Separability in Asymmetric Phase-Covariant Cloning

Abstract

Here, asymmetric phase-covariant quantum cloning machines are defined and trade-off between qualities of their outputs and its impact on entanglement properties of the outputs are studies. In addition, optimal families among these cloners are introduced and also their entanglement properties are investigated. An explicit proof of optimality is presented for the case of qubits, which is based on the no-signaling condition. Our optimality proof can also be used to derive an upper bound on trade-off relations for a more general class of optimal cloners which clone states on a specific orbit of the Bloch sphere. It is shown that the optimal cloners of the equatorial states, as in the case of symmetric phase-covariant cloning, give rise to two separable clones, and in this sense these states are unique. For these cloners it is shown that total output is of GHZ-type.