Multi-mode twin-beam states in the mesoscopic intensity domain

•This review paper is on the characterization of twin-beam states in the mesoscopic intensity regime.•Multi-mode twin-beam states are entangled in the number of photons.•We show that mesoscopic twin-beam states are robust against noise sources and losses.•Mesoscopic twin-beam states are useful for the implementation of Quantum Technologies. Parametric down conversion is a quantum nonlinear process of paramount importance in the Quantum Optics context since it produces entangled states of light. Such states, usually called twin-beam (TWB) states, exhibit perfect correlations in many degrees of freedom, among which the number of photons. In this review paper, we focus on their generation and characterization in the rather unusual mesoscopic intensity regime. The use of photon-number-resolving detectors allows the experimental reconstruction of the statistical properties of TWB states and the demonstration of their nonclassical nature. This characterization is preliminary and necessary in view of the application of TWB states to the development of Quantum Technologies. In this work we focus on the robustness of mesoscopic TWB states against losses and noise sources for their exploitation in the field of Quantum Communication.