How to observe and quantify quantum-discord states via correlations

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.

Geometric Discord for Multi-qubit Systems

“…This measure, which has been designated as quantum discord, is able to capture not only quantum correlations in entangled states but also in separable states [1]. The concept of quantum discord was initially introduced by Ollivier and Zurek [2], which is attracting increasing interest [3][4][5][6][7][8][9][10][11][12].…”

“…FIG.1: Geometric discord of states(9),(11) and(20) under phase noise acting on the first qubit of the quantum system. (a)Geometric discord of two-qubit states (9) under phase noise acting on the first qubit of the quantum system.…”

Engineering two-qubit mixed states with weak measurements

“…. Quantum correlations are commonly quantified via concurrence (a measure of entanglement) [21,39] and quantum discord [19][20][21][22]. Calculating discord Q and concurrence C for an arbitrary state is a challenging task, but for the state in Eq.…”

“…Open quantum systems are usually described by mixed states. The quantumness of the latter is expressed through their discord [18][19][20][21]. Discorded quantum states have been proposed as resources for achieving quantum speedup [22,23], for remote state preparation [24], and for quantum purification protocols [25,26].…”

“…For example, using Peres-Horodecki criterion [36,37], one shows that ρ is separable (not entangled) if and only if α ≥ 3β−1 3β+1 . Quantum correlations are commonly quantified via concurrence (a measure of entanglement) [20,38] and quantum discord [18][19][20][21]. Calculating discord Q and concurrence C for an arbitrary state is a challenging task, but for the state in Eq.…”