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  • Open Access

Quantum memory precludes mixed-unitary dynamics

Charlotte Bäcker1,*, Konstantin Beyer2,†, and Walter T. Strunz1,‡

  • *Contact author: charlotte.baecker@tu-dresden.de
  • †Contact author: kbeyer1@stevens.edu
  • ‡Contact author: walter.strunz@tu-dresden.de
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Phys. Rev. Research 8, 033288 – Published 8 September, 2026

DOI: https://doi.org/10.1103/dn6t-y9ky

Abstract

Unital quantum channels, defined by their property of leaving the maximally mixed state invariant, form an important class of quantum operations. A distinguished subset of these channels can be represented as a probabilistic mixture of unitary evolutions. Characterizing channels that do not admit such a decomposition is in general a hard problem with significant implications for noise mitigation in quantum technologies and for fundamental problems in quantum information theory. Here, we establish a link between mixed unitarity of unital channels and the (quantum) nature of the memory effects in non-Markovian dynamics. Translating the problem into the language of process tensors, this connection yields a hierarchy of semidefinite programs that provides numerically efficient witnesses for non-mixed-unitary behavior, outperforming existing criteria. We demonstrate the power of this approach through illustrative examples of unital channels in dimensions 3 and 4, and show how the method can detect non-mixed-unitary behavior based on incomplete channel tomography.

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Outline

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Phys. Rev. Research 8, 033288– Published 8 September, 2026
  • Received 9 April 2026
  • Accepted 11 August 2026
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