Ultimate entanglement robustness of two-qubit states against general local noises

TL;DR

This paper investigates how to prepare bipartite entangled states that retain entanglement the longest under local qubit noises, revealing that maximally entangled states are optimal for unital noises but not for nonunital noises, with practical examples.

Contribution

It introduces a decomposition technique linking nonunital and unital channels and explicitly finds the most robust states under general local noises.

Findings

Maximally entangled states are optimal under unital noise.

For nonunital noise, the most robust states are not maximally entangled.

Robust states can retain entanglement twice as long as conventional states.

Abstract

We study the problem of optimal preparation of a bipartite entangled state, which remains entangled the longest time under action of local qubit noises. We show that for unital noises such a state is always maximally entangled, whereas for nonunital noises, it is not. We develop a decomposition technique relating nonunital and unital qubit channels, based on which we find the explicit form of the ultimately robust state for general local noises. We illustrate our findings by amplitude damping processes at finite temperature, for which the ultimately robust state remains entangled up to two times longer than conventional maximally entangled states.