Prototype of a Free Space Optical Link for Quantum Communications

Abstract

This Master’s thesis presents the development of a free-space optical link for quantum communications, followed by the design and characterization of the single-photon source used to power it. A laboratory-scale link was established using Ritchey–Chrétien telescopes. To overcome the central obstruction of the telescopes, two different methods were tested: the use of axicon lenses to shape the beam into a toroidal beam with a Bessel-Gaussian profile and the lateral displacement of the input beam. The latter enabled successful photon transmission, revealing a bunching measurement in the second order cross-correlation function at the expected arrival time difference between the heralding photons and the signal ones measured at the receiver of 44 coincident counts per minute. To provide the required quantum light source, an entangled-photon source based on spontaneous parametric down-conversion in beta barium borate crystals was developed. Pairs of polarization-entangled photons at 810 nm were produced and verified using a CHSH inequality test, which resulted in a Bell parameter of S = 2.55 ± 0.15, which confirms the presence of quantum entanglement