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

TL;DR

This paper introduces a symmetric 1 to 2 quantum cloning machine that achieves high-fidelity copies for specific qubit states, optimizing eavesdropping strategies in quantum key distribution protocols.

Contribution

A new symmetric quantum cloning machine is proposed that provides high-fidelity copies for states on a specific meridian of the Bloch sphere, including optimal cloning for those states.

Findings

Achieves fidelity between 0.90 and 0.95 for targeted states.

Optimal cloning transformation for the Eastern meridian states.

Enhanced eavesdropping success rate in B92 protocol.

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 $\ketm{0}, \ketm{1}$ together with the diagonal state $\ketm{+} = \frac{1}{\sqrt{2}} (\ketm{0} + \ketm{1})$, 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.