Quantum Algorithms for Computing Maximal Quantum $f$-divergence and Kubo-Ando means
Trung Hoa Dinh, Nhat A. Nghiem
TL;DR
This work develops two quantum algorithms for computing fundamental quantum information measures by embedding matrices and operations into quantum circuits via block-encoding and Quantum Singular Value Transformation (QSVT). It first targets maximal quantum $f$-divergences $D_f^{\max}(\rho\|\sigma)$ for operator-convex functions $f$, deriving procedures that block-encode density operators, form normalized composites like $\gamma$, and approximate $f(\gamma)$ (notably $\gamma\log\gamma$ for $f(x)=x\log x$) using direct QSVT or Stieltjes/Pade-type representations; trace estimation then yields the divergence with explicit complexity bounds tied to condition numbers. The second major thrust computes Kubo--Ando means $\sigma_f$ of PD operators, using harmonic–mean mixtures and Löwner–Stieltjes representations to realize $A\sigma_f B$ as block-encodings of sums or resolvents, respectively, with quadrature approximations over representation measures. By unifying entropic quantities and matrix means under a single block-encoding framework, the paper provides universal, programmable quantum subroutines that can adapt to the specific $f$ or mean of interest and offers explicit resource scaling in terms of block-encoding costs, conditioning, and desired accuracy. The approach promises practical impact for quantum information geometry, state analysis, and related optimization tasks, and opens pathways for applying quantum speedups to a broad class of operator-valued functionals.
Abstract
The development of quantum computation has resulted in many quantum algorithms for a wide array of tasks. Recently, there is a growing interest in using quantum computing techniques to estimate or compute quantum information-theoretic quantities such as Renyi entropy, Von Neumann entropy, matrix means, etc. Motivated by these results, we present quantum algorithms for computing the maximal quantum $f$-divergences and the operator-theoretic matrix Kubo--Ando means. Both of them involve Renyi entropies, matrix means as special cases, thus implying the universality of our framework.
