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arXiv:2504.04723 · Physical Review Letters

Quantum Incompatibility in Parallel versus Antiparallel Spins

Ram Krishna Patra, Kunika Agarwal, Biswajit Paul, Snehasish Roy Chowdhury, Sahil Gopalkrishna Naik, Manik Banik

Published Feb 23, 2026• 3 citations

Abstract

We explore the joint measurability of incompatible qubit observables on ensembles of parallel and antiparallel spin-1/2 pairs. In parallel configuration, both spins are prepared in the same state, whereas in antiparallel case, each spin is paired with its flipped counterpart. We show that the antiparallel configuration uniquely enables exact simultaneous prediction of three mutually orthogonal spin components-an advantage not achievable with parallel states. Extending beyond three observables, we examine joint measurability for larger sets of spin measurements and further generalize our analysis to state configurations beyond the parallel and antiparallel cases. As we show, our results reveal a deep connection to the "mean King retrodiction task" proposed by Vaidman, Aharonov, and Albert and have implications for a cryptographic protocol introduced by Jeffrey Bub. We further demonstrate how the enhanced compatibility in the antiparallel configuration can facilitate efficient estimation of unknown measurement devices. Finally, we discuss prospects for experimentally realizing the enhanced measurement compatibility in antiparallel configuration by analyzing the effect on finite subensembles of states.

DOI: 10.1103/tqrb-4m9p

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