JP2005268958A - Quantum cipher communication device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum cipher communication device enabling a fast pulse repetition speed although a BB84-"plug&play" quantum cipher system is used as a basis. <P>SOLUTION: In the configuration of the conventional BB84-"plug&play", a light phase modulator 120 driven by an RF frequency signal of a fixed frequency f<SB>1</SB>is added to Alice 100, and filters 250, 260 for transmitting only a specified frequency each are added to the prestage of two photon detectors 230, 240 of Bob 200. The light phase modulator 120 driven by the frequency f<SB>1</SB>generates the modulation sideband waves of an optical pulse frequency f<SB>0</SB>and a frequency (f<SB>0</SB>±f<SB>1</SB>), when an optical pulse advances in one direction. The frequency of Rayleigh scattering light is f<SB>0</SB>, thus detecting the modulation sideband waves as a signal without any noise by setting the specified frequency of the filters 250, 260 to be the frequency (for example, f<SB>0</SB>+f<SB>1</SB>) of one modulation sideband wave by Rayleigh scattering. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a quantum cryptography communication device, and more particularly to a technique for solving the problem that the encryption key generation speed of a BB84- “plug & play” (quantum cryptography communication device) equipped with a quantum cryptography communication protocol BB84 is low.

  A quantum cryptographic communication device is a device that provides absolutely secure cryptographic communication based on the basic principle of quantum mechanics. Conventionally, in encrypted communication, there is an encryption method in which a transmission side and a reception side have a common encryption key, and using this, transmission data is encrypted on the transmission side and decrypted on the reception side. This encryption method is known as an absolutely secure encryption system unless the encryption key is known to the outside, but it is important to supply the encryption key to the sender / receiver without being known to the outside.

  Quantum cryptographic communication is a system that supplies a sender / receiver with an encryption key that is guaranteed not to be wiretapped based on the principle of quantum mechanics. Several encryption methods are known as quantum cryptography communication. Since the present invention uses an encryption method improved from the encryption method (Non-Patent Document 1) called BB84- “plug & play” (quantum encryption communication device) that implements the quantum encryption communication protocol BB84, this encryption method is used as the prior art. Will be described.

  Here, BB84 is the name of the quantum cryptography communication protocol for the basic configuration / principle, and “plug & play” is the name of the quantum cryptography communication device of the embodiment for stably operating the protocol. First, the basic configuration / principle BB84 will be described, followed by the “plug & play” configuration.

  FIG. 2 is a diagram showing a basic configuration of the BB84 quantum cryptography. The transceivers that share the encryption key are referred to as Alice (transmitter) 1000 and Bob (receiver) 2000 according to convention. Alice 1000 transmits one photon to Bob 2000 as one of four polarization states of a vertical straight line, a horizontal straight line, a clockwise circle, and a counterclockwise circle.

  Two photon measurement systems 2200 and 2300 are provided on the Bob 2000 side. One of them is a system 2200 for measuring a linear polarization state. Photons input to the linear polarization measurement system 2200 are input to a polarization beam splitter 2220 provided with photon detectors 2240 and 2260 at two output ends. If the input photon is in the vertical linear polarization state, the first photon detector 2240 detects the photon, and if the input photon is in the horizontal linear polarization state, the second photon detector 2260 detects the photon.

  Another of the photon measurement systems is a system 2300 that measures a circular polarization state. Photons input to the circular polarization measuring system 2300 pass through the λ / 4 wave plate 2310 and then input to the polarization beam splitter 2340 provided with photon detectors 2320 and 2360 at two output ends, respectively. If the input photon is in the clockwise circular polarization state, the first photon detector 2320 detects the photon, and if the input photon is in the counterclockwise circular polarization state, the second photon detector 2360 detects the photon. An optical switch 2100 is provided in the front stage of the two measurement systems, and the transmitted photons are randomly switched and input to the linear polarization measurement system 2200 or the circular polarization measurement system 2300.

  When linearly polarized photons are input to the linear polarization measuring system 2200, Bob 2000 can correctly determine whether the input photons are longitudinally polarized or transversely polarized with 100% probability. . However, when circularly polarized photons are input to the linear polarization measurement system 2200, the photon detector 2240 or the photon detector 2260 detects the photons with a probability of 1/2 for Bob 2000. That is, Bob 2000 cannot determine whether the input photon is a clockwise circle or a counterclockwise circle depending on the linear polarization measurement system 2200.

  Similarly, when a circularly polarized photon is input to the circularly polarized wave measuring system 2300, Bob 2000 can correctly determine whether the input photon is a clockwise circle or a counterclockwise circle with a probability of 100%. When linearly polarized photons are input to the circular polarization measurement system 2300, the polarization state cannot be determined.

  Using the above system, Alice 1000 and Bob 2000 generate an encryption key in the following procedure. First, after transmitting and receiving photons as many times as necessary through the transmission line 3000, Alice 1000 informs Bob 2000 whether each photon has been sent with linear polarization or circular polarization. However, Alice 1000 does not inform Bob 2000 whether the polarization state is vertical or horizontal, clockwise or counterclockwise. On the other hand, Bob 2000 informs Alice 1000 whether each photon has been switched to the linear polarization measurement system 2200 or the circular polarization measurement system 2300. If the transmission state of each photon matches the measurement system, Bob 2000 can correctly determine the polarization state of the transmitted photon. Therefore, for example, if the vertical straight line and the clockwise circle are bits “1” and the horizontal straight line and the counterclockwise circle are bits “0”, Alice 1000 and Bob 2000 obtain common bit information.

  Here, information exchanged between Alice 1000 and Bob 2000 is only information about linear polarization or circular polarization, and information about whether the polarization is vertical or horizontal, clockwise or counterclockwise is Since it does not go out to the outside transmission line 3000, the acquired bit information does not leak outside. Therefore, the bit information obtained in this way is adopted as an encryption key.

  On the other hand, when the polarization state (straight or circular) sent by Alice 1000 and the measurement system switched by Bob 2000 do not match, Bob 2000 cannot determine the polarization state of the transmitted photon. In this case, the photon detection event is ignored.

  The basic configuration and operation principle of the BB84 quantum cryptography are as described above. The problem with this method is that it is not suitable for optical fiber transmission. In general, an optical fiber is birefringent although it is minute due to external stress or bending. Therefore, the polarization state of light changes as the fiber propagates. Furthermore, since these external influences change over time, the way of changing the polarization also changes over time. Since the BB84 system transmits information according to the polarization state of photons, a polarization control means that compensates for polarization fluctuations is required to operate the system.

  An implementation apparatus called “plug & play” has been developed to solve the problem of compensating for this polarization fluctuation. FIG. 3 shows the configuration of the “plug & play” device. A laser light source 210 is provided on the Bob 200 side, and pulsed light emitted from the laser light source 210 passes through an optical circulator 215 and is branched into two optical paths 274 and 276 by a 2 × 2 optical coupler 220. . The optical pulse output to the path 274 (hereinafter referred to as pulsed light S) is output to the fiber transmission path 300 via the polarization beam splitter (PBS) 270. At this time, the input polarization state to the polarization beam splitter 270 is set to the horizontal linear polarization by the polarization controller (PC) 280 placed in front of the polarization beam splitter 270, and the input pulse light S Is output to the fiber transmission line 300 without loss as horizontal linearly polarized light.

  The pulsed light S transmitted from Bob 200 reaches Alice 100 via the fiber transmission line 300. The pulsed light S input to Alice 100 passes through an optical attenuator (ATT) 130 and an optical phase modulator (PM) 140, is reflected by a Faraday reflector (FM) 110, and then is again reflected by the optical phase modulator 140, optical light. The signal is returned to the fiber transmission line 300 through the attenuator 130. Here, the optical attenuator 130 is set so as to adjust the optical power of the optical pulse to be less than one photon on average per pulse when the optical pulse is output in a folded manner. The optical phase modulator 140 randomly modulates the phase of the input pulsed light S with 0, π / 2, π, or 3π / 2. Further, the Faraday reflector 110 acts to convert the polarization state of the input light into a state orthogonal to the reflected state and reflect it. The pulsed light S output from Alice 100 in this way propagates through the same fiber transmission path 300 as the forward path and reaches Bob 200.

  The pulsed light S input from the fiber transmission line 300 to the polarization beam splitter 270 of Bob 200 is automatically output to the reflection port of the polarization beam splitter 270, that is, the optical path 276. This is because in Alice 100, the polarization is converted into an orthogonal state by the Faraday reflector 110.

  In the fiber path 300, the polarization state of light changes as it propagates, but the orthogonal relationship is maintained. For example, when longitudinally linearly polarized light and transversely linearly polarized light are input to the fiber path 300, they are generally elliptically polarized at the output end of the fiber path 300, but both are orthogonally polarized waves. Also, the fiber path 300 is reversible with respect to polarization changes. Therefore, when the optical pulse S input from the Bob 200 to the fiber transmission line 300 as a horizontal linearly polarized wave is converted into orthogonal polarization on the Alice 100 side and traces the same fiber transmission line 300 in the opposite direction, Bob 300 When it reaches, it becomes longitudinally linearly polarized wave. For this reason, the optical pulse S returned from Alice 100 is output from the polarization beam splitter 270 to the optical path 276. The pulsed light L output to the optical path 276 is input to the optical coupler 220 through the polarization controller (PC) 285, the delay fiber 295, and the optical phase modulator (PM) 290. At this time, the optical phase modulator 290 is assumed not to modulate the pulsed light S.

  On the other hand, in Bob 200, pulsed light emitted from the laser light source 210 and branched to the optical path 276 by the optical coupler 200 (hereinafter referred to as pulsed light L) follows a path opposite to that of the pulsed light S. That is, the pulsed light L output to the optical path 276 is input to the polarization beam splitter 270 via the optical phase modulator 290, the delay fiber 295, and the polarization controller 285. At this time, the optical phase modulator 290 randomly applies 0 or π / 2 phase modulation to the pulsed light L. As described above, the optical phase modulator 290 is not modulated with respect to the pulsed light S, but the pulsed light S and the pulsed light L pass through the optical phase modulator 290 at different times. It is possible to make a modulation state. Further, the polarization controller 285 is set so that the input state to the polarization beam splitter 270 is vertical linear polarization. As a result, the pulsed light L is output to the fiber transmission line 300 as a vertically linearly polarized light without loss.

  The pulsed light L that has propagated through the fiber transmission line 300 reaches Alice 100, returns to Bob 200 through the optical attenuator 130, the optical phase modulator 140, and the Faraday reflector 110, and again through the same path. At this time, the optical phase modulator 140 is assumed not to modulate the pulsed light L. As described above, the optical phase modulator 140 applies phase modulation to the pulsed light S at 0, π / 2, or 3π / 2. However, the pulsed light S and the pulsed light L are optical signals at different times. Since the light passes through the phase modulator 140, such a modulation state is possible.

  The pulsed light L that has returned to Bob 200 via the fiber transmission line 300 is input to the polarization beam splitter 270. At this time, since the polarization is converted into the orthogonal state in the Faraday reflector 110 of Alice 100, the pulsed light L input to the polarization beam splitter 270 is a horizontal straight line for the same reason as described for the pulsed light S. The polarization is output to the optical path 274 and reaches the optical coupler 220.

  As described above, the pulsed light L and the pulsed light S that reciprocated along the fiber transmission line 300 are input to the two ports of the optical coupler 220 of Bob 200, respectively. Since these two pulse lights L and S have propagated in the opposite directions on the same path, they reach the coupler 220 at the same time. Further, regarding the polarization, the light path 274 and the light path 276 indicate that when the same polarization light first branched by the optical coupler 220 reaches the polarization beam splitter 270, the light from the path 274 is a horizontal straight line. Since the light from 276 is set to be a vertical straight line and the polarization change is reversible, the pulsed light L and the pulsed light S returned from the polarization beam splitter 270 to the coupler 220 have the same polarization. ing. Therefore, interference between both the pulsed light L and S occurs in the coupler 220, and light is output to the first photon detector 230 or the second photon detector 240 according to the phase difference.

  Since the pulsed lights L and S that have reached the optical coupler 220 after reciprocating along the fiber transmission line 300 basically follow the same path in the opposite direction, the propagation phases experienced are the same. However, in the optical phase modulator 140 and the optical phase modulator 290, both the pulse lights L and S are subjected to different phase modulations. One pulse light S is subjected to position modulation of 0, π / 2, π, or 3π / 2 in the optical phase modulator 140, and is not modulated in the optical phase modulator 290. The other pulsed light L is unmodulated in the optical phase modulator 140 and 0 or π / 2 phase modulated in the optical phase modulator 290. Accordingly, the phase difference between both of the pulsed lights L and S is π / 2, or 0, or π / 2, or π, or 3π / 2. The phase differences for the respective modulation states are summarized in Table 1 below.

  If the phase difference between the pulse lights L and S is 0, the interference light is always output to the first photon detector 230. If the phase difference is π, the interference light is always output to the second photon detector 240. On the other hand, when the phase difference is π / 2, π / 2, or 3π / 2, it is output to the first photon detector 230 or the first photon detector 230 with half probability. Therefore, the photon detection event in the case where the shading in Table 1 is applied is indeterminate, while the photon detection event in the case where the shading is not applied is deterministic.

  When the above embodiment described as the prior art is used, an operation equivalent to the BB84 quantum cryptography can be effectively performed as described below. First, Bob 200 and Alice 100 transmit and receive the required number of pulse lights.

  Thereafter, Alice 100 informs Bob 200 whether (i) the pulsed light S has been modulated by 0 or π, or (ii) has been modulated by π / 2 or 3π / 2 by the optical phase modulator 140. However, it is not notified whether it is 0, π, π / 2, or 3π / 2. On the other hand, Bob 200 notifies the optical phase modulator 290 whether the pulsed light L is modulated by 0 or π / 2. If the modulation of Alice 100 is case (i) and the modulation of Bob 200 is 0, or the modulation of Alice 100 is case (ii) and the modulation of Bob 200 is π / 2, then Alice 100 is the photon of Bob 200 It can be known which of the detectors 230 and 240 has detected the photon.

  Thus, for example, if the detection event by the first photon detector 230 is bit “0” and the detection event by the second photon detector 240 is bit “1”, Alice 100 and Bob 200 have common bit information. Will get. On the other hand, in other cases, the photon detection event is indeterminate, and Alice 100 cannot know which photon detector 230, 240 of Bob 200 has detected the photon. Therefore, the detection event in this case is ignored.

  In the above procedure, information indicating which photon detector 230, 240 has detected a photon does not appear outside. Therefore, the obtained bit information can be used as an encryption key.

  This embodiment is configured such that two pulse lights automatically propagate in the opposite directions in the opposite directions and automatically enter a predetermined interference state by the effect of the Faraday reflector 110 provided on the Alice 100 side. ing. For this reason, there is no need for the polarization control required in the original BB84 system, and stable operation is possible.

G. Ribordy, J-D, Gautier, N. Gisin, O. Guinnard and H. Zbinden, “Automated,‘ plug & play ’quantum key distribution,” Electronics Letters 29th October 1998, Vol. 34, No. 22, PP. 2116-2117

  In the conventional BB84- “plug & play” method described above, Rayleigh scattering in the fiber transmission line 300 is a factor that hinders the speeding up of the system. Rayleigh scattering here is a phenomenon in which a part of propagating light is scattered backward due to density fluctuation of the glass medium. Due to this phenomenon, part of the pulsed light emitted from Bob 200 to Alice 100 is backscattered in the fiber transmission line 300 and returns to the photon detectors 230 and 240 of Bob 200. The reflected light due to this backscattering acts as noise and causes the system to malfunction.

  In order to avoid this malfunction, the photon detectors 230 and 240 are configured so that the desired signal photons and the Rayleigh reflected photons reach the photon detectors 230 and 240 at different times, and the photon detectors 230 and 240 are set only at the time when the desired signal photons arrive. Make it work. Specifically, after sending out one pulsed light, Bob 200 waits until the pulsed light returns, and sends out the next pulsed light when the pulsed light returns. If comprised in that way, since a signal photon is input after a Rayleigh reflected photon is input within waiting time, both can be distinguished. A delay line may be inserted into Alice 100 as necessary in order to keep the arrival time between the reflected photon and the signal photon.

  However, if a technique for avoiding the influence of such Rayleigh scattering is employed, the repetition rate of the pulsed light transmitted by Bob 200 is limited. That is, since Bob 200 sends out the next pulse light after the pulse light once sent back, the pulse interval cannot be made shorter than the round-trip propagation time. For this reason, it is difficult to increase the generation speed of the encryption key.

  The present invention has been made in view of the above-described problems, and an object thereof is to provide a quantum cryptography communication apparatus that enables a high pulse repetition rate while being based on the BB84- “plug & play” quantum cryptography. There is.

  The problem that the encryption key generation speed of the BB84- “plug & play” (quantum encryption communication device) that implements the quantum encryption communication protocol BB84 is slow is that the lower limit of the pulse transmission interval of the optical pulse that is the physical medium for transmitting and receiving the encryption key is This happens because it is constrained by some value. The lower limit value is a sufficient pulse transmission interval to avoid the influence (noise) of Rayleigh scattering that occurs when an optical pulse is transmitted through a fiber.

The present invention has the conventional BB84- “plug & play” configuration shown in FIG. 3 with an A) optical phase modulator (120 in FIG. 1) driven by an RF (radio frequency) signal of a certain fixed frequency f ₁ . B) It is characterized in that a filter (250, 260 in FIG. 1) that transmits only a specific frequency is added before the two photon detectors (230, 240 in FIG. 1). The optical phase modulator (120) driven by the RF signal having the added frequency f ₁ has an optical pulse frequency f ₀ and a frequency (f ₀ ± f ₁ ) when the optical pulse travels in one direction. ) Modulation sidebands. Since the frequency of the Rayleigh scattered light is f ₀ , the specific frequency of the filter (250, 260) is set to one modulation sideband frequency (f ₀ + f ₁ or f ₀ −f ₁ ), The photon detector (230, 240) can detect the modulated sideband as a signal without noise (light pulse frequency f ₀ ) due to Rayleigh scattering.

  With the above configuration, according to the present invention, it is possible to remove the lower limit value of the optical pulse transmission interval that is restricted by the influence of Rayleigh scattering, thereby enabling short interval pulse transmission and high-speed encryption key generation. Thus, it is possible to provide a BB84- “plug & play” quantum cryptography communication device that operates at a high pulse repetition rate.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a system configuration of an embodiment of the present invention. A fiber transmission line 300 connects between a transmission side device (hereinafter referred to as Alice) 100, which is usually called Alice, and a reception side device (hereinafter referred to as Bob) 200, which is usually called Bob.

Alice 100 is driven by a Faraday reflector 110, a path 115 that connects the fiber transmission line 300 and the Faraday reflector 110, and a frequency f ₁ that is arranged on the path 115 and generates a modulated band-wave. 1 optical phase modulator 120, optical attenuator 130 disposed on path 115, and second optical phase modulator 140 disposed on path 115.

  Bob 200 includes a laser light source 210, a circulator 215, an optical coupler 220, a first photon detector 230, a second photon detector 240, a first optical filter 250, and a second optical filter 260. A polarization beam splitter 270, a first optical path 274 connecting the third terminal of the optical coupler 220 and the first terminal of the polarization beam splitter 270, and a fourth terminal of the optical coupler 220. A second optical path 276 that connects the second terminal of the wave beam splitter 270, a first polarization controller 280 disposed on the first optical path 274, and a second optical path 276. A second polarization controller 285 and an optical phase modulator 290 arranged; a delay line 295 inserted in the second optical path 276; a fiber transmission line 300; and a third terminal of the polarization beam splitter 270. The And a path 298 to continue to.

The laser light source 210 generates a pulse of light frequency _{f 0.} The optical coupler 220 has 2 × 2 input / output terminals, and branches and outputs the pulsed light from the laser light source 210 input to the first terminal to the third terminal and the fourth terminal. The first photon detector 230 receives the output light from the first terminal of the optical coupler 220. The second photon detector 240 receives the output light from the second terminal of the optical coupler 220. The first optical filter 250 is placed immediately in front of the first photon detector 230 and transmits only light of _one frequency (for example, f ₀ + f ₁ ) of the modulation band (f ₀ ± f ₁ ). To do. The second optical filter 260 is placed immediately before the second photon detector 240 and transmits only light of _one frequency (for example, f ₀ + f ₁ ) of the modulation band.

  The polarization beam splitter 270 has a 1 × 2 input / output terminal, and combines the horizontally linearly polarized light input to the first terminal and the vertically linearly polarized light input to the second terminal into the third terminal. Wave and output.

  When the pulsed light emitted from the laser light source 210 branched to the third terminal of the optical coupler 220 is input to the first terminal of the polarization beam splitter 270, the first polarization controller 280 is a horizontal straight line. The polarization state is set. When the pulsed light emitted from the laser light source 210 branched to the fourth terminal of the optical coupler 220 is input to the second terminal of the polarization beam splitter 270, the second polarization controller 285 is vertically polarized. It is set to be in a state.

  The second optical phase modulator 140 in Alice 100 is 0 or π / 2, or π for one of the pulsed light from the laser light source 210 branched into two by the optical coupler 220 in Bob 200, Alternatively, 3π / 2 phase modulation is applied. An optical phase modulator 290 in Bob 200 applies phase modulation of 0 or π / 2 to the other of the pulsed light from the laser light source 210 branched into two by the optical coupler 220 in Bob 200.

  As shown in FIG. 1, in one embodiment of the present invention, in addition to the prior art configuration of FIG. 3, an optical phase modulator 120 is provided in Alice 100 and two photon detectors in Bob 200 are provided. Optical filters 250 and 260 are provided in front of 230 and 240.

Optical phase modulator 120 is driven by an RF signal of frequency f ₁ of the fixed. When the light of the carrier frequency f ₀ is input to the phase modulator 120 that is driven in this way, the light in which the modulation sideband is generated at the frequency position of f ₀ ± f ₁ is output from the phase modulator 120. The modulation sideband has a fixed phase relationship with respect to the original carrier frequency component. The pulsed light travels back and forth through the optical phase modulator 120 by the Faraday reflector 110. Usually, the phase modulator 120 operating at high speed has directionality, and phase modulation is applied to light propagating in a certain direction. In some cases, the light propagating in the opposite direction is unmodulated. Therefore, even if the pulsed light reciprocates through the optical phase modulator 120 driven by f ₁ , no double modulation is applied. The optical filters 250 and 260 in Bob 200 transmit only one of the modulation sidebands, for example, only f ₀ + f _1, and block the other components (f ₀ , f ₀ -f ₁ ).

  With such a configuration, the BB84- “plug & play” quantum cryptography can be implemented without being affected by Rayleigh scattering. That is, Bob 200 sends out two branched pulsed light, and these pulsed light returns to Bob 200 again through Alice 100, interferes with optical coupler 220 in Bob 200, and a photon detector according to the interference Photons are input to 230 and 240 in the same manner as in the prior art of FIG. However, in this embodiment, the signal pulse light transmitted from Bob 200 and input to Alice 100 is divided into a carrier component and a modulation sideband component in Alice 100, and the filtering action of the optical filters 250 and 260 in Bob 200. Thus, only one photon of the modulation sideband component is input to the photon detectors 230 and 240 of Bob 200.

On the other hand, Rayleigh scattered light, since the optical signal pulse comes is reflected to Bob 200 occurring in the transmission line 300 directed from Bob 200 to Alice 100, the optical frequency is the original frequency f _0. Since this frequency light is blocked by the optical filters 250 and 260, no Rayleigh scattered light is input to the photon detectors 230 and 240, and the Rayleigh scattered light does not act as noise as in the prior art. Therefore, in this embodiment, Bob 200 can send the next pulse light without waiting for the pulse light once sent back and forth to return. In other words, the BB84- “plug & play” quantum cryptography can be implemented at a high pulse repetition rate.

(Other embodiments)
The preferred embodiment of the present invention has been described by way of example, but the embodiment of the present invention is not limited to the above-described example, and its constituent members and the like are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, and change in shape are all included in the embodiment of the present invention. For example, in the above-described embodiment of the present invention, the polarization controllers 280 and 285 are used to correctly set the polarization state to the polarization beam splitter 270 in the receiver. By using the wave holding fiber, it is possible to set the polarization in a predetermined state without the polarization controller. In the embodiment of the present invention, the Faraday reflector is used, but the present invention is not limited to this, and a reflective member having the same function may be used.

It is a block diagram which shows the structure of one Embodiment of the quantum cryptography communication apparatus of this invention. It is a conceptual diagram which shows the basic composition of BB84 quantum cryptography. It is a block diagram which shows the structure of the conventional BB84- "plug & play" quantum cryptography communication apparatus.

Explanation of symbols

100 Alice 110 Faraday reflector 115 path 120 first optical phase modulator 130 optical attenuator 140 second optical phase modulator 200 bob 210 laser light source 215 circulator 220 optical coupler 230 first photon detector 240 second photon Detector 250 First optical filter 260 Second optical filter 270 Polarization beam splitter 274 First optical path 276 Second optical path 280 First polarization controller 285 Second polarization controller 290 Optical phase Modulator 295 Delay line 298 Path 300 Fiber transmission path 1000 Alice (transmitter)
2000 Bob (receiver)
2100 Optical switch 2200 Linear polarization measurement system 2220 Polarization beam splitter 2240 First photon detector 2260 Second photon detector 2300 Circular polarization measurement system 2310 λ / 4 wavelength plate 2320 Polarization beam splitter 2340 First photon Detector 2360 Second photon detector 3000 Transmission path

Claims (2)

A quantum cryptography communication device that encrypts data transmitted and received between a transmitter and a receiver connected via a fiber transmission path and supplies an encryption key for decryption,
The receiver
A laser light source that generates pulsed light having an optical frequency f ₀ ;
An optical coupler having a 2 × 2 input / output terminal and branching and outputting the pulsed light from the laser light source input to the first terminal to the third terminal and the fourth terminal;
A first photon detector;
Means for inputting output light from the first terminal of the optical coupler to the first photon detector;
A second photon detector for inputting output light from the second terminal of the optical coupler;
A first optical filter that is placed immediately before the first photon detector and transmits only one frequency light of the modulated band wave;
A second optical filter that is placed immediately in front of the second photon detector and transmits only one frequency light of the modulated bandpass wave;
Polarized light having a 1 × 2 input / output terminal and combining the horizontally linearly polarized light input to the first terminal and the vertically linearly polarized light input to the second terminal into the third terminal for output. A beam splitter,
A first optical path connecting the third terminal of the optical coupler and the first terminal of the polarization beam splitter;
When the pulsed light emitted from the laser light source branched to the third terminal of the optical coupler is input to the first terminal of the polarization beam splitter, it is set so as to be in a horizontal linear polarization state. Means,
A second optical path connecting the fourth terminal of the optical coupler and the second terminal of the polarization beam splitter;
When the pulsed light emitted from the laser light source branched to the fourth terminal of the optical coupler is input to the second terminal of the polarization beam splitter, it is set to be in a vertically linearly polarized state. Means,
An optical phase modulator placed on the second optical path;
A delay line inserted on the second optical path;
An optical path connecting the third terminal of the polarization beam splitter and the fiber transmission path;
The transmitter is
A reflecting member that converts and reflects the polarization state of the input light into a polarization state that is orthogonal to the polarization state;
An optical path connecting the fiber transmission path and the reflecting member;
A first optical phase modulator disposed on the optical path of the transmitter and driven at a frequency f ₁ that generates a modulated band wave;
An optical attenuator disposed on the optical path of the transmitter;
A second optical phase modulator disposed on the optical path of the transmitter;
The second optical phase modulator in the transmitter is 0 or π / 2, or π with respect to one of the pulsed light from the laser light source branched into two by the optical coupler in the receiver, Or add 3π / 2 phase modulation,
The optical phase modulator in the receiver applies phase modulation of 0 or π / 2 to the other of the pulsed light from the laser light source branched into two by the optical coupler in the receiver. A quantum cryptography communication device. The quantum cryptography communication device according to claim 1, wherein the reflection member is a Faraday reflector.

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