Evolution of tripartite entangled states in a decohering environment and their experimental protection using dynamical decoupling. (arXiv:1705.03432v2 [quant-ph] UPDATED)

We embarked upon the task of experimental protection of different classes of
tripartite entangled states, namely the maximally entangled GHZ and W states
and the ${\rm W \bar{W}}$ state, using dynamical decoupling. The states were
created on a three-qubit NMR quantum information processor and allowed to
evolve in the naturally noisy NMR environment. Tripartite entanglement was
monitored at each time instant during state evolution, using negativity as an
entanglement measure. It was found that the W state is most robust while the
GHZ-type states are most fragile against the natural decoherence present in the
NMR system. The ${\rm W \bar{W}}$ state which is in the GHZ-class, yet stores
entanglement in a manner akin to the W state, surprisingly turned out to be
more robust than the GHZ state. The experimental data were best modeled by
considering the main noise channel to be an uncorrelated phase damping channel
acting independently on each qubit, alongwith a generalized amplitude damping
channel. Using dynamical decoupling, we were able to achieve a significant
protection of entanglement for GHZ states. There was a marginal improvement in
the state fidelity for the W state (which is already robust against natural
system decoherence), while the ${\rm W \bar{W}}$ state showed a significant
improvement in fidelity and protection against decoherence.

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