2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/92256The purpose of this short tutorial paper is to review various criteria that have been used to characterize the quantum character of correlations in optical systems, such as "gemellity", QND correlation, intrication, EPR correlation and Bell correlation, to discuss and compare them. This discussion, restricted to the case of measurements of continuous optical variables, includes also an extension of known criteria for "twin beams" to the case of imbalanced correlations.Submitted to the special issue of Laser Physics in honour of Prof. Herbert WaltherQuantum Physicstext