Counterfactual Quantum Key Distribution
The task of distributing a secret key could be achieved even when the particle (on which the secret information, e.g. polarization, has been encoded) does not traverse through the quantum channel. A protocol developed by Tae-Gon Noh. Here Alice generates a photon which randomly takes either path (a) or path (b). Path (a) stays inside Alice's secure device and path (b) goes to Bob. By rejecting the photons that Bob receives and only accepting the ones he doesn't receive, Bob & Alice can set up a secure channel, i.e. Eve's attempts to read the counterfactual photons would still be detected. This protocol uses the quantum phenomenon whereby the possibility that a photon can be sent has an effect even when it isn't sent. So-called Interaction-free measurement also uses this quantum effect, as for example in the bomb testing problem, whereby you can determine which bombs are not duds without setting them off, except in a counterfactual sense.
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