Telling a smile from a veil: Weak values, Cheshire cats, and counterfactual communication

Quantum communication employs quantum sources of light and therefore rises interesting possibilities for improved security in various communication protocols. One of the most mysterious schemes for quantum communication is called "counterfactual communication". It allegedly allows several parties to communicate without the existence of information carriers (such as photons) in the transmission channel. We have recently analyzed these scenarios and quantified the traces left by the photons in various schemes - not all were shown to fulfill their goal, just one [1]. Moreover, we have shown that in some cases it is true that the particle as a whole is not transmitted, but its properties may leave it and travel in space [2]. This is called the "Quantum Cheshire cat" paradox [3]. I will discuss all the above using the framework of weak measurements [4], which allow to probe quantum systems without necessarily collapsing them.


[1] Wander A., Cohen E., Vaidman L., "Three approaches for analyzing the counterfactuality of counterfactual protocols", Phys. Rev. A 104, 012610 (2021).
[2] Aharonov Y., Cohen E., Popescu S., "A Dynamical Quantum Cheshire Cat effect and Implications for Counterfactual Communication", Nat. Commun. 12, 4770 (2021).
[3] Aharonov Y., Cohen E., “Weak Values and Quantum Nonlocality”, in Quantum Nonlocality and Reality: 50 Years of Bell's Theorem, Mary Bell and Shan Gao (Eds.), Cambridge University Press (2016), 305-313, arXiv:1504.03797.
[4] Tamir B., Cohen E., “Introduction to Weak Measurements and Weak Values”, Quanta 2, 7-17 (2013).