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npj Quantum Information

Counterfactual communication not achieved yet – A Comment on Salih et al. (2022)

Sandu Popescu

Published Jul 24, 2025• 1 citation
Publisher

Abstract

In this comment I argue that there are two major issues with the protocol presented in “The laws of physics do not prohibit counterfactual communication”, Hatim Salih, Will McCutcheon, Jonte R. Hance and John Rarity, npj Quantum Information (2022) 8:60. One is an erroneous interpretation of the role of clicks in detector D 0 and the second is the way in which post-selection is used. I argue that these invalidate the claims of that paper.

DOI
10.1038/s41534-025-00975-5

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Reply to “Comment to ‘The laws of physics do not prohibit counterfactual communication’ ”

Hatim Salih, Jonte R. Hance, Will McCutcheon +1 more

In his Comment on our recent paper “The laws of physics do not prohibit counterfactual communication”, npj Quantum Information (2022) 8:60, Popescu argues that the claims of the paper are invalid. Here, we refute his argument, showing that it is based on ignoring the specifics of what we set out to prove (that counterfactual communication is possible for post-selected particles, and more specifically in these cases is not prohibited by the weak trace or consistent histories criteria for particle path), followed by an unwarranted simplification of the protocol. Moreover, the Comment’s excursion into interpretation is misplaced. Our communication protocol is a precisely defined one that allows two remote parties, albeit rarely, to communicate an arbitrarily long binary message, with arbitrarily high accuracy. This is not a matter of interpretation—as the concrete example given in our paper in question illustrates. As for our overarching claim that no particles are exchanged in the course of this communication, we have already demonstrated this both theoretically and experimentally, in the postselected case we consider, as per the weak trace and consistent histories criteria for path of a quantum particle.

arXiv:2507.19563•npj Quantum Information•Jun 10, 2025•2 citations

Comment on “Protocol for Direct Counterfactual Quantum Communication”

Lev Vaidman A

I argue that recent "Protocol for Direct Counterfactual Quantum Communication" by Salih et al. [PRL 110, 170502 (2013), arXiv:1206.2042] is counterfactual only for one type of the information bit.

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In a recent paper, Griffiths [Phys. Rev. A 107, 062219 (2023)] analyzed a protocol for transmission of information between two parties introduced by Salih et al. [Phys. Rev. Lett. 110, 170502 (2013)]. There is a considerable controversy about the counterfactuality of this protocol, and Griffiths suggested to resolve it by introducing a new measure of channel usage, which he called ``Cost.'' I argue that this measure is not appropriate because the original interaction-free measurement protocol which triggered the definition of the concept of counterfactuality is not counterfactual according to this measure.

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Reply to “Comment to ‘The laws of physics do not prohibit counterfactual communication’ ”

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•Jan 1, 2025•0 citations

Reply to "Comment to 'The laws of physics do not prohibit counterfactual communication'"

H Salih, Jonte R. Hance, W McCutcheon +1 more

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Reply to "Comment on Protocol for Direct Counterfactual Quantum Communication"

Hatim Salih, Li, Zheng-Hong, M. Al-Amri +1 more

In reply to Vaidman's Comment [arXiv:1304.6689], we show that his claim that our Protocol for Direct Counterfactual Quantum Communication [PRL 110, 170502 (2013), arXiv:1206.2042] is counterfactual only for one type of information bit is wrong.

arXiv:1404.5392•Apr 22, 2014•4 citations
In this comment I argue that there are two major issues with the protocol presented in “The laws of physics do not prohibit counterfactual communication”, Hatim Salih, Will McCutcheon, Jonte R. Hance and John Rarity, npj Quantum Information (2022) 8:60. One is an erroneous interpretation of the role of clicks in detector D 0 and the second is the way in which post-selection is used. I argue that these invalidate the claims of that paper.
In his Comment on our recent paper “The laws of physics do not prohibit counterfactual communication”, npj Quantum Information (2022) 8:60, Popescu argues that the claims of the paper are invalid. Here, we refute his argument, showing that it is based on ignoring the specifics of what we set out to prove (that counterfactual communication is possible for post-selected particles, and more specifically in these cases is not prohibited by the weak trace or consistent histories criteria for particle path), followed by an unwarranted simplification of the protocol. Moreover, the Comment’s excursion into interpretation is misplaced. Our communication protocol is a precisely defined one that allows two remote parties, albeit rarely, to communicate an arbitrarily long binary message, with arbitrarily high accuracy. This is not a matter of interpretation—as the concrete example given in our paper in question illustrates. As for our overarching claim that no particles are exchanged in the course of this communication, we have already demonstrated this both theoretically and experimentally, in the postselected case we consider, as per the weak trace and consistent histories criteria for path of a quantum particle.
In a recent paper, Griffiths [Phys. Rev. A 107, 062219 (2023)] analyzed a protocol for transmission of information between two parties introduced by Salih et al. [Phys. Rev. Lett. 110, 170502 (2013)]. There is a considerable controversy about the counterfactuality of this protocol, and Griffiths suggested to resolve it by introducing a new measure of channel usage, which he called ``Cost.'' I argue that this measure is not appropriate because the original interaction-free measurement protocol which triggered the definition of the concept of counterfactuality is not counterfactual according to this measure.