JP5020999B2 - Quantum cryptographic communication device and quantum cryptographic communication method - Google Patents

Description

  The present invention relates to a quantum cryptography communication device and a quantum cryptography communication method.

  In recent years, research on quantum cryptography communication in which physical safety is guaranteed by using the quantum mechanical properties of light has been underway. Quantum cryptography is a method of supplying a secret key for performing cryptographic communication between two parties present at distant points, and is also called quantum key distribution. Here, the secret key is a random bit string of “0” or “1” as an entity. Although there are various types of quantum key distribution, here, as a conventional technique, a quantum key distribution system (see Non-Patent Document 1) that performs homodyne detection of four coherent states will be described.

  FIG. 1 is a diagram showing a basic configuration of a conventional coherent / homodyne detection quantum key distribution system. The transmitter transmits coherent light having a phase of {0, π} and {π / 2, 3π / 2} with extremely weak power. The receiver combines the received light with the local light having the same frequency as the received light, and detects the intensity of the combined light by the two photodetectors. A demodulated signal is obtained by differential detection of the two detection signals. Here, the phase of local light is 0 or π / 2. This receiver configuration is a configuration called optical homodyne detection.

  The demodulated signal of the receiver in the above configuration is as follows. First,

It expresses. A is the amplitude, ω is the angular frequency, θ is the phase, and the received signal light (S) or local light (L) is indicated by a subscript. Using this expression, the photoelectric field after combining the signal light and the local light is

For the photodetector 2

And each light intensity I is

It becomes. Here, ω _S = ω _L.

  A signal proportional to the light intensity is output from the photodetector, and is differentially detected. From the equations (1) and (2), the demodulated signal S after differential detection is expressed as follows.

Here, the phase of the signal light is θ _S = {0, π} {π / 2, 3π / 2}, and the phase of the local light is θ _L = {0, π / 2}. Therefore, as shown in Table 1, the value that the demodulated signal S can take is 0 or ± 2A _S A _L depending on each phase value.

The above is the reception characteristics when there is no fluctuation between the signal light and the local light. However, in the case of coherent light, fluctuations that are inevitable in quantum mechanics exist due to the uncertainty principle. Therefore, the demodulated signal fluctuates around the above three values, and its distribution is as shown in FIG. In this case, since the signal light power is extremely weak, A _S is very small. Therefore, the signal distribution centered on S = 0 and the distribution centered on S = ± 2A _S A _L partially overlap each other. For this signal distribution, the receiver sets a threshold d where d> 2A _S A _L and a threshold −d where −d <2A _S A _L. The reason why the peak of the distribution of S = 0 is higher than the other two is that the difference in the number of phase combinations is reflected.

Using the above configuration, the transmitter and the receiver obtain a secret key bit by the following procedure. First, the receiver detects the signal light transmitted from the transmitter and transmitted through the transmission path by the above receiving configuration. At this time, when the demodulated signal is greater than or equal to the threshold value d or less than or equal to the threshold value -d, it is recorded whether the local light emission phase is greater than or equal to d or less than or equal to -d. After performing the above transmission / reception a required number of times, the receiver notifies the transmitter of the time slot when the demodulated signal exceeds the threshold and the local light emission phase at that time. However, it does not tell if it was greater than d or less than -d. The transmitter informs the receiver whether the signal light phase of the notified time slot was {0, π} or {π / 2, 3π / 2}. However, only one set is notified, and the phase value itself is not taught. Next, when the signal light phase is θ _S = {0, π} and the local light emission phase is θ _L = π / 2, the receiver sets bit “1” for the time slot that is equal to or greater than the threshold value d. A bit “0” is assigned to each time slot that is equal to or less than the threshold −d. In addition, when the signal light phase is θ _S = {π / 2, 3π / 2} and the local light emission phase is θ _L = 0, bit “1” is set to the threshold − Bit “0” is assigned to each time slot that was less than or equal to d. On the other hand, the transmitter generates bits for the time slot exceeding the threshold from its own phase modulation data and the local light emission phase of the receiver. Specifically, when {θ _L = 0 and θ _S = π / 2} or {θ _L = π / 2 and θ _S = π}, bit “1”, {θ _L = 0 and θ _S = − Bits “0” are set when π / 2} or {θ _L = π / 2 and θ _S = 0}. The bits generated in this way are almost the same in the transmitter and the receiver. Although there are some mismatches due to the tailing of the signal distribution, a small number of bit mismatches can be removed by a procedure called error correction, and the bit string after error correction can be used as a secret key.

  The security of the secret key generated by the above configuration and procedure is ensured for the following reason. The most basic eavesdropping method is to branch / save part of the transmission signal and wiretap the local light emission phase that the receiver notifies the transmitter, and then use the same local light emission phase as the receiver for the stored signal. This is a method for measuring homodyne. However, even if the threshold value is set in the same way as the receiver and the bit value is obtained, there is no correlation between the fluctuation of the receiver and the fluctuation of the eavesdropper due to quantum noise, and the same signal is set to the threshold value for both the receiver and the eavesdropper. The probability of exceeding is small. Therefore, the probability that an eavesdropper obtains the same bit value as that of the receiver is small, and slight information leakage is removed by a data compression process called confidentiality enhancement. Alternatively, a method of generating a wiretapping bit by setting a threshold value at signal level = 0 and generating a bit “1” if it is larger than that and bit “0” if it is smaller can be considered, but the signal distribution is widened as shown in FIG. Therefore, a bit value different from that of the transceiver may be generated. Then, the transmitter / receiver can estimate the bit error rate of the eavesdropper and extend the error by increasing the confidentiality to generate a secure secret key. That is, an eavesdropper cannot obtain a secret key by any of the above eavesdropping bit generation means.

As a more advanced wiretapping method, there is a method called an impersonation method. Impersonation eavesdropping is an eavesdropping method in which an eavesdropper receives all transmission signals and transmits a dummy signal to an original receiver using an optical transmitter based on the reception result. If the eavesdropper can correctly recognize the transmission signal, the same dummy signal as the original transmission signal can be sent, and the secret key can be obtained without being noticed by the receiver. Here, in order to send a complete dummy signal, it is necessary to correctly identify whether the signal light phase θ _S is {0, π / 2, π, 3π / 2}. In order to identify these four phase values, the eavesdropper branches the received signal into two, and performs homodyne detection with the local light emission phase θ _L = 0 for one and θ _L = π / 2 for the other. Each of the homodyne detections is performed, and the results of both are comprehensively judged. However, since the signal-to-noise ratio is deteriorated in the transmission signal due to the two branches in addition to the fluctuation due to the quantum noise, the four phase values cannot always be correctly identified. Therefore, when the receiver generates a key bit from the dummy signal, a mismatch occurs with the key bit of the transmitter.

  Therefore, the transmitter / receiver obtains a secret key according to a normal procedure, and then answers several test bits. If the system is operating normally, the bit information of both will match, but if there is spoofing, a mismatch bit will appear. If there is a mismatch bit, it is determined that there is a possibility of eavesdropping, and the secret key is discarded. In other words, if the test bits match, it can be determined that there was no wiretapping, and the secret key is guaranteed to be secure.

Ryo Namiki and Takuya Hirano, “Security of quantum cryptography using balanced homodyne detection,” Physical Review A, vol. 67, 022308 (2003).

  In the above prior art, the receiver generates a secret key bit by homodyne detection of extremely weak light. However, in order to correctly perform homodyne detection, the frequency and phase of the local light must be controlled stably and precisely. This is very difficult to achieve in the optical frequency region, and this is a problem in implementing the above-described conventional technology.

  The present invention has been made in view of the above problems, and an object thereof is to provide a quantum cryptography communication device and a quantum cryptography communication method for supplying a secret key without using homodyne detection.

In order to achieve such an object, the first aspect of the present invention provides a secret key in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ. A quantum cryptography communication device comprising: a transmitter that transmits an optical pulse train at a constant time interval; and a receiver that demodulates the secret key from the optical pulse train, wherein the optical power level and δ of the optical pulse train are quantum noises. Are selected so that some of the pulses phase-modulated with a phase of ± δ overlap with each other, and some of the pulses phase-modulated with a phase of π / 2 ± δ overlap with each other. Branching means for bifurcating the optical pulse train from the first path into a first path and a second path, and a delay means for giving a time difference equal to the pulse interval of the optical pulse train between the first path and the second path And one of the first and second paths A phase modulating means for modulating the propagation phase at 0 or [pi / 2, has input and output terminals of the 2 × 2, and the optical coupler the first and second paths to the respective input terminals are connected, the optical coupler First and second light detection means connected to output terminals of the first and second light detection means, differential detection means for generating a demodulated signal from the outputs of the first and second light detection means by differential detection, and the demodulated signal possess a threshold processing means for performing threshold processing with respect to the receiver, the time slots and the phase modulation means when said demodulated signal is less than the threshold value d is greater than or threshold -d is phase modulated Whether the transmission phase difference of the pulse related to the notified time slot is (a) ± 2δ, or (b) ± It was π / 2 ± 2δ (C) Notifying the receiver whether it was the other, and based on the notification from the transmitter, the receiver has the transmission phase difference of the above (a) and the demodulated signal is greater than or equal to a threshold value d, or A bit “0” is assigned to a time slot in which the transmission phase difference is (b) and the demodulated signal is equal to or less than the threshold −d, the transmission phase difference is (a) and the demodulated signal is the threshold −d. Or assigning bit “1” to a time slot in which the transmission phase difference is (b) and the demodulated signal is equal to or greater than a threshold value d, and the transmitter, based on the notification from the receiver, In a time slot where the phase modulated by the phase modulation means is 0 and the transmission phase difference is ± π / 2−2δ, or the phase modulated by the phase modulation means is π / 2 and the transmission phase difference is −2δ. Bit "0" is assigned to A bit for a time slot in which the phase modulated by the phase modulation means is 0 and the transmission phase difference is ± π / 2 + 2δ, or the phase modulated by the phase modulation means is π / 2 and the transmission phase difference is 2δ. A secret key bit is generated by assigning “1” .

Further, the second aspect of the present invention provides an optical pulse train having a constant time interval representing a secret key in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ. A quantum cryptography communication device comprising a transmitter for transmission and a receiver for demodulating the secret key from the optical pulse train, wherein the optical power level and δ of the optical pulse train are phase-modulated with a phase of ± δ by quantum noise Are selected such that some of the generated pulses overlap, and some of the pulses phase-modulated with a phase of π / 2 ± δ overlap, and the receiver sets the phase of each pulse of the optical pulse train to 0 or a phase modulation means that modulates by π / 2, an optical branching means that bifurcates the phase-modulated optical pulse train into first and second paths, and a gap between the first path and the second path. A delay giving a time difference equal to the pulse interval of the optical pulse train. And an optical coupler having a 2 × 2 input / output terminal, the first and second paths being connected to each input terminal, and a first and a first connected to the output terminal of the optical coupler, respectively. and second light detecting means, a differential detection means for generating a demodulated signal by differential detection from the output of the first and second light detecting means, and a threshold processing means for performing threshold processing on the demodulated signal Yes, and the receiver, the time slot and the or phase of the phase modulation means is modulated was 0, the transmission or was [pi / 2 when the demodulated signal is less than the threshold value d is greater than or threshold -d The transmission phase difference of the pulse related to the notified time slot is (a) ± 2δ, (b) ± π / 2 ± 2δ, or (c) To the receiver. Then, based on the notification from the transmitter, the receiver determines that the transmission phase difference is (a) and the demodulated signal is greater than or equal to the threshold d, or the transmission phase difference is (b) and is demodulated. A bit “0” is assigned to a time slot where the signal is equal to or less than the threshold −d, the transmission phase difference is (a) and the demodulated signal is equal to or less than the threshold −d, or the transmission phase difference is (b). And a bit “1” is assigned to a time slot whose demodulated signal is equal to or greater than a threshold value d, and the transmitter has a phase modulated by the phase modulation means of 0 based on the notification from the receiver, and A bit “0” is assigned to a time slot having a transmission phase difference of ± π / 2−2δ or a phase modulated by the phase modulation means of π / 2 and a transmission phase difference of −2δ, and the phase modulation means The phase modulated by is 0 And by assigning bit “1” to a time slot having a transmission phase difference of ± π / 2 + 2δ or a phase modulated by the phase modulation means of π / 2 and a transmission phase difference of 2δ, the secret key bit is generated and characterized in that.

The third aspect of the present invention provides an optical pulse train having a constant time interval representing a secret key in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ. A quantum cryptography communication device comprising a transmitter for transmission and a receiver for demodulating the secret key from the optical pulse train, wherein the optical power level and δ of the optical pulse train are phase-modulated with a phase of ± δ by quantum noise The selected pulses are selected such that a part of the pulses overlapped and a part of the pulses phase-modulated with a phase of π / 2 ± δ overlap, and the receiver selects the optical pulse train from the transmitter as a first And a first optical branching unit bifurcating into a second path, and a second optical branching unit bifurcating one of the optical pulse trains branched by the first optical branching unit into the third and fourth paths And the optical path between the third path and the fourth path. A first delay means for providing a time difference equal to the pulse interval of the scan row, the third and the first phase modulating means for either of the propagation phase and 0 of the fourth path, the 2 × 2 output terminals A first optical coupler having the input terminals connected to the third and fourth paths, and first and second optical detection means connected to the output terminals of the first optical coupler, respectively. When the a first differential detection means for generating a first demodulated signal by differential detection from the output of the first and second light detecting means, said first differential detecting means is first generated First threshold processing means for performing threshold processing on the demodulated signal, and third optical branching means for branching the other optical pulse train branched by the first optical branching means into fifth and sixth paths And the pulse interval of the optical pulse train between the fifth path and the sixth path. A correct a second delay means for providing a time difference, the second phase modulating means to said fifth and 6 [pi / 2 to one of the propagation phase of the path of the input and output terminals of the 2 × 2, A second optical coupler in which the fifth and sixth paths are connected to each input terminal; third and fourth photodetecting means connected to output terminals of the second optical coupler; and from the output of the third and fourth optical detection means and the second differential detection means for generating a second demodulated signal by differential detection, for the second demodulated signal and the second differential detecting means is generated have a second threshold value processing means for performing threshold processing Te, the receiver, the time slot and it said first demodulated signal when the demodulated signal is less than the threshold value d is greater than or threshold -d Or the second demodulated signal is notified to the transmitter. And whether the transmitter has (a) ± 2δ, (b) ± π / 2 ± 2δ, or (c) other than that, with respect to the notified time slot. To the receiver, based on the notification from the transmitter, the transmission phase difference is (a) and the demodulated signal is greater than or equal to the threshold d, or the transmission phase difference is (b) ) And a bit “0” is assigned to a time slot in which the demodulated signal is equal to or less than the threshold −d, and the transmission phase difference is (a) and the demodulated signal is equal to or less than the threshold −d, or the transmission phase difference is Bit (1) is assigned to a time slot that is (b) above and the demodulated signal is equal to or greater than the threshold value d, and the transmitter is the first demodulated signal based on the notification from the receiver. And the transmission phase difference is ± π / 2-2δ, Alternatively, bit “0” is assigned to a time slot of the second demodulated signal and the transmission phase difference is −2δ, and the first demodulated signal and the transmission phase difference is ± π / 2 + 2δ, or the first A secret key bit is generated by assigning bit “1” to a time slot having a demodulation signal of 2 and a transmission phase difference of 2δ .

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the demodulated signal has a received light amplitude of A, {0, ± Asin (2δ), ± Acos (2δ), The distribution is spread around 7 values of ± A}.

  According to a fifth aspect of the present invention, in the fourth aspect, the threshold processing sets the first threshold value to a value larger than Asin (2δ) and sets the second threshold value to a value smaller than −Asin (2δ). It is a process for setting a threshold value and determining whether the demodulated signal is larger than the first threshold value or smaller than the second threshold value.

Further, the sixth aspect of the present invention provides an optical pulse train having a constant time interval representing a secret key in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ. A quantum cryptography communication method for generating a secret key bit common to a transmitter to transmit and a receiver to demodulate the secret key from the pulse train , wherein the optical pulse train is a first and a second in the receiver . An optical branching step for bifurcating into two paths, a delay step for giving a time difference equal to a pulse interval of the optical pulse train between the first path and the second path, and the first and second paths A phase modulation step for modulating one of the propagation phases by 0 or π / 2, and an optical pulse train that has passed through the first and second paths are combined to be combined with the first and second photodetectors. A multiplexing step for outputting the optical pulse train A differential detection step of generating a demodulated signal by differential detection from the outputs of the first and second light detection means, and a threshold processing step of performing a threshold processing on the demodulated signal, wherein the light of the optical pulse train The power level and δ are selected so that some of the pulses modulated by quantum noise with a phase of ± δ overlap, and some of the pulses phase modulated with a phase of π / 2 ± δ overlap. In the receiver, the transmitter determines whether the time slot when the demodulated signal is larger than the threshold value d or smaller than the threshold value −d and the phase modulated by the phase modulation step is 0 or π / 2. In the transmitter, whether the transmission phase difference of the pulse related to the notified time slot is (a) ± 2δ, or (b) ± π / 2 ± 2δ, c) notifying the receiver whether it was other than that, and in the receiver, based on the notification from the transmitter, the transmission phase difference is (a) and the demodulated signal is equal to or greater than a threshold value d, or A bit “0” is assigned to a time slot in which the transmission phase difference is (b) and the demodulated signal is equal to or less than the threshold −d, the transmission phase difference is (a) and the demodulated signal is the threshold −d. Or assigning bit “1” to a time slot in which the transmission phase difference is (b) and the demodulated signal is greater than or equal to the threshold d, and based on the notification from the receiver in the transmitter, In the time slot where the phase modulated by the phase modulation step is 0 and the transmission phase difference is ± π / 2−2δ, or the phase modulated by the phase modulation step is π / 2 and the transmission phase difference is −2δ. Then, bit “0” is assigned, and the phase modulated by the phase modulation step is 0 and the transmission phase difference is ± π / 2 + 2δ, or the phase modulated by the phase modulation step is π / 2 and the transmission phase difference There by assigning bit "1" for the time slot of the 2.delta., characterized Rukoto further comprising the step of generating a secret key bits.

Further, according to a seventh aspect of the present invention, an optical pulse train having a constant time interval representing a secret key, in which each pulse is phase-modulated in any one of four phases ± δ and π / 2 ± δ , A quantum cryptography communication method for generating a secret key bit common in a transmitter for transmitting and a receiver for demodulating the secret key from the pulse train , wherein the phase of each pulse of the optical pulse train is determined in the receiver. A phase modulation step of modulating the phase- modulated optical pulse train into two first and second paths, a first path and a second path, A delay step that gives a time difference equal to the pulse interval of the optical pulse train between the first and second optical detection means by combining the optical pulse train that has passed through the first and second paths Multiplexing step to output pulse train A differential detection step of generating a demodulated signal by differential detection from outputs of the first and second light detection means, and a threshold processing step of performing threshold processing on the demodulated signal, The optical power level and δ are selected so that some of the pulses that are phase-modulated with a phase of ± δ due to quantum noise overlap, and some of the pulses that are phase-modulated with a phase of π / 2 ± δ overlap. In the receiver, the time slot when the demodulated signal is larger than the threshold value d or smaller than the threshold value −d and whether the phase modulated by the phase modulation step is 0 or π / 2. The transmission phase difference of the pulse related to the notified time slot is (a) ± 2δ or (b) ± π / 2 ± 2δ. (C) Informing the receiver whether it was other than that, and in the receiver, based on the notification from the transmitter, the transmission phase difference is (a) and the demodulated signal is equal to or greater than a threshold value d, Alternatively, bit “0” is assigned to a time slot in which the transmission phase difference is (b) and the demodulated signal is equal to or less than the threshold −d, and the transmission phase difference is (a) and the demodulated signal is the threshold − bit “1” is assigned to a time slot equal to or less than d or the transmission phase difference is (b) and the demodulated signal is equal to or greater than a threshold value d, and the transmitter is configured to receive the bit based on the notification from the receiver. A time slot in which the phase modulated by the phase modulation step is 0 and the transmission phase difference is ± π / 2−2δ, or the phase modulated by the phase modulation step is π / 2 and the transmission phase difference is −2δ. Bit “0” is assigned to the phase modulation step is 0 and the transmission phase difference is ± π / 2 + 2δ, or the phase modulation step modulation phase is π / 2 and the transmission phase difference. Generating a secret key bit by assigning bit “1” to a time slot of 2δ .

Further, an eighth aspect of the present invention provides an optical pulse train having a constant time interval representing a secret key, in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ. A quantum cryptography communication method for generating a secret key bit common to a transmitter to transmit and a receiver to demodulate the secret key from the pulse train , wherein the optical pulse train is a first and a second in the receiver . A first optical branching step that branches into two paths, and a second optical branching step that splits one of the optical pulse trains optically branched by the first optical branching step into the third and fourth paths. A first delay step that gives a time difference equal to the pulse interval of the optical pulse train between the third path and the fourth path, and the propagation phase of any of the third and fourth paths is 0 a first phase modulating step of the said A first combining step for combining the optical pulse trains that have passed through the third and fourth paths and outputting the combined optical pulse train to the first and second light detection means; and the first and second light components a first differential detection step of generating a first demodulated signal by differential detection from the output of the detection means, performs threshold processing on the first demodulated signal generated by the first differential detection step A first threshold value processing step, a third optical branching step for bifurcating the other optical pulse train optically branched by the first optical branching step into fifth and sixth paths, and the fifth path A second delay step for giving a time difference equal to the pulse interval of the optical pulse train to the sixth path, and a propagation phase of any one of the fifth and sixth paths to be π / 2 Phase modulation step and passed through the fifth and sixth paths A second combining step for combining the optical pulse trains and outputting the combined optical pulse trains to the third and fourth light detection means; and differential detection from the outputs of the fifth and sixth light detection means. A second differential detection step for generating a second demodulated signal; and a second threshold processing step for performing threshold processing on the second demodulated signal generated by the second differential detection step. The optical power level and δ of the optical pulse train are such that some of the pulses that are phase-modulated with a phase of ± δ due to quantum noise overlap, and some of the pulses that are phase-modulated with a phase of π / 2 ± δ In the receiver, a time slot when the demodulated signal is greater than a threshold d or less than a threshold −d and whether it was the first demodulated signal or the second demodulated signal at the receiver. Whether it was a signal (B) ± π / 2 ± 2δ, (c) Whether the transmission phase difference of the pulses related to the notified time slot is (a) ± 2δ, or (c) ) Notifying the receiver of whether it was other than that, and in the receiver, based on the notification from the transmitter, the transmission phase difference is (a) and the demodulated signal is greater than or equal to the threshold d, or A bit “0” is assigned to a time slot in which the transmission phase difference is (b) and the demodulated signal is equal to or less than the threshold −d, and the transmission phase difference is (a) and the demodulated signal is equal to or less than the threshold −d. Or assigning bit “1” to a time slot in which the transmission phase difference is (b) and the demodulated signal is greater than or equal to a threshold value d, and the transmitter, based on the notification from the receiver, First And a bit “0” is assigned to a time slot with a transmission phase difference of ± π / 2-2δ or the second demodulation signal and a transmission phase difference of −2δ, A secret key bit is generated by assigning bit “1” to a time slot that is a demodulated signal and has a transmission phase difference of ± π / 2 + 2δ or the second demodulated signal and has a transmission phase difference of 2δ. And further comprising a step .

  A ninth aspect of the present invention is the demodulated signal according to any one of the sixth to eighth aspects, wherein the demodulated signal has a received light amplitude of A, {0, ± Asin (2δ), ± Acos (2δ), The distribution is spread around 7 values of ± A}.

  According to a tenth aspect of the present invention, in the ninth aspect, the threshold processing sets the first threshold value to a value larger than Asin (2δ), and sets the second threshold value to a value smaller than −Asin (2δ). It is a process for setting a threshold value and determining whether the demodulated signal is larger than the first threshold value or smaller than the second threshold value.

  According to the present invention, a secret key for cryptographic communication can be safely supplied using a photodetector that directly detects light intensity. Unlike the prior art, since it is not necessary to perform homodyne detection of extremely weak light, a highly practical quantum cryptography communication device and quantum cryptography communication method can be provided.

It is a figure which shows the basic composition of the conventional coherent / homodyne detection quantum key distribution system. It is a figure which shows the demodulation signal distribution in a prior art. 1 is a configuration diagram of a quantum cryptography communication device according to a first embodiment of the present invention. It is a figure for demonstrating the state of the transmission light in 1st Embodiment. (A) is a diagram showing a demodulated signal distribution in the first embodiment when θ _b = 0, and (b) is a diagram showing a demodulated signal distribution in the first embodiment when θ _b = π / 2. . It is a figure for demonstrating the setting of the threshold value in 1st Embodiment. It is a figure which shows the receiver of the quantum cryptography communication apparatus which concerns on the 2nd Embodiment of this invention. It is a figure which shows the receiver of the quantum cryptography communication apparatus which concerns on the 3rd Embodiment of this invention.

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

(First embodiment)
FIG. 3 is a configuration diagram of the quantum cryptography communication device according to the first embodiment of the present invention. In FIG. 3, the transmitter 310 includes a light source 311 and a phase modulator 312. The light source 311 outputs a coherent optical pulse train at regular intervals. The phase modulator 312 modulates the phase of each pulse with -δ or δ, or (π / 2-δ) or (π / 2 + δ). Here, it is assumed that the optical power level (the square of a real amplitude A described later) and the phase δ satisfy the relationship described below.

  In general, the state of the photoelectric field is represented by a point on a complex amplitude space. However, as described in the description of the prior art, since coherent light inevitably involves quantum noise, it is strictly expressed by a circle extending around a certain point. In this case, the outputs are A × exp [−iδ], A × exp [iδ], A × exp [i (π / 2−δ)], A × exp [i (π / 2 + δ). )] (A is a constant representing the real number amplitude), and these satisfy the relationship shown in FIG. That is, A × exp [−iδ] and A × exp [iδ] partially overlap with each other due to quantum noise, and A × exp [i (π / 2−δ)] and A × exp [i (π / 2 + δ)] partly overlaps due to quantum noise. Here, the four states need only overlap as shown in FIG. 4, and if δ is appropriately selected, it is not necessary to be extremely weak light as in the prior art.

  The receiver 320 includes an optical branching unit 321, a modulating unit 322 that modulates one branched propagation phase, a 2 × 2 optical coupler 323, and two photodetectors 324 and 325. The light input to the receiver 320 is branched into two by the optical branching means 321, given a delay time to one, and then multiplexed again by the optical coupler 323. It can be said that one of the paths functions as a delay means. Here, the delay time is assumed to be equal to the pulse interval of the input pulse train. Further, a modulation means 322 for modulating the phase is provided on one of the two branched paths, whereby the phase of the propagation light is modulated by 0 or π / 2. Photodetectors 324 and 325 are connected to two output terminals of an optical coupler 323 in which two paths branched by the optical branching means 321 are connected to input terminals. The outputs from the two photodetectors 324 and 325 are differentially detected by the differential detection means 326.

  With the above configuration, the transmitter 310 transmits an optical pulse train with a constant interval (pulse interval) that is randomly phase-modulated with -δ or δ or (π / 2-δ) or (π / 2 + δ). The receiver 320 inputs the pulse train transmitted from the transmitter 310 via the optical transmission path to the optical branching unit 321. The branched optical pulse trains are combined again by the optical coupler 323 and detected by the photon detectors 324 and 325, respectively.

  When the receiving circuit is configured in this manner, the optical coupler 323 of the receiver 320 causes the front and rear pulses to overlap and cause interference. The state of interference is described as follows.

  Now, consider the light pulse 1 that passes through the long path and the light pulse 2 that passes through the short path.

It expresses. The light pulse 1 reaches the photodetector 324 and the photodetector 325 through the optical coupler 323 from the long path. The photoelectric field E ₁ (D1) in the photodetector 324 of the light pulse 1 and the photoelectric field E ₁ (D2) in the photodetector 325 are expressed as follows.

Here, θ _b is a phase applied on the long path. On the other hand, the optical pulse 2 reaches the photodetector 324 and the photodetector 325 through the optical coupler 323 from the short path. The photoelectric field E ₂ (D1) in the photodetector 324 of the light pulse 2 and the photoelectric field E ₂ (D2) in the photodetector 325 are expressed as follows.

From the above, the photoelectric field E (D1) input to the photodetector 324 is

The photoelectric field E (D2) input to the photodetector 325 is

It becomes. Here, Δθ = θ ₁ −θ ₂ .

From Expressions (3) and (4), the output I ₁ from the photodetector 324 and the output I ₂ from the photodetector 325 are respectively expressed as follows.

Furthermore, this differential detection output S is

It becomes.

As described above, the value of the differential detection output (= demodulated signal) is determined by Δθ and theta _b. In this case, θ _1,2 = {-δ, δ, π / 2-δ, π / 2 + δ}, so Δθ is {0, ± 2δ, -π / 2, -π / 2 ± 2δ, π / 2, π / 2 ± 2δ}. Here, the probability that Δθ will be determined is determined by the number of combinations of θ ₁ and θ ₂ . Table 2 summarizes the appearance ratio of each value. Further, θ _b is any one of {0, π / 2}, and the ratio is 1: 1. Accordingly, S is any one of {0, ± Asin (2δ), ± Acos (2δ), ± A}. Table 3 summarizes the combinations of Δθ and θ _b that give S.

The above S = {0, ± Asin (2δ), ± Acos (2δ), ± A} is the reception characteristic when there is no fluctuation. Actually, the demodulated signal distribution has these values due to quantum noise. It will spread to the center. 5A shows a demodulated signal distribution when θ _b = 0, and FIG. 5B shows a demodulated signal distribution when θ _b = π / 2. Since {A, δ} is set as shown in FIG. 4, the signal distributions centered on 0 and ± Asin (2δ) partially overlap.

  For this demodulated signal distribution, the receiver sets a threshold ± d as shown in FIG. 6 by threshold processing means (not shown). That is, the threshold value d is set to a value within a range in which a signal larger than Asin (2δ) and centered on Asin (2δ) is distributed, and the signal smaller than -Asin (2δ) and centered on -Asin (2δ). The threshold value -d is set to each value within the range in which is distributed.

Using the above configuration and operating characteristics, the transmitter and the receiver obtain a common bit by the following procedure. (1) The transmitter 310 and the receiver 320 transmit and receive an optical pulse train having a required length. (2) The receiver 320 informs the transmitter 310 whether the demodulated signal is larger than the threshold value d or smaller than the threshold value −d, and whether the modulation phase θ _b on the delay path is 0 or π / 2. , Notify. (3) The transmitter 310 informs the receiver 320 that the phase difference of the pulse related to the time slot notified from the receiver 320 is (a) {± 2δ} or (b) {± π / 2 ± 2δ}. (C) It is notified whether it was other than that. However, only one of the three patterns (a) to (c) is notified, and a more detailed value (for example, 2δ or −2δ) is not taught. (4) The receiver 320 is configured such that the phase difference of the transmitter 310 is the pattern (a) and the demodulated signal is greater than or equal to the threshold value d, or the phase difference of the transmitter 310 is the pattern (b) and the demodulated signal is the threshold value −d or less. Bit “0” is assigned to the time slot. Also, a bit is set for a time slot in which the phase difference of the transmitter 310 is the pattern (a) and the demodulated signal is equal to or less than the threshold −d, or the phase difference of the transmitter 310 is the pattern (b) and the demodulated signal is equal to or greater than the threshold d. Assign “1”. (5) For the time slot notified from the receiver 320, the transmitter 310 has an interferometer phase difference of 0 and a transmission phase difference of ± π / 2 −2 δ , or an interferometer phase difference of π / 2 and a transmission position. If the phase difference is −2 δ , bit “0” is assigned. If the interferometer phase difference is 0 and the transmission phase difference is ± π / 2 + 2 δ , or the interferometer phase difference is π / 2 and the transmission phase difference is 2 δ , bit “1” is assigned. The bit values generated by the above procedure are almost the same in the transceiver. This is a secret key bit.

  The security of the secret key generated as described above can be considered in the same way as in the prior art.

  In the case of an eavesdropping method in which a part of a transmission signal is branched / stored, an eavesdropper distinguishes a signal centered on Asin (2δ) and a signal centered on Asin (2δ), as shown in FIG. In addition, since the distributions of both overlap, identification errors occur and a correct secret key cannot be obtained.

In the case of spoofing, it is necessary for the eavesdropper to identify the four states shown in FIG. 4. However, if this is performed by homodyne detection, the signal is split into two and then the local light emission phases θ _L = 0 and θ _The measurement is performed with _L = π / 2, and the four states cannot always be correctly identified due to deterioration of the signal-to-noise ratio due to quantum noise and branching. Therefore, when the receiver 320 generates a key bit from the dummy signal, a mismatch occurs with the key bit of the transmitter 310, and wiretapping is detected from this.

  As described above, in the present embodiment, a secure secret key is generated by the same mechanism as in the conventional technology. However, the major difference in mounting is that, while the prior art has received very weak light with homodyne, this embodiment directly detects the intensity of signal light of a certain power level. Therefore, an advanced homodyne detection technique is not necessary, and it can be said that the method is more practical.

(Second Embodiment)
In the first embodiment, the receiver 320 adds {0, π / 2} modulation to the phase difference between the preceding and succeeding pulses according to the phase θ _b on the delay path. It can also be realized by the configuration. The receiver 720 of the second embodiment is configured to apply {0, π / 2} phase modulation to each pulse at the front stage of the interferometer. Since the purpose of the phase modulation of the receiver 320 is to give a phase difference of 0 or π / 2 to the phase difference between the preceding and succeeding pulses, the same function as that of the first embodiment is obtained even with such a configuration. be able to.

(Third embodiment)
The function of the receiver similar to that of the first and second embodiments can also be realized by the configuration of FIG. The receiver 820 of this embodiment is configured to split a transmitted signal into two by an optical coupler or an optical switch, and to pass through an interferometer having a phase difference of 0 and an interferometer having a phase difference of π / 2, respectively. Yes. With this configuration, the demodulated signal exceeds the threshold at the output of the interferometer with a phase difference of 0, which is equivalent to θ _b = 0 and exceeds the threshold in the first embodiment, and the phase difference π / This is equivalent to the fact that the demodulated signal exceeds the threshold at the output of 2 interferometers and that θ _b = π / 2 and exceeds the threshold in the first embodiment. Therefore, according to the present embodiment, a secure secret key can be obtained in the same manner as in the first embodiment.

310 Transmitter 311 Light source 312 Phase modulator 320 Receiver 321 Optical branching means 322 Modulating means 323 2 × 2 optical coupler (corresponding to optical coupler)
324, 325 Photodetector (corresponding to photodetection means)
326 Differential detection means

Claims (10)

A transmitter for transmitting an optical pulse train having a constant time interval representing a secret key, in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ;
A quantum cryptography communication device comprising a receiver for demodulating the secret key from the optical pulse train,
Regarding the optical power level and δ of the optical pulse train, a part of pulses modulated by quantum noise with a phase of ± δ overlap, and a part of pulses modulated by a phase of π / 2 ± δ overlap. Selected as
The receiver
Optical branching means for bifurcating the optical pulse train from the transmitter into first and second paths;
Delay means for providing a time difference equal to a pulse interval of the optical pulse train between the first path and the second path;
Phase modulation means for modulating the propagation phase of any of the first and second paths by 0 or π / 2;
An optical coupler having a 2 × 2 input / output terminal and having the first and second paths connected to each input terminal;
First and second light detection means respectively connected to output terminals of the optical coupler;
Differential detection means for generating a demodulated signal from the outputs of the first and second light detection means by differential detection;
It possesses a threshold processing means for performing threshold processing on the demodulated signal,
The receiver notifies the transmitter whether the time slot when the demodulated signal is larger than the threshold value d or smaller than the threshold value −d and the phase modulated by the phase modulation means was 0 or π / 2. At the same time, whether the transmitter has (a) ± 2δ, (b) ± π / 2 ± 2δ, or (c) other than that, the transmission phase difference of the pulse related to the notified time slot To the receiver,
Based on the notification from the transmitter, the receiver determines that the transmission phase difference is (a) and the demodulated signal is greater than or equal to the threshold value d, or the transmission phase difference is (b) and the demodulated signal is Bit “0” is assigned to a time slot equal to or less than a threshold −d, the transmission phase difference is (a) and the demodulated signal is equal to or less than the threshold −d, or the transmission phase difference is (b) and A bit “1” is assigned to a time slot in which the demodulated signal is equal to or greater than a threshold value d, and the transmitter, based on the notification from the receiver, has a phase modulated by the phase modulation means of 0 and a transmission level. A bit “0” is assigned to a time slot in which the phase difference is ± π / 2−2δ or the phase modulated by the phase modulation means is π / 2 and the transmission phase difference is −2δ, and the phase modulation means modulates Phase is 0 and By assigning bit "1" to the signal phase difference ± π / 2 + 2δ or the phase modulation means is a phase [pi / 2 was modulated and time slot difference transmission phase 2.delta.,, Generating a secret key bit A quantum cryptography communication device. A transmitter for transmitting an optical pulse train having a constant time interval representing a secret key, in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ;
A quantum cryptography communication device comprising a receiver for demodulating the secret key from the optical pulse train,
Regarding the optical power level and δ of the optical pulse train, a part of pulses modulated by quantum noise with a phase of ± δ overlap, and a part of pulses modulated by a phase of π / 2 ± δ overlap. Selected as
The receiver
Phase modulation means for modulating the phase of each pulse of the optical pulse train by 0 or π / 2;
Optical branching means for branching the optical pulse train phase-modulated by the phase modulation means into first and second paths;
Delay means for providing a time difference equal to a pulse interval of the optical pulse train between the first path and the second path;
An optical coupler having a 2 × 2 input / output terminal and having the first and second paths connected to each input terminal;
First and second light detection means respectively connected to output terminals of the optical coupler;
Differential detection means for generating a demodulated signal from the outputs of the first and second light detection means by differential detection;
It possesses a threshold processing means for performing threshold processing on the demodulated signal,
The receiver notifies the transmitter whether the time slot when the demodulated signal is larger than the threshold value d or smaller than the threshold value −d and the phase modulated by the phase modulation means was 0 or π / 2. At the same time, whether the transmitter has (a) ± 2δ, (b) ± π / 2 ± 2δ, or (c) other than that, the transmission phase difference of the pulse related to the notified time slot To the receiver,
Based on the notification from the transmitter, the receiver determines that the transmission phase difference is (a) and the demodulated signal is greater than or equal to the threshold value d, or the transmission phase difference is (b) and the demodulated signal is Bit “0” is assigned to a time slot equal to or less than a threshold −d, the transmission phase difference is (a) and the demodulated signal is equal to or less than the threshold −d, or the transmission phase difference is (b) and A bit “1” is assigned to a time slot in which the demodulated signal is equal to or greater than a threshold value d, and the transmitter, based on the notification from the receiver, has a phase modulated by the phase modulation means of 0 and a transmission level. A bit “0” is assigned to a time slot in which the phase difference is ± π / 2−2δ or the phase modulated by the phase modulation means is π / 2 and the transmission phase difference is −2δ, and the phase modulation means modulates Phase is 0 and By assigning bit "1" to the signal phase difference ± π / 2 + 2δ or the phase modulation means is a phase [pi / 2 was modulated and time slot difference transmission phase 2.delta.,, Generating a secret key bit A quantum cryptography communication device. A transmitter for transmitting an optical pulse train having a constant time interval representing a secret key, in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ;
A quantum cryptography communication device comprising a receiver for demodulating the secret key from the optical pulse train,
Regarding the optical power level and δ of the optical pulse train, a part of pulses modulated by quantum noise with a phase of ± δ overlap, and a part of pulses modulated by a phase of π / 2 ± δ overlap. Selected as
The receiver
First optical branching means for bifurcating the optical pulse train from the transmitter into first and second paths;
Second optical branching means for bifurcating one optical pulse train branched by the first optical branching means into third and fourth paths;
First delay means for providing a time difference equal to a pulse interval of the optical pulse train between the third path and the fourth path;
First phase modulation means for setting the propagation phase of any of the third and fourth paths to 0;
A first optical coupler having a 2 × 2 input / output terminal and having the third and fourth paths connected to each input terminal;
First and second light detection means respectively connected to output terminals of the first optical coupler;
A first differential detection means for generating a first demodulated signal by differential detection from the output of the first and second light detecting means,
A first threshold value processing means for performing threshold processing on the first demodulated signal of the first differential detection means is generated,
Third optical branching means for bifurcating the other optical pulse train branched by the first optical branching means into fifth and sixth paths;
Second delay means for providing a time difference equal to a pulse interval of the optical pulse train between the fifth path and the sixth path;
Second phase modulation means for setting the propagation phase of any of the fifth and sixth paths to π / 2;
A second optical coupler having a 2 × 2 input / output terminal and having the fifth and sixth paths connected to each input terminal;
Third and fourth light detection means respectively connected to output terminals of the second optical coupler;
A second differential detection means for generating a second demodulated signal by differential detection from the output of said third and fourth optical detection means,
Have a second threshold value processing means for performing threshold processing with respect to the second demodulated signal and the second differential detecting means is generated,
Said receiver, said one demodulated signals was the time slot and it said first demodulated signal when smaller than the threshold value d is greater than or threshold -d, whether a second demodulation signal to the transmitter And the transmitter transmits (a) ± 2δ, (b) ± π / 2 ± 2δ, or (c) otherwise, the transmission phase difference of the pulses related to the notified time slot. Notify the receiver if it was,
Based on the notification from the transmitter, the receiver determines that the transmission phase difference is (a) and the demodulated signal is greater than or equal to the threshold value d, or the transmission phase difference is (b) and the demodulated signal is Bit “0” is assigned to a time slot equal to or less than a threshold −d, the transmission phase difference is (a) and the demodulated signal is equal to or less than the threshold −d, or the transmission phase difference is (b) and A bit “1” is assigned to a time slot whose demodulated signal is equal to or greater than a threshold value d , and the transmitter is the first demodulated signal and has a transmission phase difference of ± π based on the notification from the receiver. / 2-2δ, or the second demodulated signal and a bit slot “0” is assigned to a time slot having a transmission phase difference of −2δ, and the first demodulated signal and the transmission phase difference is ± π / 2 + 2δ, or the second demodulated signal There and by difference transmission phase assigns a bit "1" for the time slot of the 2.delta., Quantum cryptography communication device and generating a secret key bits.   The distribution of the demodulated signal is spread around seven values of {0, ± Asin (2δ), ± Acos (2δ), ± A}, where A is a received light amplitude. The quantum cryptography communication apparatus in any one of -3.   The threshold value processing sets a first threshold value to a value larger than Asin (2δ), sets a second threshold value to a value smaller than −Asin (2δ), and the demodulated signal is larger than the first threshold value. The quantum cryptography communication apparatus according to claim 4, wherein the process is a process of determining whether or not the second threshold value is smaller than the second threshold value. A transmitter for transmitting an optical pulse train having a constant time interval representing a secret key, in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ, and A quantum cryptography communication method for generating a secret key bit common to a receiver that demodulates a secret key ,
In the receiver,
An optical branching step of bifurcating the optical pulse train into first and second paths;
A delay step for providing a time difference equal to a pulse interval of the optical pulse train between the first path and the second path;
A phase modulation step of modulating the propagation phase of any of the first and second paths by 0 or π / 2;
A combining step of combining the optical pulse train that has passed through the first and second paths and outputting the combined optical pulse train to the first and second light detection means;
A differential detection step of generating a demodulated signal by differential detection from the outputs of the first and second light detection means;
A threshold processing step for performing threshold processing on the demodulated signal,
Regarding the optical power level and δ of the optical pulse train, a part of pulses modulated by quantum noise with a phase of ± δ overlap, and a part of pulses modulated by a phase of π / 2 ± δ overlap. It has been selected as,
In the receiver, the time slot when the demodulated signal is larger than the threshold value d or smaller than the threshold value −d and whether the phase modulated by the phase modulation step is 0 or π / 2 is notified to the transmitter. In addition, in the transmitter, whether the transmission phase difference of the pulse related to the notified time slot is (a) ± 2δ, (b) ± π / 2 ± 2δ, or (c) other than that Informing the receiver of:
In the receiver, based on the notification from the transmitter, the transmission phase difference is (a) and the demodulated signal is greater than or equal to the threshold d, or the transmission phase difference is (b) and the demodulated signal is Bit “0” is assigned to a time slot equal to or less than a threshold −d, the transmission phase difference is (a) and the demodulated signal is equal to or less than the threshold −d, or the transmission phase difference is (b) and A bit “1” is assigned to a time slot whose demodulated signal is equal to or greater than a threshold value d, and the transmitter modulates the phase modulated by the phase modulation step based on the notification from the receiver and transmits the bit. A bit “0” is assigned to a time slot having a phase difference of ± π / 2−2δ or a phase modulated by the phase modulation step of π / 2 and a transmission phase difference of −2δ, and the phase modulation step A bit “1” is assigned to a time slot in which the modulated phase is 0 and the transmission phase difference is ± π / 2 + 2δ, or the phase modulated by the phase modulation step is π / 2 and the transmission phase difference is 2δ. it makes quantum cryptography communication method comprising Rukoto further comprising the step of generating a secret key bits. A transmitter for transmitting an optical pulse train having a constant time interval representing a secret key, in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ, and the secret from the pulse train A quantum cryptography communication method for generating a secret key bit common to a receiver that demodulates a key,
In the receiver,
A phase modulation step of modulating the phase of each pulse of the optical pulse train by 0 or π / 2;
An optical branching step of bifurcating the phase-modulated optical pulse train into first and second paths;
A delay step for providing a time difference equal to a pulse interval of the optical pulse train between the first path and the second path;
A combining step of combining the optical pulse train that has passed through the first and second paths and outputting the combined optical pulse train to the first and second light detection means;
A differential detection step of generating a demodulated signal by differential detection from the outputs of the first and second light detection means;
A threshold processing step for performing threshold processing on the demodulated signal,
Regarding the optical power level and δ of the optical pulse train, a part of pulses modulated by quantum noise with a phase of ± δ overlap, and a part of pulses modulated by a phase of π / 2 ± δ overlap. It has been selected as,
In the receiver, the time slot when the demodulated signal is larger than the threshold value d or smaller than the threshold value −d and whether the phase modulated by the phase modulation step is 0 or π / 2 is notified to the transmitter. In addition, in the transmitter, whether the transmission phase difference of the pulse related to the notified time slot is (a) ± 2δ, (b) ± π / 2 ± 2δ, or (c) other than that Informing the receiver of:
In the receiver, based on the notification from the transmitter, the transmission phase difference is (a) and the demodulated signal is greater than or equal to the threshold d, or the transmission phase difference is (b) and the demodulated signal is Bit “0” is assigned to a time slot equal to or less than a threshold −d, the transmission phase difference is (a) and the demodulated signal is equal to or less than the threshold −d, or the transmission phase difference is (b) and A bit “1” is assigned to a time slot in which the demodulated signal is equal to or greater than a threshold value d, and in the transmitter, the phase modulated by the phase modulation step is 0 based on the notification from the receiver, and the transmission level A bit “0” is assigned to a time slot having a phase difference of ± π / 2−2δ or a phase modulated by the phase modulation step of π / 2 and a transmission phase difference of −2δ, and the phase modulation step is changed. Bit “1” is assigned to a time slot in which the phase is 0 and the transmission phase difference is ± π / 2 + 2δ, or the phase modulated by the phase modulation step is π / 2 and the transmission phase difference is 2δ. And a step of generating a secret key bit . A transmitter for transmitting an optical pulse train having a constant time interval representing a secret key, in which each pulse is phase-modulated in any one of four phases of ± δ and π / 2 ± δ, and the secret from the pulse train A quantum cryptography communication method for generating a secret key bit common to a receiver that demodulates a key,
In the receiver,
A first optical branching step for bifurcating the optical pulse train into first and second paths;
A second optical branching step of branching one optical pulse train optically branched by the first optical branching step into a third and a fourth path;
A first delay step for providing a time difference between the third path and the fourth path equal to a pulse interval of the optical pulse train;
A first phase modulation step in which the propagation phase of any of the third and fourth paths is 0;
A first combining step of combining the optical pulse trains that have passed through the third and fourth paths and outputting the combined optical pulse train to the first and second light detection means;
A first differential detection step of generating a first demodulated signal by differential detection from the output of the first and second light detecting means,
A first threshold value processing step of performing threshold processing on first demodulated signal generated by the first differential detection step,
A third optical branching step for bifurcating the other optical pulse train optically branched by the first optical branching step into fifth and sixth paths;
A second delay step providing a time difference equal to a pulse interval of the optical pulse train between the fifth path and the sixth path;
A second phase modulation step in which the propagation phase of any of the fifth and sixth paths is π / 2;
A second combining step of combining the optical pulse trains that have passed through the fifth and sixth paths and outputting the combined optical pulse train to the third and fourth photodetecting means;
A second differential detector generating a second demodulated signal by differential detection from the output of said fifth and sixth light detection means,
And a second threshold value processing step of performing threshold processing with respect to the second demodulated signal generated by the second differential detection step,
Regarding the optical power level and δ of the optical pulse train, a part of pulses modulated by quantum noise with a phase of ± δ overlap, and a part of pulses modulated by a phase of π / 2 ± δ overlap. It has been selected as,
In the receiver, the time slot when the demodulated signal is larger than the threshold value d or smaller than the threshold value −d and whether it is the first demodulated signal or the second demodulated signal are sent to the transmitter. At the transmitter, whether the transmission phase difference of the pulse related to the notified time slot is (a) ± 2δ, (b) ± π / 2 ± 2δ, or (c) otherwise A step of notifying the receiver of the occurrence,
In the receiver, based on the notification from the transmitter, the transmission phase difference is (a) and the demodulated signal is greater than or equal to the threshold d, or the transmission phase difference is (b) and the demodulated signal is Bit “0” is assigned to a time slot equal to or less than a threshold −d, the transmission phase difference is (a) and the demodulated signal is equal to or less than the threshold −d, or the transmission phase difference is (b) and A bit “1” is assigned to a time slot in which the demodulated signal is equal to or greater than a threshold value d, and the transmitter is the first demodulated signal and the transmission phase difference is ± π based on the notification from the receiver. / 2-2δ, or the second demodulated signal and a bit slot “0” is assigned to a time slot having a transmission phase difference of −2δ, and the first demodulated signal and the transmission phase difference is ± π / 2 + 2δ or the second And generating a secret key bit by assigning a bit “1” to a time slot having a transmission phase difference of 2δ .   The distribution of the demodulated signal is spread around seven values of {0, ± Asin (2δ), ± Acos (2δ), ± A}, where A is a received light amplitude. The quantum cryptography communication method according to any one of?   The threshold value processing sets a first threshold value to a value larger than Asin (2δ), sets a second threshold value to a value smaller than −Asin (2δ), and the demodulated signal is larger than the first threshold value. The quantum cryptography communication method according to claim 9, wherein the quantum cryptography communication method is a process of determining whether or not the second threshold is smaller.

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