Quantum discord in quantum computation

TL;DR

This paper explores quantum discord as a measure of quantum correlations, examining its properties and implications for quantum information processing, including the roles of entanglement and initial correlations in quantum computations and process tomography.

Contribution

It provides a detailed analysis of quantum discord's role in quantum computation and clarifies its limitations in quantum process tomography.

Findings

Changes in discord indicate the need for entanglement in quantum gates.

Zero system-environment discord is necessary for standard quantum evolution.

Absence of initial correlations may be required for consistent process tomography.

Abstract

Quantum discord is a measure of the quantumness of correlations. After reviewing its different versions and properties, we apply it to the questions of quantum information processing. First we show that changes in discord in the processed unentangled states indicate the need for entanglement in the distributed implementation of quantum gates. On the other hand, it was shown that zero system-environment discord is a necessary and sufficient condition for applicability of the standard completely positive description of the system's evolution. We demonstrate that this result does not translate into useful quantum process tomography. Depending on the details of the preparation procedure only absence of any initial correlations may guarantees consistency of the process tomography.