"Counterfactual" quantum protocols

Abstract

The counterfactuality of recently proposed protocols is analyzed. A definition of `counterfactuality' is offered and it is argued that an interaction-free measurement of the presence of an opaque object can be named `counterfactual', while proposed "counterfactual" measurements of the absence of such objects are not counterfactual. The quantum key distribution protocols which rely only on measurements of the presence of the object are counterfactual, but quantum direct communication protocols are not. Therefore, the name `counterfactual' is not appropriate for recent "counterfactual" protocols which transfer quantum states by quantum direct communication.