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  • Open Access

Multistate imaginarity and coherence in qubit systems

Mao-Sheng Li1, Rafael Wagner2,3,4,*, and Lin Zhang5

  • *Contact author: rafael.wagner@uni-ulm.de
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Phys. Rev. A 113, 012428 – Published 20 January, 2026

DOI: https://doi.org/10.1103/tpgw-v6ht

Abstract

Traditionally, the characterization of quantum resources has focused on individual quantum states. Recent literature, however, has increasingly explored the characterization of resources in multistates—ordered collections of states indexed by a varying parameter. In this work, we provide a unitary-invariant framework to pinpoint imaginarity and coherence in sets of qubit states: We prove that Bloch vectors must be coplanar to be imaginarity free and collinear to be incoherent, yielding exact rank-based tests of coherence and imaginarity and closed-form bounds for existing robustness quantifiers, all based on two-state overlaps only. We also show that the set of imaginarity-free multistates is not convex, and that third-order invariants completely characterize multistate imaginarity of single-qubits but not of higher-dimensional systems. As our main technical result, we show that every Bargmann invariant of single-qubit states is determined (up to conjugation) by two-state overlaps. Beyond qubits, we give purity and system-agnostic coherence witnesses from equality constraints on higher-order invariants and connect our results to practical protocols—characterization of partial distinguishability, spin-chirality detection, and subchannel discrimination.

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Phys. Rev. A 113, 012428– Published 20 January, 2026
  • Received 13 August 2025
  • Accepted 5 January 2026
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