Putting Chaos to Work: Multi-Photon Entanglement in Complex Scattering Media

长期以来，纠缠一直被认为是量子物理学的一大谜团，它激发了科学家和哲学家的想象力。纠缠的量子粒子以一种完全相关的方式表现，无论它们相距多远！最初研究的对象是被称为光子的光的极化粒子，如今，缠结已经在单个原子、超导电路甚至小钻石上得到了证明。虽然纠缠确实告诉了我们一些奇怪的、违反直觉的自然行为方式，但它最近成为现代量子技术的基石，这些技术承诺了牢不可破的加密、超敏感的成像和前所未闻的计算能力。从复杂的量子模拟器到大规模的量子加密网络，纠缠光子将在几乎所有基于量子物理的未来技术中发挥作用。在量子光学中，产生许多光子的纠缠态仍然是一项极其艰巨的任务。为了创造最简单的这种状态，多对纠缠光子通过一系列光学元件（如镜子和分束器）复杂地结合在一起。随着这些状态的复杂性增加，创建它们所需的元素网络也变得相当大。这种网络中的每个元素都必须精确控制，这对量子物理学家来说是一个艰巨的挑战。当一束光照射到一层油漆或一块方糖上时，它会经历数百万次反射和变换。这些日常物品与复杂的光学元件网络惊人地相似，不同之处在于通过它们传输的任何信息通常都会丢失。然而，近年来，塑造光的波前技术的进步，结合快速计算算法，已经导致了前所未有的控制光如何通过这种无序介质传播。利用这些技术，科学家们已经取得了非凡的成就，比如通过一根头发厚度的光纤传输完整的图像！在这个项目中，我提议利用无序介质作为微型“量子光学实验室”的潜力来产生、操纵和传输大型、复杂的纠缠态光。通过仔细控制进入这种介质的光子的量子态，通常会扰乱量子信息的数百万次散射事件可以用来操纵它！通过这种方式，可以使复杂散射介质具有与大型量子光学元件网络相同的功能，同时克服了通常困扰此类网络的控制和可扩展性问题。在我的研究中，我将专注于一种称为多模光纤（MMF）的特定类型的散射介质，它通常用于高速互联网连接。mmf具有某些独特的优势。首先，它们便宜、小巧、易得。其次，由于它们在光通信中的天然应用，它们不仅可以用于产生纠缠，还可以用于传输纠缠。这将使我能够极大地扩展现代量子密码系统的信息容量，并开发生物组织深处超敏感量子成像的实用技术！最后，大的、多光子纠缠态的产生将帮助我进一步突破量子力学的极限，并更好地理解纠缠的复杂关联之舞。

Long considered one of the big mysteries of quantum physics, entanglement has captured the imaginations of scientists and philosophers alike. Entangled quantum particles behave in a perfectly correlated way, regardless of how far apart they may be! First studied with polarised particles of light called photons, entanglement has today been been demonstrated with individual atoms, superconducting circuits, and even small diamonds. While entanglement certainly tells us something about the strange, counterintuitive ways in which nature behaves, it has recently emerged as a cornerstone of modern quantum technologies that promise unbreakable encryption, ultra-sensitive imaging, and yet-unheard-of computing power. From complex quantum simulators to large-scale quantum cryptographic networks, entangled photons will play a role in almost every future technology based on quantum physics.Generating an entangled state of many photons remains an extremely demanding task in quantum optics. To create even the simplest such states, multiple pairs of entangled photons are intricately combined via a series of optical elements such as mirrors and beam splitters. As the complexity of these states is increased, the network of elements required to create them also becomes rather large. Each element in such a network must also be precisely controlled, which presents a difficult challenge to quantum physicists.When a beam of light impinges on a layer of paint or a sugar cube, it undergoes millions of reflections and transformations. Such everyday objects are surprisingly analogous to a complex network of optical elements, with the difference being that any information transmitted through them is usually lost. In recent years however, advances in technology for shaping the wavefront of light, combined with fast computational algorithms have resulted in unprecedented control over how light propagates through such disordered media. Using these techniques, scientists have achieved remarkable feats such as sending an entire image down an optical fibre the thickness of a human hair!In this project, I am proposing to harness the potential of disordered media as miniature "quantum optics laboratories" for generating, manipulating, and transporting large, complex entangled states of light. By carefully controlling the quantum states of photons entering such media, the millions of scattering events that would normally scramble their quantum information can be put to work for manipulating it instead! In this manner, complex scattering media can be made to serve the same function as large networks of quantum optical elements, while overcoming the problem of control and scalability that normally plague such networks.In my research, I will focus on a specific type of scattering medium called a multi-mode fibre (MMF), which is commonly used in high-speed internet connections. MMFs have certain unique advantages. First, they are cheap, compact, and readily available. Second, due to their natural application in optical communications, they can be used not only for creating entanglement, but also for transporting it. This will allow me to vastly expand the information capacity of modern quantum cryptographic systems and develop practical techniques for supersensitive quantum imaging deep inside biological tissue! Finally, the generation of large, multi-photon entangled states will help me further push the limits of quantum mechanics and gain a better understanding of the complex dance of correlations that is entanglement.