Continuous Variable Quantum Key Distribution in presence of emulated atmospheric turbulence and with passive eavesdropping

Atmospheric turbulence has the potential to disrupt quantum communication. Therefore, it is important to research the impact of such turbulence on free-space quantum key distribution. In this experiment the correlations between the two users, Alice and Bob (ρA,B), and between Alice and an eavesdropper, Eve, (ρA,E) were compared for a satellite to ground continuous variable quantum key distribution system, emulated in the lab. Here the atmospheric turbulence was emulated using a deformable mirror. Bob is considered to be at the centre of the beam at a ground station with passive Eve being displaced from the centre of the beam at a another ground station. It was found that Alice and Bob had better correlation than Alice and Eve, with both ρA,B and ρA,E decreasing with increasing atmospheric turbulence and ρA,E decreasing the more Eve was displaced from the centre of the beam. The less correlated Alice and Eve are, the less information Eve has about the raw key. When Eve’s information about the raw key is below a certain threshold, Alice can increase the signal power. Therefore the channel distance and key rate can be increased.