Adjusting phase-covariant qubit channel performance with non-unitality

Abstract

We analyze quantum communication properties of phase-covariant channels depending on their degree of non-unitality. In particular, we derive analytical formulas for minimal and maximal channel fidelity on pure states and maximal output purity. Next, we introduce a measure of non-unitality and show how to manipulate between unital and maximally non-unital maps by considering classical mixtures of quantum channels. Finally, we prove that maximal fidelity and maximal output purity increase with non-unitality and present several examples. Interestingly, non-unitality can also prolong quantum entanglement and lead to its rebirth.

Improving classical capacity of qubit dynamical maps through stationary state manipulation

“…To exclude the influence of other effects, assume that the phase-covariant qubit channel Λ has fixed eigenvalues and only the parameter λ * can be varied. According to [13], there is a simple connection between λ * and the non-unitality measure…”

“…Incorporating non-local noises through memory kernel addition has been analyzed e.g. in [6,9,10,13]. However, physical justification for such addition has not yet been studied on a microscopical level.…”

“…An improved performance measured by the channel fidelity, output purity, and the ability to preserve quantum entanglement was attained through adding noises to the Markovian evolution on the level of memory kernel master equations [11,12]. Recently, it has also been shown how to engineer non-unitality in order to enhance communication properties of qubit channels [13].…”