Quantum Discord Witness With Uncharacterized Devices

TL;DR

This paper introduces a new method to witness quantum discord in bipartite states using uncharacterized measurements, avoiding full state tomography and demonstrating robustness against device imperfections.

Contribution

It proposes a dimension witness-based approach for detecting quantum discord with uncharacterized devices, applicable to arbitrary-dimensional bipartite states.

Findings

Maximum witness occurs with orthogonal basis measurements for two-qubit states

Method is robust against device imperfections

Only 2x2 measurements needed for large-dimensional states

Abstract

Quantum discord represents a correlation beyond classicality that could be useful for many quantum information tasks, and therefore is viewed as a valuable quantum resource. Theoretically, whether a bipartite quantum state has a non-zero amount of quantum discord or not can be determined once its density matrix is given. Experimentally, however, it is not easy to perform a reliable tomography on the state, especially when considering the imperfection of devices and the high dimensionality of system. Here, inspired by the so-called dimension witness, we propose a new approach using uncharacterized measurements to witness quantum discord of an unknown bipartite state within arbitrary dimension system. Furthermore, $2 \times 2$ measurements are sufficient for any bipartite state of arbitrarily large dimension. For any two-qubit state, we show that the maximum of witness happens when one performs mutually orthogonal basis measurement on each qubit. Our method exhibits high robustness against device imperfections, such as error tolerance, indicating its experimental feasibility.