JP4591972B2 - Quantum key distribution system - Google Patents

Description

  The present invention relates to a quantum encryption key distribution system, and more particularly to a quantum encryption key distribution system that supplies a secure encryption key using a relative phase difference of a phase-modulated optical pulse train.

  Conventionally, as a cryptographic technique, an encryption method based on mathematical calculation difficulty of a mathematical formula (for example, a long time is required for calculation for decryption) has been used, but recently, a single photon level is used. Research on quantum cryptography that uses light to guarantee physical security is ongoing.

  Quantum cryptography is a cryptographic technique that uses the theory of quantum mechanics, and is known as the ultimate cryptographic technique in which the content becomes meaningless even if eavesdropping, and the eavesdropping is understood.

  In the field of quantum communication, there is known a system that supplies a secret key for performing cryptographic communication between two parties present at points distant from each other, and this system is also called a quantum key distribution system.

  There are various methods for quantum key distribution. In this specification, a “differential phase shift quantum key distribution method” (Non-Patent Document 1), which is a typical prior art, will be described.

  FIG. 1 shows the basic configuration of the prior art differential phase shift quantum key distribution system. In FIG. 1, the transmitter 100 includes a control unit (not shown), storage means (not shown), and communication means (not shown). Similarly, the later-described receiver 110 also includes a control unit (not shown), storage means (not shown), and communication means (not shown).

  The transmitter 100 stores in the storage means (not shown) the phase modulation data of the optical pulse train 106 transmitted by itself, and at the same time the coherent optical pulse train 106 with a constant interval, which is arbitrarily phase-modulated with 0 or π, per pulse. It is transmitted to the optical transmission line 107 at an average of less than 1 photon (for example, 0.1 photon / pulse). The state where the average number of photons is less than 1 can be realized by greatly attenuating normal laser light.

  When such a light pulse train is detected by the receiver 110, the detection result is that a photon is detected in a certain pulse and nothing is detected in a certain pulse. The pulse at which a photon is detected is uncertain until it is measured.

  As shown in FIG. 1, the optical pulse train 106 transmitted from the transmitter 100 reaches the receiver 110 via the optical transmission path 107. First, the receiver 110 uses the optical branching means 111 to split the optical pulse train received from the transmitter 100 into two so as to be equally divided in energy, and the branched optical pulse trains are divided into the long path 112 and Send to short path 113. In the long path 112, a certain delay (in this specification, time T) is added to the optical pulse train. Thereafter, the optical pulse train that has passed through the long path 112 and the short path 113 is combined again by the 2 × 2 multiplexing coupler 114. The 2 × 2 multiplexing coupler 114 includes two input terminals (connected to the long path 112 and the short path 113, respectively), and receives an optical pulse train that has passed through the long path 112 and the short path 113. The 2 × 2 multiplexing coupler 114 also includes two output terminals, each of which has a first photon detector (photon detector 1) 115 and a second photon detector (photon detector 2). ) 116.

  Assume that the constant delay time T given to one optical pulse train in the long path 112 is equal to the constant interval T of the optical pulse train input from the transmitter 100 to the receiver 110 via the optical transmission path 107. In the 2 × 2 multiplexing coupler 114 (indicated by T in FIG. 1), the preceding and following pulses overlap and are multiplexed. The manner in which the pulses overlap is illustrated below the receiver 110 in FIG.

  The optical pulse train input to the receiver 110 is phase-modulated by 0 or π. Therefore, if the propagation phase of the branching / combining path in the receiver 110 is appropriate, the phase difference between the overlapping pulses is 0 or π.

  As a result of multiplexing by the 2 × 2 multiplexing coupler 114, both pulses interfere. If the phase difference is 0, the first detector 115 detects the photon, and if the phase difference is π, the second detector 116 detects the photon. Will be detected.

  Using the above configuration, the transmitter 100 and the receiver 110 obtain a secret key by the following procedure.

  First, the receiver 110 detects a photon from a pulse transmitted from the transmitter 100 through the optical transmission path 107 with the above-described configuration. At this time, the receiver 110 records the photon detection time and the photon detector (115 or 116) in a built-in storage means (not shown) by a control unit (not shown). After a predetermined number of photons are received, the receiver 110 detects the time when the photons are detected by the transmitter 100 via a communication means (not shown) and a predetermined communication path (not shown) (photon detection). Time).

  The transmitter 100 uses the first detector 115 or the second photon detector 116 to detect a photon from the photon detection time notified from the receiver 110 and the phase modulation data of the transmitter itself. The receiver 110 can know.

  Therefore, if the photon is detected by the first photon detector 115 as bit “0” and the photon detected by the second photon detector 116 is determined as bit “1” in advance, The receiver 110 can obtain the same bit string on both sides.

  In the above procedure, since only the photon detection time is notified from the receiver 110 to the transmitter 100, the bit information does not go outside the receiver 110 and is not eavesdropped.

  In order to implement the key distribution system, a photon detector capable of detecting one photon is required. In such a photon detector, an avalanche-photo-diode (APD) to which a high bias voltage is applied is usually used. However, the performance of InGaAs-APD for the fiber communication wavelength band (1.3-1.5 μm band) is not good, especially after detecting a photon once, and subsequently detecting an erroneous pulse due to the detection effect. This phenomenon is a cause of system performance degradation. Therefore, in order to avoid the occurrence of the after pulse, the APD is normally used in a gate operation mode in which a high bias voltage is applied intermittently with a time interval. The upper limit of the repetition frequency of the gate operation is several MHz. At a frequency higher than this upper limit, the false detection rate due to the after pulse increases. For this reason, the upper limit of the gate frequency of the APD is a factor that limits the key generation rate of the key distribution system.

  On the other hand, Si-APD operating in a short wavelength band (0.5-0.8 μm) has excellent performance in both photon detection efficiency / noise characteristics. The after-pulse generation probability is also small, so there is no need for gate operation. If this characteristic can be used for detection of photons in the communication wavelength band, the key generation rate of the quantum key distribution system can be greatly improved. Then, research of the photon detector which converts the wavelength of a communication wavelength band photon into a short wavelength band and detects this with Si-APD has been advanced in recent years (Non-Patent Document 2).

FIG. 2 shows a basic configuration of such a wavelength modulation photon detector. A signal photon having a wavelength of 1.5 μm is combined with pump light having a wavelength of 1.3 μm by the optical multiplexer 201 and input to the optical nonlinear element 202. In the optical nonlinear element 202, a signal photon having a wavelength of 1.5 μm is converted into a photon having a wavelength of (1 / 1.5 + 1 / 1.3) ⁻¹ = 0.7 μm by a light mixing phenomenon called optical parametric interaction. The The wavelength-converted photon (wavelength-converted photon) is taken out by the optical filter (pump light blocking filter) 203 and detected by the Si-APD 204. Thereby, it is possible to detect the 1.5 μm signal photon by the Si-APD 204 without performing the gate operation, and the key generation rate of the quantum key distribution system can be increased.

K. Inoue, E. Waks, Y. Yamamoto, `` Differential-phase-shift quantum key distribution using coherent light '' 2003, Physical Review A, vol.68, paper number 022317 C. Langrock, E. Diamanyi, RV Roussev, Y. Yamamoto, MM Fejer, H. Takesue, `` Highly efficient single-photon detection at communication wavelength by use of upconversion in reverse-proton-exchanged periodically poled LiNbO3 waveguide '', 2005 , Optics Letters, vol.30, pp.1725-1727

  The wavelength conversion type photon detector as described above has an advantage that non-gate operation is possible, but has a disadvantage of being polarization dependent. The efficiency of the optical parametric interaction that occurs in the optical nonlinear element 202 depends on the polarization state of the pump light and the signal light photon. Therefore, the detection efficiency as a photon detector depends on the polarization state of the signal photon. The signal photon has propagated through the optical fiber (for example, the optical transmission line 107), and its polarization state varies depending on the transmission environment. In order to cope with this, a control system that makes the input polarization state of the signal photon to the photon detector constant is required, which complicates the transmission apparatus.

  In order to solve the above problems, an object of the present invention is to provide a quantum key distribution system that uses a wavelength conversion photon detector and does not require complicated polarization control of signal light. .

  To achieve the above object, according to the present invention, in a quantum key distribution system including a transmitter and a receiver, the transmitter transmits a light pulse train composed of pulses having a constant time interval, and is transmitted from the light source. A phase modulator that phase-modulates the optical pulse train with 0 or π, and a polarization modulator that alternately outputs the optical pulse train phase-modulated with the phase modulator as two polarization states orthogonal to each pulse. And an attenuation means for transmitting the optical pulse train output from the polarization modulation means as an optical pulse train having an average of less than one photon per pulse, and the receiver transmits from the attenuation means of the transmitter. Branching means for receiving the received optical pulse train and branching it into a first optical pulse train and a second optical pulse train, and a time of the optical pulse train sent from the transmitter for the first optical pulse train. A delay unit that delays by a time twice as long as the interval; and an optical multiplexing unit that combines the first optical pulse train and the second optical pulse train delayed by the delay unit to generate a combined optical pulse. The relative phase difference between the delayed pulse of the first optical pulse train and the second optical pulse train of the optical pulse generated by the wave means and the optical multiplexing means is zero. In this case, the first photon detection means for detecting photons, and the delayed first optical pulse train and the second optical pulse train of the optical pulses generated by the optical multiplexing means are shifted by two pulses. Second photon detection means for detecting photons, photon detection means for detecting photons among the first and second photon detection means, and a detection time thereof, when the relative phase difference between the pulses is π. Recording means for recording The features.

  Here, the polarization modulation means branches the optical pulse train from the phase modulator, delays it by half of the time interval of one of the branched pulse trains, and converts it to an orthogonal polarization state and converts it. It is also possible to combine the pulse train and the other branched pulse train.

  Further, the polarization modulation means includes a birefringence element arranged so that a fast axis and a slow axis are 45 degrees with respect to a polarization state of the optical pulse train from the phase modulator. You can also.

  The transmitter transmits an optical pulse train having a predetermined length to the receiver, the receiver notifies the transmitter of the photon detection time when the photon is detected, and the transmitter has its own phase. From the modulated data and the photon detection time received from the receiver, it is determined which one of the first and second photon detection means has detected a photon, and the transmitter and the receiver For a photon detected event, a bit “0” is provided for photon detection by the first photon detection means, and a bit “1” is provided for photon detection by the second photon detection means, The common bit value obtained as a result can be used as a secret key bit.

  With the above configuration, according to the present invention, in the quantum key distribution system using the photon detector having polarization dependency, the polarization and multiplexing of the optical pulse are controlled in order to correct the disturbance of the polarization state. Therefore, the influence of the polarization state can be easily removed without performing complicated polarization control, and a stable key generation operation can be obtained.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 3 shows the configuration of the quantum key distribution system according to the first embodiment of the present invention. In FIG. 3, the transmitter 300 includes a light source 301, a phase modulator 302, a polarization modulator 303, an attenuation unit 304, and a control unit (CPU 1) 305.

  The light source 301 is connected to the phase modulator 302. The phase modulator 302 is connected to the polarization modulator 303, and the polarization modulator 303 is further connected to the attenuation means 304. That is, the phase modulator 352 and the polarization modulator 303 exist between the light source 301 and the attenuation means 303.

  The control unit 305 is a CPU (Central Control Unit) that performs the main control of the entire transmitter 300. The transmitter 300 also stores various data in addition to a ROM (not shown) storing a light source 301, a phase modulator 302, a polarization modulator 303, an attenuation unit 304, a control program for the control unit 305, and the like. In addition, a memory (not shown) such as a RAM used as a temporary workstation is provided. With such a configuration, the control unit 305 can perform main control of the entire transmitter 300.

  The light source 301 of the transmitter 300 transmits an optical pulse train at a constant time interval T. This optical pulse train is input to the phase modulator 302.

  The phase modulator 302 phase-modulates each pulse included in the optical pulse train input from the light source 301 with 0 or π, and then passes the optical pulse train to the polarization modulator 303. That is, the phase of each pulse included in the optical pulse train output from the phase modulator 302 is 0 or π.

  The polarization modulator 303 modulates the input optical pulse alternately into two orthogonal polarization states for each pulse, and then passes the modulated optical pulse train in the polarization state to the attenuation unit 304. Thereby, for example, an optical pulse train in which the odd-numbered pulse is in the longitudinal polarization state (V) and the even-numbered pulse is in the transverse polarization state (H) is output to the attenuation unit 304.

  The attenuating means 304 may be any means as long as it is a means for greatly attenuating light incident from a light source such as a laser beam, such as an ND filter (Neutral Density Filter). . In the present invention, an optical pulse train having an average of less than one photon per pulse is transmitted from the transmitter 300. However, an optical pulse train having an average of less than one photon per pulse greatly attenuates ordinary laser light and realizes a state where the average number of photons is less than one. be able to.

  More specifically, “an optical pulse train of less than one photon per pulse” refers to a state of a predetermined number of optical pulse trains in which the number of photons is smaller than the number of pulses.

  Thereafter, the transmitter 300 transmits the optical pulse train 306 that has passed through the attenuation means 304 to the receiver 310 via the optical transmission path 307.

  The receiver 310 as a light detection device includes an optical branching unit (C1) 311, a long path 312, a short path 313, a 2 × 2 multiplexing coupler (C2) 314, and a first photon detector (photon detector 1) 315. , A second photon detector (photon detector 2) 316, and a control unit (CPU 2) 317. As the first photon detector 315 and the second photon detector 316, for example, a wavelength modulation type photon detector having the configuration of FIG. 2 described above can be applied.

  The optical branching unit 311 is connected to the long path 312 and the short path 313. The 2 × 2 multiplexing coupler 314 has two input terminals and two output terminals, the input terminals are connected to the long path 312 and the short path 313, and the output terminal is the first photon detector. 315 and a second photon detector 316.

  The 2 × 2 multiplexing coupler 314 receives the optical pulse train from the long path 312 at one input terminal, and receives the optical pulse train from the short path 313 at the other input terminal. Thereafter, when the phase difference between the two optical pulse trains is 0, the 2 × 2 multiplexing coupler 314 outputs a photon from one output terminal to the first photon detector 315, and when the phase difference is π. Has a characteristic of outputting photons from the other output terminal to the second photon detector 316.

  For the receiver 310, a control unit 317 on the receiver side performs main control of the entire receiver 310. It is the same as the transmitter 300 in that it has a ROM (not shown) that stores a control program of the control unit 317 and a memory (not shown) that performs the same function as the memory that the transmitter 300 has.

  The optical pulse train 306 input to the receiver 310 is branched into the long path 312 and the short path 313 energetically equally (for example, 50 to 50) by the optical branching unit 311. The optical pulse train branched to the long path 312 is delayed by a certain delay time, and then combined again with the optical pulse train that has passed through the short path 313 by the 2 × 2 optical multiplexing coupler 314. Here, the fixed delay time is assumed to be equal to twice the pulse interval T of the input optical pulse train 306 as shown in FIG.

  Under the above configuration, the transmitter 300 generates an optical pulse train having a constant interval (pulse interval) T arbitrarily phase-modulated by 0 or π at an average of less than one photon (for example, 0.1 photon / pulse). It is sent to the optical transmission line 307.

  In the following description, for convenience of explanation, the description will be made under the above conditions, but the present invention is not intended to be limited to the above conditions.

  The receiver 310 receives the optical pulse train 306 transmitted from the transmitter 300 via the optical transmission path 307 by the optical branching unit 311. The optical pulse train branched into the long path 312 and the short path 313 by the optical branching means 311 equally (for example, 50 to 50) is multiplexed by the 2 × 2 multiplexing coupler 314. The combined optical pulse train is detected by the first photon detector 315 or the second photon detector 316 according to the phase difference between the pulses as will be described later.

  If the circuit of the receiver 310 as described above is configured, the 2 × 2 multiplexing coupler 314 of the receiver 310, as shown in FIG. 4, every other pulse (odd number pulses / even number pulses). ) Overlap. In FIG. 4, V represents the longitudinal polarization state of the optical pulse, and H represents the lateral polarization state of the optical pulse. That is, each pulse of the optical pulse train 306 sent from the transmitter 300 is sent out at a constant time interval T, and the optical pulse train passing through the long path 312 is delayed by a constant time interval 2T. The optical pulse train that has passed through the path 312 is combined with the optical pulse train that has passed through the short path 313 while being delayed by exactly two pulses. Further, since the optical pulse train 306 transmitted from the transmitter 300 is transmitted in the orthogonal polarization state for each pulse, every other pulse has the same polarization state. Therefore, overlapping pulses cause interference.

  If the phase difference between the pulses is 0 as a result of the interference by the 2 × 2 multiplexing coupler 314, the first photon detector 315 detects the photon, and if the phase difference between the pulses is π, the first photon detector 315 detects the photon. Two photon detectors 316 detect the photons. However, since the light transmitted from the transmitter 300 is less than one photon on average per pulse, photons are rarely detected.

  When the control unit 317 of the receiver 310 obtains a signal that a photon is detected from the photon detector, the time when the photon is detected in a storage unit (not shown) such as a memory provided in the receiver 310 (photon detection). Time).

  FIG. 4 shows how the received pulses overlap when the optical pulse train 306 described in FIG. 3 is transmitted. The right oblique rectangle (1, 3, 5) and the left oblique rectangle (2, 4, 6) are shown. Although pulses are shown, the right hatched rectangle represents a pulse transmitted with vertical polarization (V), and the left hatched rectangle represents a pulse transmitted with horizontal polarization (H). The polarization state changes during fiber transmission of the optical transmission line 306, and the input state to the receiver 310 is not necessarily longitudinal / transversely polarized. Retained after transmission.

Using the above configuration and operation characteristics, the transmitter 300 and the receiver 310 obtain a common bit by the following procedure. As described above, the control units 305 and 317 also control the corresponding transmitter 300 and receiver 310 to obtain a common bit.
(1) The transmitter 300 transmits an optical pulse train 306 having a predetermined length to the receiver 310 via the optical transmission path 307.
(2) The receiver 310 notifies the transmitter 300 of a time slot (photon detection time) in which a photon is detected via a predetermined communication path 308. An arbitrary communication path can be used as the communication path 308 and is not limited to an optical transmission path, and may be either a wireless or wired communication network.
(3) The transmitter 300 detects either the first photon detector 315 or the second photon detector 316 in the receiver 310 from its own phase modulation data and the photon detection time received from the receiver 310. Determine if the instrument has detected a photon.
(4) The transmitter 300 and the receiver 310 each detect a photon detected by the first photon detector 315 if the photon is detected, and the bit “0” is detected by the second photon detector 316. If there is, bit “1” is assigned. At this time, since the photon detector that has detected the photon has already been determined, the transceivers 300 and 310 obtain the same bit value. This bit value is a secret key bit.

  Hereinafter, in this embodiment, even when the detection efficiency of the photon detector used depends on the signal light polarization, it will be described that a stable system operation can be obtained without polarization control.

In general, the polarization state is represented by a two-dimensional complex unit vector. Here, it is assumed that the photon detector detects light in the polarization state of e ₁ , but is insensitive to light in the polarization state of e ₀ orthogonal to this light. The fact that both e ₁ and e ₀ are orthogonally polarized is
(E ₁ ^* · e ₀ ) = 0
Or (e ₀ ^* · e ₁ ) = 0
It is expressed as Where (·) is the inner product of vectors and ^* is the complex conjugate.

On the other hand, the signal light is in a polarization state where the odd-numbered pulse is e _{V and in} which the odd-numbered pulse is e _H. Since the orthogonal relationship is maintained after fiber transmission,
(E _V ^* · e _H ) = 0
Or (e _H ^* · e _V ) = 0
It is. Here, in general, {e _V , e _H } can be expressed by {e ₁ , e ₀ } as follows.

θ and φ are constants determined by how the polarization state changes during fiber transmission, but {e _V , e _H } and {e ₁ , e ₀ } are independent of θ and φ according to the above table. Indicates that an orthogonal relationship holds.

When the signal light is input to the photon detector (315 or 316), polarization components overlapping the polarization state e ₁ with the detection sensitivity is photon detection. The degree of polarization overlap is quantitatively represented by | (e _V ^* · e ₁ ) | ² and | (e _H ^* · e ₁ ) | ² . The reason for this is the square of the absolute value because it is not the amplitude but the intensity (light intensity) that is observed. Using the above equation (1), the degree of polarization overlap is | (e _V ^* · e ₁ ) | ² = cos ² θ for odd-numbered pulses.
For even-numbered pulses | (e _H ^* · e ₁ ) | ² = sin ² θ
It becomes.

The above consideration is that the detection efficiency of the photon detectors 315 and 316 is a value proportional to cos ² θ for interference between odd-numbered pulses, and sin ² θ for interference between even-numbered pulses. It shows that it becomes a proportional value. When both are added together,
cos ² θ + sin ² θ = 1
Thus, the value does not depend on θ. This means that the sum of the probability that interference between odd-numbered pulses is detected by the photon detector and the probability that interference between even-numbered pulses is detected by the photon detector is always constant. Is shown. That is, signal detection independent of polarization fluctuations in the fiber can be performed.

  Therefore, in the present embodiment, the key generation is performed using the photon detection signal for interference between odd-numbered pulses and the photon detection signal for interference between even-numbered pulses. A stable key generation operation can be achieved without having to

(Second Embodiment)
FIG. 5 shows a configuration of a transmitter in the quantum key distribution system according to the second embodiment of the present invention. The configuration of the entire system of this embodiment is the same as that of the first embodiment of FIG. 3, but the means for generating a pulse train of polarization states that are alternately orthogonal in the transmitter is different.

  As shown in FIG. 5, the light source 301 of the transmitter 300 transmits an optical pulse train at a constant time interval 2T, and this optical pulse train is input to the phase modulator 302. The phase modulator 302 phase-modulates each pulse included in the optical pulse train input from the light source 301 by 0 or π. That is, the phase of each pulse included in the optical pulse train 501 output from the phase modulator 302 is 0 or π.

  The output light from the phase modulator 302 is split in energy equally into the long path 503 and the short path 504 by the optical branching means 502 (for example, 50 to 50). The optical pulse train branched into the long path 503 is converted into an orthogonal state by the polarization rotation element 505, and further delayed by ½ of the time interval of the pulse train from the light source 301, that is, T, The polarization beam splitter 506 recombines the optical pulse train that has passed through the short path 504.

  At the input to the polarization bit splitter 506, the polarization state of the optical pulse passing through the long path 503 and the polarization state of the optical pulse train passing through the short path 504 are orthogonal. In the polarization beam splitter 506, these two pulse trains are combined into the same path 507. As a result, the polarization beam splitter 506 outputs the optical pulse train that has passed through the long path 503 and the optical pulse train that has passed through the short path 504 as orthogonal polarization states. Furthermore, since the pulse train of time interval 2T is delayed and combined by time T, the time interval is T, and the odd-numbered pulse (V) and the even-numbered pulse (H) are in the orthogonal polarization state. An optical pulse train 508 is output from the polarization beam splitter 506.

  An output 508 from the polarization beam splitter 506 is input to the attenuation unit 304. Since the subsequent configuration and operation from the attenuating means 304 are the same as those in the first embodiment, description thereof will be omitted.

(Third embodiment)
FIG. 6 shows a configuration of a transmitter in the quantum key distribution system according to the third embodiment of the present invention. The configuration of the entire system of this embodiment is the same as that of the first embodiment of FIG. 3, but means for generating a pulse train that is a polarization state alternately orthogonal in the transmitter is the first embodiment and the second embodiment. This is different from the embodiment.

  As shown in FIG. 6, the light source 301 of the transmitter 300 transmits an optical pulse train at a constant time interval 2T, and the optical pulse train is input to the phase modulator 302. The phase modulator 302 phase-modulates each pulse included in the optical pulse train input from the light source 301 by 0 or π. That is, the phase of each pulse included in the optical pulse train 601 output from the phase modulator 302 is 0 or π.

  Output light from the phase modulator 302 is input to the birefringent element 602. Here, the polarization state of the input pulse train 601 is set to be oblique 45 ° with respect to the two refractive index axes (fast axis and slow axis) of the birefringent element 602. Then, in the birefringent element 602, the light propagates separately into a fast axis component and a slow axis component. The fast axis and the slow axis have different refractive indexes and therefore different propagation speeds. If the refractive index difference between the fast axis and the slow axis and the length of the birefringent element 602 are appropriate, the propagation time difference between the fast axis component and the slow axis component is half of the time interval of the input pulse train 601 due to the difference in propagation speed. , T. Since the fast axis component and the slow axis component are orthogonal to each other in polarization state, the time interval from the birefringent element 602 is T, and the odd-numbered and even-numbered pulses are in the orthogonal polarization state. An optical pulse train 603 is output.

  The output from the birefringent element 602 is input to the attenuation means 304. Since the subsequent configuration and operation from the attenuating means 304 are the same as those in the first embodiment, description thereof will be omitted.

(Other embodiments)
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example, and the constituent members thereof are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, change in shape design, etc. are all included in the embodiments of the present invention.

It is a block diagram which shows the basic composition of the differential phase shift quantum key distribution system of a prior art. It is a block diagram which shows the basic composition of the well-known wavelength modulation type photon detector which can be utilized by this invention. 1 is a configuration diagram of a quantum key distribution system according to a first embodiment of the present invention. FIG. 4 is a timing diagram showing how received pulses overlap when the pulses described in FIG. 3 are transmitted. It is a block diagram of the transmitter in the quantum key distribution system which concerns on the 2nd Embodiment of this invention. It is a block diagram of the transmitter in the quantum key distribution system which concerns on the 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Transmitter 106 Pulse train 107 Optical transmission line 110 Receiver 111 Optical branching means 112 Long path 113 Short path 114 2 × 2 multiplexing coupler 115 First photon detector (photon detector 1)
116 Second photon detector (photon detector 2)
201 Optical multiplexer 202 Optical nonlinear element 203 Optical filter (pump light blocking filter)
204 Si-APD
DESCRIPTION OF SYMBOLS 300 Transmitter 301 Light source 302 Phase modulator 303 Polarization modulator 304 Attenuating means 305 Control unit (CPU1)
306 Optical pulse train 307 Optical transmission line 310 Receiver 311 Optical branching means (C1)
312 Long path 313 Short path 314 2 × 2 multiplexing coupler (C2)
315 First photon detector (photon detector 1)
316 Second photon detector (photon detector 2)
317 Control unit (CPU2)
318 Communication path 501 Optical pulse train 502 Optical branching means 503 Long path 504 Short path 505 Polarization rotating element 506 Polarization beam splitter 507 Path 508 Optical pulse train 601 Optical pulse train 602 Birefringent element 603 Optical pulse train

Claims (4)

In a quantum key distribution system comprising a transmitter and a receiver,
The transmitter is
A light source that transmits an optical pulse train composed of pulses at regular time intervals;
A phase modulator that phase-modulates the optical pulse train transmitted from the light source with 0 or π;
Polarization modulation means for alternately outputting the optical pulse train phase-modulated by the phase modulator as two polarization states orthogonal to each other;
Attenuating means for transmitting the optical pulse train output from the polarization modulation means as an optical pulse train having an average of less than one photon per pulse;
With
The receiver
Branching means for receiving the optical pulse train transmitted from the attenuation means of the transmitter and branching into a first optical pulse train and a second optical pulse train;
Delay means for delaying the first optical pulse train by a time twice as long as the time interval of the optical pulse train sent from the transmitter;
An optical multiplexing unit that multiplexes the first optical pulse train and the second optical pulse train delayed by the delay unit to generate a combined optical pulse; and an optical pulse generated by the optical multiplexing unit First photon detection means for detecting a photon when a relative phase difference between the delayed pulse of the first optical pulse train and the second optical pulse train by two pulses is zero. When,
When the relative phase difference between the delayed pulse of the first optical pulse train and the second optical pulse train of the optical pulse generated by the optical multiplexing means by two pulses is π, Second photon detection means for detecting photons;
A quantum key distribution system comprising: a photon detection unit that detects a photon out of the first and second photon detection units; and a recording unit that records the detection time.   The polarization modulation unit branches the optical pulse train from the phase modulator, delays it by half the time interval of one of the branched pulse trains, and converts it into an orthogonal polarization state, and the converted pulse train 2. The quantum key distribution system according to claim 1, wherein the other pulse train branched is multiplexed.   2. The polarization modulation means includes a birefringence element disposed so that a fast axis and a slow axis are 45 degrees with respect to a polarization state of an optical pulse train from the phase modulator. Quantum key distribution system described in 1. The transmitter transmits an optical pulse train of a predetermined length to the receiver;
The receiver notifies the transmitter of the photon detection time when the photon is detected,
The transmitter determines, based on its phase modulation data and the photon detection time received from the receiver, which one of the first and second photon detection means has detected a photon. ,
The transmitter and the receiver have a bit “0” for photon detection by the first photon detection means, and a bit “0” for photon detection by the second photon detection means. 1 ”and the common bit value obtained as a result is used as a secret key bit.
The quantum key distribution system according to any one of claims 1 to 3, wherein

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