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arXiv · 2510.01888

Counterfactual quantum measurements

Ingita Banerjee·Kiarn T. Laverick·Howard M. Wiseman
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Abstract

Counterfactual reasoning plays a crucial role in exploring hypothetical scenarios, by comparing some consequent under conditions identical except as results from a differing antecedent. David Lewis' well-known analysis evaluates counterfactuals using a hierarchy of desiderata. These were, however, built upon a deterministic classical framework, and whether it could be generalized to indeterministic quantum theory has been an open question. In this paper, we propose a formalism for quantum counterfactuals in which antecedents are measurement settings. Unlike other approaches, it non-trivially answers questions like: "Given that a photon-detector, observing an atom's fluorescence, clicked at a certain time, what would a field-quadrature detector have measured, if it had been used instead?"

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Ingita Banerjee, Kiarn T. Laverick, Howard M. Wiseman. 2025-10-02. Counterfactual quantum measurements. https://doi.org/10.1103/9836-q2q3

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