iAQC: The Intensity-Aware Quantum Cryptography Protocol

This paper reports a variant of the three-stage quantum cryptography protocol which can be used in low intensity laser output regimes. The variant, which tracks the intensity of the laser beam at the intermediate stages, makes the task of the eavesdropper harder than the standard K06 protocol. The constraints on the iAQC protocol are much less than those on BB84 and in principle it can not only be used for key distribution but also for direct bitwise encryption of data. The iAQC protocol is an improvement on the K06 protocol in that it makes it harder for the eavesdropper to monitor the channel. Introduction We present here a variant of the three-stage quantum cryptography protocol [1], sometimes called the K06 protocol, which has protection against eavesdropping built into it. The 3-stage quantum cryptography protocol is based on random rotations (or other commutative operators) which can better protect duplicate copies of the photons than in non-single qubit transmissions of BB84. This protocol can use attenuated pulse lasers rather than single-photon sources in the quantum key exchange, which makes it possible to transmit the pulses over a greater distance. The theoretical basis of the K06 protocol is the fact that unknown pure states carry information [2],[3],[4] even though the von Neumann entropy of such states equals zero. In this new protocol, which we call iAQC protocol, both Alice and Bob monitor the intensity of the light beam coming to them in the intermediate stages of the protocol. Doing so makes it possible for Alice and Bob to determine if any photons have been siphoned off by the eavesdropper. This tracking can be done by measuring a pre-set fraction of the incoming beam. This makes the task of Eve, the eavesdropper, harder than it would be without the intensity awareness feature. If intensity is not tracked then iAQC is identical to K06.