JP4348540B2 - Quantum cryptography equipment - Google Patents

Description

  The present invention relates to a quantum cryptography device for transmitting quantum cryptography between a transmission unit and a reception unit connected by a transmission line, and in particular, distributes an encryption key from a transmission unit to a reception unit by optical fiber communication. The present invention relates to a quantum cryptography device that performs quantum cryptography key distribution.

  With the explosive spread of the Internet and the commercialization of electronic commerce, there is an increasing social need for cryptographic technologies such as confidentiality and tampering prevention of communications and personal authentication. Currently, common key systems such as DES (Data Encryption Standard) ciphers and public key systems such as RSA (Rivest, Shamir, Adleman) ciphers are widely used. However, these are based on “computational safety”. In other words, such conventional encryption methods are constantly threatened by advances in computer hardware and decryption algorithms. In particular, in fields where extremely high security is required, such as transactions between banks and information related to military and diplomacy, the impact will be great if practically secure cryptographic methods become practical.

  There is a one-time pad method as an encryption method whose unconditional security is proved by information theory. The one-time pad method is characterized in that an encryption key having the same length as the communication text is used, and the encryption key is discarded once. Non-Patent Document 1 proposes a specific protocol for securely delivering an encryption key used in the one-time pad method. As a result, research on quantum cryptography has become active. In quantum cryptography, the laws of physics guarantee the security of cryptography, so it is possible to guarantee the ultimate security that does not depend on the limits of computer capabilities. Currently studied quantum cryptography transmits 1-bit information as a single photon. For this reason, when the state of a photon is changed by an optical fiber that is a transmission path, the security of the quantum cryptography is greatly impaired.

  In the conventional quantum cryptography apparatus, as described in FIG. 1 of Patent Document 1, a photon pulse is temporally divided into two using an interference system having an optical path difference on the transmission side, and the mutual phase difference is modulated. Random number bits that serve as an encryption key are expressed, and the transmitted random number bits are reproduced by causing interference on the photon pulse divided in two on the receiving side. For this reason, the optical path difference between the interference systems used on the transmission side and the reception side must be completely equal. Also, if the polarization state fluctuates in the transmission path, the intelligibility of interference decreases, leading to an increase in reception error rate. In quantum cryptography, an increase in reception error rate is used as a means of detecting eavesdroppers, so an increase in reception error rate due to changes in the polarization state in the transmission path reduces the probability of detection of eavesdroppers, resulting in a decrease in security of quantum cryptography. Let

  In order to solve the above-described problem, a quantum cryptography device that invents a variation in the polarization direction using a Faraday mirror has been invented as described in Patent Document 2 or Non-Patent Document 2 in which this is simplified. Yes. In this conventional quantum cryptography device, optical pulses that are temporally divided and whose polarization directions are orthogonal are sent from the receiver to the transmitter, and the transmitter reverses the traveling direction of the light transmitted using the Faraday mirror, and at the same time After the polarization direction is rotated by 90 degrees, a phase difference is given between the divided photon pulses by a phase modulator and sent back to the receiving side. With such a folding configuration, the interference system that splits the photon pulse in time and the interference system that recombines in time are the same, so the optical path difference of the interference system is constant for a time longer than the round-trip time of the photon pulse. If this is maintained, interference with high clarity can be obtained. As is well known, the polarization direction of the light reflected by the Faraday mirror is returned to the initial state if the initial state is a linear polarization state, regardless of any polarization state disturbance in the transmission path on the way. Since they are orthogonal, the clarity of the interference signal is not impaired even when the polarization state in the transmission path is disturbed, and the security of the quantum cryptography is guaranteed.

  However, the quantum cryptography device disclosed in Patent Document 1 has a problem. The most serious problem is the stability of the system. The quantum cryptography device disclosed in Patent Document 1 encodes and transmits information in a relative phase between two photon pulses having a fixed phase relationship, which is divided into two by an interference system on the transmission side, and transmits the interference system on the reception side. The information encoded in the relative phase is decoded by observing the component of the double photon pulse combined in step. The relative phase depends extremely sensitively on the optical path difference of the interference system of the transceiver. Interference systems of transceivers are independent and are generally placed in different environments. For this reason, in order to stabilize the system, it is necessary to stabilize the optical path difference of each interference system with the accuracy of the wavelength of light used as a communication carrier. Such stabilization of the interference system can be solved by using a solid-state interference system using an integrated optical system technology or by performing active control, but it still requires advanced technology and reduces system cost. Causes increase. Further, the light source used requires a short pulse light source, which also causes a cost increase.

The quantum cryptography device disclosed in Patent Document 2 also has the following problems. In this method, a duplex photon pulse having a fixed phase relationship is generated by an interference system on the receiver side and transmitted to the transmitter. The transmitter folds back and simultaneously transmits information to the relative phase between the duplex photon pulses. Encode and transmit to receiver. The receiver decodes the information encoded in the relative phase by observing the double photon pulse combined in its own interference system. In this system, the path through which the two photon pulses involved in interference propagate completely matches, so that the system has high system stability. Furthermore, in this system, by using a Faraday mirror for folding, it is possible to realize a system that does not depend on the polarization state disturbance in the transmission path. However, since this method is a folded type, when a light pulse having a large intensity in the forward path crosses a weak light pulse of a single photon level in the return path, stray light is generated due to backscattering in the fiber, and the system There is a problem of reducing performance. By intermittently transmitting the burst pulse train, it is possible to avoid the influence of stray light, but at the cost of this, the key generation rate is lowered and the system performance is not improved. Such stray light due to backscattering is known as a serious problem that restricts the quantum cryptography key transmission distance.
Japanese Patent No. 2951408 Special Table 2000-517499 Bennett and Brassard, IEEE Int. Conf. On Computers, Systems, and Signal Processing, Bangalore, India, p. 175 (1984) Ribordy, Gautier, Gisin, Guinnard and Zbinden, Electronics Letters, Vol. 34 No.22, p.2116-211, 29th October 1998

The conventional quantum cryptography apparatus described above has the following problems.
(1) In the technique described in Patent Document 1, since it is necessary to provide interference systems on the transmission side and the reception side, the stability of the system is lowered and the system cost is increased. Further, since it is necessary to use an expensive pulse light source, the system cost increases.
(2) In the technique described in Patent Document 2, the stability of the system is high because an interference system only needs to be provided on the receiving side, but stray light due to backscattering in the fiber is generated and the performance of the system is lowered.

  An object of the present invention is to provide a quantum cryptography device that can perform stable operation, has no performance restrictions due to backscattered stray light in a fiber, has a simple configuration, and is low in cost.

In order to achieve the above object, a quantum cryptography device of the present invention is a quantum cryptography device for transmitting quantum cryptography between a transmission unit and a reception unit connected by a transmission path,
An interference system having two optical paths having optical path differences; a photon receiver for identifying which output port of the two output ports of the interference system outputs a photon; and the transmission via the transmission path A first phase that is time-selectively phase-modulated for a time shorter than a delay time determined by an optical path difference between two optical paths of the interference system and is input to the interference system. A receiver comprising a modulator;
A light source that outputs an optical signal having a narrower frequency width than the reciprocal of the delay time of the interference system, and an optical signal output by the light source is time-selective only for a time shorter than the delay time of the interference system. A second phase modulator capable of applying phase modulation to the first phase modulator, and the first phase modulator and the photon receiver so that the number of photons that can arrive within an operation time of the first phase modulator is 1 or less on average A transmitter configured by an optical attenuator that attenuates the light intensity of the optical signal after the phase modulation is performed by one phase modulator and emits the light to the transmission path;
Time synchronization means for operating the first and second phase modulators and the photon receiver synchronously for a time shorter than the delay time of the interference system.

  According to the present invention, since the necessary interference system can be reduced to only one reception unit by omitting the interference system of the transmission unit, the apparatus configuration can be simplified and the cost can be reduced. Also, using a light source that outputs an optical signal having a narrower frequency width than the reciprocal of the delay time of the interference system, the first and second phase modulators and the photon receiver are moved from the delay time of the interference system by the time synchronization means. However, since they are operated in synchronism only for a short time, it is not necessary to use an expensive pulse light source, it is possible to use a light source that outputs continuous light, and it is possible to reduce costs. Furthermore, in the present invention, since the transmission direction of the optical signal is unidirectional from the transmission unit to the reception unit, it is possible to prevent performance from being deteriorated due to stray light due to backscattering in the optical fiber serving as the transmission path. .

In another quantum cryptography apparatus of the present invention, the first phase modulator is
A polarization scrambler that randomly modulates the polarization of an optical signal incident from the transmission path;
For an optical signal input from the polarization scrambler, a polarizer that passes only an optical signal having a certain polarization plane; and
You may make it comprise from the phase modulation part which performs phase modulation with respect to the optical signal which passed the said polarizer.

Further, the first phase modulator is
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
A Faraday mirror for terminating a branch transmission line branched from the trunk transmission line by the circulator;
A phase which is provided in the middle of the branch transmission line and performs phase modulation on the optical signal when the optical signal guided to the branch transmission line is terminated by the Faraday mirror and makes a round trip through the branch transmission line. You may make it comprise from a modulation | alteration part.

Further, the first phase modulator is
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
A loop mirror configured by combining a polarization beam splitter and a Faraday rotator with a polarization-polarized optical fiber, and terminating a branch transmission line branched from a main transmission line by the circulator;
An equal amplitude that is provided in the middle of the branch line transmission line and is separated by a certain time in time when the optical signal guided to the branch line transmission line is terminated by the loop mirror and makes one round trip through the branch line transmission line. A phase modulation unit that performs phase modulation on the optical signal by being provided with the two electric pulses.

Further, the first phase modulator is
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
A polarization beam splitter, a Faraday rotator, and a phase modulator are coupled by a constant polarization optical fiber. The polarization beam splitter, the Faraday rotator, and the phase modulator are configured by a loop mirror that terminates a branch transmission line branched from the trunk transmission line by the circulator. May be.

Further, the first phase modulator is
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
A polarization beam splitter, a half-wave phase plate, and a phase modulator are coupled by a constant polarization optical fiber, and are configured by a loop mirror that terminates a branch transmission line branched from a main transmission line by the circulator. It may be.

Further, the first phase modulator is
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
The polarization beam splitter and the phase modulator may be coupled by a constant polarization optical fiber, and may be configured by a loop mirror that terminates a branch transmission line branched from the main transmission line by the circulator.

  As described above, according to the present invention, the following effects can be obtained.

  (1) The first effect is that, according to the present invention, the interference system of the transmission unit can be omitted and the necessary interference system can be reduced to only one reception unit as compared with the quantum cryptography device disclosed in Patent Document 1. It is possible to simplify the device configuration and reduce the cost.

  (2) According to the present invention, the second effect is that it is not necessary to use an expensive pulse light source as in the quantum cryptography device disclosed in Patent Document 1, and a continuous light source is sufficient. Therefore, the cost of the apparatus can be reduced.

  (3) The third effect is that, unlike the quantum cryptography device disclosed in Patent Document 2, according to the present invention, the transmission direction of the optical signal is unidirectional from the transmitter to the receiver. There are no performance limitations due to stray light.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows the configuration of a quantum cryptography apparatus according to an embodiment of the present invention. The quantum cryptography apparatus of the present embodiment includes a transmission unit 10 that transmits quantum cryptography, a reception unit 20 that receives quantum cryptography from the transmission unit 10, a transmission path 4 that connects the transmission unit 10 and the reception unit 20, and And time synchronization means 8.

The transmitter 10 includes a light source 1 that outputs an optical signal having a constant frequency with a frequency width Δν that is narrower than the reciprocal of the delay time τ _d of the interference system 6, a phase modulator 2, and an optical attenuator 3. The unit 20 receives the photon received from the output port of the phase modulator 5, the interference system 6 having two optical paths with optical path differences, and the two output ports of the interference system 6. And a container 7.

  The time synchronization means 8 operates the phase modulators 2 and 5 and the photon receiver 7 in synchronization for a time shorter than the delay time of the interference system 6.

The light source 1 is a light source having a narrow line width and frequency stability so that the frequency width Δν of the output light satisfies Δν << 1 / τ _d , and the frequency is stabilized by a feedback mechanism using an atomic absorption line. A semiconductor laser can be used. Here, τ _d is a delay time determined by the optical path difference between the two optical paths of the interference system 6.

The phase modulator 5 selects the optical signal transmitted from the transmission unit 10 to the reception unit 20 via the transmission line 4 for a time shorter than the delay time τ _d determined by the two optical paths of the interference system 6. Phase modulation is applied to the interference system 6.

  The phase modulator 2 of the transmission unit 10 can phase-selectively apply phase modulation to the optical signal output from the light source 1 only for a time shorter than the delay time of the interference system 6. It is a modulator.

  As this phase modulator 2, a phase modulator using a commercially available lithium niobate waveguide can be used. These phase modulators have polarization plane dependency in their modulation characteristics. Since the polarization plane of the incident light can be freely controlled by the light source 1, the best polarization plane may be selected and incident. On the other hand, the transmission line 4 generally does not preserve the polarization plane. Therefore, there arises a problem that the magnitude of the electric signal corresponding to the phase modulation given by the phase modulator 5 is not uniquely determined. There are the following two methods for solving this problem.

  The first method is a method of making the light non-polarized plane. An example of the configuration of the phase modulator 5 using this method is shown in FIG. In this example, the phase modulator 5 includes a polarization scrambler 32, a polarizer 33, and a phase modulation unit 34.

  The polarization scrambler 32 applies random polarization plane modulation to the optical signal incident from the transmission path 4. The polarizer 33 allows only the optical signal having the same polarization plane as the optimum polarization plane in the phase modulation unit 34, that is, having a constant polarization plane, to the optical signal input from the polarization scrambler 32. The phase modulation unit 34 performs phase modulation on the optical signal that has passed through the polarizer 33.

  In FIG. 2, the light incident from the transmission path 4 is given random polarization plane modulation by the polarization scrambler 32. Only photons having the same polarization plane as the optimal polarization plane of the phase modulator 34 are selected by the polarizer 33, and then modulated by the phase modulator 34 and emitted. This method has the disadvantage that half of the incident photons are lost by the polarizer 33. In addition, a cost increase factor such as the polarization scrambler 32 remains.

  The second method is a method for making the polarization plane independent of the phase modulator. In the quantum cryptography device of this embodiment, the plane of polarization need not be preserved by the input / output light of the phase modulator 5. Some examples of the configuration of the phase modulator 5 using the second method are shown in FIGS.

  FIG. 3 is a diagram illustrating a configuration example for realizing a phase modulator having polarization plane independence.

  This phase modulator includes a circulator 42, a phase modulator 43, and a Faraday mirror 44.

  The circulator 42 branches the optical signal incident from the main transmission line 41 to the branch transmission line, and returns the optical signal from this branch transmission line to the main transmission line. The Faraday mirror 44 terminates the branch transmission line branched from the main transmission line 41 by the circulator 42. The phase modulation unit 43 is provided in the middle of the branch transmission line. When the optical signal guided to the branch transmission line is terminated by the Faraday mirror 44 and makes a round trip through the branch transmission line, Perform phase modulation.

  Next, the operation of this phase modulator will be described with reference to FIG. The optical signal incident from the main transmission line 41 is branched by the circulator 42 into a branch optical path (branch transmission path). The light guided to the branch optical path makes one round trip through the branch optical path terminated by the Faraday mirror 44, passes through the phase modulator 43 in the process, and returns to the main transmission path by the circulator 42. By providing the phase modulation unit 43 with two electric pulses of equal amplitude separated in time by the round trip time between the phase modulation unit 43 and the Faraday mirror 44, phase modulation independent of the polarization plane of the input light can be provided. Is possible.

  FIG. 4 is a diagram showing another configuration example for realizing a phase modulator having polarization plane independence similar to the phase modulator shown in FIG. Light incident from the main transmission line 51 is guided to the branch optical path by the circulator 52. The light guided to the branch optical path makes one round trip through the branch optical path terminated by a loop mirror composed of a polarization beam splitter 54, a Faraday rotator 55 having a polarization rotation angle of 90 degrees, and constant polarization optical fibers 56 and 57 that couple them. In this process, the signal passes through the phase modulator 53 twice and returns to the main transmission line by the circulator 52. It is known that a loop mirror having this configuration operates in the same manner as a Faraday mirror. By providing the phase modulation unit 53 with two electric pulses of equal amplitude separated in time by the round trip time between the phase modulation unit 53 and the loop mirror, it is possible to provide phase modulation independent of the polarization plane of the input light. It is.

  FIG. 5 is a diagram showing another configuration example for realizing a phase modulator having polarization plane independence. Light incident from the main transmission line 61 is guided to the branch optical path by the circulator 62. The light guided to the branch optical path is composed of a polarization beam splitter 63, a phase modulation unit 64, a Faraday rotator 65 having a polarization rotation angle of 90 degrees, and constant polarization optical fibers 66, 67, 68 that couple them. Is returned to the trunk transmission line by the circulator 52. In this configuration, there is an advantage that a single pulse is sufficient for the electric pulse applied to the phase modulation unit 64.

  FIG. 6 is a diagram showing another configuration example for realizing a phase modulator having polarization plane independence similar to the phase modulator shown in FIG. In the configuration shown in FIG. 6, the Faraday rotator 65 is replaced with a half-wavelength phase plate 75 in the configuration shown in FIG. The optical axis of the half-wave phase plate 75 is arranged to be rotated by 45 ° from the optical axes of the constant polarization optical fibers 77 and 78. The phase modulator having such a configuration also has the same function as the phase modulator shown in FIG.

  The main transmission line 71, circulator 72, polarization beam splitter 73, and constant polarization optical fibers 76 to 78 in FIG. 6 are the main transmission line 61, circulator 62, polarization beam splitter 63, constant polarization optical fiber 66 in FIG. ˜68 respectively.

  FIG. 7 is a diagram showing another configuration example for realizing a phase modulator having polarization plane independence similar to the phase modulator shown in FIG. 5, and an element corresponding to the Faraday rotator 65 in FIG. Not present in the loop mirror. Instead, the terminals 831 and 832 of the polarization beam splitter 83 are respectively coupled to constant polarization optical fibers 85 and 86 in which the direction of the electric field vector is aligned with the slow axis. Also with this configuration, a phase modulator having polarization plane independence can be realized.

  The main transmission line 81, circulator 82, polarization beam splitter 83, and constant polarization optical fibers 85 and 86 in FIG. 7 are the main transmission line 61, circulator 62, polarization beam splitter 63, and constant polarization optical fiber 66 in FIG. ˜68 respectively.

  From the viewpoint of optical loss, these second methods cannot be said to be superior to the first method unless the optical loss due to various optical elements placed in the optical path is smaller than 3 dB. There is an advantage that the cost can be reduced by eliminating the need for a dynamic element such as a scrambler.

  The optical attenuator 3 attenuates the optical signal that has passed through the phase modulator 2 to the light intensity necessary for ensuring the security of the quantum cryptography. Specifically, the optical attenuator 3 is subjected to phase modulation by the phase modulator 2 so that the average number of photons that can arrive within the operating time of the phase modulator 5 and the receiver 7 is 1 or less. The light intensity of the optical signal is attenuated.

  Although the interference system 6 having an optical path difference can be realized by an optical fiber and a fiber coupler, it is better in terms of stability to be implemented by an integrated optical element in the application of the planar optical circuit technology. In order for the quantum cryptography device to operate without depending on the polarization rotation of the light in the transmission line 4, the birefringence characteristics of the long optical path and the short optical path of the interference system 6 are balanced by a polarization controller or a technique equivalent thereto. It is necessary to be.

  The photon receiver 7 can operate in a gate mode that operates only for a short time by external control. When the gate open signal is input from the time synchronization means 8, the photon receiver 7 enters the gate open state. 6 has a function of discriminating to which of the two output ports the photon is output and recording the result. As the photon detector 7 used for this, a gate mode operation avalanche photodiode can be used, but even various photon detectors can be used as long as the gate mode operation can be added.

  The time synchronization means 8 includes a clock of the transmitter, a clock of the receiver, a communication means for sharing time information between the transmitter and the receiver, a phase modulator 2, a phase modulator 5, and a photon receiver 7. It is constituted by means for generating a signal to be operated at an appropriate time. The timepiece and the signal generating means can be implemented by appropriately designed electronic circuits. The means for sharing time information can be implemented by an optical signal using the transmission line 4, but the present invention is not limited to this means.

  Next, the operation of the quantum cryptography apparatus of the present embodiment will be described in detail with reference to the drawings.

  First, the general operation of the quantum cryptography apparatus of the present embodiment will be described.

In the transmission unit 10, the light emitted from the light source 1 passes through the phase modulator 2, is attenuated by the optical attenuator 3 to the light intensity necessary for ensuring the security of the quantum cryptography, and then enters the transmission line 4. Incident. At that time, the phase modulator 2 uses the quaternary electric signal output from the time synchronization means 8 to apply pulsed phase modulation to the light component being passed. At this time, the electric signal output value is programmed so that the phase modulation amount P is selected as P−P ₀ ∈ [0, π / 2, π, 3π / 2] and at random.

Then, the optical signal that has passed through the transmission path 4 and is input to the receiver 20 passes through the phase modulator 5. At this time, the phase modulator 5 time-selectively applies phase modulation only to the light component that has been phase-modulated by the phase modulator 2 or the light component that is in the time domain before and after τ _d thereof. The phase modulation amount Q when performing phase modulation in the phase modulator 5 is given a binary electric signal from the time synchronization means 8 to the phase modulator 5 so that Qε [0, π / 2] and randomly selected. It is done. The light that has passed through the phase modulator 5 is incident on an interference system 6 having an optical path difference, and, as will be described below, the phase modulation amount difference PQ between the phase modulator 2 and the phase modulator 5 is sinusoidal. , And only one of the output light components that fluctuates in time is selectively detected by the optical receiver 7.

  The photon receiver 7 identifies to which of the two output ports of the interference system 6 the corresponding interference light component is output, and records the result.

  If the protocol given in Non-Patent Document 1 is executed based on the recording obtained by the photon receiver 7 and the recording of the phase modulation amounts P and Q of the phase modulator 2 and the phase modulator 5, it is safe. A final encryption key can be generated.

  A synchronization signal for synchronizing the phase modulator 2 and the phase modulator 5 is given from the time synchronization means 8. When adjusting the synchronization signal in time, the attenuation amount of the optical attenuator 3 is set to zero. Adjustment can be easily performed by using a strong optical signal.

  Furthermore, detailed operations in the quantum cryptography apparatus of the present embodiment will be described below.

First, let us consider a state where the actions of the phase modulator 2 and the phase modulator 5 are ignored, that is, the phase modulation is not performed. If the delay time determined by the optical path difference between the two optical paths of the interference system 6 is τ _d and the frequency width of the output light of the light source 1 is Δν, a photon receiver when Δν << 1 / τ _d is satisfied. The ratio of the optical output to the two output ports 7 depends on the optical path length difference of the interference system 6 and the center frequency of the output light of the light source 1, and for each of these parameters, the periods vτ _d , 1 It varies with / τ _d . Here, v is the phase velocity of light in the optical path of the interference system 6. This is because the light components separated by time τ _d have a clear phase relationship and interfere with each other. This is a well-known optical interference phenomenon and is applied to devices such as optical filters.

  Next, operations of the phase modulator 2 and the phase modulator 5 will be described with reference to the timing chart of FIG. First, pulsed phase modulation is applied to the continuous light emitted from the light source 1 only by the phase modulator 2.

  FIG. 8A is a diagram showing a time change of the phase modulation amount in the phase modulator 2, and FIG. 8B is a diagram showing a time change of the phase modulation amount in the phase modulator 5. 8C and 8D are diagrams showing temporal changes in the light intensity of the two output ports of the photon receiver 7, and FIG. 8E is a gate open signal given to the photon receiver 7. It is a figure which shows the time change of.

When there is no phase modulation, the light output to the photon receiver 7 is in a steady state determined by the optical path length difference of the interference system 6 and the center frequency of the output light of the light source 1. When pulsed phase modulation is applied to the optical signal only by the phase modulator 2, the optical output of the photon receiver 7 fluctuates in a pulsed manner due to the action. As shown in FIGS. 8 (c) and 8 (d), this pulse-like variation is delayed in two time regions by an optical propagation time (time t ₁ -t ₃ ) from the phase modulator 2 to the photon receiver 7. Appears on. This is because the light component in the time domain τ _d before and after interferes with the light component subjected to phase modulation. Since this variation is caused by optical interference, when the waveform shown in FIG. 8C and the waveform shown in FIG. 8D are added, a constant value is obtained, which are complementary to each other. As shown in FIG. 8, it is always possible to adjust the phase modulation amount P ₀ so that the optical output intensity to one output port is reduced to zero, and the optical output intensity to the other output port is reduced to zero. Phase modulation amount

It is always possible to adjust to. The difference between these two phase modulation amounts

Becomes π. Therefore, if the photon receiver 7 is operated synchronously for a short time using a gate open signal synchronized with one of these two time domains, and only the optical signal in this time domain is observed (time t ₄ ) It is possible to obtain a photon receiver 7 output that depends sinusoidally on the phase modulation amount P of the phase modulator 2. In addition to this, when the phase modulator 5 is operated by delaying the optical propagation time (time t ₁ -t ₂ ) between the phase modulator 2 and the phase modulator 5 to give the phase modulation amount Q, the phase modulation is performed. It is possible to obtain the output of the photon receiver 7 that depends sinusoidally on the difference PQ between the phase modulation amounts of the detector 2 and the phase modulator 5.

The output characteristic of the photon receiver 7 that depends sinusoidally on the difference P-Q between the phase modulation amounts of the phase modulator 2 and the phase modulator 5 is a characteristic necessary for mounting the quantum cryptography device. It is possible to configure a quantum cryptography apparatus using the action. Specifically, the attenuation amount of the optical attenuator 3 is set so that the average number of photons that can arrive within the operation time of the photon receiver 7 is 1 or less, and the phase modulation amount P of the phase modulator 2 is P−. The pulse modulation is randomly and periodically given so that P ₀ ∈ [0, π / 2, π, 3π / 2], and the phase modulation amount Q of the phase modulator 5 is set to [0, It is only necessary to randomly select from π / 2], apply pulse modulation, delay the optical propagation time, and record the result obtained by operating the photon receiver 7 for a short time (which port was output). The photon receiver 7 identifies to which of the two output ports of the interference system 6 the corresponding interference light component is output, and records the result.

  After that, as described above, if the protocol given in Non-Patent Document 1 is executed on the basis of the obtained recording and the recording of the phase modulation amounts P and Q of the phase modulator 2 and the phase modulator 5, it is safe. A final encryption key can be generated.

Although the present embodiment has mainly described the case where the light emitted from the light source 1 is continuous light, the light component necessary for this method is the light component subjected to phase modulation and the light in the time domain of front and rear τ _d. Only one of the ingredients. Therefore, the present invention is not limited to such a case, and the light source may be discontinuous light from which light is emitted only in these time regions.

It is a block diagram which shows the structure of the quantum cryptography apparatus of one Embodiment of this invention. It is a figure which shows the structural example of the phase modulator 5 in the quantum cryptography apparatus of one Embodiment of this invention. It is a figure which shows the other structural example of the phase modulator 5 in the quantum cryptography apparatus of one Embodiment of this invention. It is a figure which shows the other structural example of the phase modulator 5 in the quantum cryptography apparatus of one Embodiment of this invention. It is a figure which shows the other structural example of the phase modulator 5 in the quantum cryptography apparatus of one Embodiment of this invention. It is a figure which shows the other structural example of the phase modulator 5 in the quantum cryptography apparatus of one Embodiment of this invention. It is a figure which shows the other structural example of the phase modulator 5 in the quantum cryptography apparatus of one Embodiment of this invention. The operation timing of the phase modulator 2 (FIG. 8A), the operation timing of the phase modulator 5 (FIG. 8B) of the quantum cryptography apparatus according to the embodiment of the present invention, and the two outputs of the photon receiver 7 FIG. 9 is a timing chart for explaining the operation of a port light intensity change (FIGS. 8C and 8D) and a gate open signal (FIG. 8E) applied to the photon receiver 7. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Phase modulator 3 Optical attenuator 4 Transmission path 5 Phase modulator 6 Interference system 7 Photon receiver 8 Time synchronization means 10 Transmitter 20 Receiver 32 Polarization scrambler 33 Polarizer 34 Phase modulator 41 Transmission path 42 Circulator 43 Phase modulator 44 Faraday mirror 51 Transmission path 52 Circulator 53 Phase modulator 54 Polarization beam splitter 55 Faraday rotator 56 Constant polarization optical fiber 57 Constant polarization optical fiber 61 Transmission path 62 Circulator 63 Polarization beam splitter 64 Phase modulation section 65 Faraday Rotor 66 to 68 Constant polarization optical fiber 71 Transmission path 72 Circulator 73 Polarization beam splitter 74 Phase modulator 75 Half wavelength phase plate 76 to 78 Constant polarization optical fiber 81 Transmission path 82 Circulator 83 Polarization beam splitter 84 Phase Modulator 85, 86 Constant polarization optical fiber 831, 832 Polarization beam splitter terminal

Claims (7)

A quantum cryptography device for transmitting quantum cryptography between a transmission unit and a reception unit connected by a transmission path,
An interference system having two optical paths having optical path differences; a photon receiver for identifying which output port of the two output ports of the interference system outputs a photon; and the transmission via the transmission path A first phase that is time-selectively phase-modulated for a time shorter than a delay time determined by an optical path difference between two optical paths of the interference system and is input to the interference system. A receiver comprising a modulator;
A light source that outputs an optical signal having a narrower frequency width than the reciprocal of the delay time of the interference system, and an optical signal output by the light source is time-selective only for a time shorter than the delay time of the interference system. A second phase modulator capable of applying phase modulation to the first phase modulator, and the first phase modulator and the photon receiver so that the number of photons that can arrive within an operation time of the first phase modulator is 1 or less on average A transmitter configured by an optical attenuator that attenuates the light intensity of the optical signal after the phase modulation is performed by one phase modulator and emits the light to the transmission path;
Time synchronization means for operating the first and second phase modulators and the photon receiver synchronously for a time shorter than a delay time of the interference system;
A quantum cryptography device. The first phase modulator comprises:
A polarization scrambler that randomly modulates the polarization of an optical signal incident from the transmission path;
For an optical signal input from the polarization scrambler, a polarizer that passes only an optical signal having a certain polarization plane; and
A phase modulator that performs phase modulation on the optical signal that has passed through the polarizer;
The quantum cryptography device according to claim 1, comprising: The first phase modulator comprises:
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
A Faraday mirror for terminating a branch transmission line branched from the trunk transmission line by the circulator;
A phase which is provided in the middle of the branch transmission line and performs phase modulation on the optical signal when the optical signal guided to the branch transmission line is terminated by the Faraday mirror and makes a round trip through the branch transmission line. A modulation unit;
The quantum cryptography device according to claim 1, comprising: The first phase modulator comprises:
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
A loop mirror configured by combining a polarization beam splitter and a Faraday rotator with a polarization-polarized optical fiber, and terminating a branch transmission line branched from a main transmission line by the circulator;
An equal amplitude which is provided in the middle of the branch transmission line and is separated in time by a fixed time when the optical signal guided to the branch transmission line is terminated by the loop mirror and makes a round trip through the branch transmission line. A phase modulation unit that performs phase modulation on the optical signal by being provided with two electric pulses of
The quantum cryptography device according to claim 1, comprising: The first phase modulator comprises:
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
A polarization mirror, a Faraday rotator, and a phase modulator that are coupled by a polarization optical fiber; a loop mirror that terminates a branch transmission line branched from a main transmission line by the circulator;
The quantum cryptography device according to claim 1, comprising: The first phase modulator comprises:
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
A polarization mirror, a half-wave phase plate and a phase modulator, which are configured by being coupled by a constant polarization optical fiber, and a loop mirror for terminating a branch transmission line branched from a main transmission line by the circulator;
The quantum cryptography device according to claim 1, comprising: The first phase modulator comprises:
A circulator for branching the optical signal incident from the main transmission line to the branch transmission line and returning the optical signal from the branch transmission line to the main transmission line;
A loop mirror configured by coupling a polarization beam splitter and a phase modulator by a polarization-polarized optical fiber, and terminating a branch transmission line branched from a main transmission line by the circulator;
The quantum cryptography device according to claim 1, comprising:

Images