Detecting the quantum discord of an unknown state by a single observable

Abstract

We propose a single observable to witness the nonzero quantum discord of an unknown quantum state provided that we have four copies of the state. The expectation value of this observable provides a necessary and sufficient condition for nonzero quantum discord in $2\times N$ systems and a necessary condition in higher finite-dimensional bipartite systems. Furthermore, a nontrivial lower bound of quantum discord can be obtained from this expectation value. The proposed observable can be experimentally measured in exactly the same easy manner as that of the entanglement witness, moreover a quantum circuit is designed to determine the expectation value of our observable with four simultaneous local qubit-measurements.

8 pages, 2 figures

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• On the quantumness of correlations in nuclear magnetic resonance
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• Witnessing non-objectivity in the framework of strong quantum Darwinism
• Measuring nonclassical correlations of two-photon states
• Experimental Verification of Quantum Discord in Continuous-Variable States
• How to observe and quantify quantum-discorded states
• How to observe and quantify quantum-discorded states via correlations
• Theoretical insights on measuring quantum correlations
• Geometric measure of quantum discord and the geometry of a class of two-qubit states
• Quantum Discord Witness With Uncharacterized Devices
• The local detection method: Dynamical detection of quantum discord with local operations