High-fidelity copies from a symmetric 1 to 2 quantum cloning machine

Abstract

A symmetric 1 to 2 quantum cloning machine (QCM) is presented that provides high-fidelity copies with $0.90\le F\le 0.95$ for all pure (single-qubit) input states from a given meridian of the Bloch sphere. \cor{Emphasize is placed especially on the states of the (so-called) Eastern meridian, that includes the computational basis states $\text{ketm}0,\text{ketm}1$ together with the diagonal state $\text{ketm}+=\frac{1}{\sqrt{2}}(\text{ketm}0+\text{ketm}1)$, for which suggested cloning transformation is shown to be optimal.} In addition, we also show how this QCM can be utilized for eavesdropping in Bennett's B92 protocol for quantum key distribution with a substantial higher success rate than obtained for universal or equatorial quantum copying.