Robustness of Quantum Cryptography Systems with Phase–Time Coding against Active Probing Attacks

Abstract

The robustness of a quantum cryptography system is considered that uses a protocol with phase–time coding on attenuated coherent states. This protocol admits an effective fiber optic implementation, which does not require a phase modulator on the receiver side and adjusting the polarization state at the input of the receiver side. The absence of a phase modulator on the receiver side excludes a side channel of information leakage associated with the active probing of the phase modulator at the receiving station, thus making the system more robust against such attacks compared to other systems. The nonstrict single-photon nature of information states, as well as information leakage through side channels, is considered by the generalized decoy state method, which takes into account joint collective measurements of information quantum states and quantum states in side channels. An estimate for the secret key length is obtained that is expressed only in terms of observed quantities at the receiving station and the parameters of quantum states in side channels.