Intensity and State Estimation in Quantum Cryptography

This paper describes how the communicating parties can employ intensity and state estimation to detect if the eavesdropper has siphoned off and injected photons in the received communication. This is of relevance in quantum cryptography based on random rotations of photon polarizations. Introduction The paper investigates the use of state and intensity measurements by Alice and Bob to detect the presence of the active eavesdropper and the use of mixed states rather than pure states in the transmission process. This use can be applied to variations of the three-stage quantum cryptography protocols based on random unitary transformations [1]-[6]. Apart from the faked-state attack [7], [8], the main weakness of BB84 protocol is that single photons are not easy to produce, and the duplicate photons can be used by the eavesdropper to reconstruct the key. The attacker can siphon off the photons when they are transferred between Alice and Bob. Moreover, as the photons are siphoned off only at one step, the intensity of the output at the receiver’s end is not affected. There is also the problem of generating single photons [9] as well as lack of availability of single photon detectors. The polarization rotation protocols are based on fundamental cryptographic primitives [10]. The protocol called iAQC [5] is a variant of three-stage in which Alice and Bob track the intensity of the laser beam at each stage making it possible to detect the intruder. But the drawback of iAQC lies in the scenario when Eve removes some photons and replaces them with the same number of photons with different polarization back into the stream where the intensity remains constant. Such siphoning can be done by fiber tapping [11] or using half silvered mirrors if the communication media is free space. In this case the intensity is not changed and hence Eve will not be discovered and the changes in the state will be ascribed to noise. In order to overcome this threat from Eve, an additional step of detecting the state of photons using tomography [12] is