Witnessing nonclassical correlations via a single-shot experiment on an ensemble of spins using NMR

TL;DR

This paper demonstrates a single-shot experimental method using NMR to detect nonclassical correlations in a bipartite quantum system, simplifying the process compared to traditional quantum discord measurements.

Contribution

The authors develop a positive map as a witness operator that can detect nonclassicality in a single experimental run on an ensemble of spins.

Findings

The witness map successfully detects nonclassicality in the system.

Quantum discord measurements align well with the witness detection.

Nonclassical correlations evolve over time and are observable with the proposed method.

Abstract

A bipartite quantum system in a mixed state can exhibit nonclassical correlations, which can go beyond quantum entanglement. While quantum discord is the standard measure of quantifying such general quantum correlations, the nonclassicality can be determined by simpler means via the measurement of witness operators. We experimentally construct a positive map to witness nonclassicality of two qubits in an NMR system. The map can be decomposed so that a single run of an experiment on an ensemble of spins suffices to detect the nonclassicality in the state, if present. We let the state evolve in time and use the map to detect nonclassicality as a function of time. To evaluate the efficacy of the witness operator as a means to detect nonclassicality, we measure quantum discord by performing full quantum state tomography at each time point and obtained a fairly good match between the two methods.