Tight Characterization of Error Exponent for Classical-Quantum Channel Coding with Activated Non-Signaling Assistance

The optimal error exponent for classical-quantum channel coding assisted by activated non-signaling correlations is equal to the well-known sphere packing bound, which can be written as a single-letter formula optimized over Petz-Rényi divergences. This characterization remains tight for arbitrarily low rates below the channel capacity.