Evolution of quantum discord and its stability in two-qubit NMR systems

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.

Contributions of different parts of spin–spin interactions to quantum correlations in a spin ring model in an external magnetic field

“…The quantum discord does not vanish [21] in mixed states in NMR experiments in contrast to the entanglement and indicates that quantum correlations exist in many-qubit systems. Experimental studies [21] using NMR-tomography methods [4] support this conclusion. The development of methods for computing the quantum discord in many-qubit systems is therefore very relevant.…”

Quantum Correlations in NMR Systems

“…For various theoretical and experimental aspects of quantum discord and related measures reader can refer to review [67]. Investigations on the evolution of quantum discord under decoherence [68] and under decoherence-suppression sequences [66] have also been reported. In the following we briefly describe some aspects related to discord and geometric discord.…”

“…al. [66] have studied discord and its evolution in certain NMR systems. After preparing the pseudopure state ρ 0 = (1 − )1/2 + |00 00| they applied the pulse sequence shown in Fig.…”

Geometric quantum discord and non-Markovianity of structured reservoirs

“…In the following, by fixing s = 3 (i.e., we consider the superOhmic reservoir) and ω c = 2ω 0 , we solved Eqs. (12) and (15) numerically, and displayed the corresponding results with η ∈ [0.02, 1] in Fig. 3.…”

“…Particularly, there has been an increasing interest of people on quantifying quantum discord from different perspectives, and to date there are a number of discord-like correlation measures being proposed [2,[4][5][6][7][8][9]. On the other hand, the behaviors of quantum discord in the spin chain [10], the two-level atoms [11], and the NMR system [12] have also been studied extensively.…”