# How to observe and quantify quantum-discorded states via correlations

## TL;DR

This paper introduces a simple protocol for detecting and quantifying quantum discord in unentangled bipartite systems, enabling experimental studies of quantum correlations beyond entanglement.

## Contribution

The authors propose a novel, experimentally feasible protocol to measure quantum discord directly from correlation measurements without full state tomography.

## Key findings

- Protocol allows detection of quantum discord via repeated correlation measurements.
- Applicable to electronic and quantum-optical systems.
- Enables experimental exploration of quantum correlations beyond entanglement.

## Abstract

Quantum correlations between parts of a composite system most clearly reveal themselves through entanglement. Designing, maintaining, and controlling entangled systems is very demanding, which raises the stakes for understanding the efficacy of entanglement-free, yet quantum, correlations, exemplified by quantum discord. Discord is defined via conditional mutual entropies of parts of a composite system, and its direct measurement is hardly possible even via full tomographic characterization of the system state. Here we design a simple protocol to detect and quantify quantum discord in an unentangled bipartite system. Our protocol relies on a characteristic of discord that can be extracted from repeated direct measurements of certain correlations between subsystems of the bipartite system. The proposed protocol opens a way of extending experimental studies of discord to electronic systems but can also be implemented in quantum-optical systems.