bi stored in the reception-side buffer 201 and the conventional θ _A and θ _B ,
Difference in modulation amount between bases A and B, Δθ,
That is, Δθ = | θ _A −θ _B ± θ ′ _Bi |
The side detected from the coupler 11 is determined by the value of.

As described above, the distinction between the normal state (the state where the optical pulses do not overlap in the forward path and the backward path as shown in FIG. 9) and the state where the timings overlap as shown in FIG. This can be detected as an increase in the quantum error rate (QBER) due to the modulation of the forward optical pulse that should not be modulated. However, if the quantum error rate is not improved even when the forward data (θ ′ _Bi ) is taken into account, it can be determined that this is due to wiretapping. Therefore, it is preferable to discard the corresponding data and interrupt the processing. Further, when the modulation data (θ ′ _Bi ) that would have been received in the forward path is taken into account, it is conceivable that some deviation occurs in the corresponding data (corresponding inbound optical pulse). In this case, the stored data (θ ′ _Bi ) is replaced with data (θ ′ _{B, i−1} ) one pulse before (or data θ ′ _{B, i + 1} one pulse ahead), etc. A mechanism to find timing by improving the system will be incorporated.

  A more specific case of FIG. 4 will be described with reference to FIG. FIG. 5 is a simplified version of FIG. The light pulse Q emitted from the laser 9 passes through the circulator 14 and enters the coupler 11. The incident optical pulse Q is split by the coupler 11 into an optical pulse a passing through a short path S and an optical pulse b passing through a long path L provided with a delay generating optical fiber 15. As described in the background art, it is divided into the preceding “light pulse a” and “light pulse b” that follows the light pulse a with a delay of about the time passing through the delay generating optical fiber 15.

(About pulse b)
In PMB8, the reciprocating optical pulses are in a state of overlapping. For this reason, when the PMB 8 performs phase modulation on the optical pulse (b-1) in the return path, the optical pulse b also undergoes phase modulation in the forward path. Optical pulse b in FIG. 5, an optical pulse (b-1) shows the case of receiving the phase modulation of theta _'B by PBS 8. The reception-side buffer 201 (FIG. 3) stores the phase modulation amount θ ′ _B for the forward optical pulse b under the control of the reception-side control device E _B (7). theta _'B only a phase-modulated optical pulse b optical pulse b (theta' written as _B). Optical pulse b (θ _'B) is input to the station A via the communication optical fiber 16. The optical pulse b (θ ′ _B ) is reflected by the FM 4 and further phase-modulated by the PMA 3. In this case, the phase modulation amount is theta _A. An optical pulse b (θ ′ _B ) phase-modulated by the PMA 3 is written as an optical pulse b (θ ′ _B + θ _A ). The optical pulse b (θ ′ _B + θ _A ) travels from the communication optical fiber 16 to the coupler 11 via the PBS 10.

(About preceding pulse a)
On the other hand, the preceding optical pulse a passes through the PBS 10 to the communication optical fiber 16 as it is. The pulse a is reflected by the FM 4 and sent to the communication optical fiber 16 without being phase-modulated by the PMA 3. Then, the optical pulse a is reflected by the PBS 10 toward the path L, and undergoes phase modulation by the PMB 8. At this time, under the control of the reception side control device E _B (7), the PMB 8 phase-modulates the pulse a based on the phase modulation amount θ ′ _B stored in the reception side buffer 201. That is, the PMB 8 phase-modulates the optical pulse a by the phase modulation amount “θ _B + θ ′ _B ”. An optical pulse a phase-modulated by “θ _B + θ ′ _B ” is written as an optical pulse a (θ _B + θ ′ _B ).
in this case,
Δθ = | (θ ′ _B + θ _A ) − (θ _B + θ ′ _B ) |
= | Θ _A -θ _B |
Thus, it is possible to obtain the same result as in a normal state (a state in which light pulses do not overlap in the forward path and the return path).

  As described above, in the quantum cryptography receiving device 200, the PMB 8 modulates the optical pulse based on the phase modulation amount stored in the reception-side buffer 201. Therefore, the phase modulation is performed after ensuring the security of the quantum cryptography. The difficulty of timing control can be alleviated.

(Control of phase modulation timing at site A)
6 and 7 are diagrams for explaining the phase modulation timing at the site A by the PMA 3. The phase modulation timing at the site A by the PMA 3 will be described with reference to FIGS. Regarding the phase modulation timing of the site A, the influence of the temperature change is not a significant problem. This is because, since the distance between PMA 3 and FM 4 is short, once the modulation timing is adjusted, the expansion and contraction of the communication path due to the influence of temperature is considered to be small thereafter. However, as shown in FIG. 6, the forward and backward pulses also exist at site A. When FIG. 6 is described in comparison with FIG. 1, the forward path is a path from PMA 3 to FM 4. The return path is a path from FM4 to PMA3. As shown in FIG. 6, since there are pulses for the forward path and the backward path also at the site A, it is necessary to adjust the phase modulation timing by distinguishing the optical pulses for the forward path and the backward path when setting the phase modulation timing. This is hard. Therefore, similarly to the site B side, the phase modulation amount in the forward path is also stored in the site A, and the phase modulation amount in the return path is determined based on the stored phase modulation amount in the forward path. This increases the degree of freedom in timing adjustment within the base A, and it is considered that adjustment is easy. However, as a price, the buffer to be stored and some calculations are increased.

The operation will be described below with reference to FIG. The specific operation is the same as that of the base B. FIG. 7 shows a state in which light pulses in the forward path and the return path overlap during modulation by the PMA 3. FIG. 7 corresponds to FIG. In FIG. 7, optical pulses 41x to 44x indicate forward optical pulses. Optical pulses 41y to 44y show the case where the optical pulses 41x to 44x travel on the return path, and the optical pulses 41y to 44y are the same as the optical pulses 41x to 44x, respectively. In addition, the present phase modulation data 48 and phase modulation data 47 preceding the time c (optical pulse reciprocation period) from the time at which the optical pulse is to be modulated are shown. In this case, the period c is a time required for the light pulse to travel from PMA 3 to FM 4 (outward path), to be reflected by FM 4 and to return to PMA 3 (return path). As shown in FIG. 7, the state in which the optical pulses of the forward path and the backward path overlap during modulation by PMA3 is caused by the control device E _A (1) as an increase in the quantum error rate (QBER) as in FIG. It can be detected.
In FIG.
At time “t-2-c”, the control device E _A (1) stores the phase modulation amount (θ ′ _A1 ) to the optical pulse 41x by the PMA 3 in the transmission side buffer 101.
Similarly,
The control device E _A (1) stores the phase modulation amount (θ ′ _A2 ) to the optical pulse 42x by the PMA 3 in the transmission side buffer 101 at the time “t-1-c”.
Similarly,
The control device E _A (1) stores the phase modulation amount (θ ′ _A3 ) to the optical pulse 43x by the PMA ₃ in the transmission side buffer 101 at time “t−c”.
Similarly,
The control device E _A (1) stores the phase modulation amount (θ ′ _A4 ) to the optical pulse 44x by the PMA 3 in the transmission side buffer 101 at the time “t + 1−c”.
Similarly, the control device E _A (1) stores all the phase modulation amounts in the transmission side buffer 101.
And by control of control device E _A (1),
The PMA 3 performs phase modulation on the optical pulse 41y based on the phase modulation amount (θ ′ _A1 ) stored in the transmission buffer 101 at time “t−2” after the period c. That is, when the original modulation amount is (θ _A1 ), PMA 3 performs phase modulation on the optical pulse 41 y on the return path with a phase modulation amount that cancels the phase modulation (θ ′ _A1 ) with respect to the optical pulse 41 x on the forward path.
Similarly,
The PMA 3 performs phase modulation on the optical pulse 42y based on the phase modulation amount (θ ′ _A2 ) stored in the transmission buffer 101 at time “t−1” after the period c.
Similarly,
The PMA 3 performs phase modulation on the optical pulse 43y based on the phase modulation amount (θ ′ _A3 ) stored in the transmission side buffer 101 at time “t” after the period c.
Similarly,
The PMA 3 performs phase modulation on the optical pulse 44y based on the phase modulation amount (θ ′ _A4 ) stored in the transmission side buffer 101 at time “t + 1” after the period c.

That is, the transmission side buffer 101 causes the optical pulse to reciprocate along the path at the time T (for example, t−2, t−1, t, t + 1, etc.) when the PMA 3 attempts to phase modulate the optical pulse on the return path. The phase modulation data (corresponding modulation amount) at the time “Tc”, which is the time before the period c (light pulse reciprocation period) required for, is stored. Based on θ ′ _Ai stored in the transmission-side buffer 101, the PMA 3 performs phase modulation on the optical pulse in the return path with a phase modulation amount that cancels the phase modulation amount received by the optical pulse in the forward path.

  As described above, in the quantum cryptography transmission device 100, the PMA 3 modulates the optical pulse based on the phase modulation amount stored in the transmission side buffer 101. Therefore, the phase modulation is performed after ensuring the security of the quantum cryptography. The difficulty of timing control can be alleviated.

  The operations of the quantum cryptography reception device 200 and the quantum cryptography transmission device 100 have been described above. However, these operations can also be grasped as optical pulse phase modulation methods.

(About the security of the quantum cryptography of Embodiment 1)
In quantum cryptography, the absolute security feature is the biggest selling point. However, much research has been done so far, but it is still not enough. In particular, the major factor in safety discussions is the light source, but much of the discussion is based on an ideal single-photon source that does not actually exist.

  Most of the actual systems are coherent light sources, and the appearance distribution of photons follows a Poisson distribution. That is, the probability that two or more photons are output at the same timing instead of a single photon is not zero. At present, many quantum cryptography systems use a weak light source having an average photon number of 0.1 (a probability that one photon is output in 1 out of 10 pulses) as the intensity of a coherent light source. . In this case, the probability that two or more photons are transmitted in the same pulse is 0.5% of the total. If the average number of photons is 1, which is ten times as strong as 0.1, the probability of two or more transmissions increases to about 50% of all pulses. Therefore, at this rate, theoretically, the eavesdropping of the raw key is successful. The amount of information that may have been taken by this eavesdropper is estimated, and the security is enhanced by an information-theoretic procedure such as confidentiality enhancement processing.

  In the past, the inventors considered a method of performing phase modulation across adjacent pulses because of the strict time interval of phase modulation when aiming at high-speed driving at the GHz level. However, in this method, two photons encoded with the same modulation amount at the same time exist in the communication path, which is problematic from the viewpoint of safety, and there is a background to sealing this idea. However, as in the first embodiment, the photon that undergoes phase modulation is a single photon, and the single photon is subjected to a plurality of operations (actions), and the phase modulation that is applied last. However, if there is an influence capable of destructively overwriting all the previous modulations, the safety is the same as the safety of the conventional single photon phase modulation, and the conventional argument can be applied as it is. Therefore, the system of the first embodiment is considered to satisfy the safety requirement based on the conventional single photon nature.

(Pipeline processing, subtle sending timing problems)
In the first embodiment, the method of storing the phase modulation amount in the forward path and determining the phase modulation for one photon by the sum total with the phase modulation in the backward path has been described. It should be noted here that “the security of the quantum cryptography according to the first embodiment” is also described, but the information needs to be distributed to two or more photons, and these photons need not be sent to the communication path. is there. For this purpose, the pulse width of the voltage applied to perform phase modulation must be within the photon pulse interval. Otherwise, when the phase modulation timing is shifted, the same information is encoded into two photons, which is not safe (from the viewpoint of the control circuit such as duty ratio, the photon pulse About half of the interval is preferable for modulation pulse width because there is no bias of voltage fluctuation). Strictly speaking, photons carrying the same phase information should not be transmitted before the observation at the site B is completed. (However, it is assumed that the site is safe.) Under this restriction, processing can be performed in a pipeline manner in order to increase the speed. As a result, it is possible to realize twice the efficiency compared to the intermittent operation mode (Patent Document 2), to be resistant to temperature fluctuations, to be able to operate at high speed even in the Plug & Play system, and to realize a stable system.

(Protocol modified from BB84 protocol)
In the system of the first embodiment, phase modulation following the BB84 system has been described. However, it is also possible to perform modulation of the BB84 protocol as a sum by assuming that the phase modulation is performed reciprocally in the first place. That is, in addition to modulation such as π and π / 4, modulation such as 3π / 7 on the forward path, 4π / 7 on the return path, and total π is possible. That is, a system that realizes the BB84 protocol with the total sum being 0, π / 4, π / 2, and 3π / 4 can be easily considered.

1 is a diagram illustrating a configuration of a quantum cryptography system in Embodiment 1. FIG. 1 is a block diagram of a quantum cryptography transmission device 100 according to Embodiment 1. FIG. 2 is a block diagram of the quantum cryptography reception device 200 according to Embodiment 1. FIG. FIG. 6 is a diagram for explaining the phase modulation operation of the PMB 8 in the first embodiment. FIG. 4 is a diagram for explaining a specific operation of phase modulation of the PMB 8 in the first embodiment. FIG. 3 is a diagram for explaining phase modulation timing by PMA 3 in the first embodiment. FIG. 4 is another diagram illustrating phase modulation timing by PMA 3 in the first embodiment. The figure which shows a prior art example. Another figure which shows a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Transmission side control apparatus E _A , 2 Attenuator, 3 Phase modulator, 4 Faraday mirror, 5 Photo detector, 6 Coupler, 7 Reception side control apparatus E _B , 8 Phase modulator, 9 Laser, 10 Polarization beam splitter, 11 coupler, 12, 13 photon detector, 14 circulator, 15 optical fiber for delay generation, 16 optical fiber for communication, 101 transmission side buffer, 201 reception side buffer, 100 quantum cryptography transmission device, 200 quantum cryptography reception device.

Claims (7)

In a quantum cryptography receiving device that receives the quantum cryptography by the plug and play method from a quantum cryptography transmission device that sends the quantum cryptography by a plug and play method in the quantum cryptography method,
A reception-side phase modulation unit that receives an optical pulse reflected by the quantum cryptography transmission device and phase-modulates the received optical pulse;
A reception-side phase modulation amount storage unit that stores a phase modulation amount of an optical pulse phase-modulated in the past by the reception-side phase modulation unit;
The reception-side phase modulation unit is
A quantum cryptography reception device that modulates a phase of an optical pulse based on a phase modulation amount stored in the reception-side phase modulation amount storage unit when phase-modulating the optical pulse. The reception-side phase modulation amount storage unit is
The reception-side phase modulation unit stores a corresponding modulation amount that is a phase modulation amount corresponding to an optical pulse to be phase-modulated,
The reception-side phase modulation unit is
2. The quantum cryptography receiving device according to claim 1, wherein when the optical pulse is phase-modulated, the optical pulse is phase-modulated based on the corresponding modulation amount stored in the reception-side phase modulation amount storage unit. The corresponding modulation amount stored in the reception-side phase modulation amount storage unit is
A quantum cryptography receiving apparatus in which the optical pulse corresponding to the corresponding modulation amount is self for a preset period with respect to the time at which the reception-side phase modulation unit attempts to phase-modulate the optical pulse corresponding to the corresponding modulation amount 3. The phase modulation amount at a past time during an optical pulse reciprocation period indicating a period of time from transmission to transmission after being reflected by the quantum cryptography transmission device and returning to the self quantum cryptography reception device again. Quantum cryptography receiver. The quantum cryptography receiver further includes:
A reception-side quantum error rate detection unit that detects a quantum error rate in quantum cryptography communication,
The reception-side phase modulation unit is
Based on the phase modulation amount stored in the reception-side phase modulation amount storage unit when the optical pulse is phase-modulated when the detection result of the quantum error rate by the reception-side quantum error rate detection unit exceeds a predetermined value. The quantum cryptography receiver according to claim 1, wherein the optical pulse is phase-modulated. In a quantum cryptography receiving device that receives quantum cryptography by a plug and play method in a quantum cryptography method, in a quantum cryptography transmission device that sends the quantum cryptography by the plug and play method,
A transmission-side phase modulation unit that receives an optical pulse transmitted from the quantum cryptography reception device and phase-modulates the received optical pulse;
A transmission-side phase modulation amount storage unit that stores a phase modulation amount of an optical pulse phase-modulated in the past by the transmission-side phase modulation unit;
The transmission-side phase modulation unit is
A quantum cryptography transmitting device characterized in that, when phase-modulating an optical pulse, the optical pulse is phase-modulated based on the phase modulation amount stored in the transmission-side phase modulation amount storage unit. In a phase modulation method of optical pulses performed by a quantum cryptography receiving device that receives the quantum cryptography by the plug and play method from a quantum cryptography transmission device that transmits the quantum cryptography by a plug and play method in the quantum cryptography method,
The receiving phase modulation unit
Receiving an optical pulse reflected by the quantum cryptography transmitter, phase modulating the received optical pulse,
The receiving-side phase modulation amount storage unit
Storing a phase modulation amount of an optical pulse phase-modulated in the past by the reception-side phase modulation unit;
The reception-side phase modulation unit is
A phase modulation method of an optical pulse, characterized in that when an optical pulse is phase-modulated, the optical pulse is phase-modulated based on the phase modulation amount stored in the reception-side phase modulation amount storage unit. In a phase modulation method of optical pulses performed by a quantum cryptography transmission device that transmits the quantum cryptography by the plug and play method to a quantum cryptography reception device that receives the quantum cryptography by a plug and play method in the quantum cryptography method,
The transmission-side phase modulation unit
Receiving an optical pulse transmitted from the quantum cryptography receiver, phase-modulating the received optical pulse,
The transmission-side phase modulation amount storage unit is
Stores the phase modulation amount of the optical pulse phase-modulated in the past by the transmission-side phase modulation unit,
The transmission-side phase modulation unit is
A phase modulation method for an optical pulse, characterized in that when an optical pulse is phase-modulated, the optical pulse is phase-modulated based on the phase modulation amount stored in the transmission-side phase modulation amount storage unit.

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