A Note on Four-State BB84 Protocol.

Pedro Juan Roig, Salvador Alcaraz, Katja Gilly +2 more

Han Bao-bin

Four polarization states are used for key distribution in the BB84 protocol,but the polarization states will be rotated for magneto-optic effect during the transmission.Po- larization variation will effect distribution of the key,so a means for polarization restoration is required.After the introduction of BB84 protocol and the four states the effect of polar- ization rotation on quantum key distribution is analyzed.The simulation by Matlab shows the rotation of polarization state will effect key distribution.At last,a feasible method is presented.

Kern, Oliver, Renes, Joseph M.

Ying Sun, Qiaoyan Wen, Fei Gao +1 more

The security of the decoherence-free version of the Bennett-Brassard 1984 (BB84) protocol [A. Cabello, Phys. Rev. A 75, 020301 (2007)] is analyzed and shown to be vulnerable under the intercept-resend attack. We propose two improved versions of this protocol. Both improvements remain the performance of robustness against collective noise and refuse the security flaw. Especially, the second improvement, which is called four-qubit decoherence-free (DF) BB84 protocol, not only remains all characteristics of the original protocol but also has a higher efficiency. We also give a detailed security proof of four-qubit DF BB84 protocol.

Arpita Maitra, Goutam Paul

Abstract. The BB84 protocol is used by Alice (the sender) and Bob (the receiver) to settle on a secret classical bit-string by communicating qubits over an insecure quantum channel where Eve (the Eavesdropper) can have access. In this paper, we revisit a well known eavesdropping technique against BB84. We claim that there exist certain gaps in understanding the existing eavesdropping strategyin terms of cryptanalytic view and we try to bridge those gaps in this paper. First we refer to the result where it is shown that in the six-state variant of the BB84 protocol (Bruß, Phys. Rev. Lett., 1998), the mutual information between Alice (the sender) and Eve (the eavesdropper) is higher when two-bit probe is used compared to the one-bit probe and hencethetwo-bitprobeprovidesastrongereavesdroppingstrategy.However, from cryptanalytic point of view, we show that Eve has the same success probability in guessing the bit transmitted by Alice in both the cases of the two-bit and the one-bit probe. Thus, we point out that having higher mutual information may not directly lead to obtaining higher probability in guessing the key bit. It is also explained in the work of Bruß that the six-state variant of the BB84 protocol is more secure than the traditional four-state BB84. We look into this point in more detail and identify that this advantage is only achieved at the expense of communicating more qubits in the sixstate protocol. In fact, we present different scenarios, where given the same number of qubits communicated, the security comparison of the four and six-state protocols is evaluated carefully.

Ohta Fumio, Hisayoshi SATO, Takaragi Kazuo