Quantum correlAtions in TerAhertz qcl COMBs (QATACOMB)

量子技术（QT）平台，能够利用原子，光和固态系统的非经典状态，最近已经在通信，计算，信息，传感和计量等各种战略领域实现。在电磁波谱的可见光和近红外部分的成功成就导致了最近在小型化和紧凑几何结构方面的进展。这反过来又使得一方面能够实现高度集成的量子平台，另一方面能够扩展到非传统的光谱区域，这些区域的特殊功能仍然没有得到充分利用。在这方面，QT迁移到太赫兹（THz）频率范围在技术上具有挑战性，尽管具有巨大的技术潜力。事实上，连续变量太赫兹纠缠态的制备可以成为未来实现量子计算协议、量子隐形传态或提高所选自由空间量子通信信道的容量、鲁棒性和安全性的基础。例如，太赫兹辐射的独特特征，透过其他不透明材料的透射性，或相对于瑞利散射的鲁棒性，可以潜在地允许大量的前沿应用，例如在不透明或恶劣环境（灰尘，烟雾，颗粒）中的量子安全快速数字数据传输，或者在光谱和太赫兹设置中的量子增强灵敏度。QATACOMB的目标是开发一个小型化的固态平台，用于太赫兹频率光的非经典压缩态的产生，检测和完整表征。这将利用太赫兹量子级联激光器（QCL）频率梳（FC）作为非线性源，与石墨烯纳米级量子传感器和腔耦合超快相干探测器相结合。QCL是迄今为止最有效的THz频率小型化激光器。他们的广泛的增益和受控的群速度色散最近使紧凑的FC发电，基于四波混频（FWM）过程中发生的增益介质。因此，QCL是产生多模压缩态光的理想候选者，由于量子相关的边带模式的存在。特别是，由双模压缩态提供的量子增强的灵敏度确实已经在可见/NIR光谱和计量设置中以及在引力波和目标检测的应用中得到了证明，从而激发了用于THz量子感测的类似方法，其中双模压缩态可以通过采用THz QCL谐波梳来实现。此外，通过太赫兹辐射控制分子中的旋转自由度可以为QT开发冷分子样品提供新的方法。该项目目标的成功实现将是颠覆性的，评估太赫兹光子学和QT战略领域的基础知识。

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.