# Evolution of tripartite entangled states in a decohering environment and their experimental protection using dynamical decoupling

## TL;DR

This study experimentally investigates the robustness of different tripartite entangled states in a noisy environment and demonstrates how dynamical decoupling can protect entanglement, especially for more fragile states like GHZ.

## Contribution

It provides the first experimental comparison of the robustness of GHZ, W, and ${\rm W \bar{W}}$ states in a natural decoherence environment and shows effective dynamical decoupling protection.

## Key findings

- W state is most robust against decoherence.
- GHZ states are most fragile, but can be protected with dynamical decoupling.
- ${\rm W \bar{W}}$ state shows unexpected robustness and improved fidelity.

## Abstract

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.