  A novel study on harmonic frequency combs emitted by Quantum Cascade Lasers
(QCLs) is here presented, demonstrating the presence of intensity correlations
between twin modes characterising the emission spectra. These originate from a
Four-Wave Mixing (FWM) process driven by the active medium's third-order
non-linearity. The study of such correlations is essential for the engineering
of a new generation of semiconductor devices with the potential of becoming
integrated emitters of light with quantum properties, such as squeezing and
entanglement. Starting from experimental results, the limits of
state-of-the-art technology are discussed as well as the possible methodologies
that could lead to the detection of non-classical phenomena, or alternatively
improve the design of QCLs, in the compelling perspective of generating quantum
correlations in mid-infrared light.
