Communication without particles in the transmission channel
In 1993 Elitzur and Vaidman introduced Interaction-Free Measurement, a quantum method for detecting an object without any particle being near it.
Penrose in his book Shadows of the Mind coined the word "counterfactual" for this idea and it led to numerous theoretical and experimental works. Recently, it was claimed that not only the presence of an object can be detected in a counterfactual way, but also its absence, thus allowing communication of bits 0 and 1 without particles in the transmission channel.
Prof. Lev Vaidman's recent paper shows how these protocols fail to be counterfactual for one of the bits. The failure is highly non-trivial: although none of the particles passes through the transmission channel, the particles leave a trace there and, therefore, are present in the channel. The presence of the particles in the channel has a natural explanation in the framework of the time-symmetric quantum mechanics pioneered by Prof. Yakir Aharonov and developed at Tel Aviv University.