Discording power of quantum evolutions

TL;DR

This paper introduces a measure called discording power to evaluate how effectively two-qubit quantum gates generate quantum discord, revealing that some gates like sqrt(SWAP) are perfect discorders across all purities, unlike others such as CNOT.

Contribution

The paper defines and analyzes the discording power of two-qubit unitaries using Cartan decomposition, identifying gates that maximally generate discord at various purity levels.

Findings

Certain gates like sqrt(SWAP) are perfect discorders for all purities.

CNOT is a perfect discorder only at low or full purity, not intermediate.

The measure generalizes quantum discord generation to all purity levels.

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 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{{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.