Classicality witness for two-qubit states

Abstract

In the last few years one realized that if the state of a bipartite system can be written as ${\sum }_{i,j}{p}_{ij}|a_i><a_i|\otimes |b_j><b_j|$, where $\{|a_i>\}$ and $\{|b_j>\}$ form orthonormal basis for the subsystems and $\{p_{ij}\}$ is a probability distribution, then it possesses at most classical correlations. In this article we introduce a nonlinear witness providing a sufficient condition for classicality of correlations (absence of quantum discord) in a broad class of two-qubit systems. Such witness turns out to be necessary and sufficient condition in the case of Bell-diagonal states. We show that the witness introduced here can be readily experimentally implemented in nuclear magnetic resonance setups.

To appear in IJQI

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• Quantum discord and its allies: a review
• Measuring bipartite quantum correlations of an unknown state
• Verification of quantum discord
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• On the quantumness of correlations in nuclear magnetic resonance
• Quantum discord in nuclear magnetic resonance systems at room temperature
• 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
• Quantum discord under system-environment coupling: the two-qubit case
• Beating detection loophole in nonlinear entanglement witnesses
• Witnessing nonclassical correlations via a single-shot experiment on an ensemble of spins using NMR
• von Neumann measurement-related matrices and the nullity condition for quantum correlation
• Computing coherence vectors and correlation matrices, with application to quantum discord quantification
• Quantum Discord Witness With Uncharacterized Devices
• Detecting Two Qubit Both-Way Positive Discord States
• Measurement-device-independent nonlinear entanglement witnesses
• Any Two Qubit State Has Non Zero Quantum Discord Under Global Unitary Operations