Editing Quantum key distribution (section)

==Counterfactual quantum key distribution==
{{See also|Counterfactual quantum computation}}
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 using a protocol developed by Tae-Gon Noh.<ref>{{cite journal | last=Noh | first=Tae-Gon | title=Counterfactual Quantum Cryptography | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=103 | issue=23 | date=2009-12-01 | issn=0031-9007 | doi=10.1103/physrevlett.103.230501 | pmid=20366133 | page=230501|arxiv=0809.3979| bibcode=2009PhRvL.103w0501N | s2cid=9804265 }}</ref> Here Alice generates a photon which, by not taking a measurement until later, exists in a superposition of being in paths (a) and (b) simultaneously. 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 is not sent. So-called [[interaction-free measurement]] also uses this quantum effect, as for example in the [[Elitzur-Vaidman bomb-testing problem|bomb testing problem]], whereby an experimenter can conceptually determine which bombs are not duds without setting them off, except in a [[counterfactual definiteness|counterfactual]] sense.