Fig. 1: Schematic setup for our one cycle implementation of Salih et al.’s 2013 counterfactual communication protocol.

See ref. ¹. All beamsplitters are polarising beamsplitters (PBSs), transmitting H-polarised and reflecting V-polarised light. We want to know if photons detected at Alice’s D₀ have been to Bob on the right-hand side. We place detector D₀ at the bottom, rather than immediately after the topmost PBS, so that the setup exactly includes the equivalent of one cycle of Salih et al.’s Michelson-type protocol laid-out sequentially in time¹². This allows the conclusions drawn for this one cycle to be applicable to any of the concatenated cycles from the 2013 counterfactual communication protocol.