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Laser light pulses can be modulated to pulse pair with 400 ps delay by going through two light paths of dierent lengths and by multiplexing them (by going through asym-metric interferometer). is pulse pair is expressed as |α>F ⊗|α>S. Here, suxes F and S mean temporally rst pulse (First) and the second pulse (Second) that imply the posi-tions of pulses, respectively. en, the pulse pair are input to a dual drive optical modulator with two electrodes. en, the two pulses are multiplexed aer modulated with phases φ1 and φ2 that correspond to bit information and base information at each electrode, according to the ran-dom number sequence applied by random number source A of the control block shown in Fig. 2. e states are de-scribed by the following equations.00ZFS (3)10ZFS (4)44022iiYFSee (5)44022iiYFSee (6)Here, |ΨZ0> and |ΨZ1> the states correspond to the Z-bases, and |ΨY0> and |ΨY1> are the states corresponding to the Y-base. ese four states have equivalent eect concerning the Z-base, X-base and the function of QKD of the polarization mode explained above. Here, the reason for using Y for the base is only for convenience because we use input power for the modulator.Aer that, the time-bin signals are input to the second dual drive optical modulator. en, the signals are modu-lated randomly into several types of intensity according to the requirement (iii) “Decoy method” and pass through the attenuator to become a weak optical pulse. Lastly, it is input into the optical ber of quantum communication channel. One of the examples of the Decoy method is the two set-tings of signal intensity |α|2 = 0.5 photon/time-bin, decoy intensity |α|2 = 0.2 photons/time-bin, and |α|2 = 0 photon/time-bin (vacuum). It is not possible for real laser light to decrease the probability that two or more photons are contained in a pulse to zero. Hence, the state with two or more photons in the time-bin pulse still remains. In such a case, a so-called “photon number splitting attack” is pos-sible for Eve by stealing a photon and transmitting the others photons to Bob keeping the states.In this case, a bit error never occurs because the base and contents of the bit do not change. erefore, the eavesdropping cannot be detected. e Decoy method can improve tolerance to such an attack and can extend trans-mission distance. If the photon distribution of the light source is known, the detection rate and error rate of signal are determined by the loss of the transmission channel. However, if there is an attack in which a multi-photon state FiF3Composition of encoder used for device implementation of the BB84 protocol for optical fiber transmission and outline of time-bin signalf1f2encoderAn-symmetricMach-ZehnderinterferometerfiberLight sourceattenuatorZ0Z1Y0Y150 ps400 psmodulatorRandom numbergeneratorRandom numbergeneratormodulator・bit (0,1)・basis (Y or Z)Decoy intensityPhase randomizationPulse-pairTime-bin signalSignal: 0.5 photon/time-binDecoy: 0.2photon/time-binVacuum: 0.0photon/time-binFirst pulseFSecond pulseSsasaki-fig_time-bin-encoder_X_Y_waveforms3 Quantum Key Distribution Network16　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)