Generation of quasi-single-mode twin-beam states in the high-intensity domain

Single-mode quantum optical states represent an interesting resource for the implementation of quantum information protocols. Here, we address the generation of twin-beam (TWB) states characterized by a quasi-single-mode thermal statistics. In more detail, the selection procedure leading to single-mode states is achieved with a strong filtering applied to intense TWBs generated in a single crystal by parametric downconversion. We investigate the role of pump power in the production of the single-mode states and compare the results obtained with nonlinear crystals having different lengths.