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quantum signals by using an appropriate measuring method. Quantum signals must contain at least more than two non-orthogonal states. Hereaer in this article, I ex-plain the representative QKD protocol of BB84 as an ex-ample. e outline of the protocol is shown in Fig. 1. In the eld of cryptography, traditionally, we call the autho-rized transmitter Alice and the authorized receiver Bob and the eavesdropper Eve. Hereaer, I follow this tradition. Also, in the outline of the BB84 protocol, the case that the polarization state of a single photon as a quantum signal is used as an example.In the BB84 protocol, two sets of polarization states, that is, the Z-base of horizontal and vertical polarization {|H>, |V>}, and the X-base of dextrorotation and levorota-tion polarization {|45°>, |−45°>} are used. {|H>, |V>} is expressed as {|Z0>, |Z1>}, and {|45°>, |−45°>} is expressed as {|X0>, |X1>} to describe the protocol. e transmitter (Alice) encodes 1s and 0s to the corresponding polarization state selecting one of the two from the Z-base and the X-base when she encodes each bit of information of 1s and 0s of a random number sequence to a photon. erefore, the quantum signal to be transmitted consist of four com-ponents of {|Z0>, |Z1>, |X0>, |X1>}. ough the state vector in each base crosses the other orthogonally, the state vectors between the Z and X bases do not cross orthogo-nally. In reality, the inner products are as follows.100012ZXZX (1)110112ZXZX (2)Figure 2 shows a simple example of a correspondence table of bit information and base information used between the transmitter and receiver.e receiver (Bob) selects one from the Z-base or X-base at random and independently from the transmitter to measure photons. Sometimes no photon is detected due to optical loss in the quantum communication channel. ere is some noise in the quantum communication chan-nel so that the state detected is dierent from that of transmission. Such case is not shown in Fig. 1.Aer transmission of this quantum signals, Alice and Bob do not disclose the bit information but exchange only the information on the base they actually used (base infor-mation) via a public transmission channel, and select the slot where the bases of Alice and Bob coincide (base comparison). e bit sequence thus remaining is called the sieving key. en, a part of the sieving key is selected as a test bit for comparison between Alice and Bob to evaluate the bit error rate.If eavesdropping exists in a quantum transmission channel, the bit error rate increases. at is because the sequence retransmitted to Bob always generates errors due to the non-discriminability theorem or impossibility of duplication even if Eve tries to copy the sequence of non-orthogonal states in a quantum transmission channel by FiF2　Block diagram of QKD linkLightsourceAliceBobSynchronization signalsynchronizationsynchronizationControl blockPhoton detectorsdecoderencoderQuantum signalQuantum channelQuantum communication blockKey distillationKey distillation public transmission channel・basis collation・test bit・syndrome forerror collectionSecure keySecure keyKey distillation blockRandom number Generator ARandom number（basis）Random number（bit）Random number（basis）detection signalRandom number Generator B3 Quantum Key Distribution Network14　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)