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Nonclassicality of a macroscopic qubit-ensemble via parity measurement induced disturbance

Lorenzo Braccini1,*, Debarshi Das2,†, Ben Zindorf1, Stephen D. Hogan1, John J. L. Morton3,4, and Sougato Bose1

  • *Contact author: lorenzo.braccini.18@ucl.ac.uk
  • †Contact author: dasdebarshi90@gmail.com
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Phys. Rev. Research 8, 043092 – Published 26 October, 2026

DOI: https://doi.org/10.1103/kwpf-hm77

Abstract

We propose an experimental scheme to test the nonclassicality of a macroscopic ensemble of qubits through the violation of the classical notion of macrorealism (MR) via the fundamental measurement-induced disturbance of quantum systems. An electromagnetic resonator is used to probe the parity of the qubit ensemble. The action of sequential measurements allows the nonclassicality of the whole ensemble to manifest itself, in the ideal case, irrespective of its size. This enables to probe the macroscopic limits of quantum mechanics, as the qubit ensemble is, effectively, a single large spin of many ℏ units. Even as ℏ→0 in comparison to the total angular momentum of the ensemble, a constant amount of violation of MR is found in the noiseless case. However, environmental decoherence and inhomogeneity of qubit-electromagnetic field couplings precipitate the quantum-to-classical transition. This illustrates that Bohr's correspondence principle is not fundamental, but a consequence of practical limitations. We outline an implementation with a variety of qubits (superconducting qubits, spins in semiconductors, and Rydberg atoms) coupled to a coplanar waveguide resonator, and—via the corresponding noise analysis—find that violation of MR is detectable up to 100 qubits via current technology.

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Phys. Rev. Research 8, 043092– Published 26 October, 2026
  • Received 6 March 2026
  • Accepted 17 September 2026
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