Local discrimination of orbital angular momentum in entangled states

In this paper, we address the use of a calcite crystal-based local detector for the discrimination of the orbital angular momentum of quantum radiation produced by parametric down-conversion, using only a portion of the beam. Specifically, we propose and experimentally demonstrate that discrimination can be achieved by exploiting the introduction of a precise and controlled spatial shift between two replicas of the state within the crystals. This approach utilizes a robust and intrinsically stable monolithic configuration, obviating the need for feedback mechanisms or thermal drift compensation. Our method offers a promising avenue for enhancing the reliability and efficiency of quantum communication systems, and we believe that this technology could significantly advance the development of quantum communication techniques, where information encoding is based on orbital angular momentum, or spatially distributed orbital angular momentum detection.