Quantumness of quantum channels

Quantum coherence is a fundamental aspect of quantum physics and plays a central role in quantum information science. This essential property of the quantum states could be fragile under the influence of the quantum operations. The extent to which quantum coherence is diminished depends both on the channel and the incoherent basis. Motivated by this, we propose a measure of nonclassicality of a quantum channel as the average quantum coherence of the state space after the channel acts on, minimized over all orthonormal basis sets of the state space. Utilizing the squared $l_1$-norm of coherence for the qubit channels, the minimization can be treated analytically and the proposed measure takes a closed form of expression. If we allow the channels to act locally on a maximally entangled state, the quantum correlation is diminished making the states more classical. We show that the extent to which quantum correlation is preserved under local action of the channel cannot exceed the quantumness of the underlying channel. We further apply our measure to the quantum teleportation protocol and show that a nonzero quantumness for the underlying channel provides a necessary condition to overcome the best classical protocols.

Figure 1

The quantumness and geometric discord of the amplitude damping channel as a function of the channel's parameter $\gamma$. For quantumness and geometric discord, the optimized $\widehat{\mathit{n}}$ shows the sudden change from the $xy$ plane to the $z$ axis on $\gamma =1/6$ and $\gamma =1/2$, respectively.

Figure 2

The channel's quantumness and fidelity of the teleportation, as a function of the parameter $w$, when the Werner state is shared between Alice and Bob. The horizontal line shows the best classical fidelity $2/3$.