Date Added: Oct 2011
Photonic quantum key distribution is commonly implemented using interferometers, devices that inherently cause the addition of vacuum ancillas, thus enlarging the quantum space in use. This enlargement sometimes exposes the implemented protocol to new kinds of attacks that have not yet been analyzed. The authors consider several quantum key distribution implementations that use interferometers, and analyze the enlargement of the quantum space caused by the interferometers. While, they prove that some interferometric implementations are robust (against simple attacks), they also show that several other implementations used in QKD experiments are totally insecure. This result is somewhat surprising since although they assume ideal devices and an underlying protocol which is proven secure (e.g., the Bennett-Brassard QKD), the realization is insecure.