Given the ability to apply an unknown quantum channel acting on a $d$-dimensional system, we develop a quantum algorithm for transforming its singular values. The spectrum of a quantum channel as a superoperator is naturally tied to its Liouville representation, which is in general non-Hermitian. Our key contribution is an approximate block-encoding scheme for this representation in a Hermitized form, given only black-box access to the channel; this immediately allows us to apply polynomial transformations to the channel's singular values by quantum singular value transformation (QSVT). We then demonstrate an $O(d^3/\delta )$ upper bound and an $\mathrm{\Omega }(d/\delta )$ lower bound for the query complexity of constructing a quantum channel that is $\delta$-close in diamond norm to a block encoding of the unnormalized Hermitized Liouville representation. We show that our method applies practically to the problem of learning the $q$-th singular value moments of unknown quantum channels for arbitrary $q>2,q\in \mathbb{R}$, which has implications for testing whether a quantum channel is entanglement breaking. Our results establish a general framework for manipulating black-box quantum channels with QSVT and related methods, enabling direct evaluation of the channel's spectral properties without tomography or significant classical postprocessing.

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• Entanglement detection
• Quantum algorithms
• Quantum channels