engleski

In Quantum Direct Communication an Undetectable Eavesdropper Can Always Tell Ψ from Φ Bell States in the Message Mode

We show that in any quantum direct communication protocol that is based on Psi and Phi Bell states, an eavesdropper can always tell Psi from Phi states without altering the transmission in any way in the message mode. This renders all protocols that make use of only one Psi state and one Phi state completely insecure in the message mode. All four-Bell-state protocols require a revision and this might be of importance for new implementations of entanglement-based cryptographic protocols. The detection rate of an eavesdropper is 25% per control transmission, i.e., a half of the rate in the two-state (ping-pong) protocol. An eavesdropper can detect control probes with certainty in the standard control transmission without a photon in the Alice-to-Bob's travel mode and with near-certainty in a transmission with a fake photon in the travel mode. Resending of measured control photons via the travel mode would make an eavesdropper completely invisible.

quantum direct communication

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