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

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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.