Quantum nonlinear optics with 2D materials

当两束来自手电筒的光束交叉时，它们并不像《星星战争》中的军刀那样发生冲突，而是简单地继续各自的方式。这是因为自由光子不相互作用。然而，当放置在适当的介质中时，光子可以有效地感觉到彼此的存在，使得光学系统的响应取决于光子的数量。在这种情况下，我们说它具有由介质提供的光学非线性。通常，体积越大，非线性越强，目标是在尽可能小的尺度上实现突出的非线性。非线性与“军刀效应”一起，最终可以提供对光子量子态的有效操纵。这样就可以制备出具有高非线性的单光子态并用于量子信息处理。这将导致超快量子计算和通信平台，作为量子应用的基础，包括安全的通信网络，增加计算能力和传感器在没有量子技术的情况下不可能达到的水平。当光被限制在光学腔（例如，由两个镜子设置）中时，它与介质的相互作用将大大增强。如果光子往返的平均次数变大，它们就可以与介质中的激发杂交，导致半光半物质准粒子--极化激元。杂化使得受限光和所产生的极化激元能够相互作用。这种能力落后于经典非线性光学的许多应用，包括用于快速宽带通信的光孤子。然而，寻找一个最佳系统的任务，其中大的非线性极化激元在几个量子的限制，仍然是一个悬而未决的问题。在该项目中，我将发现如何提高光学非线性在微小的规模。通过研究二维（2D）材料中的强光-物质耦合，这将成为可能，其中单层厚度可以小于纳米。考虑到几层的组合，我将证明极化激元的非线性响应可以提升到单光子过程变得可观察的水平。因此，这项研究将使这些易于生产的量子光学处理微型系统能够作为负担得起的量子技术的平台。

When two beams from light torches cross, they do not clash like sabres from "Star Wars", but simply continue each its own way. This follows from the fact that free photons do not interact. However, when placed in an appropriate medium, photons can effectively feel the presence of each other, making the response of the optical system dependent on the number of photons. In this case, we say it has an optical nonlinearity provided by the medium. Typically the larger the volume, the stronger the nonlinearity, and the goal is to achieve prominent nonlinearity at the smallest possible scale. Together with the "sabre-effect", nonlinearity can ultimately provide the efficient manipulation of quantum states for photons. Thus with high level of nonlinearity single photon states can be prepared and used in quantum information processing. This would result in ultrafast quantum computing and communication platforms, serving as the basis for quantum applications that include secure communication networks, increased computational power and sensing at a level impossible to reach without quantum technologies.When light is confined in an optical cavity (for instance, set by two mirrors), its interaction with the medium is greatly enhanced. If the average number of roundtrips made by photons becomes large, they can hybridize with excitations in the medium, leading to half-light half-matter quasiparticles - polaritons. The hybridization makes confined light and the resulting polaritons able to interact. This ability stays behind the progress in numerous applications of classical nonlinear optics, including optical solitons for fast broadband communication. However, the task of finding an optimal system, where large nonlinearity for polaritons is achieved in the limit of few quanta, remains an open question.In the project, I will discover ways to increase optical nonlinearity at the minuscule scale. This will become possible by studying strong light-matter coupling in two-dimensional (2D) materials, where monolayer thickness can be smaller than a nanometer. Considering combinations of a few layers, I will show that the nonlinear response for polaritons can be elevated to the level where single photon processes become observable. The research will thus enable these easy-to-produce miniature systems for quantum optical processing to function as a platform for affordable quantum technologies.