Entropic regularised optimal transport in a noncommutative setting

Abstract: This survey has been written in occasion of the School and Workshop about Optimal Transport on Quantum Structures at Erd\"os Center in September 2022. We discuss some recent results on noncommutative entropic optimal transport problems and their relation to the study of the ground-state energy of a finite-dimensional composite quantum system at positive temperature, following the work [FGP23]. In the first part, we review some of the classical primal-dual formulations of optimal transport in the commutative setting, including extensions to multimarginal problems and entropic regularisation. We discuss the main features of the entropic problem and show how optimisers can be efficiently computed via the so-called Sinkhorn algorithm. In the second part, we discuss how to apply these ideas to a noncommutative setting, in particular on the space of density matrices over finite dimensional Hilbert spaces. In this framework, we present equivalences between primal and dual formulations, and use them to characterise the optimisers. Despite the lack of explicit formulas due to the noncommutative nature of the problem, one can also show that a suitable quantum version of the Sinkhorn algorithm converges to the minimiser of the entropic problem. In the final part of this work, we discuss similar results for bosonic and fermionic systems.