Quantum / CONCEPT

The Choi spectrum of unital product-unitary-equivariant maps

For all positive n₁ and n₂, the Choi matrix of the unital map that acts on the four isotypic components of n₁n₂ × n₁n₂ matrices with weights 1, λ₀₁, λ₁₀ and λ₁₁ has its eigenvalues among four values given by explicit formulas, taken with multiplicities 1, n₁² − 1, n₂² − 1 and (n₁² − 1)(n₂² − 1); coinciding values add their multiplicities.