To maintain the detection statistics of coherent states with different intensities, Eve is not able to change the transmittances of single-photon and multi-photon state freely without being noticed by Alice and Bob. Instead of sending one coherent state, Alice sends pulses with different intensities, so that she can obtain more information to monitor the quantum channel. To improve the performance of the weak coherent state QKD, Hwang proposed the decoy-state method 12. As a result, the performance of QKD is very poor. In a conventional security analysis 11, Alice and Bob have to assume all the losses and errors come from the single-photon components in the worst scenario case. ![]() Since Eve could have a full control of the quantum channel, she can make the transmittance of multi-photon states to be 100% in the PNS attack. When multi-photon states are used for QKD, Eve can launch attacks, like the photon-number-splitting (PNS) attack 9, 10, to break the security. A weak coherent state source contains multi-photon components (details shown in Methods). In practical QKD systems, a highly attenuated laser or a weak coherent state source is widely used to substitute for a perfect single-photon source which is beyond state-of-the-art technology. The efficient BB84 scheme is experimentally demonstrated in 2009 8. In the efficient scheme, Alice and Bob put a bias in the probabilities of choosing the Z basis and X basis, which can make the basis sift-factor close to 100% in the infinitely long key limit. This factor can be improved by the efficient BB84 scheme proposed by Lo et al. That is, the basis-sift factor is 1/2 in the original BB84 protocol. The key can only be extracted from the pulses where they use the same basis and this results in that on average half of the raw data is discarded. After that, they compare the basis through an authenticated classical channel. ![]() Bob measures the received pulses in two bases randomly. In BB84, Alice encodes the key information randomly into the X and Z bases and sends quantum pulses to Bob. The best known protocol of QKD is the BB84 protocol 1 presented by Bennett and Brassard in 1984. The unconditional security has been proven even when an eavesdropper, Eve, has unlimited computation power permitted by quantum mechanics 3, 4, 5, 6. It aims at extending a secret key between two distant parties, commonly noted as Alice and Bob. Quantum key distribution (QKD) 1, 2 is one of the most realistic applications in quantum information.
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