JP2010233123A - Optical communication system and method, transmitter and receiver, and quantum encryption key distribution system and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive and simplified system configuration by reducing the quantity of random numbers to be used without sacrificing a signal processing speed. <P>SOLUTION: A quantum encryption key distribution system includes: phase modulators (modulators) 14, 31 provided in both Alice (transmitter) 1 and Bob (receiver) 3 for applying modulation of the same value to a plurality of light pulses within a predetermined block (spatial and temporal range); light pulse detectors 35, 36 provided in Bob 3 for detecting a light pulse to which encryption key information has been given as bits; and key information discarding means processors (key information discarding means) 17, 37 provided in both Alice 1 and Bob 3 for discarding, when a plurality of light pulses are detected within the same block by the light pulse detectors 35, 36, all or a part of the bits given to these light pulses. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical communication system and method for distributing a quantum encryption key from a transmitter to a receiver by modulating the phase or polarization of a weak light pulse, a transmitter and a receiver, a quantum encryption key distribution system and method, and a program. About.

  In the current situation where the rapid growth of Internet traffic continues, improving the transmission bandwidth is one of the most important issues. This is no exception even in the backbone optical network, and various research institutions have increased the signal speed per carrier in order to expand the transmission capacity using the existing infrastructure without adding transmission lines and repeaters. In actual commercial transmission systems, the signal speed continues to increase.

  In general optical communication technology, mainly binary amplitude modulation (Amplitude Shift Keying, hereinafter referred to as ASK) is adopted, so the signal speed per carrier is improved by shortening the time slot per bit. The approach to make it was mainstream. However, since the signal speed per carrier exceeded 10 Gb / s, it seems that the development of high-speed development that relies solely on ASK has faced a wall that cannot be exceeded.

  One of the factors hindering the speeding up by the ASK method is waveform deterioration due to wavelength dispersion peculiar to the optical transmission line. Chromatic dispersion is a phenomenon in which the propagation delay time that occurs in a transmission line varies depending on the signal light wavelength. Since the signal light spectrum has a specific wavelength range, short wavelength components and long wavelength components in the same signal light accumulate different chromatic dispersion values during transmission, and after transmission, propagation delay difference is caused by this accumulated dispersion. That is, waveform distortion occurs. Considering the ASK signal, since the signal light spectrum is proportional to the modulation speed, the waveform distortion due to chromatic dispersion increases as the signal speed increases. On the other hand, as the signal speed increases, one time slot becomes shorter. Therefore, even when the same amount of waveform distortion (difference in propagation delay) is applied, the higher the signal, the greater the influence. Therefore, the ASK system has a problem that the transmission characteristics deteriorate in proportion to the square of the signal speed.

  In view of this, as another alternative to the ASK method, a technique for realizing a high speed by increasing the transmission band by multi-leveling the state represented by one time slot has attracted attention. For example, in the technique of Non-Patent Document 1, the transmission band is improved by quaternary modulation of the phase of light, and in the techniques of Non-Patent Documents 2 and 3, amplitude transfer for modulating both the intensity and phase of light. A large number of digital bits are expressed by one symbol (one carrier and one time slot) using a modulation (APSK: Amplitude Phase Shift Keying) method.

Here, when modulating the phase of an optical signal, phase information cannot be extracted simply by directly detecting the optical signal with a photoelectric conversion module such as a photodiode (Photo Diode). After that, it is necessary to adopt a detection method such as reading the light intensity after interference.
FIG. 7 is a conceptual diagram for explaining a differential phase shift (DPS) system in which information is placed on a phase difference between adjacent bits as a first related technique. In particular, FIG. Decoding is shown. In this DPS system, a Mach-Zehnder interferometer 71 for causing a two-beam bundle having a 1-bit delay relationship to interfere with a receiving side, and a delay amount of the Mach-Zehnder interferometer 71 by temperature. A temperature controller 72 to be controlled is provided, and phase information of “0” or “π” is extracted by interfering with a continuous 2-bit optical signal pulse Pa (frame A in the figure). Here, if the phase difference between the front and rear 2-bit optical signal pulses Pa is 0, after interference, the optical signal pulse Pb is output to port 0 as shown in frame B of FIG. If the phase difference of the signal pulse Pa is π, it is output to the port 1 as an optical signal pulse Pc after interference as shown in a frame C of FIG.

  As a communication system using phase modulation of an optical signal, a quantum key distribution (QKD: Quantum Key Distribution) technique can also be cited (Patent Documents 1, 2, and 3). In the quantum key distribution method, a single photon is generally used as a communication medium, and information is transmitted in the quantum state of the photon for transmission. At this time, it is assumed that an eavesdropper on the transmission path steals information by tapping a photon being transmitted. By the way, if the quantum state of a photon is observed (wiretapped) once, it always changes to another quantum state by Heisenberg's uncertainty principle. Change occurs. Therefore, by detecting this change, the receiver can reliably detect the presence or absence of eavesdropping.

  In the quantum key distribution (QKD) method, for example, an optical interferometer is organized by the sender Alice and the receiver Bob, and phase modulation is randomly performed on each photon by the sender Alice and the receiver Bob. By obtaining output data of “0” or “1” according to the difference of the modulation phase depth, and then comparing a part of the condition when the output data is measured with the sender Alice and the receiver Bob, Finally, the same bit string can be shared as a quantum key between the sender Alice and the receiver Bob.

FIG. 8 is a schematic diagram showing a schematic configuration and an operation principle of a unidirectional quantum encryption key distribution system as a second related technique.
In this system, as shown in the figure, a schematic configuration is composed of an Alice 8a which is a transmitter, an optical transmission line 9, and a Bob 9a which is a receiver that receives distribution of a quantum encryption key from the Alice 8a via the optical transmission line 7a. Has been.
The Alice 8a includes a pulse light source 81, a 1-bit delay interferometer 82, a temperature controller (TEC) 83, a phase modulator (Mod) 84, two random number sources (RND) 85 and 86, and a DAC ( Digital-to-analog converter) 87. Bob 9a includes a phase modulator (Mod) 91, a 1-bit delay interferometer 92, a temperature controller (TEC) 93, photon detectors (PD) 94 and 95, and a random number source (RND) 96. It is roughly structured.

  The 1-bit delay interferometers 82 and 92 included in both of Alice 8a and Bob 9a are unidirectional optical interferometers, and include, for example, a 2-input 2-output Mach-Zender interferometer 813. Temperature controllers (TEC) 83 and 93 control the temperature of the 1-bit delay interferometers 82 and 92 to set the delay amount to 1 bit. With this configuration, one-way communication is possible from Alice 8a to Bob 9a via the optical transmission line 7a (Patent Document 2). In the second related technique, as shown in FIG. 8, the Alice 8a side is also provided with the 1-bit delay interferometer 82 because the information between adjacent bits is the same as in the first related technique employing the DPS method. This is because the eavesdropper who has prepared the local oscillator may be eavesdropped, and if so, the security of encryption key distribution will be impaired.

  In Alice 8a, as shown in FIG. 8, the pulse light source 81 generates an optical pulse train (ΔT interval) Pd. The optical pulse train (ΔT interval) Pd is converted by the 1-bit delay interferometer 82 into a double pulse train Pe with a delay amount ΔT, and the phase modulator 84 gives a phase difference of φA between each pair of pulses. Modulation Pf is applied. For example, in a quantum cryptography key distribution (QKD) algorithm called BB84 protocol, the phase difference φA takes four values of 0, π, π / 2, and 3π / 2. Randomly assigned. In order to randomly assign four values of π, π / 2, and 3π / 2 to each pulse pair, two random number sources 85 and 86 and these random numbers are added in Alice 8a as described above. A matching DAC 87 is provided.

  In Bob 9a, the optical signal pulse Pf sent from Alice 8a via the optical transmission line 7a is subjected to modulation Pg by the phase modulator 91 so as to give a phase difference of φB between the pulse pairs again, and the delay amount A pulse pair is caused to interfere by using a 1-bit delay interferometer 93 of ΔT, and the interference results Ph and Pi are read by the photon detectors 94 and 95. The phase modulation on the Bob 9a side is performed by randomly assigning binary values of 0 and π / 2, and as described above, the random number source 96 is provided for this purpose. If the phase difference Δφ (= φA + φB) of the pulse pair Pg is 0, the optical signal pulse Ph after interference is output to the photon detector 94, and if the phase difference Δφ of the pulse pair is π, the optical signal pulse Pi after interference is Are output to the photon detector 95.

  However, in the second related technology, although the operating speed of the photon detectors (PD) 94 and 95 is currently limited, the system repetition frequency (1 / ΔT) has been increased to about 1 GHz. The delay amount of the 1-bit delay interferometer corresponding to this operating speed is as long as ˜500 ps (˜10 cm), and it is necessary to control the temperature of the interferometer in units of 0.01K in order to obtain stable characteristics. , Etc. have problems.

Non-Patent Document 4 discloses a quantum encryption key distribution algorithm called a BB84 protocol as a third related technique.
FIG. 9 is an explanatory diagram for explaining the BB84 protocol (quantum encryption key distribution algorithm) as the third related technique.
In this method, four kinds of quantum states are used. Alice 8b has a first random number source (not shown) that generates random number 1 and a second random number source (not shown) that generates random number 2, and the random number 1 represents encryption key data of “0” or “1”. The method (base) for coding the information of the random number 1 with the random number 2 is determined.
In the quantum cryptography key distribution method in which the four-state coding is performed using the phase difference between two coherent pulses, as shown in FIG. 9, the phase 0 is the encryption key “0” and the phase π is the encryption key “1”. A base representing a set (coding set) (the base is referred to as an “X base”), a base having a phase π / 2 representing an encryption key “0”, and a phase 3π / 2 representing an encryption key “1” (the base is represented by “ 2 sets of bases (referred to as “Y bases”) are selected with a random number 2.
That is, Alice 8b randomly modulates one photon in four ways of 0, π / 2, π, and 3π / 2, and transmits it to Bob 9b via optical transmission line 7b. On the other hand, as shown in the figure, Bob 9b has a random number source (not shown) that generates a random number 3 corresponding to the base, and decodes photons sent from Alice 8b.

  In Bob9b, when the value of the random number 3 is “0”, the phase 0 (X basis) is modulated on the photon, while when the value of the random number 3 is “1”, the phase π / 2 (Y (Basic) modulation. The random number 4 of Bob 9b illustrates a random number obtained as an output of the optical interferometer of Bob 9b. When the basis of modulation performed by both Alice 8b / Bob 9b is the same (random number 2 on Alice 8b side = random number 3 on Bob 9b side), Bob 9b correctly detects the value of random number 1 as shown in FIG. (Random number 1 on the Alice 8b side = random number 4 on the Bob 9b side). On the other hand, when the basis of the modulation performed by both Alice 8b / Bob 9b is different (random number 2 on the Alice 8b side ≠ random number 3 on the Bob 9b side), Bob 9b is set to the random number 4 regardless of the value of the random number 1 on the Alice 8b side. A value of “0” or “1” is randomly obtained.

  Since the random numbers 1, 2, and 3 are random numbers that change for each bit, the probability that the bases match and the probability that they do not match are both 50%, but the basis reconciliation process in the latter stage In order to delete bits whose bases do not match, Alice 8b and Bob 9b can share a 0/1 bit string corresponding to random number 1 as a quantum key. Here, the encryption key before deleting the base mismatch bit is referred to as “raw key”, and the encryption key after the base mismatch bit is deleted is referred to as “shift key”.

JP 2006-229608 A JP 2007-286551 A JP 2008-294946 A `` Gb / s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs / AlGaAs Integration '', R. A. Griffin et al., OFC2002, PD-FD6 `` Proposal of 8-state Symbol (Binary ASK and QPSK) 30-bit / s Optical Modulation / Demodulation Scheme '', S. Hayase et al., ECOC2003, Th.2.6.4 `` Quaternary Optical ASK-DPSK and Receivers With Direct Detection '', M. Ohm et al., IEEE Photonics Technology Letters, Vol.15, No.1, p.159 Quantum Cryptography: Public Key Distribution and Coin Tossing, IEEE Int. Conf.on Computers, Systems, and Signal Processing, Bangalore, India, p.175, Bennet, Brassard `` Quantum Key Distribution with High Loss: Toward Global Secure Communication '', W. Y. Hwang, Physical Review Letters, Vol. 91, 057901 `` Reduced randomness in quantum cryptography with sequences of qubits encoded in the same basis '', L.-P. Lamoureux et al., Physical Review A, Vol. 73, 032304

  By the way, the key generation speed improvement of quantum cryptography key distribution can increase the pulse repetition frequency for transmitting photon signals, or send multiple photon transmissions in parallel using wavelength division multiplexing (WDM) technology or the like. However, there is a problem that the amount of random numbers to be used increases. As an example, if the repetition frequency of the optical pulse is F [Hz], a random number generator of 2 * F [Hz] on the Alice side and F [Hz] on the Bob side is required as shown in FIG. When the decoy method described in Non-Patent Document 5 is applied, the amount of random numbers required further increases. In the decoy method, at least a 2-bit random number is required to represent four-level intensity (average photon number), and therefore the amount of random number required on the Alice side is 4 * F [Hz]. On the other hand, since these random numbers must not be predictable in advance, the pseudo random numbers are not desirable and are preferably complete random numbers. However, although a physical random number module using thermal noise or photon detection is commercially available as a complete random number source, its speed is only a few MHz, so that the repetition frequency F of the optical pulse reaches a few GHz. It is difficult to implement a system, and therefore a method for reducing the amount of random numbers used is desired.

  Non-Patent Document 6 proposes a method of reducing the amount of random numbers by dividing a bit string into blocks (N bits are set as one block) and setting the base modulation to a constant value for each block. However, in the method of Non-Patent Document 6, it is assumed that N bits is sufficiently small with respect to the code length (M bits are code length) when performing signal processing. In other words, the code length is sufficiently long. Signal processing is required. On the other hand, in the signal processing, depending on the code length such as random permutation as shown in FIG. 10 as the fourth related technique, and confidentiality enhancement as shown in FIG. 11 as the fifth related technique. Therefore, the code length needs to be as short as possible. Further, Non-Patent Document 6 does not mention the condition that the relationship between the N bits and M bits should satisfy. Therefore, with the technology of Non-Patent Document 6, although the amount of random numbers can be reduced, excessive time is required for random permutation and confidentiality enhancement processing, and it is difficult to realize a high-speed system as a whole. is there.

  The present invention has been made in view of the above-mentioned circumstances, and does not cause an increase in code length during signal processing (thus, without increasing the signal processing time), and in addition, without reducing system performance. An object of the present invention is to provide an optical communication system and method, a transmitter and a receiver, a quantum key distribution system and method, and a program that can reduce the amount of random numbers to be used, and can be easily realized at low cost and in low cost.

  In order to solve the above problems, a first configuration of the present invention relates to an optical communication system that modulates the phase or polarization of a weak light pulse to communicate information from a transmitter to a receiver. A modulator that is provided in one or both of the devices and modulates the same value to a plurality of the optical pulses within a predetermined spatial and temporal range, and is provided in the receiver to detect the optical pulses. When the plurality of optical pulses are detected in the same spatial and temporal range by the optical pulse detector, these optical pulses are provided in both the transmitter and the receiver. And a discarding unit for discarding all or a part of the bits given to the optical pulse.

  The second configuration of the present invention relates to an optical communication method for communicating information from a transmitter to a receiver by modulating the phase or polarization of a weak light pulse. In one or both of the transmitter and the receiver, A modulation process for modulating the same value to a plurality of the optical pulses within a predetermined spatial and temporal range, an optical pulse detection process for detecting the optical pulse at the receiver, the transmitter and the reception When both of the optical pulses are detected within the same spatial and temporal range by the optical pulse detection process, both or all of the bits added to these optical pulses are discarded. And a discarding process.

  A third configuration of the present invention is characterized in that a receiver includes the modulator, the optical pulse detector, and the discarding unit that constitute the optical communication system.

  According to a fourth configuration of the present invention, a transmitter includes the modulator configuring the optical communication system and the discarding unit that operates based on an operation result of the discarding unit of the receiver. It is a feature.

    According to the configuration of the present invention, the amount of random numbers to be used is greatly reduced by applying the same base modulation to a plurality of photon pulses, so that the mounting amount and the number of components of the random number source module can be reduced. Can do. In particular, when WDM technology is applied to a quantum cryptography key distribution (quantum cryptography communication) system, the system can be constructed with an inexpensive configuration.

It is a block diagram which shows the structure of the quantum key distribution system which is 1st Embodiment of this invention. It is a conceptual diagram for demonstrating a mode that an average 1 photon is detected for every block in the same system. It is a block diagram which shows the structure of the quantum cryptography key distribution system which is 2nd Embodiment of this invention. FIG. 4 is a conceptual diagram for explaining how photons are independently detected for each wavelength in the system. It is a block diagram which shows the structure of the quantum encryption key distribution system which is the 3rd Embodiment of this invention. It is a block diagram which shows the structure of the quantum encryption key distribution system which is 4th Embodiment of this invention. It is a figure for demonstrating the differential phase shift (DPS) system which puts information on the phase difference between adjacent bits as a 1st related technique, and is a conceptual diagram which shows especially the decoding of a DPS signal. It is a schematic diagram which shows schematic structure and the principle of operation of a unidirectional quantum encryption key distribution system as a second related technique. It is explanatory drawing for demonstrating BB84 protocol (quantum encryption key distribution algorithm) as a 3rd related technique. It is a conceptual diagram explaining random permutation among signal processing accompanying a quantum cryptography key distribution method as explanation of the 4th related art. As a description of the fifth related technique, it is a conceptual diagram illustrating the enhancement of confidentiality in the signal processing associated with the quantum cryptography key distribution method.

As a preferred embodiment (part 1) of the present invention, one or both of the transmitter and the receiver is driven by a random number source, and the plurality of optical pulses in a block consisting of a plurality of bit times are modulated with the same value. (Preferably, base modulation) is provided, and the receiver is provided with an optical pulse (photon) detector for detecting the optical pulse (photon) to which the encryption key information is attached. In the present invention, both the receiver and the receiver are provided with key information discarding means for discarding a plurality of optical pulses in the same block detected by the optical pulse detector. Realized the purpose.
As a preferred embodiment (part 2) of the present invention, one or both of the transmitter and the receiver is driven by a random number source, and a plurality of optical pulses in a block consisting of a plurality of bit times are modulated with the same value. The optical pulse (photon) detector for detecting the optical pulse (photon) to which the encryption key information is given as a bit is provided in the receiver, both in the transmitter and the receiver, When the optical pulse detector detects a plurality of the optical pulses in the same block, the key information discarding means for discarding the other bits from the block, leaving the bits assigned to one optical pulse. By realizing the above, the object of the present invention is realized.

  As a preferred embodiment (part 3) of the present invention, one or both of the transmitter and the receiver are driven by a random number source, and within the same time slot (bit time), the optical pulses of the plurality of wavelengths have the same value. And a plurality of optical pulses for detecting the optical pulses (photons) of the plurality of wavelengths, to which the encryption key information is assigned as bits, in the receiver. (Photon) detector is provided, and when the optical pulse detector simultaneously detects the optical pulses of the plurality of wavelengths in both the transmitter and the receiver, all bits added to these optical pulses are discarded. By providing the key information discarding means, the object of the present invention is realized.

  As a preferred embodiment (part 4) of the present invention, one or both of the transmitter and the receiver are driven by a random number source, and the optical pulses of the plurality of wavelengths are modulated with the same value in the same time slot. Provided with a modulator, and provided with a plurality of optical pulse (photon) detectors for detecting the optical pulses (photons) of the plurality of wavelengths, to which the encryption key information is assigned as bits, in a receiver, a transmitter and a receiver In both cases, when the optical pulses of the plurality of wavelengths are detected at the same time by the optical pulse detector, all the bits assigned to these optical pulses are left with one wavelength, and others are discarded. By providing the key information discarding means, the object of the present invention is realized.

  Preferably, the transmitter includes an optical attenuator that suppresses the light intensity of the wavelength emitted from the light source to 1 photon / pulse or less and generates an optical pulse composed of a single photon.

Embodiment 1

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram showing the configuration of a quantum cryptography key distribution system according to the first embodiment of the present invention, and FIG. 2 explains how an average of one photon is detected for each block in the system. It is a conceptual diagram for.
This embodiment relates to a system supporting the BB84 protocol. On the receiver (Bob3) side, a plurality of photon pulses in a block defined by the length of the time axis are subjected to the same base modulation, and the photon pulse When the optical pulse detectors (gate mode photon detectors 35, 36) for detecting a plurality of photon pulses in the same block, the block is discarded. In order to facilitate the following description, in this embodiment, 10 bits are set as one block, and the base modulation in the block is fixed.

As shown in FIG. 1, the quantum cryptographic key distribution system of this embodiment includes a transmitter Alice 1, an optical transmission path 2, and a receiver that receives the distribution of the quantum cryptographic key from Alice 1 via the optical transmission path 2. And Bob3.
As shown in the figure, Alice 1 includes a laser diode (LD) 11 that generates a photon pulse, an optical attenuator (not shown), a 2-input 2-output Mach-Zender interferometer 12, and a temperature adjustment. 13, a phase modulator 14, a clock source 15 that drives the laser diode 11, a random number source 16, a quantum encryption key distribution program (hereinafter simply referred to as a processing program), and a processor 17 that performs signal processing. It is composed of
The optical attenuator suppresses the light intensity of the wavelength emitted from the laser diode 11 to 1 photon / pulse or less, and generates a single photon pulse. The Mach-Zender interferometer 12 time-separates the photon pulse attenuated by the optical attenuator and outputs a double photon pulse. The temperature controller 13 controls the temperature of the delay amount of the Mach-Zender interferometer 12. The phase modulator 14 has a function of adding a predetermined phase difference to a double photon pulse that is time-separated into two.

Since the random number source 16 uses four random quantum states in this embodiment, the random number source 16 generates a random number 1 representing encryption key data “0” or “1” and a random number 2 that determines a basis. Random number 2, for example, “X base” representing a set of encryption key “0” for phase 0 and encryption key “1” for phase π, encryption key “0” for phase π / 2, and encryption for phase 3π / 2 Two sets of bases, “Y base” representing the key “1”, are selectively determined (see FIG. 9).
By driving the random number source 16, Alice 1 randomly modulates four photons of 0, π / 2, π, and 3π / 2 with respect to one photon, and transmits it to Bob 3 via the optical transmission line 2.

The processing program is read into the processor 17 from an internal storage device such as a ROM or an external storage device such as a magnetic disk, and controls the operation of the processor 17.
The processor 17 performs step S11 for frame synchronization under the control of the processing program, and when the photon detectors 35 and 36 (on the Bob3 side) detect a plurality of bits (photon pulses) in the block, the processor 17 discards the block. Step S12 (block discarding process), step S13 for performing base collation, step S14 for performing error correction, and step S15 for enhancing confidentiality are executed.

As shown in the figure, Bob 3 includes a phase modulator 31, a Mach-Zender interferometer 32, a temperature regulator 33, a random number source 34, two gate mode photon detectors (hereinafter simply referred to as a photon detector). 35, 36, a processing program (not shown), and a processor 37 for performing signal processing. The random number source 34 generates a random number 3 corresponding to the base and decodes the photon pulse transmitted from Alice 1 (see FIG. 9).
The phase modulator 31 gives a phase difference again to the double photon pulse sent from Alice 1 via the optical transmission line 2 based on the driving of the random number source 34. The Mach-Zehnder interferometer 32 multiplexes the double photon pulses given the phase difference again by the phase modulator 31. The temperature regulator 33 controls the temperature of the delay amount of the Mach-Zender interferometer 32. The photon detectors 35 and 36 are made of an avalanche photo diode (APD).

The processing program is read into the processor 37 from an internal storage device such as a ROM or an external storage device such as a magnetic disk, and controls the operation of the processor 37.
The processor 37 performs step P11 for frame synchronization under the control of the processing program, and when the photon detectors 35 and 36 detect a plurality of bits (photon pulses) in the block, discards the block P12 (block Discard processing), step P13 for performing base matching, step P14 for performing error correction, and step P15 for enhancing confidentiality are executed.

Next, the operation of the above configuration will be described with reference to FIGS.
In Alice 1, as shown in FIG. 1, a photon pulse synchronized with a clock source 15 of 625 MHz is generated by a laser diode 11, and a time is generated by a Mach-Zender interferometer 12, which is an asymmetric two-beam interferometer with two inputs and two outputs. It is converted into a separated double photon pulse. Thereafter, by modulating the optical phase of one side of the double photon pulse by the phase modulator 14, a relative phase difference (φA) of the double photon pulse is randomly generated and sent out to the optical transmission line 2.

  The double photon pulse passes through the optical transmission line 2 and reaches Bob 3, and the optical phase on one side thereof is modulated by the phase modulator 31. Here, the phase modulation is performed randomly based on driving of the random number source 34 so that the relative phase difference (φB) of the double photon pulses becomes 0 and π / 2. The double photon pulses are combined using a Mach-Zender interferometer 32, which is a two-input two-output asymmetric two-beam bundle interferometer, so that the photon detectors 35, 36 are in accordance with the modulation phase applied by Alice1 and Bob3. Is detected. Specifically, based on the driving of the random number source 34, if the phase difference Δφ of the double photon pulse given again by the phase modulator 31 is 0, the photon pulse after interference is detected by the photon detector 35, If the phase difference Δφ of the double photon pulse is π, the photon pulse after the interference is detected by the photon detector 36.

  Here, as shown in FIG. 2, the random number source 34 sets 10 bits as one block and performs different base modulation for each block. That is, in this embodiment, the base modulation is fixed in the block. Actually, the probability that a photon can be detected is about 1/1000 due to factors on the optical transmission line 2, but in this embodiment, the photon can be detected with a probability of about 1/10 for simplicity of explanation. And On the receiving side, base modulation is performed with 10 bits as one block, and photon detection is performed. However, since the detection probability is about 1/10, as shown in FIG. Can be detected. However, since the detection probability is an average probability, it is possible that a plurality of bits are detected in one block as shown in the second block in FIG.

  At this time, if the eavesdropper intensively holds the information on the second block, the deterioration of the safety is increased by that amount. Therefore, in the block discarding process (steps P12 and S12 in FIG. 1), The second block is discarded (whether or not it is detected). As described above, since the probability that a photon can be detected is about 1/1000, the probability that another bit can be detected in a block in which a certain bit can be detected is about 1/100. Minutes do not cause any significant degradation in key generation speed, so there is no problem.

  As described above, according to this embodiment, since the base modulation is fixed in the block, the amount of random numbers to be used can be greatly reduced, and therefore the amount of random number source modules mounted and the number of components can be reduced. Can do. For this reason, an inexpensive and simple quantum cryptography communication system can be constructed.

Embodiment 2

FIG. 3 is a block diagram showing the configuration of a quantum cryptography key distribution system according to the second embodiment of the present invention, and FIG. 4 shows a state where an average of one photon is detected for each block for each wavelength in the system. It is a conceptual diagram for demonstrating.
This embodiment uses a wavelength division multiplexing (WDM) system and relates to a system that supports the BB84 protocol. On the receiver (Bob6a) side, the same value is used for photons of multiple wavelengths within the same time slot (bit time). The optical pulse detectors 651, 661,..., 668, 668, when simultaneously detecting photons having a plurality of wavelengths, discard all bits assigned to these photons. Yes.
For convenience of explanation, a case where eight wavelengths are combined and transmitted by wavelength division multiplexing (WDM) technology is handled.

As shown in FIG. 3, the quantum key distribution system of this embodiment includes a transmitter Alice 4a, an optical transmission line 5a, and a receiver that receives distribution of the quantum encryption key from Alice 4a via the optical transmission line 5a. And Bob 6a.
As shown in the figure, Alice 4a generates eight wavelength photon pulses having different wavelengths λ ₁ , λ ₂ ,... Λ _8, and eight laser diodes (LD) 411, 412,. 8 optical attenuators (not shown), a 2-input 2-output Mach-Zender interferometer 42, a temperature controller (TEC) 43, and 8 phase modulators (Mod) 441, 442,. 458, variable delay units 461, 462,... 468, random number source 47 that drives phase modulators 441, 442,. And a processor 49 for performing signal processing, and an optical demultiplexer 483 including a WDM filter, a quantum encryption key distribution program (hereinafter simply referred to as a processing program). To have.

Each of the laser diodes (LD) 411, 412, ..., 418 described above generates a photon pulse having a corresponding wavelength. Each of the clock sources 451, 452,... 458 drives a corresponding laser diode 411, 412,. Each of the variable delay devices 461, 462,... 468 delays the drive signal from the corresponding clock source 451, 452,. Each optical attenuator suppresses the light intensity of the wavelength emitted from the corresponding laser diode to 1 photon / pulse or less, and generates a single photon pulse. The optical multiplexer 481 combines the photon pulses of the eight wavelengths λ ₁ , λ ₂ ,... Λ ₈ attenuated by each optical attenuator and outputs them to the Mach-Zender interferometer 42.

The Mach-Zehnder interferometer 42 time-separates a single photon photon pulse attenuated by an optical attenuator, and outputs a double photon pulse of 8 wavelengths. The temperature regulator 43 controls the temperature of the delay amount of the Mach-Zender interferometer 42. The optical demultiplexer 483, Mach-Tsuenda interferometer 42 multiplexed twin photon pulses eight wavelength lambda ₁ is inputted _from, lambda _2, ... branched into twin photon pulses lambda _8, corresponding Output to the phase modulators 441, 442,. Each of the phase modulators (Mod) 441, 442,..., 448 has a function of adding a predetermined phase difference to the 8-wavelength double photon pulse in accordance with the driving of the random number source 47.

Since the random number source 47 uses four random quantum states in this embodiment, the random number source 47 generates a random number 1 representing encryption key data “0” or “1” and a random number 2 that determines the basis. Random number 2, for example, “X base” representing a set of encryption key “0” for phase 0 and encryption key “1” for phase π, encryption key “0” for phase π / 2, and encryption for phase 3π / 2 Two sets of bases, “Y base” representing the key “1”, are selectively determined (see FIG. 9).
By driving the random number source 16, the Alice 4 a randomly modulates four photons of 0, π / 2, π, and 3π / 2 with respect to one photon and transmits it to the Bob 6 a via the optical transmission line 5.
The optical multiplexer 482 multiplexes two photon pulses of 8 wavelengths λ ₁ , λ ₂ ,... Λ ₈ input from the phase modulators 441, 442,. Send to Bob 6a.

The processing program is read into the processor 49 from an internal storage device such as a ROM or an external storage device such as a magnetic disk, and controls the operation of the processor 49.
The processor 49 performs frame synchronization under the control of the processing program, and when the optical pulse detectors 651, 661,... 658, 668 (on the Bob 6a side) simultaneously detect photon pulses of a plurality of wavelengths, Step S22 (simultaneous measurement bit discard processing) for discarding all the bits given to these photon pulses, step S23 for performing base collation, step S24 for performing error correction, and step S25 for performing confidentiality enhancement Execute.

Bob 6a includes a phase modulator 61, a random number source 62 that drives the phase modulator 61, a Mach-Zender interferometer 63, a temperature regulator 64, and two photon detectors 651, 652,. , 658, 661, 662,..., 668, optical demultiplexers 671 and 672 including WDM filters, a processing program (not shown), and a processor 68 that performs signal processing. The random number source 62 generates a random number 3 corresponding to the base and decodes the photon pulse sent from the Alice 4a (see FIG. 9).
The phase modulator 61 gives a phase difference again to the multiplexed double photon pulse transmitted from the Alice 4a through the optical transmission line 5a in accordance with the driving of the random number source 62. The Mach-Zehnder interferometer 63 generates a single photon pulse by combining the multiplexed double photon pulses given the phase difference again by the phase modulator 61. The temperature adjuster 64 controls the temperature of the delay amount of the Mach-Zender interferometer 63. The optical demultiplexers 671 and 672 branch the multiple photon pulses input from the Mach-Zender interferometer 63 into single optical pulses of 8 wavelengths λ ₁ , λ ₂ ,... Λ ₈ and corresponding photon detectors. 651, 652,..., 658, 661, 662,. The optical pulse detectors 651, 661, ..., 658, 668 are made of avalanche photodiodes (APDs).

The processing program is read into the processor 68 from an internal storage device such as a ROM or an external storage device such as a magnetic disk, and controls the operation of the processor 68.
The processor 68 performs step P21 for performing frame synchronization under the control of the processing program, and the photon detectors 651, 652,... 658, 661, 662,. When it is detected in the simultaneous slot, step P22 (simultaneous measurement bit discarding process) for discarding the relevant bit, step P23 for performing base collation, step P24 for performing error correction, and step P25 for enhancing confidentiality are executed.
Here, in this embodiment, the Mach-Zender interferometer 42 on the Alice 4a side and the Mach-Zender interferometer 63 on the Bob 6a side are used in which the phase differences between the long and short paths coincide with each other with accuracy in wavelength units. Is preferred.

Next, the operation of this embodiment will be described.
In Alice 4a, 8-wavelength photon pulses synchronized with 625 MHz clock sources 451, 452,..., 458 are generated by laser diodes 411, 412,. Is converted into a double photon pulse. Thereafter, based on the driving of the random number source 16, the optical phase on one side of the double photon pulse for each wavelength is modulated by the phase modulators 441, 442,. The phase difference (φA) is randomly generated in four ways of 0, π / 2, π, and 3π / 2. The optical multiplexer 482 multiplexes two photon pulses of 8 wavelengths λ ₁ , λ ₂ ,... Λ ₈ input from the phase modulators 441, 442,. Send to Bob 6a.

  The multiplexed double photon pulse passes through the transmission path 5 and reaches the Bob 6a, and the optical phase on one side thereof is modulated by the phase modulator 61. Here, the phase modulation is performed randomly based on driving of the random number source 62 so that the relative phase difference (φB) of the multiplexed double photon pulse becomes 0 and π / 2. Next, the multiplexed dual photon pulses are combined using the Mach-Zender interferometer 63, and detected by the optical pulse detectors 651, 661,..., 658, 668 according to the modulation phase applied by the Alice 4a and Bob 6a. The

  In Bob 6a, as shown in FIG. 4, base modulation of the same value is performed on all 8-wavelength signals by a phase modulator 61 in accordance with a random number source 62. Actually, the probability that a photon can be detected is about 1/1000 due to factors on the optical transmission line. In this embodiment, for simplicity, it is assumed that a photon pulse can be detected with a probability of about 1/10. As shown in FIG. 3, since each wavelength can be received with a probability of 1/10, only one photon pulse is received in most bit times. For example, as shown in the 13th bit of FIG. It is also possible that photons of multiple wavelengths are detected at the same bit time.

At this time, if the eavesdropper intensively holds information on the 13th bit, the deterioration of the safety is increased by that amount. Therefore, in the simultaneous measurement bit discard (steps P22 and S22), (actually eavesdropping is detected). The 13th bit is discarded (regardless of whether or not it is present). The processor 49 of Alice 4a executes the simultaneous measurement bit discard based on the operation result (step P22) of the processor 68 of Bob 6a (step S22).
As described above, since the probability that photons can be actually detected is about 1/1000, the probability that a photon of another wavelength can be detected in a bit that can detect a photon of a certain wavelength is about 1/100. Since the discarding portion does not cause a significant deterioration in the key generation speed, there is no problem.

  According to this embodiment, substantially the same effect as described in the first embodiment can be obtained. In addition, on the transmitter side, the variable delay devices 461, 462,... 468 can control the timing of the photon pulses output from the laser diodes 411, 412,. Later, a single phase modulator 61 can perform simultaneous modulation on the receiver side. Therefore, since the receiving side is configured to simultaneously perform base modulation using a single phase modulator 61 (without increasing the number of phase modulators) for photon pulses of a plurality of wavelengths, it is inexpensive and simple. A simple quantum cryptography communication system can be constructed.

Embodiment 3

FIG. 5 is a block diagram showing a configuration of a quantum cryptography key distribution system according to the third embodiment of the present invention.
When WDM is applied to the quantum cryptography system, a configuration as shown in FIG. 5 can also be adopted. In this embodiment, the same base modulation is performed on a signal having a plurality of wavelengths. The difference from the second embodiment is that a plurality of phase modulators 611, 612,..., 618 in Bob 6b are provided in Alice 4b. This eliminates the need for timing control of the optical pulse. Even in the configuration shown in FIG. 5, the amount of random numbers used (the amount of random number source modules mounted and the number of components) can be reduced.

Embodiment 4

  In the above embodiment, it is assumed that the difference in delay between the interferometers held by the transmitter and the receiver is the same for each wavelength. However, as shown in FIG. 6, Alice 4c and Bob 6c By preparing Mach-Zender interferometers 421, 422,..., 428, 631, 632,. Is possible.

  As described above, the embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like within the scope not departing from the gist of the present invention. However, it is included in this invention. For example, in the above-described embodiment, a Mach-Zender interferometer is used as the 1-bit delayed two-beam bundle interferometer. However, the present invention is not limited to this, and a Michelson interferometer may be used.

In the first embodiment described above, the base modulation is fixed with 10 bits as one block. However, the block length for fixing the base modulation is not limited to 10 bits, and can be increased or decreased as necessary. In the first embodiment described above, only the base modulation on the reception side is fixed in units of blocks. Alternatively, only the base modulation on the transmission side may be fixed in units of blocks. Alternatively, the base modulation on both the reception side and the transmission side may be fixed in units of blocks.
Further, in the first embodiment described above, all blocks in which a plurality of bits are detected are discarded, but instead, only one bit may be left and the remaining bits may be discarded. That is, 1 bit is discarded from a block in which 2 bits are detected in one block, and 2 bits are discarded from a block in which 3 bits are detected in 1 block.

In the second embodiment described above, the eight-wavelength photon pulses are bundled. However, the present invention is not limited to this, and the number of wavelengths can be increased or decreased as necessary. In the second embodiment described above, only the base modulation on the reception side is simultaneously modulated. Alternatively, only the base modulation on the transmission side may be simultaneously modulated. Alternatively, Both base modulations on the transmission side may be simultaneously modulated.
In the second embodiment described above, all bits in which a plurality of wavelengths are simultaneously detected are discarded, but instead, only one wavelength may be left and the remaining wavelengths may be discarded. That is, bit discard is performed such that one wavelength is discarded for two wavelengths detected by one bit, and two wavelengths are discarded for three bits detected by one bit.

  In the above-described embodiment, the quantum cryptography system that performs key sharing by placing information on the phase of the photon signal has been described. Instead, it is possible to apply the present invention by using other parameters such as polarization. Of course. In the above-described embodiment, the quantum key distribution system using photon pulses has been described. However, the present invention is not limited to this, and light that communicates information from a transmitter to a receiver by modulating the phase or polarization of a weak light pulse. Widely applicable to communication systems.

  The present invention can be applied to an optical interference communication system using phase modulation and polarization modulation represented by quantum cryptography key distribution technology. The quantum cryptographic key distribution method is not limited to the round-trip type or the one-way type, and the protocol is not limited.

1, 4a, 4b, 4c Transmitter (Alice)
11, 411, 412, ..., 418 Laser diode (LD, light source)
12, 42, 421, 422,..., 428 Mach-Zender interferometers 13, 43, 431, 432,..., 438 Temperature regulators 14, 441, 442, ..., 448 Phase modulator (modulator)
15, 451, 452, ..., 458 Clock source 16, 47 Random number source 461, 462, ..., 468 Variable delay device 481, 482 Optical multiplexer (WDM filter)
483 Optical demultiplexer (WDM filter)
17, 49 processor (key information discarding means)
S11, S21 Frame synchronization S12 Block discard S22 Simultaneous measurement bit discard S13, S23 Base verification S14, S24 Error correction S15, S25 Concealment enhancement 2, 5 Optical transmission path 3, 6a, 6b, 6c Receiver (Bob)
32, 63, 641, 642, ..., 648 Mach-Zehnder interferometer 33, 64, 641, 642, ..., 648 Temperature regulator 31, 61, 611, 612, ..., 618 Phase modulator (modulator)
35, 36, 651, 661, ..., 658, 668 Gate mode photon detector (photon detector)
34, 62 Random number source 671, 672, 673, 675 Optical demultiplexer (WDM filter)
674 Optical multiplexer (WDM filter)
37, 68 processor (key information discarding means)
P11, P21 Frame synchronization P12 Block discard P22 Simultaneous measurement bit discard P13, P23 Base verification P14, P24 Error correction P15, P25 Increased confidentiality

Claims (28)

An optical communication system for communicating information from a transmitter to a receiver by modulating the phase or polarization of a weak light pulse,
A modulator that is provided in one or both of the transmitter and the receiver and that modulates the same value to a plurality of the optical pulses within a predetermined spatial and temporal range;
An optical pulse detector provided in the receiver for detecting the optical pulse;
Provided in both the transmitter and the receiver, when the plurality of optical pulses are detected within the same spatial and temporal range by the optical pulse detector, the bits assigned to these optical pulses An optical communication system comprising: discarding means for discarding all or part of the optical communication system. An optical communication system for communicating information from a transmitter to a receiver by modulating the phase or polarization of a weak light pulse,
A modulator that is provided in one or both of the transmitter and the receiver and that modulates a plurality of the optical pulses in a block defined by a length of time with the same value;
An optical pulse detector provided in the receiver for detecting the optical pulse;
Provided in both the transmitter and the receiver, and when the optical pulse detector detects a plurality of the optical pulses in the same block, the optical pulse detector includes a discarding unit that discards the block. An optical communication system. An optical communication system for communicating information from a transmitter to a receiver by modulating the phase or polarization of a weak light pulse,
A modulator that is provided in one or both of the transmitter and the receiver and that modulates a plurality of the optical pulses in a block defined by a length of time with the same value;
An optical pulse detector provided in the receiver for detecting the optical pulse;
When a plurality of the optical pulses are detected in the same block by the optical pulse detector provided in both the transmitter and the receiver, the bit assigned to one optical pulse from the block An optical communication system comprising: discarding means for discarding others while leaving An optical communication system for communicating information from a transmitter to a receiver by modulating the phase or polarization of light pulses of a plurality of weak wavelengths,
A modulator that is provided in one or both of the transmitter and the receiver, and that performs modulation of the same value on the optical pulses of the plurality of wavelengths in the same time slot;
A plurality of optical pulse detectors provided in the receiver for respectively detecting the optical pulses of the plurality of wavelengths;
A discarding unit provided in both of the transmitter and the receiver, and when the optical pulse detector simultaneously detects the optical pulses of the plurality of wavelengths, discarding means for discarding all the bits assigned to these optical pulses; An optical communication system comprising: An optical communication system for communicating information from a transmitter to a receiver by modulating the phase or polarization of light pulses of a plurality of weak wavelengths,
A modulator that is provided in one or both of the transmitter and the receiver, and that performs modulation of the same value on the optical pulses of the plurality of wavelengths in the same time slot;
A plurality of optical pulse detectors provided in the receiver for respectively detecting the optical pulses of the plurality of wavelengths;
Provided in both the transmitter and the receiver, and when the optical pulse detector simultaneously detects the optical pulses of the plurality of wavelengths, the bits for one wavelength among all the bits added to these optical pulses An optical communication system comprising: discarding means for discarding others while leaving   4. The optical communication system according to claim 2, wherein the block includes a plurality of predetermined bit times.   6. The optical communication system according to claim 4, wherein the time slot corresponds to a predetermined bit time.   The optical communication system according to any one of claims 1 to 5, wherein the modulator performs base modulation as the modulation.   The optical communication system according to claim 1, wherein the modulator is driven by a random number source.   The transmitter is provided with an optical attenuator that suppresses the light intensity of the wavelength emitted from the light source to 1 photon / pulse or less and generates the optical pulse composed of the single photon. The optical communication system according to any one of claims 1 to 5.   A receiver constituting the optical communication system according to any one of claims 1 to 10, wherein the receiver includes the modulator, the optical pulse detector, and the discarding unit. Machine.   A transmitter constituting the optical communication system according to any one of claims 1 to 10, comprising the modulator and the discarding unit that operates based on an operation result of the discarding unit of the receiver. The transmitter characterized by comprising.   11. The quantum key distribution system according to claim 1, wherein information communicated from the transmitter to the receiver is encryption key information. An optical communication method for communicating information from a transmitter to a receiver by modulating the phase or polarization of a weak light pulse,
A modulation process in which one or both of the transmitter and the receiver modulate the same value to a plurality of the optical pulses within a predetermined spatial and temporal range;
An optical pulse detection process for detecting the optical pulse at the receiver;
When both the transmitter and the receiver detect the plurality of optical pulses within the same spatial and temporal range by the optical pulse detection process, all of the bits added to these optical pulses are detected. Or a discarding process for discarding a part of the optical communication method. An optical communication method for communicating information from a transmitter to a receiver by modulating the phase or polarization of a weak light pulse,
A modulation process in which one or both of the transmitter and the receiver performs the same modulation on the plurality of optical pulses in the block defined by the length of time;
In the receiver, an optical pulse detection process for detecting the optical pulse provided with information;
When both the transmitter and the receiver detect a plurality of the optical pulses in the same block by the optical pulse detection process, the transmitter and the receiver include a block discard process for discarding the block. Optical communication method. An optical communication method for communicating information from a transmitter to a receiver by modulating the phase or polarization of a weak light pulse,
A modulation process in which one or both of the transmitter and the receiver performs the same modulation on the plurality of optical pulses in the block defined by the length of time;
In the receiver, an optical pulse detection process for detecting the optical pulse provided with information;
When both the transmitter and the receiver detect a plurality of the optical pulses in the same block by the optical pulse detection processing, the bits assigned to one optical pulse are left from the block. And a partial bit discarding process for discarding others. An optical communication method for communicating information from a transmitter to a receiver by modulating the phase or polarization of light pulses of a plurality of weak wavelengths,
In one or both of the transmitter and the receiver, in the same time slot, a modulation process for modulating the same value to the optical pulses of the plurality of wavelengths, and
In the receiver, a plurality of optical pulse detection processes for detecting the optical pulses of the plurality of wavelengths given information,
In both the transmitter and the receiver, when the optical pulses of the plurality of wavelengths are simultaneously detected by the optical pulse detection process, an all-bit discard process for discarding all the bits given to these optical pulses. An optical communication method comprising: An optical communication method for communicating information from a transmitter to a receiver by modulating the phase or polarization of light pulses of a plurality of weak wavelengths,
In one or both of the transmitter and the receiver, in the same time slot, a modulation process for modulating the same value to the optical pulses of the plurality of wavelengths, and
In the receiver, a plurality of optical pulse detection processes for detecting the optical pulses of the plurality of wavelengths given information,
When both the transmitter and the receiver simultaneously detect the optical pulses of the plurality of wavelengths by the optical pulse detection process, the bits for one wavelength are left out of all the bits added to these optical pulses. And a partial bit discarding process for discarding others.   The optical communication method according to claim 15 or 16, wherein the block includes a plurality of predetermined bit times.   The optical communication method according to claim 17 or 18, wherein the time slot corresponds to a predetermined bit time.   The optical communication method according to claim 14, wherein in the modulation process, base modulation is performed as the modulation.   The optical communication method according to claim 14, wherein the modulation process performs the modulation using a random number source.   15. The transmitter is provided with an optical attenuator that suppresses the light intensity of a wavelength emitted from a light source to 1 photon / pulse or less and generates the optical pulse composed of the single photon. 18. The optical communication method according to any one of 18.   The optical communication method according to any one of claims 14 to 21, wherein information communicated from the transmitter to the receiver is encryption key information.   A program causing a computer to function as a discarding unit of the optical communication system according to any one of claims 1 to 10.   A program that causes a computer to function as a discarding unit of the receiver according to claim 11.   A program for causing a computer to function as discarding means for the transmitter according to claim 12. 24. A program for causing a computer to execute each process of the optical communication method according to claim 14.

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