Evolution of Quantum Discord and its Stability in Two-Qubit NMR Systems

TL;DR

This study explores how quantum discord evolves and remains stable in two-qubit NMR systems, using experimental and theoretical methods to measure and analyze quantum correlations under various conditions.

Contribution

It introduces methods for measuring quantum discord in two-qubit systems and experimentally investigates their evolution and stability in NMR setups.

Findings

Quantum discord can be cyclically increased and decreased in Werner-like states.

Dynamical decoupling preserves parts of the quantum state but not the discord.

A relaxation model effectively describes discord evolution in spin-lock conditions.

Abstract

We investigate evolution of quantum correlations in ensembles of two-qubit nuclear spin systems via nuclear magnetic resonance techniques. We use discord as a measure of quantum correlations and the Werner state as an explicit example. We first introduce different ways of measuring discord and geometric discord in two-qubit systems and then describe the following experimental studies: (a) We quantitatively measure discord for Werner-like states prepared using an entangling pulse sequence. An initial thermal state with zero discord is gradually and periodically transformed into a mixed state with maximum discord. The experimental and simulated behavior of rise and fall of discord agree fairly well. (b) We examine the efficiency of dynamical decoupling sequences in preserving quantum correlations. In our experimental setup, the dynamical decoupling sequences preserved the traceless parts of the density matrices at high fidelity. But they could not maintain the purity of the quantum states and so were unable to keep the discord from decaying. (c) We observe the evolution of discord for a singlet-triplet mixed state during a radio-frequency spin-lock. A simple relaxation model describes the evolution of discord, and the accompanying evolution of fidelity of the long-lived singlet state, reasonably well.