JP2009225028A - Quantum encryption key delivery system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum encryption key delivery system of time-bin entangled photon requiring no polarization control to signal light. <P>SOLUTION: The quantum key delivery system includes: a photon pair generator which generates a time-bin entangled photon pair comprising a pulse train of signal photon and a pulse train of idler photon, a first receiver which receives the pulse train of signal photon, and a second receiver which receives the pulse train of idler photon. The photon pair generator sends the pulse train in which polarized states of adjoining pulses are orthogonal to each other, for each pulse train. The first and second receivers respectively branch the received pulse trains, and apply a delay between the branched paths so that pulses whose polarized states coincide with each other are interfered with each other for photon detection. Thereby, a polarization-dependent optical component and a device can be used and a polarization-independent key generation operation can be attained as an entire system. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a quantum key distribution system, and more particularly to a quantum key distribution system using time-entangled photons.

  In recent years, research on quantum cryptography communication in which safety is physically guaranteed by using light of one photon level has been advanced. The quantum cryptography is for sharing a secret key for performing cryptographic communication between two remote communication devices, and is also called quantum key distribution. There are various methods for quantum key distribution, but here, a quantum key distribution system using time-entangled photons will be described.

FIG. 1 shows an example of a conventional quantum key distribution system using time-entangled photons (Patent Document 1). The system 100 includes a photon pair generator that generates time-position entangled photon pairs and a set of receivers A and B that generate secret keys at points remote from each other. The time-entangled photon pair generator has a pump light source and an optical nonlinear medium, and the pump light from the pump light source is incident on the optical nonlinear medium that causes an optical parametric process, and thereby has a constant time interval in a entangled relationship. The T optical pulse train (signal photon and idler photon) is delivered to the receivers A and B. Here, the photon pair generator is set to a light level that averages less than one photon pair / pulse. The signal photon and idler photon are always generated in pairs in order to satisfy the energy conservation law. Furthermore, when an arbitrary continuous photon pair is seen, there is a relationship that the sum (Δθ _s + Δθ _i ) of the phase difference Δθ _s between signal photons and the phase difference Δθ _i between idler photons is always constant. A photon pair having such a correlation is called a time-position entangled photon pair.

  The signal idler pulse train output from the time-position entangled photon pair generator reaches the receivers A and B through the transmission lines, respectively. Each receiver has the same configuration, and includes an optical phase modulator, a one-pulse delay interferometer, and two photon detectors that detect photons according to the interference state of the interferometer. Each receiver receives a pulse train having a pulse interval T, and each pulse is phase-modulated by 0 or π / 2 by a phase modulator. Each phase-modulated pulse is branched into two by optical branching units C1 and C3, a delay T is added between the branching paths by an optical delay path, and then multiplexed by 2 × 2 optical couplers C2 and C4. . Since the delay T of the optical delay path is set to be equal to the pulse interval T, the preceding and succeeding pulse trains are overlapped and combined. That is, in the coupling couplers C2 and C4, the photons of the preceding and following pulse trains interfere with each other, and the photons are output to one of the two output ports of the coupling coupler according to the interference state. Therefore, photons are detected by one of the photon detectors A1 and A2, B1, and B2 connected to the two output ports according to the interference state of the photons. In addition to the phase modulation information in the phase modulator, the detection time and detection result in the photon detector are supplied to the information processing device CPU. The information processing apparatus is configured to exchange information between the two receivers A and B.

  In the above configuration, when both the receivers A and B detect photons, the photon detection result of the receiver A and the photon detection result of the receiver B are included in each receiver due to the nature of the time-position entangled photon pair. There is a correlation as shown in Table 1 depending on the phase modulation applied by.

  Since each receiver phase-modulates each pulse by 0 or π / 2, the phase difference (differential phase amount) given to two continuous pulses causing interference by this phase modulation is −π / 2, 0 or π / 2. Since there are three types of differential phase amounts given by one receiver, there are nine combinations of differential phase amounts given by two receivers. Table 1 shows the correlation of photon detection for each combination.

In Table 1, Δθ _a and Δθ _b represent the differential phase amounts given by the receivers A and B, respectively. “A1-B2” indicates that the detector B2 always detects a photon whenever the detector A1 detects the photon. The same applies to “A2-B1”, “A1-B1”, and “A2-B2”. Furthermore, “?” Indicates that there is no correlation between the photon detection events of the two receivers, and that each photon detector detects photons at random. The above correlation can be summarized as “A1-B2” or “A2-B1” when Δθ _a + Δθ _b = ± π, and “A1-B1” or “A2-B2” when Δθ _a + Δθ _b = 0. , Δθ _a + Δθ _b = ± π / 2, “?”. A detailed description using the correlation formula in Table 1 is described in Patent Document 1.

Using the above configuration, the receivers A and B can share the secret key by the following procedure.
(1) First, each receiver receives a pulse train and detects photons as described above. At this time, the time when the photon is detected and the detector with which the photon is detected are recorded in the information processing device CPU.
(2) After receiving the required predetermined number of photons, each receiver indicates the differential phase amount (or the phase modulation amount in the phase modulator) together with the time when the photons are detected (photon detection time). Are sent and received via a normal transmission line.
(3) Then, when both photons detect photons at the same time, in the case of a predetermined rule (for example, Δθ _a + Δθ _b = ± π, the detection event by the detector A1 / B2 is indicated by a bit “ 0 ”, detection event by detector A2 / B1 is bit“ 1 ”, and Δθ _a + Δθ _b = 0, detection event by detector A1 / B1 is bit“ 0 ”, detection event by detector A2 / B2 The key bit is generated according to the rule that sets the bit to “1”. No bit is generated when Δθ _a + Δθ _b = ± π / 2.

  Due to the nature of the time-entangled photon pair, the bits generated by both receivers A and B always match each other as described above. Thus, the two receivers A and B can share the same bit string as a secret key.

  In this procedure, the information transmitted and received between the receivers A and B through the normal transmission path is the photon detection time and the differential phase amount, and the bit information does not go outside. Also, another person cannot generate a key bit from the information transmitted and received. Since the light transmitted from the photon pair generator is less than one photon on average per pulse, an eavesdropper cannot branch a part of the signal to obtain bit information. This is because, since the photon is not divided into two, if the eavesdropper detects the photon by branching, the photon does not reach the receiver and does not become a bit string acquired by the two receivers.

  Next, another conventional example will be described. FIG. 2 shows another example of a conventional quantum key distribution system using time-entangled photon pairs (Non-Patent Document 1). Although basically the same as the conventional example of FIG. 1, in the system 200 of FIG. 2, each receiver does not include a phase modulator.

With this configuration, the correlation characteristics at which the receivers A and B simultaneously detect photons are the same as when Δθ _a = Δθ _b = 0 in Table 1. That is, if the detector A1 detects a photon, the detector B1 always detects a photon, and if the detector A2 detects a photon, the detector B2 always detects a photon.

Using the above configuration, the receivers A and B can share the secret key by the following procedure.
(1) First, each receiver receives a pulse train and detects photons as described above. At this time, the time at which the lattice is detected and which detector has detected the photon are recorded in the information processing device CPU.
(2) After receiving the required number of photons, each receiver transmits and receives the photon detection time (photon detection time) to each other via a normal transmission path using the information processing device CPU.
(3) Then, both receivers detect a photon detected at the same time with a predetermined rule (for example, detection event by detector A1 / B2 is bit “0”, detection event by detector A2 / B2 is detected) The key bit is generated according to the rule of “1”.

  Due to the nature of the time-entangled photon pair, the bits generated by both receivers A and B always match each other as described above. Thus, the two receivers A and B can share the same bit string as a secret key.

  In this procedure, the information transmitted and received between the receivers A and B through the normal transmission path is the photon detection time, and the bit information does not go outside. Also, another person cannot generate a key bit from this transmitted / received information.

JP 2006-179982 A K. Inoue and H. Takesue, “Quantum key distribution using entangled-photon trains with no basis selection,” Physical Review A, vol. 73, 032332, 2006.

  In the conventional example of FIG. 1, each receiver applies phase modulation to the received signal transmitted from the photon pair generator. For this, an optical phase modulator is used, but the modulation efficiency of the optical phase modulator usually depends on the polarization state of the input light. That is, in order to always give 0 or π / 2 phase modulation, the polarization state of the input light must be kept in a predetermined state. However, in general, the polarization state of the light transmitted through the optical fiber transmission line is random and fluctuates with time. Therefore, in order to operate the system stably in the conventional example of FIG. 1, although not shown, means for controlling the polarization state of the transmission signal light is required at each receiver. In order to perform such control on light having an average photon number of less than 1 / pulse, a high level of technology is required, resulting in complication of the apparatus.

  In the conventional example of FIG. 2, no phase modulator is used, but other optical components and devices may have polarization dependency. For example, as a photon detector used in a receiver, research on a wavelength conversion type photon detector shown in FIG. 3 is underway.

  In the wavelength conversion type photon detector 300, a signal photon having a wavelength of 1.5 μm is combined with pump light having a wavelength of 1.3 μm (or 0.9 μm) and input to the optical nonlinear element. In an optical nonlinear element, a signal photon having a wavelength of 1.5 μm is converted into a short wavelength photon by a light mixing phenomenon called optical parametric interaction. This is taken out with an optical filter and detected with a Si-avalanche photodiode (Si-APD). Si-APD is superior in speed to the InGaAs-avalanche photodiode (InGaAs-APD) for 1.5 μm band. Therefore, 1.5 μm photons can be detected at a high rate, and as a result, the key generation rate of the quantum key distribution system can be increased.

  However, this wavelength conversion type photon detector 300 has polarization dependency. This is because the efficiency of the optical parametric interaction that occurs in the optical nonlinear element depends on the polarization state of the pump light and the signal light. As a result, the detection efficiency as a photon detector is the polarization state of the signal photon. Will depend on. Therefore, when the wavelength conversion type photon detector is used in the conventional example of FIG. 2, as in the conventional example of FIG. 1, means for controlling the polarization state of the transmission signal light is required in each receiver.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a quantum key distribution system using time-entangled photons that does not require polarization control of signal light.

  In order to achieve the above object, the present invention provides a quantum key distribution system for generating a time-position entangled photon pair comprising a pulse train of signal photons and a pulse train of idler photons. A photon pair generator, a first receiver that receives the pulse train of the signal photons, and a second receiver that receives the pulse train of the idler photons, the photon pair generator for each pulse train, Pulse trains in which the polarization states of adjacent pulses are orthogonal to each other are sent out, and each of the receivers branches the received pulse train, and delays between the branch paths, thereby interfering pulses with the same polarization state. And detecting photons.

  The invention according to claim 2 is the quantum key distribution system according to claim 1, wherein the photon pair generator includes a pump light source that generates a pulse train of pump light, and a pulse train of the pump light. An optical nonlinear medium that generates a pulse train of the signal photon and a pulse train of the idler photon from a pulse train of pump light in which the polarization states of the adjacent pulses are orthogonal to each other. It is characterized by comprising.

  The invention according to claim 3 is the quantum key distribution system according to claim 1, wherein the photon pair generator includes a pump light source that generates a pulse train of pump light, and a pulse train of the pump light. An optical nonlinear medium that generates a pulse train of signal photons and a pulse train of idler photons, and a polarization modulation unit that orthogonalizes polarization states of adjacent pulses for each pulse train of the signal photons and idler photons. It is characterized by.

  According to the present invention, in a secret key distribution system using entangled photons, it is possible to achieve a key generation operation having no polarization dependency as a whole system while using optical components and devices having polarization dependency.

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

(First embodiment)
FIG. 4 is a configuration example of the quantum key distribution system according to the first embodiment of the present invention. Although basically the same as the conventional example of FIG. 1, the system 400 of FIG. 4 is different in the following two points. First, in the time-entangled photon pair generator, the pump light incident on the optical nonlinear medium is polarization-modulated. Second, in each receiver, the time delay amount given by the delay interferometer is twice the pulse interval (2T). That is, the interferometer in each receiver is a two-pulse delay interferometer. Further, it is assumed that an analyzer that transmits only the state of linear polarization with the highest modulation efficiency is provided in the previous stage of the phase modulation means in each receiver. This grating is not shown because it is usually built into a phase modulator.

  In the time-entangled photon pair generator, the pump light from the pump light source is polarization-modulated so that the incident polarization state to the optical nonlinear medium is orthogonal to each other for each pulse. For example, horizontal linearly polarized wave (H) −vertical linearly polarized wave (V) −horizontal linearly polarized wave (H) −vertical linearly polarized wave (V)... In an isotropic nonlinear medium such as an optical fiber, the polarization state of the signal idler light generated by the optical parametric process is the same as that of the pump light. For this reason, when the polarization-modulated pump light as described above is incident on such an optical non-linear medium, a signal idler pulse train in which the polarization state is orthogonal for each pulse is generated from the non-linear medium.

  The signal idler pulse train output from the time-position entangled photon pair generator reaches the receivers A and B through the transmission lines, respectively. Each receiver has the same configuration, and includes an optical phase modulator, a two-pulse delay interferometer, and two photon detectors that detect interference results from the interferometer. Each of the receivers A and B receives a pulse train having a pulse interval T, and phase-modulates each pulse by 0 or π / 2 by a phase modulator. Each phase-modulated pulse is branched into two by optical branching devices C1 and C3, a delay 2T is added between the branching paths by an optical delay path, and then multiplexed by 2 × 2 optical couplers C2 and C4. . Since the delay 2T of the optical delay path is set to be equal to twice the pulse interval T, as shown in FIG. 5, the pulse trains are overlapped and multiplexed every other pulse. That is, in the coupling couplers C2 and C4, the photons of the pulse train interfere every other pulse, and the photons are output to one of the two output ports of the multiplexing coupler according to the interference state. Therefore, photons are detected by one of the photon detectors A1 and A2, B1, and B2 connected to the two output ports according to the interference state of the photons. In addition to the phase modulation information in the phase modulator, the detection time and detection result in the photon detector are supplied to the information processing device CPU. The information processing apparatus is configured to exchange information between the two receivers A and B.

  As described above, in the signal idler pulse train output from the time-position entangled photon pair generator, the polarization states of adjacent pulses are orthogonal to each other. When such a pulse train passes through the 2-bit delay interferometer, the polarization states of the pulses overlapped by the multiplexing coupler coincide (FIG. 5). Therefore, the overlapping pulses cause interference, and photons are detected by one of the two photon detectors depending on the interference state.

As a result of the interference, the correlation of photon detection when the receivers A and B both detect photons is the same as in Table 1. However, in Table 1, {Δθ _a , Δθ _b } is the differential phase amount between adjacent pulses, but in the configuration of FIG. 4, it is the differential phase amount between every other pulse.

Using the above configuration, the receivers A and B can share the secret key by the following procedure.
(1) First, each receiver receives a pulse train and detects photons as described above. At this time, the time when the photon is detected and the detector with which the photon is detected are recorded in the information processing device CPU.
(2) After receiving the required predetermined number of photons, each receiver indicates the differential phase amount (or the phase modulation amount in the phase modulator) together with the time when the photons are detected (photon detection time). Are sent and received via a normal transmission line.
(3) Then, when both photons detect photons at the same time, in the case of a predetermined rule (for example, Δθ _a + Δθ _b = ± π, the detection event by the detector A1 / B2 is indicated by a bit “ 0 ”, detection event by detector A2 / B1 is bit“ 1 ”, and Δθ _a + Δθ _b = 0, detection event by detector A1 / B1 is bit“ 0 ”, detection event by detector A2 / B2 The key bit is generated according to a rule that sets the bit to “1”. If Δθ _a + Δθ _b = ± π / 2, no bit is generated.

  Due to the nature of the time-entangled photon pair, the bits generated by both receivers A and B always match each other as described above. Thus, the two receivers A and B can share the same bit string as a secret key. As in the conventional example, the obtained bit string becomes a secret key that is not known to the outside.

  When configured as described above, the photon detection characteristics of each receiver do not depend on the polarization state of the transmission signal light. The reason is described below.

First, it is assumed that the optical phase modulator of each receiver has the maximum modulation efficiency for linearly polarized light in the x direction and the minimum modulation efficiency for linearly polarized light in the y direction. However, {x, y} is an orthogonal coordinate system. The probability amplitude (corresponding to the complex amplitude of the photoelectric field) of a photon in any polarization state is represented as A (cos [θ], sin [θ] exp [iψ]) in the {x, y} orthogonal coordinate system. In addition, the fact that the two lights E ₁ and E ₂ are in an orthogonal relationship is expressed as (E ₁ · E ₂ ^* ) = 0 in the equation. Where (·) is the inner product of vectors and ^* is the complex conjugate.

The input polarization state to the optical phase modulator is generally random due to fiber transmission, but the orthogonal relationship is maintained. That is, the polarization relationship of two pulses that were orthogonally polarized at the output stage of the time-entangled photon pair generator is also orthogonal at the input stage of the optical phase modulator. Therefore, when the polarization state of the odd-numbered pulse at the input of the optical phase modulator is generally expressed as A (cos [θ ₀ ], sin [θ ₀ ] exp [iψ ₀ ]), the even-numbered polarization state Can be expressed as A (sin [θ ₀ ], −cos [θ ₀ ] exp [−iφ ₀ ]) as a state orthogonal to this.

As described at the beginning of the description of the present embodiment, the optical phase modulator is provided with an analyzer that transmits a polarization component having the maximum efficiency. Therefore, only the x component passes through the modulator. As a result, the output amplitude of the odd-numbered pulse is Acos [θ ₀ ], and the output amplitude of the even-numbered pulse is Asin [θ ₀ ]. Since the probability that a photon is detected is proportional to the square of the absolute value of the amplitude, the photon detection probability from an odd-numbered pulse is proportional to A ² cos ² [θ ₀ ], and the photon detection probability from an even-numbered pulse is It is proportional to A ² sin ² [θ ₀ ]. Therefore, the average number of detected photons of the entire pulse is proportional to A ² cos ² [θ ₀ ] + A ² sin ² [θ ₀ ] = A ² . That is, the average number of detected photons of the entire pulse is a constant that does not depend on the state of input polarization to the phase modulator, and the photon detection characteristics of the receiver are independent of polarization.

(Second Embodiment)
In the first embodiment, in the time-position entangled photon pair generator, the signal idler photon pulse train in which the polarization state is orthogonal is generated for each pulse by polarization-modulating the incident pump light to the optical nonlinear medium. However, in the second embodiment, as shown in FIG. 6, after the polarization of the signal / idler photon pair is generated, the signal light and the idler light are separated by means such as an optical filter. Also in this case, a signal idler photon pulse train in which the polarization states are orthogonal for each pulse can be generated. Moreover, it can replace with an optical filter and can also use an optical coupler.

  Even with such a configuration of the photon pair generator, a photon detection characteristic independent of the input polarization state can be obtained by the same principle as in the first embodiment.

(Third embodiment)
FIG. 7 is a configuration example of a quantum key distribution system according to the third embodiment of the present invention. Although basically the same as the conventional example of FIG. 2, the system 700 of FIG. 7 is different in the following two points. First, in the time-entangled photon pair generator, the pump light incident on the optical nonlinear medium is polarization-modulated. Second, in each receiver, the time delay amount given by the delay interferometer is twice the pulse interval (2T). That is, the interferometer in each receiver is a two-pulse delay interferometer.

  The configuration of the time position entangled photon pair generator is the same as that of the first embodiment. Thus, a signal-idler pulse train in which the polarization states are orthogonal for each pulse is generated from the time-position entangled photon pair generator.

The configuration of each receiver is obtained by removing the phase modulation means from the receiver configuration shown in the first embodiment. As a result, in the multiplexing coupler, the pulses interfere with each other, and as a result, the correlation of photon detection when the receivers A and B both detect photons is Δθ _a + Δθ _b = 0 in Table 1. It will be the same as the case. However, in Table 1, {Δθ _a , Δθ _b } is the differential phase amount between adjacent pulses, but in the configuration of FIG. 6, it is the differential phase amount between every other pulse.

Using the above configuration, the receivers A and B can share the secret key by the following procedure.
(1) First, each receiver receives a pulse train and detects photons as described above. At this time, the time when the photon is detected and the detector with which the photon is detected are recorded in the information processing device CPU.
(2) After receiving the required number of photons, each receiver transmits and receives the photon detection time (photon detection time) to each other via a normal transmission path using the information processing device CPU.
(3) Then, both receivers detect a photon detected at the same time in accordance with a predetermined rule (for example, detection event by detector A1 / B1 is bit “0”, detection event by detector A2 / B2 is detected). The key bit is generated according to the rule of “1”.

  Due to the nature of the time-entangled photon pair, the bits generated by both receivers A and B always match each other as described above. As a result, the two receivers A and B can share the same bit string as a secret key. As in the conventional example, the obtained bit string becomes a secret key that is not known to the outside.

  Next, with the above configuration, even when the photon detector of each receiver has polarization dependency, the key bit generation characteristic as a system becomes polarization independent. The reason will be described below.

First, it is assumed that the photon detector of each receiver has detection efficiency only for linearly polarized light in the x direction. On the other hand, the odd-numbered pulse is A (cos [θ ₀ ], sin [θ ₀ ] exp [iψ ₀ ]), and the even-numbered pulse is A (sin [θ ₀ ], −cos [θ ₀ ] exp [ It is assumed that a pulse train having a polarization state of −iψ ₀ ]) is input. As described in the first embodiment, in this way, the polarization states of the odd-numbered pulses and the even-numbered pulses are orthogonal to each other. Then, the photon detection probability from the odd-numbered pulse is proportional to A ² cos ² [θ ₀ ], and the photon detection probability from the even-numbered pulse is proportional to A ² sin ² [θ ₀ ]. Thus, the total number of detected average photons is proportional to A ² cos ² [θ ₀ ] + A ² sin ² [θ ₀ ] = A ² . That is, the constant does not depend on the state of input polarization to the photon detector, and the photon detection characteristic of the receiver is independent of polarization.

(Fourth embodiment)
In the third embodiment, in the time-position entangled photon pair generator, the signal idler photon pulse train in which the polarization state is orthogonal for each pulse is generated by polarization-modulating the incident pump light to the optical nonlinear medium. However, in the fourth embodiment, as shown in FIG. 8, after the polarization of the signal / idler photon pair is generated, the signal light and the idler light are separated by means such as an optical filter. Also in this case, a signal idler photon pulse train in which the polarization states are orthogonal for each pulse can be generated.

  Even with such a configuration of the photon pair generator, a photon detection characteristic that does not depend on the polarization state of the input signal light can be obtained by the same principle as in the third embodiment.

  The present invention has been described above with respect to several embodiments. However, in view of the many possible embodiments to which the principles of the present invention can be applied, the embodiments described herein are merely illustrative, It is not intended to limit the scope of the invention. The configuration and details of the embodiment exemplified here can be changed without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

It is a figure which shows an example of the quantum key distribution system by the conventional time position entangled photon. It is a figure which shows another example of the quantum key distribution system by the conventional time position entangled photon. It is a figure which shows an example of a wavelength conversion type photon detector. It is a figure which shows the structural example of the quantum key distribution system which concerns on the 1st Embodiment of this invention. It is a figure which shows a mode that a pulse train overlaps and is multiplexed every other pulse in the quantum key distribution system of this invention. It is a figure which shows the structural example of the quantum key distribution system which concerns on the 2nd Embodiment of this invention. It is a figure which shows the structural example of the quantum key distribution system which concerns on the 3rd Embodiment of this invention. It is a figure which shows the structural example of the quantum key distribution system which concerns on the 4th Embodiment of this invention.

Explanation of symbols

100, 200, 400, 600, 700, 800 Quantum key distribution system 300 Wavelength conversion type photon detector C1, C3 Optical splitter C2, C4 Optical coupler

Claims (3)

A quantum key distribution system,
A photon pair generator for generating a time-entangled photon pair consisting of a pulse train of signal photons and a pulse train of idler photons;
A first receiver for receiving the signal photon pulse train;
A second receiver for receiving the pulse train of idler photons,
The photon pair generator sends a pulse train in which the polarization states of adjacent pulses are orthogonal to each other for each pulse train,
Each of the receivers divides a received pulse train, and delays between the branch paths to cause interference between pulses having the same polarization state, thereby detecting a photon. The quantum key distribution system according to claim 1,
The photon pair generator is
A pump light source that generates a pulse train of pump light;
About the pulse train of the pump light, polarization modulation means for orthogonally crossing the polarization state of adjacent pulses,
A quantum key distribution system comprising: an optical nonlinear medium that generates the pulse train of the signal photon and the pulse train of the idler photon from the pulse train of pump light in which the polarization states of the adjacent pulses are orthogonal to each other. The quantum key distribution system according to claim 1,
The photon pair generator is
A pump light source that generates a pulse train of pump light;
An optical nonlinear medium that generates the pulse train of the signal photon and the pulse train of the idler photon from the pulse train of the pump light;
A quantum key distribution system comprising polarization modulation means for orthogonally crossing the polarization states of adjacent pulses for each pulse train of the signal photon and the idler photon.

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