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Fixed-Boost Wigner Noise: Strict Trace-Distance Contraction without Quantum Degradability
A Lorentz boost acts on the canonical spin of a massive particle through a momentum dependent Wigner rotation. We show that, for one fixed observer boost, reducing over an uncertain momentum can strictly contract every pairwise spin state trace distance without producing a channel that is degradable from the less contracted one. For spin $1/2$, we first characterize the exact inversion symmetric channel cone gener...