Discording power of quantum evolutions

Galve, F.; Plastina, F.; Paris, M. G. A.; Zambrini, R.
Physical Review Letters 110, 010501 (1-5) (2013)

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 found that there exist gates
which are perfect discorders for any value of purity, and that they belong to a class of operators that includes
the $sqrt{mbox{SWAP}}$. Other gates, even those universal for quantum computation, do not posses 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.

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