JP2008199667A - Quantum encryption key distribution system, and synchronization method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum encryption key distribution system preventing the simultaneous input of both an optical pulse to a sender unit and an optical pulse from the sender unit to a phase modulator of a receiver unit owing to the delay amount of a transmission line. <P>SOLUTION: A quantum encryption key distribution system loops back an optical pulse from a receiver unit 200 to the receiver unit side in a sender unit 100, and in the receiver unit, performs phase modulation of the optical pulse returned by loopback at the sender unit. The receiver unit includes a frequency synchronizer 300 for synchronizing the transmission and the reception of the optical pulse, based on both a clock signal to be transmitted to the sender unit and a clock signal returned by loopback at the sender unit. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a quantum encryption key distribution system and a synchronization method used therefor, and more particularly to a quantum encryption key distribution method used in a quantum encryption communication system.

  In the quantum key distribution system, wiretapping between a sender unit (hereinafter referred to as Alice) and a receiver unit (hereinafter referred to as Bob) is detected with high probability based on Hisenberg's uncertainty principle. Is possible. Therefore, since the quantum cryptography key distribution system can detect eavesdropping, it is clearly understood whether or not the distributed key is safe. That is, in the quantum cryptographic key distribution system, a high level of security can be obtained by using a distributed secure key as a Vernam cryptographic key whose absolute security is proved.

  Next, a case where phase interference is used as an example of a key sharing flow of the quantum key distribution system (BB84: Bennett Brass 84) will be briefly described. Here, the quantum channel indicates a weak communication channel in which the optical power transmitted from Alice to Bob is 1 photon / bit or less, and the classical channel indicates a communication channel in a normal optical power region.

This key sharing flow is shown in FIG.
(1) Phase modulation data is generated and stored by random data bit A, which is the original data of the encryption key in Alice, and random data, which is the base (+ base, X base) (base information A) at the time of modulation.

  (2) Alice modulates the optical pulse with phase modulation data and transmits it to Bob via the quantum channel.

  (3) Bob also phase modulates an optical pulse from Alice based on random basis (+ basis, X basis) data and receives it via an interferometer.

  (4) The optical data bit B that can be received by Bob and the base at that time are stored (basic information B), and transmitted to Alice via the classical channel.

  (5) Alice compares the base information B sent from Bob with the stored base information A, and discards the bits of the random data bits A whose bases do not match.

  (6) From Alice to Bob, transmit the bit numbers of the bits of the random data bits A that remain without being discarded through the classical channel.

  (7) Bob discards the optical data bit B other than the bit number sent from Alice.

  (8) Alice and Bob share encryption key data.

  As described above, in order to share key data between Alice and Bob, it is necessary to establish synchronization in bit units. However, the quantum key distribution system is significantly different from conventional optical communication, and the optical power level is as small as a single photon level. Therefore, clock extraction using conventional quantum communication using a quantum channel is possible. It is impossible to do.

Japanese National Patent Publication No. 08-505019 JP 63-107323 A “Automated 'plug & play' quantum key distribution” (G. Ribordy, JD Gautier, O. Guinard and H. Zbinden, ELECTRONICS LETTERS 29th Oct. 21 p. 2211. 22 V 21. 22 221 Vol. "" Plug & Play "systems for quantum cryptography" (A. Muller, T. Herzog, B. Huttner, W. Tittel, H. Zbinden and N. Gisin, Appl. Phys. , Pp. 793-795) “Interferometry with Faraday mirrors for quantum cryptography” (H. Zbinden, J. D. Gautier, N. Gisin, B. Hutner, A. Muller and W. TITEL. -588)

  Among the quantum key distribution systems, the Plug & Play method (see FIG. 5) described in Non-Patent Documents 1 to 3 and proposed by the University of Geneva can compensate for polarization fluctuations in an optical fiber transmission line. It is expected as a method for putting a quantum key distribution system sensitive to polarization into practical use.

  In this plug & play system, an optical pulse is sent from Bob to Alice, modulated by Alice, and then reflected to Bob by rotating the polarized light by 90 degrees using a Faraday mirror. Bob phase modulates the light pulse reflected from Alice, interferes with it, and receives it at the receiver. At that time, Bob needs to modulate only the optical pulse sent from Alice without modulating the optical pulse sent to Alice.

In this method,
(1) When the optical pulse sent from Bob is modulated by Alice according to the pulse (need to follow the delay variation of the optical fiber transmission line)
(2) When the optical pulse reflected from Alice is modulated by Bob (it is necessary to follow the delay variation of the optical fiber transmission line), but the optical pulse sent to Alice at the same time is not modulated, that is, the optical pulse from Bob to Alice And Alice → Bob's optical pulse is not input to Bob's phase modulator at the same timing. (3) When receiving an optical pulse with Bob, the bias applied to the receiver is matched to the received optical pulse (superior in Geiger mode). High sensitivity reception)
That synchronization is necessary.

  In the quantum key distribution system, it is necessary to establish synchronization in units of bits in the exchange of basic information. However, the quantum key distribution system is very different from the conventional optical communication, and the optical power level is as small as a single photon level. Therefore, it is not possible to extract the clock like the conventional optical communication using the quantum channel. Impossible.

  Therefore, a method of performing bit synchronization, frame synchronization, and other system calibration using a classical channel has been proposed (for example, see Patent Document 1). In this method, as shown in FIG. 6, since a calibration signal is sent from Alice 700 to Bob 800, Bob 800 follows and synchronizes with Alice 700.

  However, in the Plug & Play system, as described in (2) above, only the light pulse reflected from Alice is modulated by Bob, and the light pulse from Bob to Alice is not modulated.

  In the method of the above-mentioned patent document 1, since Bob is synchronized with Alice, even if the delay variation of the transmission path occurs and the timing at which the optical pulse reflected from Alice arrives at Bob fluctuates, Although the fluctuation can be followed at the modulation timing, the optical pulse sent from Bob to Alice cannot be controlled at all.

  For this reason, depending on the amount of delay in the transmission path, the Bob → Alice optical pulse and the Alice → Bob optical pulse may be input to Bob's phase modulator at the same timing, so it is impossible to realize the Plug & Play system. is there.

  Accordingly, an object of the present invention is to solve the above problems, establish synchronization between the sender unit and the receiver unit, and output light pulses output from the receiver unit and light returned from the sender unit to the receiver unit. It is an object of the present invention to provide a quantum key distribution system capable of always maintaining a constant phase relationship with a pulse and a synchronization method used therefor.

  In the quantum key distribution system according to the present invention, the optical pulse from the receiver unit is turned back to the receiver unit in the sender unit, and the phase of the optical pulse returned by the sender unit in the receiver unit is returned. A quantum key distribution system that modulates and receives at a receiver, wherein the receiver unit is based on both a clock signal sent to the sender unit and a clock signal returned by the sender unit. And a frequency synchronization unit for synchronizing transmission and reception of the optical pulse.

  In the synchronization method used in the quantum key distribution system according to the present invention, the light pulse from the receiver unit is returned to the receiver unit side in the sender unit, and the transmitter unit returns the light pulse in the receiver unit. A synchronization method used in a quantum key distribution system that performs phase modulation of an optical pulse and is received by a receiver, wherein the receiver unit returns with a clock signal transmitted to the transmitter unit and the transmitter unit. The transmission / reception of the optical pulse is synchronized based on both of the received clock signals.

  The receiver unit used in the quantum key distribution system according to the present invention returns the light pulse from the receiver unit to the receiver unit side in the transmitter unit, and returns to the receiver unit after returning to the receiver unit. Further, the receiver unit used in the quantum key distribution system that performs phase modulation of the optical pulse and receives it by a receiver, and returns by returning the clock signal to the sender unit and the sender unit. A frequency synchronization unit that synchronizes transmission and reception of the optical pulse based on both of the clock signals.

  According to the present invention, the transmitter unit and the receiver unit operate synchronously, and only the optical pulse from the transmitter unit can be modulated by the phase modulator in the receiver unit.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a quantum key distribution system according to an embodiment of the present invention. Referring to FIG. 1, a quantum key distribution system according to an embodiment of the present invention includes a transmission-side quantum unit [hereinafter referred to as Alice] 100 and a reception-side quantum unit [hereinafter referred to as Bob]. 200, an optical fiber transmission line 400 connecting Alice 100 and Bob 200, a frequency synchronization unit 300, and a folding unit 500.

  Alice 100 and Bob 200 generate an encryption key via the quantum channel 401. The frequency synchronization unit 300 includes a voltage-controlled oscillator [VCO (Voltage Controlled Oscillator)] 301, a low-pass filter [LPF (Low Pass Filter)] 302, and a phase detector (Phase Det) 303.

  The output clock signal 304 output from the frequency synchronization unit 300 is sent to the Alice 100 via the classical channel 402 in the optical fiber transmission line 400, and at the same time, the output clock signal 304 is folded back from the folding unit 500 and again in the optical fiber transmission line 400. Sent to Bob 200 via channel 402.

  At this time, the phase detector 303 detects the phase difference between the output clock signal 304 and the return clock signal 305 returned from Bob 200. The phase detector 303 outputs a signal corresponding to the detected phase difference, the high-frequency component is cut by the low-pass filter 302, and the voltage control transmitter 301 is controlled.

  By this system, it is possible to realize synchronization between Alice 100 and Bob 200 via the optical fiber transmission line 400, and that the clock wave number between Alice 100 and Bob 200 is a constant positive number.

  FIG. 2 shows a detailed block diagram. In this configuration, the quantum channel 401 (wavelength λ1) and the synchronous clock signal classical channel 402 (wavelengths λ2 and λ3) use different wavelengths and are connected to the optical fiber transmission line 400 via wavelength demultiplexers 601 and 602. Yes.

  Alice 100 includes a phase modulator (Phase Mod. A) 111, a modulator driving circuit (Drv.) 112, a delay adjustment circuit (D) 113, a Faraday mirror (Faraday Mirror) 120, and an optical attenuator (Att. ) 130.

  Bob 200 includes a laser (LD (Laser Diode)) 211 that is a pulse light source, a laser driving circuit 212, an optical attenuator (Att.) 213, an optical coupler 406, an optical circulator 250, a short path 404, and a long path 405. A phase modulator (Phase Mod. B) 221, a modulator driving circuit (Drv.) 222, a delay adjustment circuit (D) 223, a PBS (Polarization Beam Splitter) 230, and an APD (Avalanche Photo Diode) 241. , 242, an APD bias circuit (Drv.) 243, and a delay adjustment circuit (D) 244. The Short Path 404 and the Long Path 405 are polarization-maintaining fibers, and the optical circulator 250 and the optical coupler 406 are polarization-maintaining types.

  A laser 211 as a pulse light source outputs an optical pulse in synchronization with a clock supplied from the frequency synchronization unit 300. This optical pulse is separated into a short path 404 and a long path 405 by an optical coupler 406, multiplexed by a PBS 230, and reaches the Alice 100 via a wavelength multiplexer / demultiplexer 602, an optical fiber transmission line 400, and a wavelength multiplexer / demultiplexer 601. At this time, the delay modulator 223 is set so that the phase modulator 221 does not perform phase modulation on the optical pulse.

  In Alice 100, only the optical pulse that has passed the Long Path 405 of Bob 200 is modulated by the phase modulator 111. The signal for driving the phase modulator 111 is synchronized with the clock supplied from the synchronous clock return unit 500. Since this clock is synchronized with the frequency synchronization unit 300 of Bob 200 and follows the delay variation of the optical fiber transmission line 400, phase modulation can be performed in synchronization with the arrived optical pulse.

  The optical pulse is reflected with the phase rotated by 90 ° by the Faraday mirror 120, attenuated to a power of 1 photon / bit or less by the optical attenuator 130, and sent to the optical fiber transmission line 400 through the wavelength multiplexer / demultiplexer 601. . The optical pulse that has passed through the optical fiber transmission line 400 is input to Bob 200 via the wavelength multiplexer / demultiplexer 602.

  Although the optical pulse is separated into Short Path 404 and Long Path 405 by PBS 230, the phase is rotated by 90 ° by Faraday mirror 120. Therefore, when sent from Bob 200 to Alice 100, the optical pulse that has passed through Short Path 404 is Long Path 405. The light pulse passing through the long path 405 passes through the short path 404.

  Since the phase modulator 221 is inserted in the long path 405, the optical pulse that has passed through the short path 404 is modulated by the phase modulator 221 when being sent from Bob 200 to Alice 100. That is, when sending from Bob 200 to Alice 100, an optical pulse passing through Long Path 405 is phase-modulated by Alice 100, and when sending from Bob 200 to Alice 100, an optical pulse passing through Short Path 404 is phased by Bob 200. Will be modulated.

  When sending from Bob 200 to Alice 100, the optical pulse passing through the long path 405 and the light passing through the short path 404 have the same path length through both directions. become. If the modulation phase at Alice 100 and the modulation phase at Bob 200 are the same, the optical pulse is output to APD 241, and if the phase difference is π, it is output to APD 242.

  In these states, the bases of Alice 100 and Bob 200 are the same. On the other hand, when the phase difference is π / 2 or 3π / 2, the output will not be uniquely determined. In these states, the bases of Alice 100 and Bob 200 are inconsistent.

  Since the signal for driving the phase modulator 221 of Bob 200 is synchronized with the frequency synchronization unit 300, phase modulation can be performed in synchronization with an optical pulse having a delay variation in the optical fiber transmission line 400. Since the frequency control is performed so that the timing of the clock signal sent from Bob 100 to Alice 200 and the clock signal returned from Alice 100 to Bob 200 is always constant, the phase modulator 221 returns. If the initial setting is performed so that only the light is phase-modulated, the same state can be maintained even if the delay of the optical fiber transmission line 400 varies.

  Since the APD bias circuit 243 is also synchronized with the frequency synchronization unit 300, it is possible to apply a bias to the APDs 241 and 242 in synchronization with an optical pulse having a delay variation in the optical fiber transmission line 400.

  As described above, in this embodiment, synchronization between Alice 100 and Bob 200 is established, and the phase relationship between the optical pulse output from Bob 200 and the optical pulse returned from Alice 100 to Bob 200 is always constant. Can do. Therefore, in this embodiment, Alice 100 and Bob 200 operate in synchronization, and the phase modulator 221 in Bob 200 can modulate only the optical pulse from Alice 100. Therefore, Plug & Play It is possible to realize a quantum cryptography key generation system using a scheme.

  FIG. 3 is a block diagram showing the configuration of a quantum key distribution system according to another embodiment of the present invention. In FIG. 3, a quantum key distribution system according to another embodiment of the present invention has a configuration for realizing not only clock synchronization but also frame synchronization in the encryption key generation process.

  That is, the quantum key distribution system according to another embodiment of the present invention adds the frequency divider (1 / N) 20 and the quantum key generator 11 on the Alice side, and the quantum key generator 12 on the Bob side. 1 is the same as that of the quantum key distribution system according to the embodiment of the present invention shown in FIG. 1, and the same components are denoted by the same reference numerals. Here, the quantum key generators 11 and 12 are based on the above-described quantum key distribution method (BB84: Bennett Brass 84) (see FIG. 4).

  In this embodiment, not only clock synchronization but also frame synchronization in the encryption key generation process such as exchange of base information between Alice and Bob as shown in FIG. This is a configuration that can achieve the same).

  In this embodiment, the clock synchronized with the method described in FIG. 1 is divided by Alice's frequency divider 20 to generate a frame pulse, and this frame pulse is sent to Bob separately from the clock. Frame synchronization is performed.

  INDUSTRIAL APPLICABILITY The present invention can be applied to fields requiring high security in data communication such as defense, diplomacy, finance, etc., and is a method for detecting phase synchronization by dividing basic frequencies by N. Is also applicable.

It is a block diagram which shows the structure of the quantum key distribution system by one Example of this invention. It is a block diagram which shows the detailed structure of the quantum encryption key distribution system by one Example of this invention. It is a figure which shows the structure for implement | achieving the frame synchronization in not only the clock synchronization by the other Example of this invention but an encryption key generation process. It is a figure which shows an example of the key sharing flow of a quantum encryption key distribution system. It is a figure for demonstrating a plug & play system quantum encryption key distribution system. It is a figure which shows the method of calibrating a bit synchronization, a frame synchronization, and other systems using a classical channel.

Explanation of symbols

100 Quantum unit on the transmitting side
111,221 Phase modulator
112, 222 Modulator drive circuit 113, 223, 244 Delay adjustment circuit
120 Faraday Mirror
130,213 Optical attenuator
200 Quantum unit on the receiving side
211 laser
212 Laser drive circuit
230 PBS
241,242 APD
243 APD bias circuit
250 Optical circulator
300 Frequency synchronization part
301 Voltage controlled transmitter
302 Low pass filter
303 Phase detector
304 Output clock signal
305 Loopback clock signal
400 optical fiber transmission line
401 Quantum channel
402 Classical Channel
404 Short Path
405 Long Path
406 Optical coupler
500 Folding part
601 and 602 wavelength demultiplexers

Claims (11)

In the transmitter unit, the optical pulse from the receiver unit is turned back to the receiver unit side, and in the receiver unit, the optical pulse returned by the transmitter unit is returned and phase-modulated, and received by the receiver. An encryption key distribution system,
The receiver unit includes a frequency synchronization unit that synchronizes transmission and reception of the optical pulse based on both a clock signal transmitted to the transmitter unit and a clock signal returned by the transmitter unit. Quantum encryption key distribution system. The quantum key distribution system according to claim 1, wherein the sender unit performs phase modulation and polarization rotation of the optical pulse from the receiver unit. The frequency synchronization unit controls the synchronization so that the relationship between the phase of the clock signal sent to the sender unit and the phase of the clock signal returned by the sender unit is constant. The quantum cryptographic key distribution system according to claim 1 or 2, characterized in that: 4. The quantum key distribution system according to claim 1, wherein the receiver unit performs the phase modulation according to the synchronization obtained by the frequency synchronization. 5. In the transmitter unit, the optical pulse from the receiver unit is turned back to the receiver unit side, and in the receiver unit, the optical pulse returned by the transmitter unit is returned and phase-modulated, and received by the receiver. A synchronization method used in an encryption key distribution system,
The synchronization method characterized in that the receiver unit synchronizes transmission and reception of the optical pulse based on both a clock signal sent to the sender unit and a clock signal returned by the sender unit. 6. The synchronization method according to claim 5, wherein the sender unit performs phase modulation and polarization rotation of the optical pulse from the receiver unit. In the receiver unit, the synchronization is achieved by controlling the relationship between the phase of the clock signal sent to the sender unit and the phase of the clock signal returned by the sender unit to be constant. The synchronization method according to claim 5 or 6, characterized in that: The synchronization method according to any one of claims 5 to 7, wherein the phase modulation is performed in the transmitter unit according to the synchronization obtained by the frequency synchronization. In the transmitter unit, the optical pulse from the receiver unit is turned back to the receiver unit side, and in the receiver unit, the optical pulse returned by the transmitter unit is returned and phase-modulated, and received by the receiver. A receiver unit used in an encryption key distribution system,
A receiver unit, comprising: a frequency synchronization unit that synchronizes transmission and reception of the optical pulse based on both a clock signal transmitted to the transmitter unit and a clock signal returned by the transmitter unit. The frequency synchronization unit controls the synchronization so that the relationship between the phase of the clock signal sent to the sender unit and the phase of the clock signal returned by the sender unit is constant. 10. A recipient unit according to claim 9, characterized in that 11. The receiver unit according to claim 9 or 10, wherein the phase modulation is performed according to the synchronization obtained by the frequency synchronization.

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