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.Comment: 9 pages, 7 figures, Phys. Rev. A (in press
