Quantum correlAtions in TerAhertz qcl COMBs (QATACOMB)

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Quantum technology (QT) platforms, capable of exploiting non-classical states of atoms, light and solid-state systems, have been recently realised in a variety of strategic fields, such as communication, computation, information, sensing and metrology. Successful achievements in the visible and near-infrared parts of the electromagnetic spectrum have led to recent advancements in miniaturized and compact geometries. This, in turn, has enabled, on one hand, the implementation of highly-integrated quantum platforms and, on the other hand, the extension to non-conventional spectral regions, whose peculiar features are still underexploited. In this regard, QT migration to the terahertz (THz) frequency range is technologically challenging, although of huge technological potential. In fact, continuous-variable entangled THz states preparation can become the founding blocks for future implementation of quantum computation protocols, quantum teleportation or to increase capacity, robustness and security of selected free-space quantum communication channels. For example, the peculiar features of THz radiation, transmissivity through otherwise opaque materials, or robustness with respect to Rayleigh scattering, can potentially allow a plethora of frontier applications, such as quantum-secured fast digital data transfer in opaque or harsh environments (dust, smog, particulate) or quantum-enhanced sensitivity in spectroscopic and metrological THz setups. The goal of QATACOMB is to develop a miniaturized solid-state platform for generation, detection and complete characterization of non-classical squeezed states of THz frequency light. This will exploit THz quantum cascade laser (QCL) frequency combs (FCs) as nonlinear sources, coupled with graphene nanoscale quantum sensors and cavity-coupled ultrafast coherent detectors. QCLs are, to date, the most efficient miniaturized lasers at THz frequencies. Their broad gain and controlled group velocity dispersion has recently enabled compact FC generation, based on four-wave mixing (FWM) processes that take place within the gain medium. As a consequence, QCLs are ideal candidates for the generation of multi-mode squeezed states of light, due to the presence of quantum-correlated side-band modes. In particular, quantum-enhanced sensitivity, provided by two-mode squeezed states has been indeed demonstrated in visible/NIR spectroscopy and metrology setups, as well as in applications for gravitational waves and target detection, motivating a similar approach for THz quantum sensing, where two-mode squeezed states can be achieved by employing THz QCL harmonic combs. Moreover, control of rotational degrees of freedom in molecules by THz radiation can provide novel ways to exploit cold molecular samples for QT. The successful achievement of the project goals will be disruptive, assessing fundamental knowledge in the strategic fields of THz photonics and QT.