Teleportation through time-varying channels: threshold geometry and a complete-positivity bound on non-Markovian backflow
A Bell pair distributed through a link that combines amplitude damping with dephasing at time-dependent rates has dynamics that separate into a fixed part and a moving one. The negativity, fully entangled fraction, discord, and average teleportation fidelity depend on time only through the accumulated damping parameters $p(t)$ and $q(t)$. Every threshold is therefore a curve fixed in the unit square, and the rates select nothing but a trajectory across it. The Horodecki fidelity formula $\bar{F} = \frac{1}{2} + \frac{1}{6}\mathrm{Tr}|T|$ covers one- and two-sided exposure alike: the condition $\det T \leq 0$ under which it takes this form holds throughout the unit square for both. Under symmetric two-sided noise the entanglement vanishes when $p+q\geq1$, where the exact relation $\bar{F}^{2s}=\frac{2}{3}+\frac{1}{3}\mathcal{N}_{2s}$ ties disentanglement and the loss of quantum advantage to the same instant. One-sided exposure admits no finite-time sudden death for any rate profile, and the discord stays strictly positive throughout the open square, so the distributed state can be separable, useless for teleportation, and still nonclassical. For harmonically modulated rates, complete positivity caps the backflow at one modulation period of static decay, $\Delta\Gamma_{k}\leq2\pi\gamma_{k,0}/\Omega$, equivalently at a modulation depth $\xi_{k}\leq4.6033$ independent of $\Omega$. Inside that window the trajectory reverses, producing finite intervals of restored quantum advantage and entanglement sudden birth.