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arXiv:1305.2519 · Journal of Physics A Mathematical and Theoretical

Three-box paradox and ‘Cheshire cat grin’: the case of spin-1 atoms

A. Matzkin, A K Pan

Published Jul 19, 2013• 27 citations
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Abstract

We propose in this work a definite theoretical implementation of the three-box paradox - a scheme in which a single quantum particle appears to be present with certainty in two separate boxes - with spin-1 atoms. We further show how our setup can give rise to a "Cheshire cat grin" type of situation, in which an atom can apparently be found with certainty in one of the boxes while one of its properties (the angular momentum projection along a specifically chosen axis) appears to be in a different box. The significance of our findings are discussed relative to the status of the properties of a system obtained from weak measurements.

DOI
10.1088/1751-8113/46/31/315307

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We propose in this work a definite theoretical implementation of the three-box paradox - a scheme in which a single quantum particle appears to be present with certainty in two separate boxes - with spin-1 atoms. We further show how our setup can give rise to a "Cheshire cat grin" type of situation, in which an atom can apparently be found with certainty in one of the boxes while one of its properties (the angular momentum projection along a specifically chosen axis) appears to be in a different box. The significance of our findings are discussed relative to the status of the properties of a system obtained from weak measurements.
Based on the concept of weak measurements, in 2013 Aharonov et al. proposed an experiment in which, according to their interpretation of the predicted results, a photon and its circular polarization were spatially separated, naming this phenomenon quantum Cheshire Cat [1]. This name is after a character in Lewis Carrolls novel Alice In Wonderland, the Cheshire Cat a cat whose grin can exist without the animal itself. Denkmayr et al., in 2014, claimed they realized experimentally the quantum Cheshire Cat, separating a neutron from its intrinsic magnetic moment [3]. We show how both results follow from interference effects, and thus the conclusion that a particle is separated from one of its intrinsic properties is not a necessary one. Making use of mostly undergraduate level quantum mechanics, we rule out the apparent paradox of the quantum Cheshire Cat.
In a recent work, Aharonov et al suggested that a photon could be separated from its polarization in an experiment involving pre- and post-selection (Aharonov et al 2013 New J. Phys. 15 113015 ). They named the effect 'quantum Cheshire Cat', in a reference to the cat that is separated from its grin in the novel Alice ' s Adventures in Wonderland . Following these ideas, Denkmayr et al performed a neutron interferometric experiment and interpreted the results suggesting that neutrons were separated from their spin (Denkmayr et al 2014 Nat. Commun. 5 4492 ). Here we show that these results can be interpreted as simple quantum interference, with no separation between the quantum particle and its internal degree of freedom. We thus hope to clarify the phenomenon with this work, by removing these apparent paradoxes.
Even the subtle and apparently strange quantum effects can sometimes survive otherwise lethal influence of an omnipresent decoherence. We show that an archetypal quantum Cheshire Cat, a paradox of a separation between a position of a quantum particle, a photon, and its internal property, the polarization, in a two-path Mach–Zehnder setting, is robust to decoherence caused by a bosonic infinite bath locally coupled to the polarization of a photon. Decoherence affects either the cat or its grin depending on which of the two paths is noisy. For a pure decoherence, in an absence of photon–environment energy exchange, we provide exact results for weak values of the photon position and polarization indicating that the information loss affects the quantum Cheshire Cat only qualitatively and the paradox survives. We show that it is also the case beyond the pure decoherence for a small rate of dissipation.
Abstract We analyse the quantum Cheshire cat using contextuality theory, to see if this can tell us anything about how best to interpret this paradox. We show that this scenario can be analysed using the relation between three different measurements, which seem to result in a logical contradiction. We discuss how this contextual behaviour links to weak values, and coherences between prohibited states. Rather than showing a property of the particle is disembodied, the quantum Cheshire cat instead demonstrates the effects of these coherences, which are typically found in pre- and post-selected systems.