This paper proposes a quantum secure communication protocol using multiple photons to represent each bit of a message to be shared. The multi-photon tolerant approach to quantum cryptography provides a quantum level security while using more than a single photon per transmission. The protocol proposed is a multi-stage protocol; an explanation of its operation and implementation are provided. The multi-stage protocol is based on the use of unitary transformations known only to Alice and Bob. This paper studies the security aspects of the multi-stage protocol by assessing its vulnerability to different attacks. It is well known that as the number of photons increases, the level of vulnerability of the multi-stage protocol increases. This paper sets a limit on the number of photons that can be used while keeping the multi-stage protocol a multi-photon tolerant quantum secure method for communication. The analysis of the number of photons to be used is based on the probability of success of a Helstrom discrimination done by an eavesdropper on the channel. Limiting the number of photons up to certain threshold per stage makes it impossible for an eavesdropper to decipher the message sent over the channel. The proposed protocol obviates the disadvantages associated with single photon implementations, such as limited data rates and distances along with the need to have no more than a single photon per time slot. The multi-stage protocol is a step toward direct quantum communication rather than quantum key distribution associated with single photon approaches.
This paper introduces an m-ary version of the Three-stage Quantum Cryptography protocol. The three-stage protocol was first proposed in 2006 and implemented in 2012. The m-ary version of the three-stage protocol proposed in this paper results in enhanced data transfer between a sender Alice and a receiver Bob since each pulse carries more than one bit of information. An experimental realization of the m-ary three-stage protocol is also reported in this paper. The implementation has used free-space optics as the transmission medium and passive optical components controlled through LabView. Furthermore, analytical results that address the impact of the noise factor and its trade-off with data rate are presented. This analysis includes a study of the probability of errors and channel capacity variations in terms of the noise variance factor for the m-ary three-stage protocol using two, four and eight levels. Limits within which the m-ary three-stage protocol can be used with higher performance efficiency compared to its original version counterpart are set.
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