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Search for classical subsystems in quantum worlds

Arsalan Adil1,2, Manuel S. Rudolph3, Andrew Arrasmith4, Zoë Holmes3, Andreas Albrecht1, and Andrew Sornborger5

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Phys. Rev. D 113, 103535 – Published 26 May, 2026

DOI: https://doi.org/10.1103/vj4x-96fk

Abstract

Decoherence and einselection explain many features of the emergence classical world from an underlying quantum theory. However, the theory assumes a particular factorization of the global Hilbert space into constituent system and environment subsystems. In this work, given a fixed Hamiltonian we show that there can exist several factorizations (or tensor product structures) of a global Hilbert space that admit a quasiclassical description of subsystems in the sense that certain states (the “pointer states”) are robust to entanglement. We derive several analytical forms that the Hamiltonian may take in such factorizations, each with its unique set of features. Our approach enables us to derive the division into quasiclassical subsystems and demonstrates that decohering subsystems do not necessarily align with our classical notion of locality. These results have interesting ramifications for the interpretation of quantum mechanics and, more practically, may be useful in algorithmically characterizing decoherence free subspaces.

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Phys. Rev. D 113, 103535– Published 26 May, 2026
  • Received 30 May 2024
  • Accepted 15 April 2026
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