Complete contraction and expansion conjecture for amplitude damping channels

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Let Aγ\mathcal A_{\gamma} denote the amplitude damping channel with damping parameter γ\gamma, and for two channels define the complete contraction and expansion coefficients by

ηN,Mcb=sup⁡ρVAI(V;B1)I(V;B2),ηˇN,Mcb=inf⁡ρVAI(V;B1)I(V;B2),\eta_{\mathcal N,\mathcal M}^{cb}=\sup_{\rho_{VA}}\frac{I(V;B_1)}{I(V;B_2)},\qquad \widecheck{\eta}_{\mathcal N,\mathcal M}^{cb}=\inf_{\rho_{VA}}\frac{I(V;B_1)}{I(V;B_2)},

where the optimization ranges over all valid states ρVA\rho_{VA} on HV⊗HA\mathcal H_V\otimes\mathcal H_A. Complete contraction and expansion conjecture. If 0<γ2<γ1<10<\gamma_2<\gamma_1<1, then

ηAγ1,Aγ2cb<1,ηˇAγ1,Aγ2cb>0.\eta_{\mathcal A_{\gamma_1},\mathcal A_{\gamma_2}}^{cb}<1,\qquad \widecheck{\eta}_{\mathcal A_{\gamma_1},\mathcal A_{\gamma_2}}^{cb}>0.

This is proposed as a sufficient condition for positivity of the ratio R3R_3 for the pair of amplitude damping channels; its resolution is not established in the supplied text.

References

Primary source

Graeme Smith and Peixue Wu, “Additivity of quantum capacities in simple non-degradable quantum channels”, arXiv:2409.03927 (2025).

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