The 'quantum counterfactuality' is one of the most striking counterintuitive effects predicted by quantum mechanics. This paper shows that the counterfactual effect is not merely an interesting academic theme, but that it can also provide practical benefits in everyday life. Based on the quantum counterfactual effect, the task of a secret key distribution between two remote parties can be accomplished even when no particle carrying secret information is in fact transmitted. The secret key obtained in this way may be used for secure communications such as internet banking and military communications. This paper also shows that, in some cases, the mere possibility that an eavesdropper can commit a crime is sufficient to detect the eavesdropper, even though the crime is not in fact carried out. (It is still unclear whether the mere possibility that the eavesdropper can commit a crime is sufficient to punish him/her, even though he or she does not in fact do it!) In a sense, part of the story of the SF film Minority Report seems plausible. Quantum cryptography allows one to distribute a secret key between two remote parties using the fundamental principles of quantum mechanics. The well-known established paradigm for the quantum key distribution relies on the actual transmission of signal particle through a quantum channel. This paper shows that the task of a secret key distribution can be accomplished even though a particle carrying secret information is not in fact transmitted through the quantum channel. The proposed protocols can be implemented with current technologies and provide practical security advantages by eliminating the possibility that an eavesdropper can directly access the entire quantum system of each signal particle.

We report on single-photon interference experiments in a Michelson-type interferometer built with two 6-km-long fiber spools, as well as on the active stabilization of the interferometer. A weak coherent light signal was (de-) multiplexed with a strong reference light using wavelength-division multiplexing technique, and real-time feedback control technique was applied for the reference light to actively stabilize the phase fluctuation in the long-armed fiber interferometer. The stabilized interferometer showed phase stability of 0.06 rad, which corresponds to an optical path length fluctuation of 15 nm between the 6-km-long interfering arms. The raw visibility obtained without subtracting noise counts in the single-photon interference experiment was more than 98% for stabilized conditions.

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