Armando Pinto

In prepare & measure continuous-variable quantum key distribution (CV-QKD) systems the quantum states generated by Alice and sent to Bob is typically performed using an optical fiber that connects both entities. However, the state of polarization (SOP) of the quantum signal transmitted in the optical fiber is inevitably disturbed due to random birefringence, impacting the overall secret key rate of the CV-QKD system. We study the secret key rate of a CV-QKD system considering the polarization drift occurring in the quantum channel. Channels with low polarization drift lightly impact the secret key rate of the system. This accounting for the SOP fluctuations. Moreover, we analyze the effect of the SOP fluctuations on the estimation of the channel transmission and of the excess noise when the polarization drift in the channel is disregarded. Such assumption results in a sub-estimation of the secret key rate, decreasing the performance of the system, when comparing with the theoretical value considering the polarization drift in the channel.

Entangled states are important for quantum cryptography, being used in several protocols of quantum key distribution. These provide an alternative to single-photon-based protocols. Entangled states are also important in fundamental terms, in the way that the behavior of these states is contradictory with the predicted by local theories. This contradiction is shown by the violation of Bell’s inequalities, as the inequality proposed by Clauser, Horne, Shimony and Holt (CHSH). In this work we shown the violation of CHSH inequality using two-particle states. We discussed pure states, an arbitrary state, and the Greenberger-Horne-Zeilinger (GHZ) state. Using the spontaneous four-wave mixing process we generated entangled photon pairs in a highly non-linear fiber, and experimentally verified the violation of CHSH inequality.