Quantum cryptography, eavesdropping, and unsharp spin measurement

Abstract

The various eavesdropping strategies on a quantum cryptographic channel as discussed by Gisin et.al. can be reproduced by unsharp spin (represented by positive operator valued measure) measurement of spin-1/2 particle. An upper bound for unsharp parameter is obtained so that it still gives rise to violation of Bell’s inequality for the two particle transmission channel.