title: Discording power of quantum evolutions.
abstract: We introduce the discording power of a unitary transformation, which assesses its capability to produce quantum discord, and analyze in detail the generation of discord by relevant classes of two-qubit gates. Our measure is based on the Cartan decomposition of two-qubit unitaries and on evaluating the maximum discord achievable by a unitary upon acting on classical-classical states at fixed purity. We find that there exist gates which are perfect discorders for any value of purity μ, and that they belong to a class of operators that includes the sqrt[SWAP]. Other gates, even those universal for quantum computation, do not possess the same property: the CNOT, for example, is a perfect discorder only for states with low or unit purity, but not for intermediate values. The discording power of a two-qubit unitary also provides a generalization of the corresponding measure defined for entanglement to any value of the purity.
authors: IFISC, UIB-CSIC, Instituto de Física Interdisciplinar y Sistemas Complejos, UIB Campus, E-07122 Palma de Mallorca, Spain.
authors: Fernando
authors: F
authors: Galve
authors: Francesco
authors: F
authors: Plastina
authors: Matteo G A
authors: MG
authors: Paris
authors: Roberta
authors: R
authors: Zambrini