HTML5 Webook

19/84

contriving an eavesdropping method. Alice and Bob can discover eavesdropping by comparing test bits randomly selected from the sieving key.Even if Eve does not exist, the bit error rate will increase when there is noise in a quantum transmission channel. As there is no method to distinguish if the noise is caused by Eve or not, all noise in the quantum transmission is as-sumed to be an eect caused by Eve.Alice and Bob judge the possibility of eavesdropping from the bit error rate of the test bit. If they judge that there is no possibility of eavesdropping, they extract an ultimately safe random number sequence by processing appropriate key distillation according to the value of the bit error rate to extract a cryptographic key.2.2Composition of QKD linkA QKD link consists of a “quantum communication block” for sharing random number data via photons, a “key distillation block” for extracting safe cryptography key from shared random number data, and a “control block” that controls these blocks. e control block supplies a random number sequence to the quantum communication block and the key distillation block also supplies a synchro-nous signal to the quantum communication block for time synchronization. e rough system structure is shown in Fig. 2. In most cases, Alice multiplexes the synchronous signals with quantum signals physically and transmits them to Bob via a quantum communication channel. In the following, details of the quantum communication block and the key distillation block are explained.・Quantum communication blocke quantum communication block consists of a light source, encoder, quantum communication channel, de-crypter and photon detector. It transmits quantum signals by synchronizing time via synchronous signals.A laser light source is oen used as a light source rather than a single photon source. Actually, long-distance QKD can be realized with laser light pulses by controlling as follows. First of all, the following four procedures should be performed.(i) Weak laser light: attenuating laser light to a weak pulse with a probability of containing two or more photons per pulse to introduce the light into the communication channel.(ii) Phase disordering: controlling the light source or modulator in order to generate phase-uncorrelated bit strings.(iii) Decoy method: introducing pulses of dierent laser light intensity other than the signal pulse used for key generation (decoy pulse) randomly in order to prevent degradation of transmission performance due to several photon components not being removed.(iv) Time-bin signal: generating two pulse pair (time-bin) [23] and coding bit information and base information to the pair.(i) is a requirement to generate a state of which the main component is a single photon.(ii) and (iii) are re-quirements to strengthen transmission performance. (iv) is a requirement to be considered for the case of an optical ber quantum transmission channel. Time-bin signal can suppress the eect of perturbation generated in an optical ber more eectively than polarization signal. Actually, the eect of perturbation can be eliminated by detecting pho-tons by having two pulses by a receiver interfere because the two pulses are perturbed almost at the same time. is is explained in the following by an example of the compo-sition of encoder devices in practical use for these points (Fig. 3).Laser light can be attenuated by an attenuator just be-fore the exit of the encoder. Hence, the requirement (i) “weak laser light” is performed at the last step in the en-coder. Before the step, laser light with sucient intensity (classic signal) is coded. Although the laser is in coherent state of the same phase, correlation between input pulses must be eliminated in order to improve the transmission performance of the BB84 protocol. (requirement (ii) “phase disordering”).If there is a phase correlation between pulses, Eve presumes the phase from the pulse sequence and can perform quantum measurement so as to compensate the eects by the Decoy method. erefore, this situation will deteriorate the safety. In the example of implementation of a typical high-speed QKD device, a laser light pulse with-out such phase correlation is generated by a repeat rate of 1.244 GHz. e time interval is about 50 picoseconds (5 × 10−12 sec, 50 ps). e laser light pulses are input into the encoder one aer another with an interval of 800 ps. is pulse sequence is dened as |α1>, |α2>, α3>, ···, Here, the amplitude is expressed as α1 = |α|eiθ1, α2 = |α|eiθ2, α3 = |α|eiθ3, ···, phasesθ1, θ2, θ3,··· must change randomly without correlation each other. In this subsection, I explain how the laser light pulses of a certain phase are input to an en-coder and are output as “time-bin signals” that meet the requirement (iv) with which bit information and base in-formation are coded.153-1 Research and Development of Quantum Key Distribution Network in NICT