Generation of photon triplets via three-photon parametric down-conversion

光的非经典态在许多量子技术中是不可或缺的。目前可用的是按需单光子，压缩态和纠缠光子对。在之前的提议中，我们的目标是通过三阶自发参量下转换（TOSPDC）首次产生纠缠光子的三重态来扩展可用量子态的列表。这可以被看作是三次谐波产生（THG）的逆转，但它在实验上更具挑战性。虽然TOSPDC及其潜在用途在理论上已经得到了广泛的研究，但其在光学领域的实验实现仍然是一个挑战。在该项目的研究过程中，我们从理论和实验两个方面探索了TOPDC的各种可能来源，包括块体晶体、充气光子晶体光纤和特殊设计的混合光纤。我们对所有这些系统进行了THG，并确定了最合适的系统。对于块状晶体，我们还实施了籽晶TOPDC。结果表明，未接种的TOPDC太弱，在所有的研究系统中观察。与此同时，我们确定了两个有前途的新系统：高度非线性微层和锥形光纤嵌入气压细胞。对于这两个系统，我们取得了第一个结果：锥形光纤中的压力可调THG和微层中纠缠光子的产生。我们现在申请延长该项目的目标是在这些新系统中的非播种TOPDC。在锥形光纤中，我们还计划增加一个新的步骤，即在气体环境中使用碱蒸汽。这将使得能够对色散进行谐振控制，允许相位匹配仅融合最低阶模式并提高效率。相同的方法将允许在这样的系统中增强THG。最后，在扩展期间，我们计划使用受激发射断层扫描来研究TOPDC。

Nonclassical states of light are indispensable in many quantum technologies. Currently available are on-demand single photons, squeezed states, and pairs of entangled photons. In the previous proposal we aimed at expanding this list of available quantum states by generating, for the first time, triplets of entangled photons through third-order spontaneous parametric down conversion (TOSPDC). This can be viewed as the reversal of third harmonic generation (THG) but it is considerably more challenging experimentally. Although TOSPDC, as well as its potential uses, have been extensively studied theoretically, its experimental implementation in optics remains a challenge. It would enable heralded generation of photon pairs, direct generation a non-Gaussian states and substantially increase the toolbox of quantum information.During the work on the project, we explored, theoretically and experimentally, various potential sources of TOPDC, including bulk crystals, gas-filled photonic crystal fibres and specially designed hybrid fibre. We performed THG for all these systems and identified the most suitable ones. For bulk crystals we also implemented seeded TOPDC. The results showed that unseeded TOPDC is too weak to observe in all investigated systems. Meanwhile, we identified two promising new systems: highly nonlinear microlayers and tapered optical fibres embedded in gas-pressure cells. For both systems, we achieved first results: pressure-tunable THG in tapered fibers and generation of entangled photons in microlayers. We now apply for the extension of the project aiming at unseeded TOPDC in these new systems. In tapered fibres, we also plan an additional new step, namely the use of alkali-vapor in the gas environment. This will enable resonant control over dispersion, allow phase-matching fusing only the lowest-order modes and increase the efficiency. The same method will allow for enhancing THG in such systems. Finally, during the extension we plan to investigate TOPDC using stimulated emission tomography.