Fixed-boost Wigner noise strictly shrinks every spin-state distance without one channel being degradable from another.

Summary. The manuscript studies spin channels induced by a fixed Lorentz boost acting through momentum-dependent Wigner rotations. For spin 1/2 it characterizes the inversion-symmetric channel cone generated by a fixed Wigner angle and transverse momenta, which contains the Pauli family M_α = diag(1−α,1−α,1−2α) for 0 ≤ α < 1/2. It claims that for 0 < α < β < 1/2 every pairwise spin-state trace distance strictly contracts under M_β relative to M_α, yet the channels are pairwise incomparable under CPTP post-processing: the exact diamond-norm deficiency is (1/2) inf_Λ∈CPTP ‖Φ_β − Λ∘Φ_α‖_♦ = α(β−α)/(2−3α), while the reverse deficiency equals β−α. The ideal channels are asserted to arise as the narrow-packet limit of pure normalizable five-component momentum states, with perturbation and finite-shot tomography bounds certifying an open set of physical examples; separately, non-identity members fail embedding in a time-homogeneous Pauli-diagonal Lindblad semigroup.

Significance. If the exact deficiency formulas and the continuum-limit realization both hold, the work cleanly separates two orderings that are often conflated: the ordering of unassisted spin distinguishability (trace distance) and the quantum statistical post-processing order (CPTP degradability). A parameter-free, closed-form diamond-norm calculation together with an explicit physical construction would be a useful contribution to relativistic quantum information and to the theory of channel comparison, showing that strict contraction of all pairwise distances need not imply degradability. The claimed open-set certification via perturbation/tomography bounds would further make the phenomenon experimentally relevant rather than purely ideal.