Random numbers represent a fundamental ingredient for secure communications and numerical simulation as well as to games and in general to Information Science. Physical processes with intrinsic unpredictability may be exploited to generate genuine random numbers. The optical propagation in strong atmospheric turbulence is here taken to this purpose, by observing a laser beam after a 143 km free-space path. In addition, we developed an algorithm to extract the randomness of the beam images at the receiver without post-processing. The numbers passed very selective randomness tests for qualification as genuine random numbers. The extracting algorithm can be easily generalized to random images generated by different physical processes.
Exchanging unconditionally secure cryptographic keys by means of a free-space quantum channel is possible even under realistic conditions, that is, in the presence of environmental noise and with the transmission of a limited number of photons, as for Satellite Quantum Communications.
In their work, just published on Nature Communications, Davide Bacco, Matteo Canale, Nicola Laurenti, Giuseppe Vallone and Paolo Villoresi, all with the Department of Information Engineering at the University of Padova, have experimentally proved the feasibility of Quantum Key Distribution (QKD) in such conditions and by considering different attack models.
This result opens perspectives for scenarios where the transmission window is limited by physical constraints, as for Satellite Communications, where the passage of one terminal over the other is restricted to a few minutes.
We present an efficient method to control the spatial modes of entangled photons produced through SPDC process. Bi-photon beam propagation is controlled by a deformable mirror that shapes a 404nm CW diode laser pump interacting with a nonlinear BBO type-I crystal. Thanks to adaptive optical system, the propagation of 808nm SPDC light produced is optimized over a distance of 2m. The whole system optimization is carried out by a feedback between deformable mirror action and entangled photon coincidence counts.
The purpose of the work was to investigate the propagation of single photon beams over the inter-island link, and in particular between a transmitter on the roof of the Jacob Kapteyn Telescope at ORM, La Palma, and the ESA-Optical Ground Station at Izana, Tenerife as receiver. Indeed, the study of the free-space propagation of quantum correlations is necessary for any future application of quantum communication aiming to connect two remote locations. The problem related to the free-space propagation is represented by the atmospheric turbulence that acts as a temporal and spatial variation of the air refraction index. A turbulent channel acts as an increment of the losses on the transmitted photons due to beam wandering of the beam centroid or to scintillation, increasing the role of the noise.
