Quantum JSD Contraction for Depolarizing Channels

We determine the exact contraction coefficient (SDPI constant) of the quantum Jensen–Shannon divergence (QJSD) for the d-dimensional depolarizing channel Δ_p(ρ) = (1−p)ρ + (p/d)I. For all d ≥ 2 and p ∈ [0,1], we obtain the closed form η_QJSD(Δ_p) = (1−p)^2 / s, where s = 1 − (d−2)p/d, via a fully analytic argument based on the Bogoliubov–Kubo–Mori (BKM) metric, with no numerical verification.