Secure Key Distribution via Pre- and Post-Selected Quantum Ensembles

A quantum key distribution scheme whose security depends on the features of preand post-selected quantum ensembles is described. PACS numbers: 03.65.Bz, 42.80.Sa, 89.70.+c A wide variety of quantum key distribution schemes have been proposed, following the original Bennett and Brassard protocol [1]. Ekert [2] has described a scheme in which two parties, Alice and Bob, create a shared random key by performing spin measurements on pairs of spin2 particles in the singlet state. The particle pairs are emitted by a source towards Alice and Bob, who each measure spin along three different directions, chosen randomly and independently for each pair. After a sequence of measurements on an appropriate number of pairs, Alice and Bob announce the directions of their measurements publicly and divide the measurements into two groups: those in which they measured the spin in different directions, and those in which they measured the spin in the same direction. They publicly reveal the outcomes of the first group of measurements and use these to check that the singlet states have not been disturbed by an eavesdropper, Eve. Essentially, they calculate a correlation coefficient: any attempt by Eve to monitor the particles will disturb the singlet state and result in a correlation coefficient that is bounded by Bell’s inequality and is hence distinguishable from the ∗Email address: [email protected]